Low calcium and calcium antagonists potentiate the contraction of guinea-pig vas deferens induced by ATP: a permissive role for P2-purinoceptors.

ATP, noradrenaline and KCl induced contractions of the isolated guinea-pig vas deferens. The ATP-induced contraction was potentiated by decreasing the external calcium concentration and was reduced by increasing the external calcium concentration. The maximum potentiation was obtained at a low calcium concentration (0.8 mM), the dose-response curve for ATP was shifted to the left in a parallel fashion at this concentration. Calcium antagonists, such as verapamil and diltiazem and MnCl2, induced a similar potentiation. On the other hand, the noradrenaline- and KCl-induced contractions were reduced by calcium antagonists and by decreasing the external calcium concentration. The ATP- and KCl-induced contractions were slightly potentiated by the removal of Mg ions from the medium, but the contractile response to ATP was not potentiated by pretreatment with difluorodinitrobenzene, an ecto-ATPase inhibitor. These results suggest that the affinity of P2-purinoceptors for ATP may be regulated by calcium sites to which calcium and calcium antagonists can bind.