On the Origin of the Bioelectrical Potential Generated by the Freshwater Clam Mantle

Mantles from freshwater clams develop potential differences (PD's) between the two surfaces when they are bathed in vitro with artificial saline solutions. The magnitude and polarity of the PD is dependent on [Ca²⁺] in the solution bathing the mantle's shell surface. When the solutions are gassed with 5% CO₂ in oxygen, the PD is in the range 25 to 50 mv, shell side positive. It decreases if [Ca²⁺] in the shell solution is elevated. The concentration dependence is logarithmic with a slope of about -27 mv per 10-fold change in [Ca²⁺], slightly less than predicted by the Nernst equation for a membrane acting as a calcium electrode. Analysis of the electrical behavior both in intact mantles and in isolated epithelia indicates that most of the PD develops across the external membranes of epithelial cells on the shell side. There is no evidence that an active calcium transport system is involved in electrogenesis, and a model based on calcium diffusion across a selectively permeable membrane can explain existent data. If CO₂ is absent, the mantle PD is very small (2–10 mv), but still sensitive to change in external [Ca²⁺]. It is proposed that CO₂ alters intracellular pH, thereby changing the equilibrium between a large store of nonionized calcium and [Ca²⁺] in the cells. A role for carbonic anhydrase in the CO₂ effect is suggested by the action of a specific inhibitor of this enzyme. The diffusion model predicts that increasing ionized calcium should increase the PD as is actually observed. Some implications of this system for the physiology of calcium movement in vivo are discussed.