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Journal of Experimental Biology 149,379-394 (1990)
Published by Company of Biologists 1990

CALCIUM UTILIZATION IN CONTRACTURES INDUCED BY ACETYLCHOLINE OR HIGH-POTASSIUM SALINE IN MOLLUSCAN PROBOSCIS MUSCLES

D. D. BROOKS 1, H. HUDDART 2, R. LENNARD 3, and R. B. HILL 1

1 Department of Zoology, The University of Rhode Island, Biological Sciences Center, Kingston, Rhode Island 02881, USA; Permanent address: School of Applied Biology, Lancashire Polytechnic, Preston PR1 2TQ,UK
2 Department of Zoology, The University of Rhode Island, Biological Sciences Center, Kingston, Rhode Island 02881, USA; Permanent address: Division of Biological Sciences, Institute of Environmental and Biological Sciences, University of Lancaster, Lancaster LAI 4YQ, UK
3 Department of Zoology, The University of Rhode Island, Biological Sciences Center, Kingston, Rhode Island 02881, USA

The mechanisms by which high-K+ saline and acetylcholine (ACh) mobilize cellular calcium in molluscan muscle was studied in three proboscis muscles of Busycon canaliculatum. BAY K 8644, a calcium agonist, enhanced tonic force induced by exposure to 25 mmoll-1 K+ in the odontophore and radular retractor muscles but inhibited this response in the radular sac muscle. Its effect on the radular protractor muscle was concentration-dependent, excitatory at 10-7 mol l-1 and inhibitory at higher concentrations. The tonic response to 80 mmoll-1 1 K+ was enhanced by BAY K 8644 in the odontophore retractor and radular retractor muscles but inhibited in the radular sac muscle. In all muscles, BAY K 8644 eliminated the fast twitches induced by 80 mmolI-1 K+, and inhibited the tonic ACh responses. Sucrose-gap studies with the radular protractor muscle showed that the effects of BAY K 8644 on the depolarizations induced by K+ and ACh were similar. The effect on the ACh-induced depolarization suggested that this agent had inhibited sodium influx through the ACh receptor

Mytelase enhanced ACh-induced tonic force, but at 10-4moll-1 it inhibited the response to doses of ACh above 10-5moll-1. Mytolon reduced the maximum ACh-induced tonic force and increased the response threshold to ACh. ACh and high-K+ depolarization pathways were not synergistic for force development

The existence of two separate cellular calcium pools, independently released by high-K+ saline and ACh, seems improbable. We suggest that K+ and ACh act via separate mechanisms to release calcium from a single cellular calcium pool

Key words: Buyscon canaliculatum, proboscis muscles, BAY K 8644, K+ and ACh responses, cellular calcium

Accepted on November 12, 1989






© The Company of Biologists Ltd 1990