Electrophysiological properties of resting secretory membranes of lamellibranch mantles. Interaction between calcium and potassium.

This study concerns the effects of ions on the shell-secreting membrane of clam mantles. The average resting potentials were --47 mV for freshwater mantles and --60 mV for marine mantles. Elevation of potassium in the absence of chloride gave a maximal slope of depolarization equivalent to 59 mV for a 10-fold change in the marine form but much less in the freshwater form. In normal potassium, a 10-fold reduction in calcium produced a hyperpolarization of 6 mV for the freshwater mantle. Neither reduction nor elevation of calcium affected the potential of marine mantles in the presence of normal potassium, but a hyperpolarization of 8 mV occurred when calcium was deleted in a low-potassium medium. Elevated calcium reduced the depolarization induced by raised potassium in both species and resulted in an increased effective membrane resistance in marine mantles. Lowered calcium enhanced the hyperpolarization caused by reduction in potassium in freshwater mantles but not in the marine species. Replacement of chloride by large anions produced transient depolarization in both freshwater and marine mantles and resulted in a maintained increased effective membrane resistance in marine mantles. The effects of sodium and magnesium on the membrane potential were not significant in normal potassium. We conclude that the secretory membrane of freshwater and marine clam mantles is permeable mainly to potassium and chloride, and that responses of the membrane potential to calcium are mediated through its effect on the permeability to potassium.