The biphasic response of rat vesical smooth muscle to ATP.
1. Adenosine-5'-triphosphate (ATP) is known to exert a variety of biological effects via the activation of either ionotropic P2x- or G-protein coupled P2Y-purinoceptor subtypes. In this study the effects induced by ATP and ATP analogues on rat bladder strips were characterized at resting tone and in carbachol-prestimulated tissues. 2. ATP exerted a clear concentration-dependent biphasic response, which was maximal at 1 mM concentration and was characterized by an immediate and transient contraction, followed by a slower sustained relaxation. The receptor mediating contraction was susceptible to desensitization by ATP and by the ATP analogue, alpha,beta-methyleneATP (alpha,beta-meATP) showing the typical features of the P2x-purinoceptor; conversely, ATP-evoked relaxation did not undergo tachyphylaxis following either ATP or alpha,beta-meATP. 3. The slower and sustained relaxant phase seemed to be due to activation of P2Y-purinoceptors, based on responses obtained with the P2Y agonist, 2-methyl-thioATP (2-meSATP) and, more importantly, based on the clear involvement of the G-proteins. In fact, the G-protein activator, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) significantly potentiated and the G-protein blocking agent, guanosine 5'-O-(2-thio-diphosphate) (GDP beta S) completely abolished the ATP-induced relaxation. No effects were exerted by these two G-protein modulators on the ATP-induced contraction. 4. The relaxant component of the ATP response of bladder tissue was not significantly influenced by nitro-benzyl-thioinosine (NBTI) or by 8-phenyltheophylline (8-PT), suggesting that the contribution of the ATP metabolite adenosine to this response was negligible. Moreover, relaxation evoked by ATP and by the adenosine analogue, 5'-N-ethylcarboxamidoadenosine (NECA) was additive.5. Suramin was unable to modify either the relaxant or the contractile responses of bladder strips to ATP. However, when tested on the concentration-response curve to the slowly hydrolysable P2x-agonist alpha,beta-meATP, a rightward shift was detected, suggesting that ATP contractile responses are mediated by suramine-sensitive P2x-purinoceptors.6. Uridine-5'-triphosphate (UTP) only induced a rapid and concentration-dependent contraction of the rat bladder preparation, which was not desensitized by pre-exposure to alpha,beta-meATP, suggesting that UTP responses were not mediated by the 'classical' P2X-purinoceptor.7. It is therefore concluded that both P2x- and P2y-purinoceptors, which mediate ATP-induced contraction and relaxation, respectively, are present in rat bladder. Moreover, removal of epithelium did not affect ATP-elicited contraction, whereas ATP-induced relaxation was significantly augmented. These data suggest that P2x- and P2Y- purinoceptors are localized in smooth muscle cells and that the relaxant response is probably modulated by excitatory factor(s) released by epithelial cells.
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Institute of Pharmacological Sciences, University of Milan, Italy.
Pinna
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Abbracchio
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Cattabeni
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Puglisi
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D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000494:pharmacology*; D019830:Adenosine-5'-(N-ethylcarboxamide); D000818:Animals; D004305:Dose-Response Relationship, Drug; D006160:Guanosine Triphosphate / Q000494:pharmacology; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009126:Muscle Relaxation / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley; D018048:Receptors, Purinergic P2 / Q000187:drug effects; D013868:Thioinosine / Q000031:analogs & derivatives / Q000494:pharmacology; D014544:Uridine Triphosphate / Q000494:pharmacology; D001743:Urinary Bladder / Q000187:drug effects* / Q000294:innervation; D014665:Vasodilator Agents / Q000494:pharmacology
D018048:Receptors, Purinergic P2; D014665:Vasodilator Agents; D019830:Adenosine-5'-(N-ethylcarboxamide); D013868:Thioinosine; D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate; C001789:4-nitrobenzylthioinosine; D000241:Adenosine; D014544:Uridine Triphosphate