Cell volume regulation in nonrenal epithelia.

Cell volume regulation occurs in both tight, Na+-transporting epithelia (e.g., frog skin) and in leaky. NaCl-transporting epithelia (e.g. amphibian gallbladder). In tight epithelia volume regulation occurs only in response to cell swelling, i.e. only regulatory volume decrease (RVD) is observed, whereas in leaky epithelia cell volume regulation has been observed in response to osmotic challenges that either swell or shrink the cells. In other words, both RVD and regulatory volume increase (RVI) are present. Both volume regulatory responses involve stimulation of ion transport in a polarized fashion: in RVD the response is basolateral KCl efflux, whereas in RVI it is apical membrane NaCl uptake. The loss of KCl during RVD appears to result in most instances from increases in basolateral electrodiffusive K+ and Cl-permeabilities. In gallbladder, concomitant activation of coupled KCl efflux may also occur. The RVI response includes activation of apical membrane cation (Na+/H+) and anion (Cl-/HCO-3) exchangers. It is presently unclear whether the net ion fluxes resulting from activation of these transporters, during either RVD or RVI, account for the measured rates of restoration of cell volume. In gallbladder epithelium, RVD is inhibited by agents which disrupt microfilaments or interfere with the Ca2+-calmodulin system. These pharmacologic effects are absent in RVI. Some steps in the chain of events resulting in either RVI or RVD have been established, but the signals involved remain largely unknown. There is reason to suspect a role of intracellular pH in the case of RVI and of membrane insertion of transporters in the case of RVD, possibly with causal roles of both intracellular Ca2+ and the cytoskeleton in the latter.