Osmotic Swelling Activates two Pathways for K + Efflux in a Rat Hepatoma Cell Line

Abstract

The pathways for the efflux of K+ from osmotically-swollen HTC rat hepatoma cells were investigated using 86Rb+ as a tracer for K+. Exposure of HTC cells to a hypotonic solution (< 250 mOsm kg-1) resulted in a transient efflux of 86Rb + that reached a maximal value after ∼1 min, and inactivated within 3 min. This initial 86Rb+ efflux was inhibited by charybdotoxin, clotrimazole and Ba2+, but not by apamin or paxilline, consistent with it being via an intermediate-conductance Ca2+- activated K+ channel. For cells exposed to an extracellular osmolality < 180 mOsm kg-1 there was an additional 86Rb+ efflux component which was slower to activate, taking 4 - 6 min to reach a maximum, and remaining active for > 20 min. The second 86Rb+ efflux component was not inhibited by K + channel blockers but was inhibited by the anion channel blockers, tamoxifen, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and niflumate. The time-courses for its activation and inactivation, as well as its dependence on the extracellular osmolality, were very similar to those observed for the hypotonically-activated efflux of the organic osmolyte, taurine. The data are consistent with the second component of 86Rb+ efflux and the efflux of taurine from osmotically-swollen cells occurring via a common pathway having a marked selectivity for taurine over 86Rb +.