CALCIUM DEPENDENCY AND THE EFFECT OF CALCIUM-ANTAGONISTS ON MOLLUSCANPROBOSCIS SMOOTH MUSCLES FROM THE WHELK, BUCCINUM-UNDATUM

In all four proboscis muscles of the whelk Buccinum undatum, the potas sium-induced depolarization response was acutely dependent upon extrac ellular calcium, being eliminated in calcium-free conditions. The resp onses to acetylcholine were found to be partly dependent upon intracel lular calcium. Responses to the peptides henylalanine-methionine-argin ine-phenylalanine-NH2 and phenylalanine-leucine-arginine-phenylalanine -NH2 were much more resistant to calcium-free conditions and appeared to engage the excitation-contraction coupling mechanism by mobilizing stored intracellular calcium. Sucrose-gap studies of radular retractor muscles showed that the organic calcium ''antagonist'' nifedipine enh anced potassium-induced depolarization responses, initiating spike-lik e action potentials and associated fast twitch activity. The inorganic calcium antagonist gadolinium exerted concentration-dependent inhibit ory actions on these muscles. Basal tonus and fast twitch activity in response to potassium-induced depolarization were eliminated as were t he spike-like action potentials of the membrane electrical response. T he inorganic calcium ''antagonist'' cadmium greatly enhanced potassium -induced contractures in all four muscles, and on its own it induced t onic force and fast twitches in all the muscles. It seems likely that cadmium may have displaced stored intracellular calcium to induce myof ilament activation. While these molluscan smooth muscles appear to pos sess calcium channels with fast and slow characteristics, their behavi our and pharmacological manipulation is very different from their more well known mammalian transient and long-lasting channel counterparts.