BASOLATERAL TRANSPORT OF TAURINE IN EPITHELIAL-CELLS OF ISOLATED, PERFUSED MYTILUS-CALIFORNIANUS GILLS

We found that the basolateral surface of the gill epithelium of the ma rine mussel Mytilus californianus possesses a carrier-mediated process capable of concentrating taurine within epithelial cells. We used ret rograde perfusion of gill sections to demonstrate the kinetics, specif icity and ion-dependence of taurine transport, [H-3]taurine was concen trated relative to a space marker ([C-14]mannitol); this accumulation was blocked by the inclusion of 10 mmoll(-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 indi cated a basolateral process with a taurine concentration at which tran sport is half-maximal (K-t) of 35.3 mu moll(-1) and a maximal flux (J( max)) of 0.35 mu mol g(-1) wet mass h(-1). Taurine accumulation on the apical surface had a higher affinity (K-t=9.5 mu moll(-1)) and a high er maximum rate of transport (J(max)=1.23 mu mol g(-1) h(-1)). Basolat eral transport was inhibited by inclusion in the perfusate of 1 mmoll( -1) of another alpha-amino acid (alpha-alanine), but not by inclusion of alpha-alanine, glutamic acid or betaine. The dependence of basolate ral taurine transport on Na+ (when replaced with N-methyl-D-glucamine) was sigmoidal with an apparent Hill coefficient of 2.3, indicating th at more than one Na+ is necessary for the transport of each taurine mo lecule. 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 onl y 20 % when Cl- was reduced from 496 to 73 mmoll(-1), suggesting that taurine uptake is not significantly influenced by the changes in Cl- c oncentration accompanying the salinity fluctuations normally encounter ed by mussels. We estimate that the various Na+ and Cl- gradients natu rally encountered by epithelial cells are capable of providing ample e nergy to maintain a high intracellular concentration of taurine. We su ggest that the ability of epithelial cells to accumulate taurine acros s the basolateral surface from the hemolymph plays a significant role in the intracellular regulation of this important osmolyte and may eff ect osmolality-dependent changes in the intracellular concentration of taurine.