ATP INDUCED-RELAXATION IN THE MOUSE BLADDER SMOOTH-MUSCLE

1 The effect of adenosine 5'-triphosphate (ATP) on the free cytosolic Ca2+ concentration ([Ca2+]i) as measured with the fluorescent Ca2+-indicator fura-2, and on force was investigated in the intact smooth muscle strips of the mouse urinary bladder. 2 ATP elicited, when exogenously applied, a large increase of [Ca2+]i With limited force development resulting in a marked Ca2+-force dissociation. 3 Release of endogenous neurotransmitters by transmural electrical stimulation (TES) for 30 s induced a steady increase of [Ca2+]i and a peak contraction, followed within 15 s by a relaxation. 4 In carbachol-prestimulated preparations. ATP elicited an initial rise of [Ca2+]i followed by a return to the initial precontraction Ca2+-level. Force in contrast presented a biphasic pattern, i.e. an initial contraction was followed by a sustained relaxation. 5 In the K+-depolarized precontracted preparation, ATP elicited a slight initial rise of [Ca2+]i. The partial relaxation of the force during depolarization was not preceded by a transient contraction. 6 The ATP-induced relaxation of the K+-prestimulated preparations was not inhibited by 8-phenyltheophylline, a potent P1-purinoceptor antagonist. 7 The order of potency for relaxation of the ATP analogues was 2-MeSATP>ATP>betagammaMe-ATP, which is characteristic for P2y-purinoceptors. 8 These results indicate that, besides its activating effect, ATP also relaxes the mouse urinary bladder. It is suggested that the relaxant effect, mediated through P2y-purinoceptors, is mainly responsible for the low contractile potency of ATP in the bladder.