NICKEL INHIBITION OF CALCIUM RELEASE FROM SUBSARCOLEMMAL CALCIUM STORES OF MOLLUSCAN SMOOTH-MUSCLE

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Abstract
Replacement of Ca in artificial sea water (ASW) by Ni causes a loss of Ca at a single exponential rate (.tau. = 23.2 min) and the loss of contracture response to acetylcholine (ACh) in the anterior byssus retractor muscle of Mytilus edulis. The ACh contracture response is lost at 2 rates; 93% of the ACh contracture response is lost rapidly (.tau. = 7.43 min) and the remaining 7% of ACh contracture response is lost slowly (.tau. = 66.6 min). The rapid phase of loss of ACh contracture corresponds to the rapid exponential uptake of Ni(.tau. = 6.5 min). The loss of ACh contracture response is attributed to the displacement of Ca from membrane release sites by Ni. In O-Ca-Mg ASW, the ACh contracture responses are well maintained over a 35-min period when stimulated by ACh every 5 min. Stimulation by ACh after 1, 20 and 40 min in O-Ca-Mg ASW results in a reduction of ACh contracture response to 15% of the response in Ca ASW. Similar treatment in O-Ca-Ni ASW reduces the ACh contracture response to 4%. K and caffeine contracture responses at 1, 20 and 40 min in O-Ca-Mg ASW are reduced to 25 and 28%. Similar treatment in O-Ca-Ni ASW produces a block of K contracture response and reduces caffeine contracture response to 3%. The 45Ca uptake data demonstrate that Ca influx is not required for the ACh contracture responses of anterior byssus retractor muscle.