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.
Morishita
H
H
Department of Pharmacology, School of Medicine, Fukuoka University, Japan.
Furukawa
T
T
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D002118:Calcium / Q000172:deficiency*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D006168:Guinea Pigs; D006584:Hexamethonium Compounds / Q000494:pharmacology; D066298:In Vitro Techniques; D007213:Indomethacin / Q000494:pharmacology; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects*; D009638:Norepinephrine / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D011983:Receptors, Purinergic / Q000187:drug effects*; D013779:Tetrodotoxin / Q000494:pharmacology
D002121:Calcium Channel Blockers; D006584:Hexamethonium Compounds; D011983:Receptors, Purinergic; D013779:Tetrodotoxin; D011189:Potassium Chloride; D001285:Atropine; D000255:Adenosine Triphosphate; D011433:Propranolol; D002118:Calcium; D009638:Norepinephrine; D011224:Prazosin; D007213:Indomethacin