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 P 2X ‐ or G‐protein coupled P 2Y ‐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, α,β‐methyleneATP (α,β‐meATP) showing the typical features of the P 2X ‐purinoceptor; conversely, ATP‐evoked relaxation did not undergo tachyphylaxis following either ATP or α,β‐meATP. 3 The slower and sustained relaxant phase seemed to be due to activation of P 2Y ‐purinoceptors, based on responses obtained with the P 2Y 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) (GTPγS) significantly potentiated and the G‐protein blocking agent, guanosine 5′‐O‐(2‐thio‐diphosphate) (GDPβS) 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 P 2x ‐agonist α,β‐meATP, a rightward shift was detected, suggesting that ATP contractile responses are mediated by suramine‐sensitive P 2x ‐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 α,β‐meATP, suggesting that UTP responses were not mediated by the ‘classical’ P 2X ‐purinoceptor. 7 It is therefore concluded that both P 2X ‐ and P 2Y ‐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 P 2x ‐ and P 2y ‐ purinoceptors are localized in smooth muscle cells and that the relaxant response is probably modulated by excitatory factor(s) released by epithelial cells.