Comparison of contractions of the smooth muscle of the guinea-pig vas deferens induced by ATP and related nucleotides.

The shape of contractile responses of the isolated guinea-pig vas deferens changes as ATP concentration is increased from 10(-7) to 10(-2) M. The ATP concentration-response curve is bimodal and reflects the change in response profile. Initially spike-like (10(-7) -3 x 10(-5) M) in nature, contractions acquire a secondary, slower tonic phase in transitional ATP concentrations (greater than or equal to 3 x 10(-5) M). At high ATP concentrations (10(-2) M) the secondary phase predominates. To determine if there are structural requirements for these complex effects, responses to ATP were compared to those elicited with analogs containing phosphate-chain, ribose and adenine modifications. In general, substitution of 5'-anhydride linkages with methylene or imido bridges prolonged responses to low concentrations but at high concentrations both potentiated and abbreviated the responses. ATP gamma S, a substrate for phosphohydrolases which incorporate phosphate but which are less able to remove thiophosphate, produced responses with a greatly prolonged tonic phase. Removal of the 2'-hydroxyl of ATP resulted in reduced potency at low concentrations while removal of the 3'-hydroxyl was without effect. Modification of both the 1 and N6 positions of adenine substantially reduced agonist activity. Responses to ATP and the beta, gamma-methylene congener were unaffected by treatment with 10(-5) M indomethacin. The results indicate that more than one simple interaction of ATP with a receptor is involved in the production of responses to ATP. Of several hypotheses discussed, we favor one which suggests that the phasic responses to low concentrations of ATP are receptor-mediated and modified by tissue enzymes, while those to high concentrations are mediated, in part, by hydrolysis per se. The hypothesis, which realizes that conformational preferences may exist as well, is proposed for the vas deferens only.