Taurine transport in skate hepatocytes. I. 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 [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.
Ballatori
N
N
Department of Biophysics, University of Rochester School of Medicine, New York 14642.
Boyer
J L
JL
D000596:Amino Acids / Q000378:metabolism; D000818:Animals; D001386:Azides / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D002250:Carbon Radioisotopes; D002478:Cells, Cultured; D002794:Choline / Q000494:pharmacology; D008099:Liver / Q000378:metabolism*; D008297:Male; D011189:Potassium Chloride / Q000494:pharmacology; D012860:Skates, Fish; D019810:Sodium Azide; D012965:Sodium Chloride / Q000494:pharmacology; D013379:Substrate Specificity; D013654:Taurine / Q000378:metabolism*
D000596:Amino Acids; D001386:Azides; D002250:Carbon Radioisotopes; D013654:Taurine; D012965:Sodium Chloride; D011189:Potassium Chloride; D019810:Sodium Azide; D002794:Choline