VOLUME-ACTIVATED CALCIUM-UPTAKE - ITS ROLE IN CELL-VOLUME REGULATION OF MADIN-DARBY CANINE KIDNEY-CELLS

Immediately after osmotic swelling of Madin-Darby canine kidney (MDCK) cells, a transient (1-2 min) increase in Ca2+ influx and internal Ca2+ (Ca(i)2+) is observed. The normal Ca2+ influx appears to be mediated by the 3Na+-Ca2+ exchange system [Borle et al. Am. J. Physiol. 259 (Cell Physiol. 28): C19-C25, 1990], but the swelling-induced component is different in 1) Na+ dependence, 2) affinity for Ca2+, 3) inhibition by La3+, and 4) direction of net flux at low external Ca2+. Swelling appears to activate an uncoupled Ca2+ flow, perhaps through cation-nonspecific stretch-activated channels. The regulatory volume decrease (RVD) is dependent on the swelling-induced pulse of Ca2+ influx and associated rise in Ca(i)2+. Swelling also induces a biphasic change in membrane potential, a hyperpolarization followed by depolarization, reflecting sequential increases in K+ and Cl- permeabilities. The time dependence of the former corresponds closely with the transient peak in Ca(i)2+, but the latter does not. Ca(i)2+ appears to have a direct activating effect on K+ channels but an indirect effect on Cl- channels, mediated via other Ca2+-triggered systems. The sequence of events following cell swelling appears to be transient increases in Ca2+ permeability, Ca2+ influx, Ca(i)2+, K+ permeability, followed by triggering of a mediating system that increases Cl- permeability. The net result is KCl, osmotic water loss, and volume adjustment.