EXTERNAL CA EFFECT ON WATER PERMEABILITY, REGULATORY VOLUME DECREASE,AND EXTRACELLULAR-SPACE IN BARNACLE MUSCLE-CELLS

The effect of extracellular Ca2+ (Ca(o)) on sarcolemmal hydraulic wate r permeability (L(p)'), regulatory volume decrease (RVD), and extracel lular space (ECS) was studied in barnacle muscle cells. Absence or pre sence of Ca(o) had no effect on L(p)' [0 Ca. = 2.762 +/- 0.098 x 10(-5 ), and 11 mM Ca(o) = 2.720 +/- 0.222 X 10(-5) cm . kg . s-1 . osmol-1 . kgH2O-1]. Likewise, cells exposed to anisosmotic media (for <30 min) behaved as osmometers in 0 and 11 mM Ca(o), showing similar slopes an d intercepts in van't Hoff plots. At longer incubation times, however, hyposmotic conditions promoted a Ca(o)-dependent RVD. The relationshi p between Ca(o) and the percentage of cells responding with RVD to a h yposmotic challenge was sigmoidal (half-maximal Ca(o) = 4.83 mM). The mean rate of RVD (40 nl/min) was independent of the level of swelling in response to hyposmotic challenges. However, the magnitude of RVD in creased with larger hyposmotic challenges. Both the presence of Ca(o) and hypotonicity reduced the ''apparent'' ECS by 47 +/- 6 and 39 +/- 6 %, respectively. Three-dimensional reconstruction of autoradiographs o f the cells was made to interpret these results.