Taurine transport in skate hepatocytes. I. Uptake and efflux
Ballatori, N.; Boyer, J.L.
American Journal of Physiology 262(3 Pt 1): G445-G450
1992
ISSN/ISBN: 0002-9513 PMID: 1550234 Document Number: 389094
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 .mu.M taurine at 15.degree. 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 .cntdot. .mu.l-1 .cntdot. 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 acid 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 (.apprx.10% in 2 h) and was unaffected by isosmotic replacement of extracellularly 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.