A VOLUME-ACTIVATED TAURINE CHANNEL IN SKATE HEPATOCYTES - MEMBRANE POLARITY AND ROLE OF INTRACELLULAR ATP

Osmoregulation in isolated hepatocytes and perfused livers of the little skate (Raja erinacea), an osmoconforming marine elasmobranch, is mediated in part by the uptake or release of the intracellular osmolyte taurine. To further characterize the efflux mechanism, [C-14]taurine release and Na+-independent uptake were assessed after cell swelling in hypotonic media containing 0.1-100 mM taurine. Rate coefficients for [C-14]taurine uptake (0.016 +/- 0.002 min(-1)) and efflux (0.015 +/- 0.003 min(-1)) were similar and independent of extracellular taurine concentration, indicating that a taurine-permeable channel mediates the release of this amino acid after cell swelling. Volume-activated taurine uptake and efflux were both blocked by pretreatment with the metabolic inhibitors 2,4-dinitrophenol, antimycin A, and KCN plus iodoacetate, by the sulfhydryl-reactive compound N-ethylmaleimide and the transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. [C-14]taurine release via this channel was immediately blocked if isotonicity was restored or a membrane-permeable metabolic inhibitor (0.5 mM 2,4-dinitrophenol) was added at different times after a hypotonic stimulus. Similar, although delayed, effects were noted with antimycin A and KCN plus iodoacetate. When isolated perfused skate livers were exposed to hypotonic stimuli, all of the taurine was released into the sinusoidal circulation, but not into bile, an effect that was also blocked by restoring isotonicity. These findings demonstrate that taurine efflux from skate hepatocytes after cell swelling is mediated by a channel. This channel is localized to the basolateral membrane and appears to require the continual presence of intracellular ATP for its function.