From: arora@maths.tcd.ie (Ashish Arora)
Newsgroups: sci.med,sci.research
Subject: Pregnancy induced hypertension and prostaglandins.
Date: 29 Aug 92 21:14:48 GMT
Organization: Dept. of Maths, Trinity College, Dublin, Ireland.


	Approximately 10% of pregnant females have high blood pressure. The 
classification of this problem is rather complex, but a subgroup of these
people have what is known as pregnancy induced hypertension or pre-eclampsia
or pre-eclamptic toxemia (which is a misnomer as there is no toxin circulating).	This is a disorder involving various organ systems in the body. The 	precise cause of this problem is unknown. Apart from a higher perinatal
mortality associated with this condition, if not treated, it can progress to
eclampsia. Eclampasia is characterised by very high blood pressures leading to
death due to intracerebral haemorrhage. At present there are various
hypothesis suggesting a cause for this disorder. These including abnormalities
in the uteroplacental circulation, an abnormality in the ratios of 
prostaglandins etc. 
	With pulmonary embolism, pregnancy induced hypertension (PIH) is the
leading cause of maternal death during pregnancy in the western world.	
	Could anyone with a knowledge of prostaglandins or obstetrics give
a critical appraisal of the following essay (which I wrote about a year ago)
entitled "Imbalances in the ratio of thromboxane/prostacyclin as a cause of
pregnancy induced hypertension -- fact or fancy ?".

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	With thromboembolism, pregnancy induced hypertension (PIH), 
occuring in 5-15% of pregnancies, remains the most important cause of
maternal and fetal morbidity and mortality (1).
	Recently, there has been interest in the role of the products of
fatty acid oxygenation in the pathogenesis of P.I.H. Three major routes
of biological oxygenation of polyunsaturated icosenoic fatty acids
such as icosatrenoic (eg. arachidonic acid), icosatetraenoic and 
icosapentanoic acids has been recognized. These routes involve addition of 
molecular oxygen via the lipoxygenase pathway to produce prostaglandins,
prostacyclin and thromboxane, and via the mono-oxygenase pathway 
(cytochrome-p450) to produce epoxides & other products (2).
	The levels of the products of fatty acid oxygenation may be
controlled by changes in the activity of the biosynthetic enzymes. 
Levels can also be affected by changes in the rate of catabolism.
However, another method of controlling the levels of the products of
fatty acid oxygenation is by a changing proportion of the products 
formed. This would especially apply in organs where the products
have opposite actions eg prostayclin and thromboxane A2.
	One of the mechanisms suggested for the general vasoconstrictive
state in P.I.H. is an imbalance in the production of the vasoactive 
prostaglandins thromboxane A2 and prostacyclin (prostaglandin I2 or
epoprostenol) (3,4,5). Both these compounds are produced by the 
cyclooxygenase pathway.
	In the cyclooxyganase pathway polyunsaturated fatty acids are
used to form PGG2. PGG2 is unstable and via enzymatic action is converted
to PGH2, an endoperoxide unequally unstable. The endoperoxides PGG2 and
PGH2 are converted, by various tissue bound specific isomerases, into
the various prostaglandins, thromboxane and prostacyclin.
	           Polyunsaturated fatty acids                                                                  |
                                |
                                |
                                v
                 --------------PGG2--------------
                 |              |               |
                 v              |               v
               PGI2             |              TxA2
                 ^              |               ^
                 |              |               |
                 --------------PGH2--------------                       
                                |
                                |
                                v
                            Other products 

	Prostacyclin can be formed from PGG2 or PGH2. It is a potent
vasodilator and an inhibitor of platelet aggregation. It also inhibits uterine
activity and increases uteroplacental blood flow. It is mostly found in the
vascular tissues and the stomach (5,6).
	Prostacyclin is hydrolysed non-enzymatically to 6-keto-PG1alpha or
metabolized enzymatically with the formation of a 15-keto metabolite by 
specific NADP-dependent 15-hydroxygenase (15-PGDH). The 15-keto metabolite
is then converted to different products depending on the pathway taken.
15-PGDF is thought to be responsible for the removasl of the majority of
the biological potency of prostaglandins, it also happens to be ubiquitous in 
most mammalian organs. Its activity profile is highly organ specific (7).
	Urinary or srum 6-keto-PGF1alpha is taken as a marker for the 
in vivo production of prostacyclin. This is done even though it is not 
known what proportion of prostacyclin is degraded through enzymatic 
pathways and whether this distribution changes in different physiological
states eg pregnancy (6).
	Thromboxane A2 can be formed from PGG2 and PGH2. It is a potent 
vasoconstrictor and a stimulator of platelet aggregation. It also stimulates
uterine activity and decreases uteroplacental blood flow (4). It is rapidly
hydrolysed to thromboxane B2 at a physiological pH (TxA2 t1/2 = 30s).
Thromboxane B2 and its metabolites have been used for the monitoring of the
production of thromboxane A2.
	All pregnancy associated tissues are known to produce thromboxane A2
in vitro. The order of thromboxane A2 production is approximately 
placenta > decidua > amnion > chorion > myometrium. It is also one of the 
major products of platelets (4).
	Walsh et al (3) have shown by in vitro studies that the production of
thromboxane B2 by preeclamptic placentas is significantly greater than that
in normal placentas. The preeclamptic patients produced over three times as
much thromboxane B2 as normal placentas. In contrast, preeclamptic patients
produced less than one half as much prostacyclin as the normal placentas.
Also, in normal pregnancy, the placenta produced approximately equal 
amounts of prostacyclin and thromboxane, but the preeclamptic placentas
produced over seven as much thromboxane as prostacyclin.
	But whether the above observations reflect in vivo changes in
thromboxane and prostacyclin biosynthesis is debatable. This is due to
difficulties associated with the determination of normal thromboxane
and prostacyclin biosynthesis. Thormboxane A2 metabolism shows a diurnal
variation in that the excretion is higher during 2-3 hr after awakening
than during the rest of the day. This increase in thromboxane A2 
biosynthesis coincides with an increased platelet aggregability in healthy
humans during the morning hours. Another important finding in healthy humans is the severfold increases in the excretion of the prostacyclin metabolite
during physical exercise. Thus, when studying prostacyclin levels it is
important to minimise physical activity in the subject.
	If these confounding factors are taqken into account it is not
surprising that contradictory results were obtained by Fitzgerald et al (1).
They performed in vivo studies on patients with established, mild P.I.H.
and found no differencein the excretion of metabolites of thromboxane B2 with
respect to healthy, pregnant patients. Also, the average increase in the 
ratio of serum thromboxane B2 to serum 6-keto-PGF1alpha in placebo treated 
women who subsequently became hypertensive was similar to that for the group as a whole.
	The exact role of an imbalance in prostaglandins with respect to
P.I.H. is, thus, unknown. Regardless of this the hypothesis of an imbalance in 
thromboxane A2/prostacyclin ratio serves as a basis for attempts at 
prophylactic treatment.
	A single dose of 0.5-1.0g of aspirin causes a substantial inhibition
of thromboxane A2 synthesis for 2-3 d and a shortlasting inhibition, 3-4 h,
of prostacyclin synthesis in humans (as was first shown by Vesterqvist in
1986) (6).
	The reasons given for the prolonged reduction of thromboxane A2
synthesis is the irreversible acetylation , at serine-530 (8), of the 
enzyme cyclooxygenase. This replaces a bulky group at the binding site and
interferes with the arachidonic cyclooxygenase active site. This inhibits the
formation of thromboxane A2 in platelets, which have no nuclei. In contrast
nucleated cells like endothelial cells are capable of resynthesising new
cyclooxygenase and the endothelium can therefore recover its prostacyclin
biosynthesis after aspirin.
	The above might not apply in pregnancy as all pregnancy associated 
tissue are known to produce thromboxane A2. Whether aspirin has the same 
effect on these nucleated cells as on platelets in non-pregnancy states,
to me, is dubious. Whether aspirin has a favourable/unfavourable effect 
in pregnancy will probably become more clear when the results of the 
current Collaberative Low Dose Aspirin study in Pregnancy (CLASP) trial
are published.
	Hypothetically aspirin would be of no benefit if there were other
mechanisms for prostaglandin production not involving cyclooxygenase.	
	It has recently been reported that in vivo synthesis of prostaglandins
can occur, without cyclooxygenase activity, by free radical catalysis (9).
	Prostaglandins can also be produced from products diverted from 
other fatty acid oxidation pathways. Cytochrome P450 dependent epoxygenase
pathway is reported to metabolise aracidonic acid to four novel isomeric
epoxyeicosatrienoic acids (EET's).
	Some of these EET's due to the cellular proximity of cytochrome
P450 and cyclooxygenase in blood vessels can be further transformed by 
cyclooxygenase which results in confering different biological properties
to them eg 5,6-epoxyeicosatrienoic acid and 20-hydroxyeicosatetraenoic acids are transformed to vasoactive mediators after metabolism by cyclooxygenase.
	Furthermore, products of the cytochrome P450 system are found to be
abnormal in P.I.H. This was particularly marked with one metabolic product
which has been tentatively identified as the major P450 product of
arachidonic acid in human reproductive tissue (10).
	It is an oversimplification to consider that prostacyclin and 
thromboxane are the only compounds involved in vascular haemodynamics.
	The vascular endothelial cells lining the blood vessels are much
more than a semipermeable membrane between the blood and the vascular smooth
muscle.
	In fact the endothelial system can be regarded as a highly active
metabolic and endocrine organ in its own right. So far, it is known to 
produce two unstable vasodilator substances - prostacyclin and 
endothelium derived relaxing factor (EDRF), which is neither nitric oxide or
nitrocystine. At the moment not much is known about EDRF synthesis except that 
it is formed from arginine and needs cofactors such as NADPH, 
tetrahydrobiopterin and calmodulin (8).
	Physical or chemical perturbation of or damage to the cell membrane
results in the formation of prostacyclin or/and EDRF. Physiologically,
prostacyclin is considered to be a local hormone rather than a circulating one
(t1/2 = 2-3 m). It may also be released by arterioles sensitive to an increased
blood flow velocity (wall sheer stress) causing local vasodilation (11).
	Aspirin and similar substances prevent prostacyclin formation but
have little effect on normal blood pressure. This shows that the endothelial
prostacyclin production has litle effect on normal blood pressure. However,
inhibitors of EDRF production, such as N-monomethyl-1-L-arginine or 
N-nitro-L-arginine give an immediate increase in blood pressure, strongly
suggesting that a basal production of EDRF actively dilates the normal 
circulation (12).
	With regard to EDRF and blood pressure control it is known that 
haemoglobin and haemoglobin/heptaglobin complex bind to and inhibit the
effects of EDRF. This is suggested to mediate the vasoconstriction induced by 
haemoglobin. Haemoglobin infusion also leads to glomerular fibrin deposition
similar to that sen in the kidney of patients who are pre-eclamptic. An
increased total haemoglobin concentration is associated with 
pre-eclampsia (13).
	Another vasoconstrictive substance, apart from thromboxane A2, is 
endothelin-1 (ET-1). This is one of three vasoconstrictive substances
released by endothelium. It is a 21 amino acid peptide bound together by
two disulfide bridges so that the three dimensional structure forms a
conical spiral. The most striking property of ET-1 is its long lasting 
hypewrtensive action. ET-1 is the most active pressor substance yet discovered,
with a potency some ten times of angiotensin II. The kidney is about ten times 
more sensitive than other vascular regions to the vasoconstrictive effects of
ET-1.
	The exact physiological role of ET-1 is as yet unknown but the 
widespread distributoin of ET-1 receptors and the potent pharmacological
actions of ET-1 implies a regulatory function in the cardiovascular system.
Because of its slow release and long action it is likely that ET-1 would 
be involved in long term (hours or days) regulation of the system (13).
	All the above, I think shows that the basic theorey behind stating
that P.I.H. is related to changes in thromboxane A2/prostacyclin ratio is
flawed.
	The situation is far more complex with many other mediators involved
in vascular haemodynamics. The present knowledge about these mediators is 
scanty and until the basic mechanisms involved in normal people is obtained
the effect of these mediators in the normal or abnormal pregnancy state is
impossible to understand.

 
******************************************************************************

References :

1) Thromboxane A2 synthesis in pregnancy induced hypertension ; Fitzgerald
et al; p751-754; The Lancet, Vol. 335, 1990.
2) Classification of products of fatty acid oxidation; Advances in 
prostaglandins, Thromboxanes & Leukotriene Research (APTLR); p 1-9; Vol. 18
1989.
3) Preeclampsia : an imbalance in placental prostacyclin & thromboxane 
production; Walsh; p335-340; Am. Journal of OBS. & Gyni; Vol. 152, no. 3,
1986.
4) Prostacyclin and Thromboxanes in Gynaecology and Obstetrics ; Yikorkala
et al; p318-325, Am Journal of Obs. and Gyni.; Vol. 152, No. 3, 1985.
5) The use of aspirin to prevent pregnancy induced hypertension & lower
the ratio of thromboxane A2 to prostacyclin in relatively high risk
pregnancies; Schiff et al, p351-356, NEJM, Vol. 321, no 6, 1989.
6) Activation of thromboxane and prostacyclin biosynthesis in humans ;
Krister & Vesterqvist, p607-611, APTLR, vol 21b, 1980.
7) Biosynthesis and catabolism of prostaglandins; APTLR (exact date lost).
8) The aspirin and haeme binding site of PGGH synthetase; Smith et al;
p77-81, Vol 21a, 1990.
9) Formation of unique biologically active prostaglandins in vivo by a 
non-cycloxygenase free radical catalysing mechanisms; Morrow et al; 
p125-129, APTLR, Vol 21a, 1990.
10) Biosynthesis of P450 products of arachidonic acid in humans - increased
formation in cardiovascular disease; Castella et al; p193-197, APTLR,
vol. 21a, 1990.
11) A new role for prostaglandins in the regulation of peripheral resistance;
Koler & Kaley, p595-599, APTLR, Vol. 21b, 1990.
12) Mediators from the endothelial cell ; Vane & Botting, p 627-623, APTLR,
Vol. 21b, 1990.
13) Hypothesis : inhibition of endothelial derived relaxing factor by
haemoglobin in the pathogenesis of preeclampsia; Sarrel et al, p1030-
1031; Lancet, Vol. 336, 1990.

APTLR = Advances in Prostaglandin, Thromboxane & Leukotriene Research.