Reversible inhibition of acetylcholine contracture of molluscan smooth muscle by heavy metals: correlation to Ca++ and metal content.

The present study examined the effects of three heavy metals on the acetylcholine (ACh) contracture and Ca++ kinetics of the anterior byssus retractor muscle of Mytilus edulis. An isolated tissue bioassay using anterior byssus retractor muscle was prepared according to standard procedures and the isometric tension produced in response to ACh was measured. Ten millimolar Ni++, Co++ or Cd++ reduced the maximum contracture response to ACh in zero-Ca medium in a time-dependent manner. The inhibition was reversed upon restoration of medium containing 10 mM Ca++. The loss (and re-establishment) of contracture response to ACh corresponded to the influx (and efflux) of the heavy metal ions opposite to the direction of Ca++ flow. These results are consistent with the concept that the loss of the ACh contracture response is attributable to the displacement of tissue Ca++ from release sites by heavy metals.