Unusual responses of proboscis muscles ofBusycon canaliculatum to some calcium antagonist agents

Summary K- and ACh-induced responses of the radular sac, odontophore retractor, and radular retractor muscles ofBusycon canaliculatum were found to be strongly dependent upon Ca]₀. Diltiazem had strong positive inotropic and chronotropic actions on fast twitch activity in the odontophore retractor and radular protractor muscles. K-induced tonic force in these muscles was partly inhibited by diltiazem but only at very high concentrations. ACh responses in all muscles were eliminated by diltiazem. Nifedipine enhanced fast twitches and tonic force in response to high K, and induced persistent spontaneous fast twitch discharges. Nifedipine inhibited ACh-induced tonic force, but induced rhythmic bursts of fast twitches persisting long after nifedipine washout. Verapamil strongly inhibited K- and ACh-induced tonic force in all three muscles at high concentration, but stimulated fast twitch responses and converted ACh contractures into fast twitch activity. Sucrose gap studies showed that nifedipine and diltiazem reduced K- and ACh-induced tension and depolarization. Paradoxically, verapamil reduced K- and ACh-induced tension but significantly enhanced their induced depolarizations. Diltiazem, nifedipine and verapamil did not act like slow Ca channel antagonists in these muscles. This may reflect differences in channel structure between molluscs and mammals, or differences in the cellular calcium release pathways operated by such channels in molluscan and mammalian muscle. These Ca-ant-agonists appeared to act as agonists of fast twitch activity in these muscles and antagonists of the ACh-induced calcium release pathway for tonic force development.