On the mechanism of calcium actions on the acetylcholine receptor-channel complex at the frog end-plate membrane.

The mechanism of depressant effects of Ca2+ on the acetylcholine (ACh)-activated ion channel conductance was studied in frog muscle fibers. Increasing the extracellular Ca2+ ( (Ca2+)o) suppressed the increase in the end-plate conductance induced by iontophoretic application of ACh and shifted the reversal potential for a neurally-induced end-plate current to a more negative value. The single channel conductance obtained by current fluctuation analysis was reduced at a high (Ca2+)o. The decay phase of a neurally-induced end-plate current was slightly prolonged at a high (Ca2+)o (18 mM), while that of miniature end-plate current and the open time of the end-plate channel measured by noise analysis remained unchanged under the same condition. The Hill coefficient and apparent dissociation constant as measured by quantitative iontophoresis of ACh and comparison of the end-plate response with a theoretical model (DREYER and PEPER, 1975) were not affected by raising (Ca2+)o, while the maximum conductance increase was suppressed. These results suggest that the depressant effect of Ca2+ on the end-plate channel is not due to the effects on surface charges of the membrane, affinity of ACh to the ACh-receptor or binding to intrachannel sites, but presumably arises from the effect on another site involved in the regulation of ion permeation of the receptor-channel complex.