AJP - Gastrointestinal and Liver Physiology, Vol 262, Issue 3 445-G450, Copyright © 1992 by American Physiological Society

ARTICLES

Taurine transport in skate hepatocytes. I. Uptake and efflux

N. Ballatori and J. L. Boyer
Department of Biophysics, University of Rochester School of Medicine, New York 14642.

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 [14C]taurine was measured in freshly isolated skate hepatocytes. Uptake of 50 microM 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 (Vmax) = 0.19 +/- 0.07 and 1.65 +/- 0.42 (SE) nmol.microliter-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 [14C]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.

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