receptors

in

cardiovascular

regulation

and

disease

The

role

of

ATP

as

an

extracellular

signalling

molecule

is

now

well

established

and

evidence

is

accumulating

that

ATP

and

other

nucleotides

-LRB-

ADP

UTP

and

UDP

-RRB-

play

important

roles

in

cardiovascular

physiology

and

pathophysiology

acting

via

-LRB-

ion

channel

-RRB-

and

-LRB-

G

protein-coupled

-RRB-

receptors

In

this

article

we

consider

the

dual

role

of

ATP

in

regulation

of

vascular

tone

released

as

a

cotransmitter

from

sympathetic

nerves

or

released

in

the

vascular

lumen

in

response

to

changes

in

blood

flow

and

hypoxia

Further

purinergic

long-term

trophic

and

inflammatory

signalling

is

described

in

cell

proliferation

differentiation

migration

and

death

in

angiogenesis

vascular

remodelling

restenosis

and

atherosclerosis

The

effects

on

haemostasis

and

cardiac

regulation

is

reviewed

The

involvement

of

ATP

in

vascular

diseases

such

as

thrombosis

hypertension

and

diabetes

will

also

be

discussed

as

well

as

various

heart

conditions

The

purinergic

system

may

be

of

similar

importance

as

the

sympathetic

and

renin-angiotensin-aldosterone

systems

in

cardiovascular

regulation

and

pathophysiology

The

extracellular

nucleotides

and

their

cardiovascular

receptors

are

now

entering

the

phase

of

clinical

development

Introduction

Ever

since

the

first

proposition

of

cell

surface

receptors

for

nucleotides

-LSB-

-RSB-

it

has

become

increasingly

clear

that

in

addition

to

functioning

as

an

intracellular

energy

source

the

purines

and

pyrimidines

ATP

adenosine

diphosphate

-LRB-

ADP

-RRB-

uridine

triphosphate

-LRB-

UTP

-RRB-

and

uridine

diphosphate

-LRB-

UDP

-RRB-

can

serve

as

important

extracellular

signalling

molecules

-LSB-

-RSB-

acting

on

homo

and

heteromultimer

ionotropic

and

metabotropic

receptor

subtypes

-LSB-

-RSB-

-LRB-

Table

-RRB-

To

terminate

signalling

ectonucleotidases

are

present

in

the

circulation

and

on

cell

surfaces

rapidly

degrading

extracellular

ATP

into

ADP

AMP

and

adenosine

-LSB-

-RSB-

Evidence

is

accumulating

suggesting

an

important

role

for

the

purinergic

system

in

cardiovascular

regulation

-LSB-

-RSB-

It

stimulates

vasoconstriction

and

vasodilatation

growth

of

vascular

smooth

muscle

cells

and

endothelial

cells

angiogenesis

is

involved

in

vascular

remodelling

stimulates

platelet

aggregation

regulates

coagulation

inflammation

and

several

aspects

of

cardiac

function

It

is

involved

in

blood

pressure

regulation

development

of

myocardial

infarction

heart

failure

and

xenograft

rejection

The

physiological

effects

of

the

purinergic

signalling

system

are

dependent

on

the

release

of

extracellular

nucleotides

the

degradation

by

ectonucleotides

the

type

of

receptors

expressed

on

the

cells

their

desensitisation

rates

and

their

second

messengers

receptors

have

highly

specific

organ

distributions

and

they

can

be

rapidly

up

or

downregulated

The

purinergic

system

may

be

of

similar

importance

as

the

sympathetic

and

renin-angiotensin-aldosterone

systems

in

cardiovascular

regulation

and

pathophysiology

The

involvement

of

purinergic

signalling

in

a

variety

of

different

cardiovascular

clinical

conditions

has

been

addressed

to

a

limited

extent

previously

-LSB-

-RSB-

and

the

large

number

of

new

discoveries

calls

for

a

review

to

summarise

the

most

important

roles

of

the

purinergic

system

in

cardiovascular

regulation

and

disease

Table

classification

intracellular

signalling

ligands

and

selective

agonists

and

subtypeI.c

signallingLigandSelective

agonistSelective

antagonistNon-selective

-LRB-

ATP

-RRB-

UTPγS

-LRB-

ATP

in

rodents

-RRB-

UTPγS

cAMPADP

Clopidogrel

prasugrel

-LRB-

cangrelor

-RRB-

cAMPADP

UDP-galactoseUDP-glucose

UDP-galactose

ion

ion

channelATP

isoPPADS

ion

ion

channelATPIvermectin

potentiates

ion

channelATP

Suramin

ion

channelATP

ion

channelATP

brilliant

blue

GEctonucleotidaseApyrase

human

The

research

field

has

grown

rapidly

since

the

term

receptor

was

coined

-LSB-

-RSB-

In

preparation

of

this

review

more

than

references

on

receptors

and

cardiovascular

regulation

were

found

in

PubMed

Among

the

receptor

subtypes

one

is

the

target

for

one

of

the

most

widely

used

medical

drugs

the

platelet

inhibitor

clopidogrel

-LRB-

Plavix

-RRB-

As

will

be

reviewed

here

several

of

the

other

receptors

are

promising

drug

targets

to

prevent

cardiovascular

disease

Regulation

of

vascular

tone

Vasoconstriction

produced

by

ATP

released

as

a

cotransmitter

with

noradrenaline

-LRB-

-RRB-

from

perivascular

sympathetic

nerves

was

recognised

early

-LSB-

-RSB-

However

following

the

seminal

discovery

of

endothelium-dependent

vasodilatation

by

Furchgott

in

the

early

it

was

shown

that

ATP

acted

on

endothelial

cells

to

release

endothelial

derived

relaxing

factor

-LSB-

later

shown

to

be

largely

nitric

oxide

-LRB-

NO

-RRB-

-RSB-

resulting

in

vasodilatation

-LSB-

-RSB-

and

dual

purinergic

neural

and

endothelial

control

of

vascular

tone

established

-LSB-

-RSB-

Neuronal

regulation

of

vascular

tone

ATP

is

together

with

and

neuropeptide

Y

-LRB-

NPY

-RRB-

a

cotransmitter

in

sympathetic

neurons

-LSB-

-RSB-

and

sensory-motor

nerves

during

axon

reflex

activity

release

ATP

to

dilate

or

constrict

vessels

-LSB-

-RSB-

-LRB-

Fig.

-RRB-

The

contribution

of

ATP

to

sympathetic

contraction

varies

between

vascular

beds

-LSB-

-RSB-

For

example

it

may

mediate

up

to

of

the

neurogenic

vasoconstriction

via

receptors

as

seen

in

mesenteric

arteries

-LSB-

-RSB-

about

in

rabbit

saphenous

arteries

and

in

rabbit

jejunal

artery

while

acts

as

a

prejunctional

modulator

-LSB-

-RSB-

In

the

renal

vasculature

both

and

receptors

are

important

for

neurogenic

contraction

receptors

mediate

contraction

in

afferent

but

are

absent

in

the

efferent

renal

arterioles

-LSB-

-RSB-

The

result

is

that

extracellular

nucleotides

selectively

influence

preglomerular

resistance

without

having

an

effect

on

postglomerular

tone

ATP

is

also

released

locally

in

the

kidney

in

response

to

increased

perfusion

pressure

and

in

receptor

knockout

mice

auto-regulatory

responses

in

kidney

afferent

arterioles

are

abolished

indicating

an

important

role

in

renal

glomerular

pressure

regulation

-LSB-

-RSB-

Fig.

receptor-mediated

regulation

of

the

circulation

See

text

for

details

Purines

and

pyrimidines

are

released

on

the

luminal

side

from

endothelial

cells

platelets

and

red

blood

cells

-LRB-

RBC

-RRB-

in

response

to

hypoxia

acidosis

adrenaline

shear

stress

and

other

stimuli

When

the

endothelial

cell

layer

is

intact

the

response

is

vasodilatation

by

the

endothelial

release

of

nitric

oxide

-LRB-

NO

-RRB-

endothelium-derived

hyperpolarising

factor

-LRB-

EDHF

-RRB-

and

prostaglandins

-LRB-

Pgl

-RRB-

When

the

endothelium

is

damaged

platelets

accumulate

release

ATP

and

ADP

and

mediate

vasoconstriction

via

receptors

on

the

vascular

smooth

muscle

cell

-LRB-

VSMC

-RRB-

On

the

adventitial

side

sympathetic

and

sensory

nerves

mediate

vasoconstriction

Extracellular

nucleotides

are

rapidly

degraded

by

on

endothelium

reducing

ATP

to

AMP

followed

by

conversion

to

adenosine

by

-LRB-

not

shown

-RRB-

In

the

subendothelium

is

present

degrading

ATP

to

ADP

maintaining

the

platelet

activating

and

contractile

effects

Ado

adenosine

CGRP

calcitonin

gene-related

peptide

SP

substance

P

Sympathetic

nerves

express

inhibitory

and

-LRB-

-RRB-

receptors

indicating

a

receptor-mediated

negative

feedback

loop

both

directly

by

ATP

and

its

degradation

product

adenosine

-LSB-

-RSB-

Ectonucleotidases

are

released

from

sympathetic

nerves

together

with

its

substrate

ATP

as

a

termination

mechanism

for

the

signalling

-LSB-

-RSB-

Another

balancing

mechanism

is

the

sympathicolytic

effect

seen

in

exercising

skeletal

muscle

which

is

important

for

increased

blood

flow

in

the

working

skeletal

muscle

during

exercise

despite

sympathetic

release

that

normally

would

reduce

blood

flow

This

sympathicolytic

effect

is

mimicked

by

injection

in

the

arterial

lumen

of

ATP

and

UTP

inducing

a

vasodilatation

that

overrides

sympathetic

vasoconstrictor

activity

in

human

skeletal

muscle

an

effect

not

obtained

by

injection

of

adenosine

-LSB-

-RSB-

suggesting

that

ATP

and

UTP

may

mediate

this

important

physiological

mechanism

The

first

evidence

of

the

presence

of

receptor

subtypes

suggested

receptors

on

vascular

smooth

muscle

cells

-LRB-

VSMC

-RRB-

and

receptors

on

endothelial

cells

-LSB-

-RSB-

Rat

blood

vessel

immunohistochemistry

and

human

mRNA

quantification

have

shown

the

receptor

to

be

the

highest

expressed

subtype

in

smooth

muscle

cells

-LSB-

-RSB-

It

has

been

difficult

to

prove

the

importance

of

other

contractile

receptors

besides

due

to

the

paucity

of

specific

agonists

and

antagonists

However

contractile

effects

of

UTP

suggested

that

receptors

were

present

on

VSMCs

and

mediated

contraction

-LSB-

-RSB-

Using

selective

pyrimidines

resistant

to

degradation

it

has

been

possible

to

show

that

contractile

effects

are

mediated

by

both

UTP-sensitive

and

UDP-sensitive

receptors

-LSB-

-RSB-

Ectonucleotidases

in

the

vessel

wall

rapidly

degrade

nucleotides

and

markedly

reduce

their

contractile

effects

Pyrimidine

analogues

more

resistant

to

ectonucleotidases

are

powerful

vasoconstrictors

and

up

to

more

potent

than

the

endogenous

ligands

-LSB-

-RSB-

Similarly

a

prominent

more

potent

UDP

contraction

has

been

seen

in

knockout

mice

demonstrating

the

protective

effect

of

ectonucleotidases

against

nucleotide

contraction

-LSB-

-RSB-

In

human

saphenous

vein

grafts

used

during

coronary

bypass

a

stable

UDP

analogue

stimulates

a

strong

contraction

lasting

for

hours

which

is

not

desensitised

-LSB-

-RSB-

The

reason

why

the

receptor

is

not

desensitised

is

the

lack

of

serine

residues

in

the

C-terminal

part

of

the

receptor

-LSB-

-RSB-

Neither

nor

receptors

are

present

in

human

coronary

arteries

possibly

to

avoid

deleterious

vasospasm

during

ischaemia

-LSB-

-RSB-

VSMC

express

receptors

that

mediate

contraction

after

stimulation

with

ADP

-LSB-

-RSB-

At

the

mRNA

level

the

receptor

is

the

highest

expressed

ADP

receptor

and

the

second

highest

expressed

receptor

in

human

VSMC

-LSB-

-RSB-

The

contractions

are

not

inhibited

in

patients

medicated

with

clopidogrel

However

drugs

with

antagonistic

effects

on

receptors

that

reach

the

peripheral

circulation

-LRB-

and

-RRB-

affecting

both

platelets

and

VSMC

could

be

of

double

therapeutic

benefit

in

their

prevention

of

both

thrombosis

and

vasospasm

-LSB-

-RSB-

In

conclusion

extracellular

nucleotides

mediate

vasoconstriction

when

released

from

nerves

on

the

adventitial

side

or

when

released

in

the

lumen

when

the

endothelium

is

damaged

-LRB-

Fig.

-RRB-

The

most

important

contractile

receptors

on

the

VSMC

are

the

ATP

receptor

the

ATP/UTP

receptor

the

UDP

receptor

and

the

ADP

receptor

-LRB-

Fig.

and

Table

-RRB-

To

determine

the

relative

physiological

importance

of

these

receptor

subtypes

in

different

human

vascular

beds

is

an

important

task

for

the

future

Table

receptor

expression

in

the

cardiovascular

subtypeEndotheliumVSMCHeartPlateletsRed

blood

cellsInflammatory

cellsPerivascular

sensory

nervesPerivascular

sympathetic

High

-LRB-

Turkey

-RRB-

MediumMedium

-LRB-

neg

feedback

-RRB-

Possible

High

Medium

High

HighMediumHigh

HighMedium

-LRB-

pos

feedback

-RRB-

Low

Possible

LowHighUnless

specified

data

refer

to

human

tissue

High

medium

or

low

expression

represents

an

effort

to

summarise

the

results

of

a

large

number

of

mRNA

protein

and

pharmacological

studies

Dash

-LRB-

-RRB-

denotes

lack

of

convincing

evidence

for

expression

VSMC

vascular

smooth

muscle

cell

Endothelial

regulation

Shear

stress

and

hypoxia

are

important

stimuli

of

both

ATP

and

UTP

release

from

endothelial

cells

-LSB-

-RSB-

-LRB-

Fig.

-RRB-

Extracellular

nucleotides

have

several

important

effects

mediated

by

activation

of

endothelial

cells

Vasodilatation

and

decreased

blood

pressure

by

release

of

prostaglandins

and

NO

has

been

demonstrated

in

several

studies

-LSB-

-RSB-

but

receptors

also

mediate

release

of

endothelium-derived

hyperpolarising

factor

-LRB-

EDHF

-RRB-

which

relaxes

VSMC

by

activation

of

potassium

channels

with

subsequent

hyperpolarisation

-LSB-

-RSB-

Both

UTP

and

ATP

reduce

forearm

vascular

resistance

in

a

prostaglandin

and

NO

independent

way

-LSB-

-RSB-

indicating

an

important

role

for

EDHF

in

receptor-mediated

vasodilatation

in

man

The

receptor

seems

to

be

of

major

importance

in

most

vascular

beds

-LSB-

-RSB-

However

several

other

receptors

are

important

for

endothelial

regulation

Pharmacology

of

vasodilatation

and

mRNA

quantification

in

man

indicates

that

and

to

a

lesser

degree

also

are

important

endothelial

receptors

-LSB-

-RSB-

Knockout

mice

experiments

recently

confirmed

this

picture

and

demonstrated

that

the

receptor

does

not

mediate

dilatation

-LSB-

-RSB-

UTP

and

ATP

are

equipotent

as

vasodilators

when

infused

in

the

human

forearm

circulation

indicating

a

role

for

both

purinergic

and

pyrimidinergic

receptors

The

receptor

is

not

expressed

on

the

vascular

endothelium

and

the

evidence

for

other

receptors

has

been

scarce

except

for

the

receptor

which

is

the

highest

expressed

receptor

in

endothelium

-LSB-

-RSB-

Using

antisense

oligonucleotides

the

receptor

was

shown

to

be

important

for

shear

stress-dependent

influx

via

an

ATP-dependent

mechanism

-LSB-

-RSB-

This

indicates

that

ATP

and

receptors

may

be

of

importance

for

shear

stress-mediated

effects

which

is

in

agreement

with

the

well-established

release

of

ATP

from

endothelial

cells

during

shear

stress

-LSB-

-RSB-

Vessel

dilation

induced

by

acute

increases

in

blood

flow

is

markedly

suppressed

in

receptor

knockout

mice

-LSB-

-RSB-

Thus

endothelial

channels

are

crucial

to

flow-sensitive

mechanisms

that

regulate

blood

pressure

and

vascular

remodelling

-LSB-

-RSB-

Extracellular

ATP

in

the

circulation

is

rapidly

degraded

into

ADP

AMP

and

adenosine

by

ectonucleotidases

Vascular

-LRB-

-RRB-

is

an

endothelial

cell

membrane

protein

with

both

ecto-ATPase

and

ecto-ADPase

activities

-LSB-

-RSB-

Ectonucleotidases

are

also

released

by

shear

stress

from

endothelial

cells

-LSB-

-RSB-

Reactive

hyperaemia

is

the

massive

increase

in

blood

flow

that

starts

when

a

blood

vessel

is

opened

after

a

period

of

ischaemia

It

is

well

known

that

ATP

is

released

during

ischaemia

-LSB-

-RSB-

Adenosine

only

mediates

the

late

phase

of

the

reactive

hyperaemia

as

has

been

shown

by

infusion

of

adenosine

deaminase

or

a

selective

antagonist

-LSB-

-RSB-

ADP

has

been

shown

to

mediate

the

mid-portion

and

peak

of

coronary

reactive

hyperaemia

via

endothelial

receptors

-LSB-

-RSB-

It

has

been

proposed

that

different

purines

may

mediate

three

phases

of

the

reactive

hyperaemia

-LSB-

-RSB-

According

to

this

hypothesis

ATP

would

mediate

the

first

part

of

the

hyperaemia

via

endothelial

or

receptors

ATP

would

then

be

degraded

to

ADP

which

mediates

peak

hyperaemia

via

endothelial

receptors

followed

by

degradation

of

ADP

to

adenosine

resulting

in

late

phase

hyperaemia

mediated

via

receptors

on

smooth

muscle

cells

Red

blood

cells

as

regulators

of

vascular

tone

The

matching

of

oxygen

supply

with

demand

requires

a

mechanism

that

increases

blood

flow

in

response

to

decreased

tissue

oxygen

levels

Several

reports

suggest

that

the

red

blood

cell

-LRB-

RBC

-RRB-

acts

as

a

sensor

for

hypoxia

and

different

mechanisms

have

been

suggested

by

which

the

deoxygenated

RBC

stimulates

vasodilatation

-LSB-

-RSB-

RBCs

contain

millimolar

amounts

of

ATP

and

possess

the

membrane-bound

glycolytic

enzymes

necessary

for

its

production

-LSB-

-RSB-

ATP

is

released

in

response

to

reductions

in

oxygen

tension

and

pH

-LSB-

-RSB-

-LRB-

see

Table

-RRB-

It

has

been

shown

in

vitro

that

vessels

dilate

in

response

to

low

levels

only

when

blood

vessels

are

perfused

with

RBCs

-LSB-

-RSB-

ATP

is

released

in

working

human

skeletal

muscle

circulation

depending

on

the

number

of

unoccupied

haemoglobin

binding

sites

-LSB-

-RSB-

Similar

results

have

been

shown

in

the

coronary

circulation

of

dogs

-LSB-

-RSB-

The

released

ATP

then

binds

to

receptors

on

the

endothelium

and

stimulates

vasodilatation

Thus

the

RBC

functions

as

an

sensor

contributing

to

the

regulation

of

blood

flow

and

delivery

by

releasing

ATP

depending

on

the

oxygenation

state

of

haemoglobin

Table

of

purines

and

pyrimidines

in

the

cardiovascular

systemSourceEndotheliumVSMCHeartPlateletsRed

blood

cellsInflammatory

cellsVascular

nervesStimuliShear

stress

hypoxia

high

glucoseHigh

glucose

hypoxiaHypoxiaCollagen

thromboxane

other

platelet

activatorsHypoxia

adrenaline

deformationInfl

activatorsSympathetic

nerves

sensory

nerve

collateralsMechanismVesicularUnknownUnknownVesicularUnknownUnknown

or

vesicularVesicularNucleotideATP

UTPATP

UTPATP

UTPADP

ATPATPATPATPTargetEndothelium

VSMCVSMCCardiomyocytes

blood

vessels

intrinsic

cardiac

neuronsPlatelets

endothelium

or

VSMCEndotheliumVSMC

-LRB-

co-released

-RRB-

Stimuli

represents

stimuli

leading

to

release

Mechanism

refers

to

mechanism

of

release

at

the

cellular

level

Target

represents

the

cells

that

are

regulated

by

the

released

purine

or

pyrimidine

Degradation

refers

to

the

degrading

enzyme

responsible

for

removing

the

purine

or

pyrimidine

after

the

release

VSMC

vascular

smooth

muscle

cell

ADP

activates

a

negative

feedback

pathway

for

ATP

release

from

human

RBCs

via

receptors

-LSB-

-RSB-

Since

blood

consists

of

approximately

RBCs

that

contain

a

higher

ATP

concentration

than

plasma

-LRB-

mmol/l

vs

μmol/l

-RRB-

even

a

minor

release

of

ATP

from

the

high

intracellular

concentrations

could

have

major

circulatory

effects

A

negative

feedback

system

may

therefore

be

of

great

physiological

importance

to

mitigate

ATP

release

Another

negative

feedback

system

is

the

mechanism

by

which

NO

inhibits

ATP

release

from

erythrocytes

illustrating

how

NO

may

turn

off

ATP

release

-LSB-

-RSB-

NO

is

then

scavenged

by

haemoglobin

Furthermore

the

previous

view

of

the

RBC

as

a

passive

bag

that

transports

oxygen

is

challenged

It

now

turns

out

that

it

releases

ATP

in

response

to

stimuli

and

as

with

most

important

signalling

systems

it

has

a

negative

feedback

system

to

terminate

its

release

The

described

negative

feedback

pathway

may

be

important

to

avoid

high

extracellular

concentrations

of

ATP

At

levels

above

µmol/l

ATP

concentrations

may

exceed

the

catalytic

capacity

of

ectonucleotidases

and

could

in

fact

stimulate

ATP

release

by

increasing

permeability

of

the

RBC

-LSB-

-RSB-

probably

via

receptors

-LSB-

-RSB-

ATP

may

even

release

ATP

from

endothelial

cells

-LSB-

-RSB-

At

high

concentrations

of

ATP

a

self-sustaining

process

may

thus

be

instigated

which

may

contribute

to

the

irreversible

stage

of

circulatory

shock

that

can

develop

rapidly

in

severely

ill

patients

Similar

mechanisms

may

be

of

importance

in

malaria

because

induction

of

the

osmolyte

permeability

in

Plasmodium-infected

erythrocytes

involves

purinoceptor

signalling

-LSB-

-RSB-

A

mechanism

of

ATP

release

from

RBCs

which

has

eluded

researchers

for

many

years

has

recently

been

suggested

by

Locovei

and

co-workers

as

being

mediated

via

the

gap

junction

protein

-LSB-

-RSB-

Erythrocytes

do

not

form

gap

junctions

instead

forms

a

mechanosensitive

ATP-permeable

channel

that

mediates

osmotically

induced

ATP

release

Hypertension

Blood

pressure

regulation

by

purinergic

signalling

is

the

net

result

of

balancing

contractile

and

dilatory

effects

as

described

above

ATP

and

UTP

released

on

the

luminal

side

from

endothelial

cells

and

erythrocytes

stimulate

vasodilatation

in

contrast

to

release

from

nerves

on

the

adventitial

side

which

results

in

vasoconstriction

ATP

may

also

regulate

blood

pressure

via

renal

mechanisms

or

brain

stem

regulation

-LSB-

-RSB-

ATP

plays

a

significantly

greater

role

as

a

sympathetic

cotransmitter

in

spontaneously

hypertensive

rats

-LRB-

SHR

-RRB-

-LSB-

-RSB-

and

there

is

increased

responsiveness

of

the

renal

vasculature

of

isolated

perfused

rat

kidneys

to

α,β-methylene

ATP

in

SHR

-LSB-

-RSB-

while

mesenteric

vascular

contractile

reactivity

to

ATP

via

and

receptors

is

not

altered

in

deoxycorticosterone

acetate

-LRB-

DOCA

-RRB-

salt

hypertension

-LSB-

-RSB-

In

the

aorta

of

SHR

endothelium-dependent

relaxation

to

ATP

is

impaired

because

of

the

concomitant

generation

of

an

endothelium-derived

contracting

factor

Contractions

to

ATP

were

significantly

potentiated

in

SHR

aorta

-LSB-

-RSB-

In

hypertensive

mature

stroke-prone

rats

NPY

modulation

of

release

of

and

ATP

from

sympathetic

perivascular

nerves

in

kidney

vessels

is

impaired

which

may

account

for

the

increased

nerve-mediated

responses

-LSB-

-RSB-

Thus

animal

models

of

hypertension

reveal

several

alterations

in

purinergic

signalling

that

may

contribute

to

the

development

of

hypertension

Diadenosine

polyphosphates

such

as

and

are

combinations

of

two

adenosine

molecules

connected

with

four

to

six

phosphate

groups

They

have

been

identified

as

vasocontractile

agents

-LSB-

-RSB-

probably

via

actions

on

and

receptors

and

are

stored

at

higher

levels

in

platelets

from

patients

with

hypertension

and

may

contribute

to

their

increased

peripheral

vascular

resistance

-LSB-

-RSB-

is

a

novel

endothelium-derived

vasoconstrictive

factor

more

potent

than

endothelin

in

renal

vasoconstriction

-LSB-

-RSB-

It

is

released

upon

stimulation

of

the

endothelium

by

acetylcholine

thrombin

and

mechanical

stress

and

can

be

cleaved

into

either

ATP

or

UTP

to

stimulate

both

and

receptors

on

VSMC

resulting

in

increased

blood

pressure

-LSB-

-RSB-

As

mentioned

above

the

receptor

is

the

most

abundantly

expressed

receptor

in

the

endothelium

and

mediates

shear

stress-stimulated

vasodilatation

receptor

knockout

mice

have

higher

blood

pressures

and

excrete

smaller

amounts

of

NO

products

in

their

urine

than

do

wild-type

mice

-LSB-

-RSB-

The

receptor

protein

is

upregulated

in

the

placenta

in

preeclampsia

-LSB-

-RSB-

Thus

shear

stress-stimulated

release

of

ATP

acting

on

receptors

may

be

important

for

blood

pressure

regulation

The

importance

of

receptors

for

hypertension

will

not

be

proven

until

tested

in

man

It

would

be

of

great

interest

to

perform

a

clinical

study

on

the

effects

of

or

receptor

antagonists

in

the

treatment

of

hypertension

Pulmonary

hypertension

Erythrocyte

release

of

ATP

appears

to

regulate

pulmonary

resistance

under

some

conditions

-LSB-

-RSB-

and

patients

with

pulmonary

hypertension

have

been

found

with

impaired

release

of

ATP

from

RBC

-LSB-

-RSB-

Endothelium-dependent

relaxation

to

ATP

has

been

demonstrated

in

human

pulmonary

arteries

-LSB-

-RSB-

On

the

other

hand

ATP

is

a

mitogen

for

pulmonary

artery

smooth

muscle

cells

which

may

be

relevant

for

the

pathophysiological

basis

of

pulmonary

hypertension

-LSB-

-RSB-

Again

the

balance

between

endothelial

versus

smooth

muscle

stimulation

by

ATP

may

regulate

blood

pressure

in

different

directions

and

we

are

far

from

a

full

understanding

of

the

role

of

the

purinergic

system

in

pulmonary

hypertension

Migraine

and

vascular

pain

Classic

migraine

is

associated

with

two

distinct

vascular

changes

an

initial

vasoconstriction

followed

by

vasodilatation

associated

with

pain

It

has

been

proposed

that

purinergic

signalling

is

involved

in

these

changes

ATP

released

from

perivascular

sympathetic

nerves

may

participate

in

producing

the

initial

vasospasm

mediated

by

receptors

in

vascular

smooth

muscle

Cerebral

arteries

are

strongly

contracted

by

UTP

and

especially

UDP

via

receptors

A

UDP

agonist

might

therefore

have

similar

effects

as

the

agonists

frequently

used

to

treat

migraine

-LSB-

-RSB-

ATP

released

by

endothelial

cells

acting

on

receptors

on

endothelial

cells

mediating

release

of

NO

and

producing

vasodilatation

may

contribute

to

the

phase

of

reactive

hyperaemia

following

vasospasm

-LSB-

-RSB-

It

has

been

suggested

that

antagonists

to

receptors

which

are

located

on

nociceptive

sensory

nerve

endings

in

cerebral

vessels

may

be

promising

candidates

for

anti-migraine

drug

development

-LSB-

-RSB-

A

recent

report

demonstrates

selective

upregulation

of

nociceptive

receptors

on

trigeminal

neurons

by

calcitonin

gene-related

peptide

and

suggests

that

this

mechanism

might

contribute

to

pain

sensitisation

in

migraine

-LSB-

-RSB-

receptors

on

trigeminal

sensory

nerves

have

also

been

implicated

in

pain

in

migraine

It

has

been

claimed

that

migraine

attacks

are

characterised

by

a

relative

depletion

of

sympathetic

stores

in

conjunction

with

an

increase

in

release

of

ATP

-LSB-

-RSB-

In

a

review

in

the

Lancet

in

Burnstock

proposed

that

vascular

pain

as

in

angina

ischaemic

muscle

and

pelvic

pain

in

women

may

be

initiated

by

ATP

released

from

microvascular

endothelial

cells

during

reactive

hyperaemia

diffusing

a

short

distance

to

activate

nociceptive

receptors

on

sensory

nerves

in

the

adventitia

-LSB-

-RSB-

Atherosclerosis

Atherosclerosis

is

the

main

cause

of

ischaemic

stroke

and

cardiovascular

disease

and

is

now

considered

to

be

an

inflammatory

disease

-LSB-

-RSB-

The

formation

of

a

plaque

starts

with

the

accumulation

of

cholesterol

followed

by

invasion

of

macrophages

taking

up

cholesterol

and

becoming

foam

cells

The

plaque

can

be

stabilised

by

smooth

muscle

cell

formation

of

a

fibrous

cap

that

covers

the

lipid-rich

region

However

stimulation

of

inflammation

by

oxidised

low-density

lipoprotein

activates

macrophages

and

dendritic

cells

into

antigen-presenting

cells

activating

T

lymphocytes

resulting

in

release

of

cytokines

and

metalloproteinases

degrading

the

fibrous

cap

The

end

result

is

a

vulnerable

plaque

and

when

it

ruptures

its

highly

thrombogenic

contents

activates

platelets

and

causes

the

formation

of

a

local

thrombus

occluding

the

artery

or

embolising

resulting

in

ischaemic

stroke

or

myocardial

infarction

Evidence

both

from

basic

research

and

from

clinical

studies

indicates

important

involvement

on

several

levels

for

purinergic

signalling

in

the

atherosclerotic

process

-LRB-

Fig.

-RRB-

Interestingly

fish

oil

components

increase

the

release

of

ATP

-LSB-

-RSB-

and

a

high

cholesterol

diet

decreases

ATP

release

from

arteries

-LSB-

-RSB-

Fig.

roles

of

receptors

in

the

atherosclerotic

inflammatory

plaque

and

during

restenosis

See

text

for

details

Purines

and

pyrimidines

acting

on

receptors

stimulate

vascular

inflammation

both

by

actions

on

the

endothelial

cell

-LRB-

EC

-RRB-

and

by

effects

on

inflammatory

cells

Furthermore

they

stimulate

vascular

smooth

muscle

cell

-LRB-

VSCM

-RRB-

proliferation

the

conversion

to

synthetic

phenotype

and

production

of

matrix

proteins

Mitogenic

receptors

are

upregulated

by

growth

factors

and

cytokines

IL

interleukin

monocyte

chemoattractant

intercellular

adhesion

TSP

thrombospondin

IDO

indoleamine

Atherosclerosis

receptor-mediated

effects

on

inflammatory

cells

Inflammatory

cells

express

a

large

number

of

receptors

with

multiple

effects

-LSB-

-RSB-

The

final

result

is

difficult

to

predict

depending

on

the

subtype

of

receptor

expressed

by

a

particular

cell

type

and

on

the

differentiation

stage

of

the

cell

The

most

important

inflammatory

cells

for

atherosclerosis

are

monocytes

that

differentiate

into

macrophages

or

dendritic

cells

in

the

plaque

and

the

T-helper

and

suppressor

lymphocytes

that

coordinate

the

inflammatory

reaction

in

the

plaque

-LSB-

-RSB-

A

large

number

of

receptors

are

expressed

on

T

lymphocytes

and

macrophages

-LSB-

-RSB-

and

have

been

suggested

to

be

important

players

in

atherosclerosis

-LSB-

-RSB-

Strong

evidence

for

a

functional

role

exists

for

and

receptors

The

receptor

is

mitogenic

and

anti-apoptotic

for

T

lymphocytes

-LSB-

-RSB-

is

important

for

release

of

interleukin

-LRB-

IL

-RRB-

-LSB-

-RSB-

tumour

necrosis

factor

-LSB-

-RSB-

and

L-selectin

an

adhesion

molecule

important

for

lymphocyte

binding

to

endothelium

-LSB-

-RSB-

All

of

these

effects

are

known

to

be

important

for

atherosclerosis

development

The

severity

of

arthritis

is

reduced

in

receptor

knockout

mice

-LSB-

-RSB-

and

rheumatoid

arthritis

is

coupled

to

increased

incidence

of

myocardial

infarction

It

would

be

of

great

interest

to

examine

the

receptor

knockout

mouse

in

an

atherosclerosis

model

ATP

and

UTP

are

chemotactic

for

dendritic

cells

probably

via

the

receptor

and

may

attract

inflammatory

cells

to

the

vascular

lesion

-LSB-

-RSB-

The

receptor

enhances

the

oxidative

burst

in

human

macrophages

-LSB-

-RSB-

receptor

deletion

in

knockout

mice

reduces

the

atherosclerotic

lesions

and

the

plaque

area

occupied

by

macrophages

in

ApoE

knockout

mice

-LSB-

-RSB-

Whether

this

is

due

to

platelet

inhibition

endothelial

or

an

inflammatory

mechanism

requires

further

studies

ATP

inhibits

T

cell

activation

via

an

increase

in

cyclic

AMP

-LRB-

cAMP

-RRB-

probably

via

a

receptor

-LSB-

-RSB-

ATP

acting

on

receptors

regulates

the

maturation

of

human

monocyte-derived

dendritic

cells

and

induces

immunosuppression

by

inhibiting

T-helper

cytokines

and

promoting

T-helper

cytokines

-LSB-

-RSB-

A

polymorphism

in

the

receptor

has

been

shown

to

have

clinical

importance

by

increasing

the

risk

of

myocardial

infarction

-LSB-

-RSB-

The

polymorphism

carried

by

one

fifth

of

the

population

causes

an

substitution

in

the

ATP

receptor

and

increases

the

risk

of

myocardial

infarction

by

The

odds

ratio

increased

stepwise

depending

on

the

number

of

alleles

and

in

subgroups

in

which

the

genetic

influence

is

known

to

be

of

increased

importance

family

history

of

acute

myocardial

infarction

-LRB-

AMI

-RRB-

early

onset

AMI

or

the

combined

group

of

early

onset

AMI

with

family

history

The

mechanism

by

which

the

polymorphism

causes

AMI

seems

to

be

coupled

to

increased

inflammation

because

the

variant

of

the

receptor

was

coupled

to

elevated

C-reactive

protein

-LRB-

CRP

-RRB-

levels

CRP

is

a

marker

of

inflammation

and

an

independent

prognostic

risk

factor

for

the

development

of

AMI

-LSB-

-RSB-

Thus

the

receptor

is

important

in

the

development

of

atherosclerosis

via

modulation

of

inflammation

either

via

effects

on

T

lymphocytes

or

macrophage

cells

Proinflammatory

effects

on

the

endothelium

Even

though

receptor-mediated

activation

of

the

endothelium

stimulates

release

of

NO

which

inhibits

inflammatory

cells

it

is

now

well

established

that

important

proinflammatory

endothelial

effects

may

also

be

triggered

A

proinflammatory

dysfunctional

endothelium

is

crucial

in

the

recruitment

of

monocytes

to

the

atherosclerotic

plaque

and

in

the

general

extravasation

and

loss

of

peripheral

resistance

seen

in

sepsis

ATP

stimulates

neutrophil

adherence

to

cultured

endothelial

cells

-LSB-

-RSB-

ATP

stimulates

release

of

monocyte

chemoattractant

growth-regulated

oncogene

α

and

increased

expression

of

intercellular

adhesion

in

human

microvascular

endothelial

cells

-LSB-

-RSB-

UTP

and

ATP

stimulate

expression

of

proinflammatory

vascular

cell

adhesion

-LRB-

-RRB-

in

endothelial

cells

through

activation

of

the

receptor

and

increases

the

adherence

of

monocytic

cells

to

human

coronary

endothelial

cells

an

effect

that

was

inhibited

by

antibodies

-LSB-

-RSB-

VCAM

is

important

for

the

recruitment

of

monocytes

and

lymphocytes

The

VCAM

stimulation

is

caused

by

receptor-induced

transactivation

of

the

vascular

endothelial

growth

factor

-LSB-

-RSB-

These

effects

have

been

confirmed

in

an

in

vivo

neointima

model

in

which

perivascular

infusion

of

UTP

enhanced

infiltration

by

macrophages

-LSB-

-RSB-

In

conclusion

ATP

and

UTP

stimulate

several

inflammatory

responses

known

to

be

important

for

atherosclerosis

development

-LRB-

Fig.

-RRB-

Restenosis

and

vascular

smooth

muscle

cell

proliferative

disease

VSMC

proliferation

contributes

to

plaque

development

but

it

is

probably

beneficial

by

stabilising

the

plaque

and

forming

a

stronger

fibrinous

cap

However

there

are

several

situations

in

which

VSMC

proliferation

is

important

in

vascular

disease

development

restenosis

after

balloon

angioplasty

diabetic

microvascular

disease

chronic

allograft

rejection

pulmonary

hypertension

and

possibly

systemic

hypertension

The

latter

is

suggested

since

sympathetic

nerves

exert

a

trophic

effect

on

vascular

smooth

muscle

-LSB-

-RSB-

ATP

is

a

cotransmitter

from

sympathetic

nerves

-LSB-

-RSB-

and

has

been

shown

to

be

a

more

potent

mitogen

than

the

other

cotransmitters

and

NPY

-LSB-

-RSB-

Extracellular

ATP

is

a

potent

growth

factor

for

VSMC

by

activation

of

G

protein-coupled

receptors

-LSB-

-RSB-

UTP

is

also

mitogenic

acting

on

receptors

-LSB-

-RSB-

as

is

UDP

acting

on

receptors

-LSB-

-RSB-

ATP

is

synergistic

with

polypeptide

growth

factors

-LRB-

e.g.

platelet-derived

growth

factor

basic

fibroblast

growth

factor

-RRB-

and

insulin

-LSB-

-RSB-

The

signal

transduction

is

mediated

via

Gq

proteins

phospholipase

C

and

D

diacylglycerol

protein

kinase

C

extracellular

signal-regulated

kinase

kinase

MAPK/ERK

activity

kinase

mitogen-activated

protein

kinases

and

Rho

-LSB-

-RSB-

Several

immediate

early

genes

are

activated

and

the

cell

is

taken

through

different

phases

of

the

cell

cycle

-LSB-

-RSB-

Sometimes

as

with

UDP

acting

on

receptors

progression

is

stimulated

to

both

the

S

and

phases

that

is

through

the

whole

cell

cycle

-LSB-

-RSB-

Mitogenic

effects

have

been

demonstrated

in

rat

porcine

and

bovine

VSMC

and

cells

from

human

coronary

arteries

aorta

and

subcutaneous

arteries

and

veins

-LSB-

-RSB-

The

trophic

effects

on

VSMC

and

the

abundant

sources

for

extracellular

ATP

in

the

vessel

wall

make

a

pathophysiological

role

probable

in

the

development

of

atherosclerosis

neointima

formation

after

angioplasty

chronic

allograft

rejection

pulmonary

hypertension

diabetic

microvascular

disease

and

possibly

hypertension

In

these

processes

interaction

between

the

VSMC

and

the

matrix

is

important

It

is

therefore

interesting

that

the

human

receptor

contains

an

integrin-binding

domain

-LRB-

RGD

-RRB-

in

its

first

extracellular

loop

and

that

interaction

with

integrins

influences

receptor-mediated

activation

of

G

proteins

-LSB-

-RSB-

ATP

stimulates

release

of

matrix

-LRB-

-RRB-

from

human

aortic

smooth

muscle

cells

and

has

been

implicated

in

aortic

aneurysm

pathogenesis

-LSB-

-RSB-

MMPs

open

up

the

matrix

for

migrating

VSMC

ATP

and

UTP

are

potent

chemotactic

agents

stimulating

VSMC

migration

via

and

receptors

-LSB-

-RSB-

These

mechanisms

together

with

growth-stimulating

effects

explain

why

perivascular

infusion

of

UTP

in

a

neointima

model

enhanced

neointimal

development

-LSB-

-RSB-

Overexpression

of

the

ATP

and

UTP-degrading

enzyme

reduces

neointima

development

in

rat

aorta

-LSB-

-RSB-

Augmentation

of

activity

is

an

important

adaptive

response

for

cardiac

allograft

survival

with

increased

inflammation

platelet

deposition

and

infarction

rates

in

NTPDase-deficient

mice

-LSB-

-RSB-

The

mechanism

is

dependent

on

increased

expression

of

the

atherosclerotic

matrix

protein

osteopontin

-LSB-

-RSB-

Diabetic

microvascular

disease

Sympathetic

vascular

dysfunction

in

early

experimental

juvenile

diabetes

was

recognised

many

years

ago

Two

weeks

after

induction

of

streptozotocin

diabetes

in

rats

there

was

prejunctional

impairment

of

sympathetic

neurotransmission

and

impaired

ATP-mediated

endothelial

function

in

mesenteric

arteries

-LSB-

-RSB-

Altered

relaxant

responses

to

ATP

in

the

corpus

cavernosum

of

men

and

rats

with

diabetes

have

also

been

reported

-LSB-

-RSB-

As

mentioned

above

red

blood

cells

may

stimulate

vasodilatation

via

release

of

ATP

which

may

be

important

in

maintaining

perfusion

in

the

microcirculation

Interestingly

in

erythrocytes

from

humans

with

type

diabetes

ATP

release

is

impaired

which

is

consistent

with

the

hypothesis

that

the

defect

in

erythrocyte

physiology

could

contribute

to

the

vascular

disease

associated

with

this

clinical

condition

-LSB-

-RSB-

High

extracellular

glucose

releases

ATP

and/or

UTP

in

endothelial

cells

and

pancreatic

β

cells

-LSB-

-RSB-

An

increase

in

glucose

from

to

mmol/l

results

in

a

marked

increase

in

the

proatherogenic

nuclear

factor

of

activated

T

cells

signalling

pathway

in

VSMC

-LSB-

-RSB-

The

effect

is

mediated

via

glucose-induced

release

of

ATP

and

UTP

which

subsequently

activate

but

also

receptors

-LRB-

after

degradation

to

UDP

-RRB-

Thus

nucleotide

release

is

a

potential

metabolic

sensor

for

the

arterial

smooth

muscle

response

to

high

glucose

Diabetic

patients

experience

microvascular

disease

characterised

by

increased

wall-lumen

ratio

mainly

because

of

an

increase

in

VSMC

and

have

higher

rates

of

restenosis

after

coronary

angioplasty

High

glucose-induced

release

of

extracellular

nucleotides

acting

on

receptors

to

stimulate

VSMC

growth

via

NFAT

-LRB-

nuclear

factor

of

activated

T

cells

-RRB-

activation

may

provide

a

link

between

diabetes

and

diabetic

vascular

disease

-LSB-

-RSB-

In

conclusion

ATP/UTP/UDP

stimulate

VSMC

growth

migration

release

of

MMPs

and

osteopontin

that

may

contribute

to

the

development

of

restenosis

diabetic

microvascular

disease

chronic

allograft

rejection

pulmonary

hypertension

and

possibly

systemic

hypertension

-LRB-

Fig.

-RRB-

Vascular

remodelling

and

angiogenesis

Vascular

remodelling

receptors

may

regulate

VSMC

phenotype

and

vice

versa

and

receptor

expression

is

markedly

altered

during

phenotype

changes

-LSB-

-RSB-

The

shift

from

a

specialised

contractile

VSMC

phenotype

into

a

proliferating

matrix-producing

synthetic

phenotype

is

a

prerequisite

for

VSMC

pathogenesis

in

vascular

disease

Pacaud

and

co-workers

found

that

the

mobilising

effects

of

increased

in

VSMC

during

culture

in

serum

indicating

upregulation

of

receptors

in

the

transition

from

contractile

to

synthetic

phenotype

-LSB-

-RSB-

This

was

confirmed

at

the

mRNA

level

where

receptors

were

found

to

be

upregulated

while

receptors

were

downregulated

-LSB-

-RSB-

Mitogenic

receptors

are

upregulated

while

ion

channel

receptors

with

only

contractile

effects

are

downregulated

in

the

synthetic

phenotype

-LSB-

-RSB-

Growth

factors

cytokines

and

interestingly

also

ATP

are

potent

stimulators

of

receptor

expression

-LSB-

-RSB-

Thus

factors

of

importance

in

the

development

of

vascular

disease

increase

mitogenic

receptors

this

is

further

supported

by

their

upregulation

in

neointima

after

balloon

angioplasty

-LSB-

-RSB-

ATP

has

a

dual

concentration-dependent

effect

on

VSMC

phenotype

Low

ATP

concentrations

stimulate

expression

of

genes

specific

for

the

contractile

phenotype

while

high

ATP

concentrations

cause

a

phenotypic

shift

from

the

contractile

to

the

synthetic

phenotype

and

this

shift

is

dependent

on

a

transient

activation

of

protein

kinase

A

which

inhibits

activation

of

a

serum

response

factor

-LSB-

-RSB-

This

previously

unrecognised

mechanism

also

appears

to

be

important

for

the

profound

mitogenic

effect

of

ATP

In

intact

human

blood

vessels

examined

in

vitro

shear

stress

decreased

contractile

receptor

expression

but

increased

the

expression

of

mitogenic

and

receptors

in

VSMC

-LSB-

-RSB-

This

mechanism

could

promote

vascular

growth

and

remodelling

induced

by

shear

stress

as

an

adaptive

response

to

increased

flow

receptor

knockout

mice

are

incapable

of

adaptive

vascular

remodelling

that

is

a

decrease

in

vessel

size

in

response

to

a

chronic

decrease

in

blood

flow

-LSB-

-RSB-

Angiogenesis

Very

little

is

known

about

the

role

of

receptors

in

angiogenesis

but

there

are

some

studies

suggesting

their

involvement

Ischaemia

stimulates

release

of

ATP

-LSB-

-RSB-

and

it

has

been

shown

to

be

a

growth

factor

for

endothelial

cells

-LSB-

-RSB-

ATP

and

UTP

stimulate

cytoskeletal

rearrangements

with

consequent

cell

migration

of

human

endothelial

cells

-LSB-

-RSB-

and

ATP

stimulates

vasa

vasorum

neovascularisation

in

pulmonary

arteries

-LSB-

-RSB-

Newly

developed

vascular

endothelia

express

very

high

levels

of

also

seen

under

hypoxic

conditions

-LSB-

-RSB-

knockout

mice

exhibit

disordered

cellular

migration

and

angiogenesis

-LSB-

-RSB-

Varicose

veins

It

has

been

suggested

that

cell

lysis

consequent

to

receptor-induced

pore

formation

contributes

to

the

disorganisation

and

decrease

in

contractile

myocytes

in

the

media

of

varicose

veins

-LSB-

-RSB-

Upregulation

of

and

receptors

and

downregulation

of

receptors

on

smooth

muscle

of

varicose

veins

is

associated

with

a

shift

from

contractile

to

synthetic

and/or

proliferative

roles

this

phenotypic

change

in

smooth

muscle

leads

to

weakening

of

vein

walls

and

may

be

a

causal

factor

in

the

development

of

varicose

veins

-LSB-

-RSB-

an

interesting

parallel

to

the

receptor

changes

seen

in

response

to

increased

shear

stress

-LRB-

see

above

-RRB-

-LSB-

-RSB-

Heart

ATP

is

released

in

the

heart

as

a

cotransmitter

together

with

catecholamines

from

sympathetic

nerves

but

it

may

also

be

released

from

other

sources

in

the

heart

such

as

endothelium

platelets

RBC

and

ischaemic

myocardium

-LRB-

Table

-RRB-

-LSB-

-RSB-

receptors

are

abundantly

expressed

in

the

foetal

human

heart

-LSB-

-RSB-

as

well

as

in

the

adult

human

heart

-LSB-

-RSB-

Inotropy

In

cardiomyocytes

ATP

stimulates

a

pronounced

positive

inotropic

effect

and

may

also

act

in

synergy

with

β-adrenergic

agonists

to

augment

myocyte

contractility

-LSB-

-RSB-

ATP

stimulates

an

increase

in

cytosolic

calcium

and

evidence

for

the

involvement

of

inositol

-LRB-

-RRB-

coupled

receptors

and

ion

channel

receptors

has

been

presented

-LSB-

-RSB-

probably

after

degradation

to

ATP

increases

force

of

contraction

in

human

ventricular

trabeculae

-LSB-

-RSB-

The

inotropic

effects

of

ATP

are

dependent

on

both

and

cAMP

-LSB-

-RSB-

The

inotropic

effects

of

catecholamines

acting

on

β-receptors

are

mediated

by

an

increase

of

cAMP

and

antagonists

of

these

receptors

are

important

drugs

for

the

treatment

of

hypertension

and

reduce

mortality

in

congestive

heart

failure

Similarly

ATP

stimulates

an

increase

in

cAMP

in

cardiac

myocytes

and

may

act

in

synergy

with

the

agonist

isoproterenol

by

differential

activation

of

adenylyl

cyclase

isoforms

-LSB-

-RSB-

However

the

ATP

receptor

mediating

this

increase

in

cAMP

has

not

been

related

to

any

particular

receptor

subtype

in

cardiac

cells

-LSB-

-RSB-

It

could

be

the

receptor

that

is

additionally

coupled

to

Gs

and

activates

adenylyl

cyclase

-LSB-

-RSB-

mRNA

for

the

receptor

has

been

detected

in

the

human

heart

at

high

levels

-LSB-

-RSB-

The

unstable

agonist

UTP

has

been

shown

to

induce

a

positive

inotropic

effect

in

rat

atria

and

in

rat

and

guinea

pig

ventricular

cardiomyocytes

-LSB-

-RSB-

Stable

UTP

and

UDP

analogues

induce

a

pronounced

inotropic

effect

on

mouse

cardiomyocytes

-LSB-

-RSB-

The

receptor

is

the

most

abundantly

expressed

receptor

with

very

low

levels

of

the

receptor

in

the

human

heart

-LSB-

-RSB-

suggesting

that

the

inotropic

effects

of

UTP

are

mediated

via

receptors

while

the

UDP

effects

are

mediated

via

the

receptor

-LSB-

-RSB-

These

mechanisms

are

mediated

via

signalling

In

conclusion

the

inotropic

effects

in

man

are

mediated

via

and

receptors

and

a

receptor

-LRB-

Fig.

-RRB-

receptors

are

probably

also

involved

since

several

subtypes

are

expressed

-LSB-

-RSB-

but

it

has

not

been

possible

to

perform

a

clear

receptor

characterisation

due

to

lack

of

selective

antagonists

Fig.

roles

of

purines

and

pyrimidines

acting

on

receptors

in

the

regulation

of

the

heart

See

text

for

details

a

ATP

UTP

and

UDP

exert

inotropic

effects

on

cardiomyocytes

leading

to

increased

cardiac

output

b

UTP

and

UDP

stimulate

hypertrophy

of

cardiomyocytes

while

ATP

can

have

apoptotic

effects

c

UTP

protects

against

ischaemic

injury

and

cardiomyocyte

cell

death

ATP-degrading

enzymes

preserve

endothelial

integrity

and

protect

against

allograft

rejection

NTPDase

nucleoside

triphosphate

diphosphohydrolase

Myocardial

infarction

Using

microdialysis

ATP

in

the

interstitial

space

has

been

estimated

to

be

nmol/l

but

the

levels

may

increase

markedly

during

electrical

stimulation

ischaemia

challenge

with

cardiotonic

agents

increase

in

blood

flow

mechanical

stretch

and

increased

work

load

-LSB-

-RSB-

ATP

is

released

from

cardiomyocytes

during

reduced

oxygen

tension

-LSB-

-RSB-

UTP

and

ATP

are

released

from

the

heart

during

cardiac

ischaemia

-LSB-

-RSB-

and

patients

with

myocardial

infarction

have

higher

plasma

levels

of

both

ATP

and

UTP

-LSB-

-RSB-

There

is

a

significant

increase

in

ectonucleotidase

activity

-LRB-

-RRB-

in

the

hearts

of

patients

with

ischaemic

heart

disease

-LSB-

-RSB-

that

could

represent

a

compensatory

mechanism

against

increased

nucleotide

levels

during

chronic

ischaemia

Ischaemia-reperfusion

provokes

barrier

failure

of

the

coronary

microvasculature

leading

to

myocardial

oedema

ATP

may

protect

against

reperfusion-induced

coronary

endothelial

barrier

damage

and

inhibition

of

ATP

degradation

enhances

the

stabilising

effect

of

ATP

on

barrier

function

-LSB-

-RSB-

The

common

ATP

receptor

polymorphism

is

associated

with

both

increased

CRP

and

increased

risk

of

developing

myocardial

infarction

-LSB-

-RSB-

Based

on

this

association

we

hypothesise

that

the

receptor

plays

an

important

role

in

cardiovascular

biology

and

in

inflammatory

disease

-LRB-

see

above

-RRB-

causing

myocardial

infarction

via

a

proinflammatory

effect

However

increased

inotropic

effects

may

also

contribute

An

interesting

yin

and

yang

situation

for

ATP

and

UTP

has

been

revealed

regarding

hypertrophic

effects

UTP

but

not

ATP

causes

hypertrophic

growth

in

neonatal

cardiomyocytes

-LSB-

-RSB-

-LRB-

Fig.

-RRB-

In

contrast

ATP

inhibits

hypertrophy

and

may

even

induce

apoptosis

and

necrosis

-LSB-

-RSB-

The

reason

for

the

difference

in

effects

could

be

due

to

activation

of

receptors

or

cAMP

stimulation

by

ATP

-LSB-

-RSB-

Both

UTP

and

ATP

transactivate

epidermal

growth

factor

receptors

but

only

ATP

stimulates

the

hypertrophic

marker

genes

atrial

natriuretic

peptide

and

myosin

light

chain

-LSB-

-RSB-

Similarly

UTP

but

not

ATP

protects

cultured

cardiomyocytes

against

hypoxic

stress

-LSB-

-RSB-

Since

UTP

is

released

during

preconditioning

-LSB-

-RSB-

a

role

for

UTP

in

the

protective

effects

of

preconditioning

is

plausible

Recently

Yitzhaki

and

co-workers

were

able

to

demonstrate

prominent

reductions

in

myocardial

infarction

size

and

improved

rat

heart

function

in

vivo

by

a

single

intravenous

bolus

dose

of

UTP

before

ischaemia

-LSB-

-RSB-

-LRB-

Fig.

-RRB-

Congestive

heart

failure

There

are

several

reports

of

alterations

or

adaptations

of

purinergic

signalling

during

congestive

heart

failure

The

positive

inotropic

effect

of

ATP

is

impaired

in

heart

failure

but

reversed

by

the

angiotensin-converting

enzyme

-LRB-

ACE

-RRB-

inhibitor

imidapril

-LSB-

-RSB-

The

contractile

responses

for

the

receptor

agonist

are

decreased

in

heart

failure

suggesting

a

downregulation

of

this

receptor

function

in

cardiomyocytes

in

a

similar

manner

as

seen

for

in

congestive

heart

failure

-LSB-

-RSB-

In

human

hearts

only

the

receptor

was

altered

in

congestive

heart

failure

-LSB-

-RSB-

In

congestive

heart

failure

receptors

are

downregulated

in

VSMC

in

resistance

arteries

which

could

represent

a

protective

response

against

the

increased

sympathetic

nerve

activity

and

peripheral

resistance

seen

in

congestive

heart

failure

-LSB-

-RSB-

Several

receptors

have

been

suggested

as

targets

for

pharmacological

treatment

of

congestive

heart

failure

Overexpression

of

receptors

has

a

beneficial

life-prolonging

effect

in

a

heart

failure

model

-LSB-

-RSB-

Both

ATP

and

catecholamines

are

released

from

sympathetic

nerves

acting

through

cAMP-stimulating

receptors

to

mediate

positive

inotropic

effects

stimulating

the

same

intracellular

mechanisms

as

adrenergic

β-receptors

It

is

possible

that

agonists

for

the

receptor

could

be

used

to

improve

cardiac

output

in

patients

with

circulatory

shock

However

an

even

more

important

drug

candidate

would

be

a

receptor

antagonist

that

may

be

beneficial

in

patients

with

congestive

heart

failure

The

extracellular

pyrimidines

UTP

and

UDP

may

be

inotropic

factors

in

man

acting

on

and

receptors

stimulating

the

same

intracellular

pathways

as

angiotensin

II

Synthetic

agonists

could

thus

be

used

as

inotropic

agents

during

circulatory

shock

and

antagonists

may

have

effects

similar

to

angiotensin

II

receptor

blockers

being

beneficial

in

the

treatment

of

hypertension

and

congestive

heart

failure

Chronotropy

and

arrhythmia

It

has

been

difficult

to

determine

the

chronotropic

effects

of

ATP

due

to

the

dominating

inhibitory

effects

of

adenosine

on

the

AV

node

We

know

that

ATP

increases

the

contractile

rate

in

neuron-myocyte

co-cultures

but

the

effect

is

markedly

reduced

in

non-innervated

myocyte

cultures

-LSB-

-RSB-

ATP

could

induce

arrhythmia

based

on

its

increased

automaticity

and

early

after-depolarisations

-LSB-

-RSB-

The

receptor

is

colocalised

with

connexin

in

gap

junctions

that

transmit

the

contractile

stimulus

between

cardiomyocytes

-LSB-

-RSB-

However

so

far

no

firm

evidence

exists

of

an

important

role

for

receptors

for

chronotropy

or

arrhythmia

Platelets

and

coagulation

Platelets

ADP

released

from

erythrocytes

or

produced

after

ectonucleotidase

breakdown

of

ATP

released

from

erythrocytes

and

endothelial

cells

was

found

to

cause

platelet

aggregation

as

early

as

before

the

concept

of

receptors

was

conceived

-LSB-

-RSB-

Later

the

effects

were

attributed

to

a

single

receptor

designated

but

it

was

not

until

that

the

three

receptors

involved

were

characterised

-LSB-

-RSB-

Two

ADP

receptors

-LRB-

and

-RRB-

and

one

ion

channel

ATP

receptor

-LRB-

-RRB-

are

expressed

on

platelets

-LSB-

-RSB-

-LRB-

Fig.

-RRB-

The

receptor

is

coupled

to

inhibition

of

cAMP

The

molecular

identifications

of

the

receptor

and

generation

of

knockout

mice

revealed

highly

prolonged

bleeding

times

and

their

platelets

aggregate

poorly

in

response

to

ADP

and

display

a

reduced

sensitivity

to

thrombin

and

collagen

-LSB-

-RSB-

The

receptor

is

responsible

for

ADP-induced

shape

change

and

weak

transient

aggregation

-LSB-

-RSB-

while

the

receptor

is

responsible

for

the

completion

and

amplification

of

the

response

to

ADP

and

to

all

platelet

agonists

including

thromboxane

thrombin

and

collagen

The

platelets

also

express

receptors

that

have

been

shown

in

knockout

mice

to

be

of

similar

importance

as

receptors

-LSB-

-RSB-

Fig.

receptor-mediated

regulation

of

coagulation

and

platelet

aggregation

See

text

for

details

ADP

is

an

important

mediator

and

positive

feedback

mechanism

for

platelet

aggregation

Activated

platelets

stimulate

coagulation

i.e.

formation

of

a

fibrin

network

In

contrast

ATP

ADP

and

UTP

release

tissue

plasminogen

activator

-LRB-

t-PA

-RRB-

from

endothelial

cells

resulting

in

degradation

of

the

fibrin

network

At

the

same

time

plasminogen

activator

inhibitor

-LRB-

-RRB-

is

also

released

from

endothelial

cells

by

ATP

stimulation

in

turn

inhibiting

t-PA

Thus

receptors

regulate

several

balancing

factors

in

haemostasis

Ado

adenosine

The

physiological

importance

of

the

receptor

is

not

fully

elucidated

However

recent

studies

have

demonstrated

that

receptors

can

generate

significant

functional

platelet

responses

independently

and

in

synergy

with

other

receptor

pathways

In

addition

studies

in

transgenic

animals

indicate

an

important

role

for

receptors

in

platelet

activation

particularly

under

conditions

of

shear

stress

and

thus

during

arterial

thrombosis

-LSB-

-RSB-

Synergistic

interaction

between

ATP

and

in

stimulating

platelet

aggregation

may

have

significant

clinical

implications

and

suggests

a

prothrombotic

role

for

ATP

in

stress

-LSB-

-RSB-

is

an

important

inhibitor

of

platelet

activation

Unexpectedly

mice

had

prolonged

bleeding

times

with

minimally

perturbed

coagulation

parameters

Platelet

interactions

with

injured

mesenteric

vasculature

were

considerably

reduced

in

vivo

and

purified

mutant

platelets

failed

to

aggregate

to

standard

agonists

in

vitro

-LSB-

-RSB-

This

platelet

hypofunction

was

reversible

and

associated

with

receptor

desensitisation

In

keeping

with

deficient

vascular

protective

mechanisms

fibrin

deposition

was

found

at

multiple

organ

sites

in

mice

and

in

transplanted

cardiac

grafts

Endothelial

ADPase

activity

is

lost

following

ischaemia-reperfusion

injury

xenograft

rejection

and

inflammation

resulting

in

increased

levels

of

ATP

and

ADP

-LSB-

-RSB-

This

may

cause

platelet

aggregation

but

also

other

effects

such

as

mitogenic

effects

on

VSMC

Infusion

of

systemic

apyrase

inhibits

platelet

aggregation

and

prolongs

xenograft

survival

-LSB-

-RSB-

This

has

also

been

shown

with

adenoviral

transfer

of

-LSB-

-RSB-

is

also

lost

in

vascular

cardiac

grafts

subjected

to

oxidant

stress

only

degrades

ATP

to

ADP

and

has

a

prothrombotic

effect

It

is

present

in

the

subendothelium

and

can

shift

the

balance

in

a

prothrombotic

direction

after

endothelial

injury

-LSB-

-RSB-

-LRB-

Fig.

-RRB-

ADP

acting

on

receptors

is

not

only

important

for

platelet

activation

it

also

stimulates

vasoconstriction

Stable

drugs

with

antagonistic

effects

on

receptors

affecting

both

platelets

and

VSMC

could

be

of

double

therapeutic

benefit

in

their

prevention

of

both

thrombosis

and

vasospasm

-LSB-

-RSB-

Coagulation

The

prothrombotic

effect

of

ADP

on

the

platelets

is

counteracted

by

the

effects

of

UTP

and

ATP

on

the

endothelium

that

stimulates

a

substantial

release

of

tissue-type

plasminogen

activator

-LRB-

tPA

-RRB-

with

fibrinolytic

effects

-LSB-

-RSB-

-LRB-

Fig.

-RRB-

To

make

the

picture

even

more

complex

ATP

also

releases

plasminogen

activator

inhibitor

-LRB-

-RRB-

which

in

turn

inhibits

tPA

-LSB-

-RSB-

Thus

receptors

are

involved

at

several

stages

of

haemostasis

which

is

important

for

the

development

of

atherosclerosis

and

thrombotic

occlusions

leading

to

myocardial

infarction

and

stroke

Clinical

importance

of

receptor-mediated

regulation

of

platelets

The

first

ADP

inhibitors

ticlopidine

and

clopidogrel

were

developed

before

the

cloning

and

identification

of

the

platelet

ADP

receptors

Later

it

was

concluded

that

they

were

prodrugs

converted

in

the

liver

to

a

metabolite

that

binds

irreversibly

to

the

receptor

resulting

in

non-competitive

antagonism

Ticlodipine

has

the

disadvantage

of

a

small

risk

of

neutropaenia

so

that

clinically

so

far

the

most

important

contribution

of

drug

development

aimed

at

receptors

has

been

the

beneficial

effects

of

the

platelet

ADP

receptor

antagonist

clopidogrel

in

atherosclerotic

disease

In

the

CAPRIE

trial

clopidogrel

was

even

more

effective

compared

to

aspirin

in

preventing

myocardial

infarctions

-LSB-

-RSB-

Clopidogrel

as

an

addition

to

aspirin

turned

out

to

be

necessary

to

prevent

acute

in-stent

thrombosis

in

percutaneous

coronary

interventions

The

CURE

trial

demonstrated

that

the

addition

of

clopidogrel

to

aspirin

reduced

clinical

events

by

in

acute

coronary

syndromes

-LSB-

-RSB-

This

has

also

been

shown

for

ST-elevation

myocardial

infarctions

-LRB-

COMMIT

CLARITY

-RRB-

However

long-term

secondary

prevention

in

high-risk

groups

with

clopidogrel

added

to

aspirin

did

not

have

significant

benefits

-LRB-

CHARISMA

-RRB-

Clopidogrel

combined

with

aspirin

reduces

cerebral

emboli

in

patients

undergoing

carotid

endarterectomy

-LSB-

-RSB-

Clopidogrel

-LRB-

Plavix

-RRB-

is

now

a

blockbuster

drug

and

one

of

the

most

sold

drugs

worldwide

to

the

benefit

of

atherosclerotic

patients

Nevertheless

clopidogrel

is

a

rather

weak

antagonist

at

receptors

with

variable

effects

often

referred

to

as

clopidogrel

resistance

Newer

receptor

antagonists

such

as

prasugrel

and

have

a

stronger

more

consistent

effect

and

are

currently

being

studied

in

major

clinical

trials

-LRB-

TRITON

and

PLATO

-RRB-

Clinical

importance

of

purinergic

signalling

in

cardiovascular

disease

future

directions

The

receptor

family

provides

opportunities

for

drug

development

-LRB-

see

Table

-RRB-

The

large

number

of

receptor

subtypes

and

the

increasing

knowledge

of

their

tissue

distributions

may

make

it

possible

to

target

a

specific

cardiovascular

organ

with

limited

side

effects

So

far

only

receptor

antagonists

have

been

explored

in

patients

but

with

extraordinary

success

Clopidogrel

is

one

of

the

most

prescribed

drugs

in

the

world

for

the

treatment

of

thrombosis

stroke

and

myocardial

infarctions

in

millions

of

patients

The

following

compounds

prasugrel

and

are

even

more

potent

receptor

antagonists

with

clinical

benefits

currently

being

tested

in

phase

III

trials

Improved

platelet

inhibition

can

also

be

achieved

with

antagonists

against

the

other

two

receptors

on

platelets

Studies

in

and

receptor

knockout

mice

and

experimental

thrombosis

models

using

selective

and

receptor

antagonists

have

shown

that

depending

on

the

conditions

these

receptors

could

also

be

potential

targets

for

new

antithrombotic

drugs

Table

disease

targetsReceptor

targetAgonist/antagonist

activity

of

therapeutic

drugDiseaseVascular

smooth

muscle

restenosis

diabetic

microvascular

disease

subarachnoidal

restenosis

diabetic

microvascular

disease

subarachnoidal

migraine

subarachnoidal

xenograft

rejectionEndothelial

remodelling

-LRB-

cancer

-RRB-

heart

heart

heart

failure

myocardial

reduced

cardiac

output

cardiac

reduced

cardiac

reduced

cardiac

transplantation

myocardial

infarctionInflammatory

infarction

stroke

peripheral

artery

infarction

stroke

peripheral

artery

infarction

stroke

peripheral

artery

infarction

stroke

peripheral

artery

diseaseRed

blood

peripheral

nociceptive

sensory

vascular

vascular

painPerivascular

sympathetic

table

presents

possible

targets

for

the

development

of

new

pharmaceutical

compounds

for

the

treatment

of

cardiovascular

diseases

ATP

UTP

and

UDP

acting

on

and

receptors

are

potent

stimulators

of

inflammation

VSMC

proliferation

and

migration

Antagonists

to

these

receptors

may

be

explored

to

protect

against

atherosclerosis

restenosis

after

balloon

angioplasty

chronic

transplant

rejection

and

diabetic

microvascular

disease

It

is

possible

that

hypertension

and

pulmonary

hypertension

could

be

treated

with

or

receptor

antagonists

In

the

heart

and

receptor

antagonists

may

have

similar

beneficial

roles

as

angiotensin

inhibition

or

β-blockers

in

the

treatment

of

congestive

heart

failure

In

the

acute

phase

of

a

myocardial

infarction

a

UTP

analogue

could

protect

cardiomyocytes

and

limit

infarct

size

A

polymorphism

in

receptors

could

be

considered

as

a

prognostic

marker

to

identify

patients

at

risk

of

myocardial

infarction

and

drugs

aimed

at

the

receptor

could

be

explored

to

prevent

myocardial

infarction

A

promising

strategy

would

be

to

use

soluble

NTPDase

to

prevent

thrombosis

restenosis

and

xenograft

transplantation

protects

against

brain

damage

in

a

stroke

model

and

a

human

genetically

engineered

solNTPDase

has

been

developed

that

could

be

used

in

clinical

trials

Possible

clinical

indications

could

be

stroke

myocardial

infarction

and

transplantation

receptor

antagonists

could

be

used

against

migraine

and

vascular

pain

-LSB-

see

-RSB-

O O O B O O O O O O B O O O O O O O O O O O O O O B O O O O O O O O O O O O O O B O O B O O O B O O O O O B O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O B O O B O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O B O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O B O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O B O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 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