title: Basolateral transport of taurine in epithelial cells of isolated, perfused Mytilus californianus gills.
abstract: We found that the basolateral surface of the gill epithelium of the marine mussel Mytilus californianus possesses a carrier-mediated process capable of concentrating taurine within epithelial cells. We used retrograde perfusion of gill sections to demonstrate the kinetics, specificity and ion-dependence of taurine transport. [3H]taurine was concentrated relative to a space marker ([14C]mannitol); this accumulation was blocked by the inclusion of 10 mmol l-1 unlabeled taurine in the perfusate. The drop in [3H]taurine uptake at increasing concentrations of unlabeled taurine was fitted to Michaelis-Menten kinetics and indicated a basolateral process with a taurine concentration at which transport is half-maximal (Kt) of 35.3 mumol l-1 and a maximal flux (Jmax) of 0.35 mumol g-1 wet mass h-1. Taurine accumulation on the apical surface had a higher affinity (Kt = 9.5 mumol l-1) and a higher maximum rate of transport (Jmax = 1.23 mumol g-1 h-1). Basolateral transport was inhibited by inclusion in the perfusate of 1 mmol l-1 of another beta-amino acid (beta-alanine), but not by inclusion of alpha-alanine, glutamic acid or betaine. The dependence of basolateral taurine transport on Na+ (when replaced with N-methyl-D-glucamine) was sigmoidal with an apparent Hill coefficient of 2.3, indicating that more than one Na+ is necessary for the transport of each taurine molecule. Complete substitution of Cl- in bathing media reduced taurine accumulation by 90% and 70% on the apical and basolateral surfaces, respectively. Taurine accumulation on both surfaces was reduced by only 20% when Cl- was reduced from 496 to 73 mmol l-1, suggesting that taurine uptake is not significantly influenced by the changes in Cl- concentration accompanying the salinity fluctuations normally encountered by mussels. We estimate that the various Na+ and Cl- gradients naturally encountered by epithelial cells are capable of providing ample energy to maintain a high intracellular concentration of taurine. We suggest that the ability of epithelial cells to accumulate taurine across the basolateral surface from the hemolymph plays a significant role in the intracellular regulation of this important osmolyte and may effect osmolality-dependent changes in the intracellular concentration of taurine.
authors.affiliation: Department of Physiology, College of Medicine, University of Arizona, Tucson 85724.
authors.forename: D S
authors.initials: DS
authors.lastname: Neufeld
authors.forename: S H
authors.initials: SH
authors.lastname: Wright
mesh_terms: D000596:Amino Acids / Q000493:pharmacokinetics / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D049872:Bivalvia / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology; D002712:Chlorides / Q000494:pharmacology; D004847:Epithelial Cells; D005880:Gills / Q000166:cytology / Q000378:metabolism*; D005942:Gluconates / Q000494:pharmacology; D006458:Hemolymph / Q000378:metabolism; D007513:Isethionic Acid / Q000494:pharmacology; D010477:Perfusion; D012964:Sodium / Q000502:physiology; D013654:Taurine / Q000493:pharmacokinetics*
chemical_list: D000596:Amino Acids; D002712:Chlorides; D005942:Gluconates; D013654:Taurine; D007513:Isethionic Acid; D012964:Sodium; C030691:gluconic acid; D002118:Calcium