EFFECT OF ISOSMOTIC REMOVAL OF EXTRACELLULAR CA2+ AND OF MEMBRANE-POTENTIAL ON CELL-VOLUME IN MUSCLE-CELLS

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