Hepatocyte water volume and potassium activity during hypotonic stress

Hepatocytes exhibit a regulatory volume decrease (RVD) during hypotonic shock, which comprises loss of intracellular K⁺ and Cl^– accompanied by hyperpolarization of transmembrane potential (V _m ) due to an increase in membrane K⁺ conductance, (G _K). To examine hepatocyte K⁺ homeostasis during RVD, double-barrel, K⁺-selective microelectrodes were used to measure changes in steady-state intracellular K⁺ activity (a _K ⁱ ) and V _m during hyposmotic stress. Cell water volume change was evaluated by measuring changes in intracellular tetramethylammonium (TMA⁺). Liver slices were superfused with modified Krebs physiological salt solution. Hyposmolality (0.8×300 mosm) was created by a 50 mm step-decrease of external sucrose concentration. Hepatocyte V _m hyperpolarized by 19 mV from –27 ± 1 to –46 ± 1 mV and a _K ⁱ decreased by 14% from 91 ± 4 to 78 ± 4 mm when slices were exposed to hyposmotic stress for 4–5 min. Both V _m and a _K ⁱ returned to control level after restoring isosmotic solution. In paired measurements, hypotonic stress induced similar changes in V _m and a _K ⁱ both control and added ouabain (1 mm) conditions, and these values returned to their control level after the osmotic stress. In another paired measurement, hypotonic shock first induced an 18-mV increase in V _m and a 15% decrease in a _K ⁱ in control condition. After loading hepatocytes with TMA⁺, the same hypotonic shock induced a 14-mV increase in V _m and a 14% decrease in a _TMA ⁱ . This accounted for a 17% increase of intracellular water volume, which was identical to the cell water volume change obtained when a _K ⁱ was used as the marker. Nonetheless, hyposmotic stress-induced changes in V _m and a _K ⁱ were blocked partly by Ba²⁺ (2 mm). We conclude that (i) hepatocyte V _m increases and a _K ⁱ decreases during hypotonic shock; (ii) the changes in hepatocyte V _m and a _K ⁱ during and after hypotonic shock are independent of the Na⁺-K⁺ pump; (iii) the decrease in a _K ⁱ during hypotonic stress results principally from hepatocyte swelling.This work was supported by grant AA-08867 from the Alcohol, Drug Abuse, and Mental Health Association.