EFFECT OF ISOSMOTIC REMOVAL OF EXTRACELLULAR NA-VOLUME AND MEMBRANE-POTENTIAL IN MUSCLE-CELLS( ON CELL)

Isosmotic removal of extracellular Na+ (Na-o) is a frequently performe d manipulation. With the use of isolated voltage-clamped barnacle musc le cells, the effect of this manipulation on isosmotic cell volume was studied. Replacement of Na-o by tris(hydroxymethyl)aminomethane produ ced membrane depolarization (similar to 20 mV) and cell volume loss (s imilar to 14%). The membrane depolarization was verapamil insensitive but depended on extracellular Ca2+ (Ca-o) and was probably due to acti vation of intracellular Ca2+ (Ca-i)-dependent nonselective cation chan nels. The cell volume loss did not require membrane depolarization but depended on Ca-o. This was probably due to an increase in Ca-i, media ted by activation of Ca2+ influx via Na+/Ca2+ exchange. Na-o replaceme nt by Li+ also promoted membrane depolarization (similar to 20 mV) and cell volume loss (20%). Both effects were reduced (similar to 73%) bu t were not abolished by Ca-o removal. Under this condition, the remain ing membrane depolarization was probably due to a higher membrane perm eability of Li+ over Na+. The remaining cell volume loss was due to me mbrane depolarization, which probably induced Ca2+ release from intrac ellular stores.