Effects of extracellular sodium on calcium efflux and membrane current in single muscle cells from the barnacle

The actions of extracellular sodium (Na(o)) on membrane potential, membrane current, membrane conductance and Ca efflux were examined in single muscle cells from the giant barnacle, Balanus nubilus. The intracellular compartment was perfused to facilitate the control of intracellular constituents including calcium ions (Ca(i)2+). Changing Na(o) has no large effect on Ca efflux when free intracellular calcium activity, [Ca2+](i), is low (about 0.1 μM). However, increasing [Ca2+](i) leads to the development of Na(o)-dependent Ca efflux as well as to an augmentation in Na(o)-independent Ca efflux. Reducing Na(o) (using Li+ as a substitute cation) leads to a depolarization of the membrane when [Ca2+](i) is low (about 0.1 μM). Increasing [Ca2+](i) causes the membrane to depolarize. With [Ca2+](i) at about 10.0 μM, reduction of Na(o) produces a hyperpolarization of the membrane. Significant Na(o)-dependent inward current is seen when [Ca2+](i) is high. This current is large with respect to the Na(o)-dependent changes in Ca efflux (about 1 μA per p-mole/sec). The Ca(i)2+-activated, Na(o)-dependent changes in Ca efflux and membrane current are not sensitive to La(o)3+. However, La(o)3+ does inhibit a fraction of the Ca(i)2+-activated changes in membrane current and Ca efflux which are not dependent on Na(o). Over a limited range of membrane potential Ca efflux is not voltage-dependent. Possible relationships between the Na(o)-dependent changes in Ca efflux and Na(o)-dependent changes in membrane potential or current are discussed. We find that these changes cannot be readily interpreted in terms of a single transport mechanism.