TAURINE TRANSPORT IN SKATE HEPATOCYTES .1. UPTAKE AND EFFLUX

Taurine is an amino sulfonic acid maintained at relatively high concentrations in skate (Raja erinacea) hepatocytes (65 mmol/l intracellular water). To identify the transport mechanisms involved in intracellular taurine accumulation, uptake and efflux of [C-14]taurine was measured in freshly isolated skate hepatocytes. Uptake of 50-mu-M taurine at 15-degrees-C was essentially linear over 3 h when measured in normal elasmobranch Ringer, was diminished by replacement of extracellular Cl- with NO3-, and was almost completely abolished by replacement of Na+ with choline+ or K+. Kinetic studies suggested the presence of two saturable Na+-dependent taurine uptake systems [apparent taurine Michaelis constant = 0.089 +/- 0.028 and 4.47 +/- 0.49 mM and maximal velocity (V(max)) = 0.19 +/- 0.07 and 1.65 +/- 0.42 (SE) nmol.mu-l-1.15 min-1 for high-and low-affinity components, respectively; n = 4], as well as a small contribution from an Na+-independent uptake system. Uptake was inhibited by other beta-amino acids but not by alpha-amino acids, taurocholate, or 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (0.5 mM). In contrast to uptake, efflux of [C-14]taurine was relatively slow (approximately 10% in 2 h) and was unaffected by isosmotic replacement of extracellular Na+ with choline+ or K+ or by replacing Cl- with NO3-. These findings suggest distinct mechanisms for taurine uptake and efflux in skate hepatocytes; uptake is largely Na+ dependent and requires Cl- for maximal activity, whereas the efflux pathway is independent of transmembrane Na+, K+, or Cl- gradients. Maintenance of high intracellular taurine levels in skate hepatocytes is achieved by active Na+-dependent uptake processes and a relatively slow efflux.