Hepatocyte transmembrane potential (V(m)) during osmotic stress responds as an osmometer, in part because of changes in membrane K+ conductance. This may contribute to the electromotive force that drives transmembrane Cl- fluxes. To test this, double-barreled ion-sensitive microelectrodes were used to measure changes in steady-state intracellular Cl- activity (a(Cl)/(i)) during osmotic stress applied to mouse liver slices. Hyperosmotic and hyposmotic conditions were created by rapidly switching to a solution in which sucrose concentrations were increased or reduced, respectively. Hyperosmotic stress [1.4 x control osmolality (280 mosmol/kgH2O)] decreased hepatocyte V(m) 46% from -39 ± 1 to -21 ± 1 mV (SE; n = 16 animals). Corresponding a(Cl)/(i) increased twofold from 19 ± 2 to 38 ± 3 mM. This shifted the Cl- equilibrium potential (E(Cl)) 19 mV, from -38 ± 0.3 to -19 ± 2 mV. Hyposmotic stress [0.71 x control osmolality (290 mosmol/kgH2O)] increased hepatocyte V(m) 64% from -28 ± 1 to -46 ± 1 mV (SE; n = 13 animals). Corresponding a(Cl)/(i) decreased 0.53-fold from 17 ± 1 to 8 ± 1 mM. This shifted the E(Cl) 20 mV from -26 ± 2 to -46 ± 3 mV. Thus hepatocyte a(Cl)/(i) is in electrochemical equilibrium with V(m). The paired measurements above were repeated after addition of K+-channel blockers quinine or Ba2+. Ba2+ (2 mM) had no effect on either V(m) or a(Cl)/(i) during hyperosmotic stress; however, Ba2+ significantly inhibited changes in V(m) and a(Cl)/(i) during hyposmotic stress. Effects of quinine (0.5 mM) on V(m) and a(Cl)/(i) during both hyperosmotic stress and hyposmotic stress were similar to those of Ba2+. A previous report shows that inhibition of hyposmotic stress-induced V(m) changes results in loss of hepatocyte volume regulation and greater swelling. Thus osmotic stress-induced changes in a(Cl)/(i) are nowhere near those predicted by cell water volume changes based on transmembrane osmotic pressure differences. We conclude that these larger changes in a(Cl)/(i) resulted from voltage-driven transmembrane Cl- fluxes.

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