Effect of isosmotic removal of extracellular Ca2+ and of membrane potential on cell volume in muscle cells.

Isosmotic removal of extracellular Ca2+ (Cao) and changes in membrane potential (Vm) are frequently performed manipulations. Using isolated voltage-clamped barnacle muscle cells, we studied the effect of these manipulations on isosmotic cell volume. Replacing Cao by Mg2+ induced 1) verapamil-sensitive extracellular Na(+)-dependent membrane depolarization, 2) membrane depolarization-dependent cell volume reduction in cells whose sarcoplasmic reticulum (SR) was presumably loaded with Ca2+ [intracellular Ca2+ (Cai)-loaded cells], and 3) cell volume increase in cells whose SR was presumably depleted of Ca2+ (Cai-depleted cells) or in Cai-loaded cells whose Vm was held constant. Membrane depolarization induced 1) volume reduction in Cai-loaded cells or 2) verapamil-sensitive volume increase in Cai-depleted cells. This suggests tha, in Cai-loaded cells, membrane depolarization induces SR Ca2+ release, which in turn promotes volume reduction. Conversely, in Cai-depleted cells, the depolarization activates Na+ influx through a verapamil-sensitive pathway leading to the volume increase. This pathway is also revealed when Cao is removed in either Cai-depleted cells or in cells whose Vm is held constant.