Hypoosmotic Cell Volume Regulation in Marine Bivalves: The Effects of Membrane Potential Change and Metabolic Inhibition

Abstract

Molluscan cells volume regulate in dilute salinities by releasing amino acids from an intracellular pool. The magnitude of this efflux depends upon external divalent cation concentration. Incubation of isolated Modiolus demissus ventricles in a K⁺ solution isosmotic to 100% seawater (SW) results in membrane depolarization and an increased amino acid efflux. In the presence of Ca²⁺-Mg²⁺-free SW, the myocardial cell membrane is depolarized and an amino acid efflux occurs. Additional removal of Na⁺ repolarizes the membrane and reduces the amino acid release. Cyanide or 2,4-DNP in 50% SW result in a prolonged amino acid efflux from the isolated hearts. Decreased temperature markedly reduces the hypoosmotically induced amino acid efflux. These results indicate that the control mechanism of the amino acid release from molluscan cells stressed by hypoosmotic salinities depends upon external divalent cation concentration, membrane permeability, and ATP.