Evidence for the presence of both pre‐ and postjunctional P2‐purinoceptor subtypes in human isolated urinary bladder
Authors: Palea S. , Pietra C. , Trist D.G. , Artibani W. , Calpista A. , Corsi M. 
Published: July 2012

In order to characterize P2‐purinoceptor(s) in human urinary bladder the contractile effects of ATP and its slowly‐hydrolyzable analogues α,β‐methylene ATP (α,β‐MeATP) and β,γ‐methylene ATP (β,γ‐MeATP) were investigated on human detrusor strips taken from patients undergoing cystectomy for bladder carcinoma.

Serial concentration‐response curves (SCRC) for ATP, α,β‐MeATP and β,γ‐MeATP were constructed with an interval of 25 min between two successive doses to avoid tachyphylaxis. ATP (10 μm–10 mm) induced a phasic contraction, which was very rapid in onset. The dose‐response curve to ATP appeared not to be monophasic: at the lower concentrations (10–300 μm) the curve was shallow, whilst at high concentrations (1–10 mm) the curve was steeper. The magnitude of the response obtained at the highest concentration tested (10 mm) was only 21.1 ± 2.8% (mean ± s.e. mean; n = 4) of the KCl (100 mm)‐induced contraction.

α,β‐MeATP (0.3 μm–1 mm) and β,γ‐MeATP (10 μm–1 mm) elicited a phasic contraction with a time course similar to that exhibited by ATP. The magnitude of the response obtained at the highest concentration tested (1 mm) was 70.3 ± 6.3% for α,β‐MeATP (n = 10) and 27.9 ± 4.5% for β,γ‐MeATP (n = 8) of KCl (100 mm)‐induced contraction. The rank order of potency was α,β‐MeATP > β,γ‐MeATP > ATP. A plateau of response could not be achieved by any of these agonists.

The P2‐purinoceptor antagonist, suramin (10–300 μm), dose‐dependently antagonized only the lower part of α,β‐MeATP dose‐response curve. Data were analysed in terms of dose‐ratio estimated at two levels of response (10% and 35% of KCl 100 mm‐induced contraction). At 10% of KCl response the Schild plot slope was 0.98 and the estimated pKB was 5.85, whereas using the dose‐ratio at the 35% level of the KCl response, the Schild plot was not linear suggesting an interaction of α,β‐MeATP with a heterogeneous receptor population.

The putative P2‐purinoceptor antagonist, Coomassie Brilliant Blue G (CB‐G) at 0.3 and 1 μm (n = 5), shifted to the left the α,β‐MeATP SCRC. The response at the highest concentration of agonist was potentiated, being equal to 78.8 ± 11.7% of the KCl (100 mm) response (n = 5). CB‐G at 0.3 μm also shifted to the left the β,γ‐MeATP SCRC and significantly potentiated the response at 1 mm up to 46.3 ± 5.6% of KCl 100 mm response (n = 4).

Pretreatment with terodotoxin (TTX) at 1 μm shifted to the left the α,β‐MeATP SCRC but the response to the highest concentration of the agonist was not potentiated, being 73.6 ± 9.9% of the KCl (100 mm) response (n = 5). TTX (1 μm) shifted to the left the β,γ‐MeATP SCRC and significantly potentiated the response at 1 mm (61.6 ± 3.1% of KCl response; n = 4).

The NO synthase inhibitor NG‐nitro‐l‐arginine methyl ester (l‐NAME) at 100 μm did not modify the SCRC to either α,β or β,γ‐MeATP.

We conclude that in human detrusor muscle there is a heterogeneity of purinoceptors. The complex antagonism exhibited by suramin suggests the presence not only of P2x‐purinoceptors but also of another contractile P2‐purinoceptor subtype insensitive to suramin. Moreover, the activity of CB‐G and TTX seems to support the existence of a prejunctional P2‐purinoceptor subtype inducing the release of one or more inhibitor neurotransmitters.