CHARACTERIZATION OF THE NA-DEPENDENT TAURINE INFLUX IN FLOUNDER ERYTHROCYTES()

About 92% of the taurine influx in flounder erythrocytes at physiologi cal conditions in vitro (330 mosmol.l-1, 145 mmol-l-1 Na+, 0.30 mmol.l -1 taurine) is Na+-dependent. This influx is highly specific for tauri ne. The beta-amino compounds hypotaurine and 8-alanine were the only c ompounds which mimicked the inhibitory effect of taurine on influx of [C-14]taurine, the former more than the latter. Counterexchange of tau rine was also mediated by the taurine transporters. Reduction of osmol ality per se did not affect the activity of these transporters. Non-li near regression analysis of the influx values revealed the presence of two different influx systems: a system with high affinity and low cap acity and another with low affinity and high capacity. However, we can not exclude the possibility that this influx of taurine was mediated b y only one transporter which operated in different modes depending on the extracellular Na+ concentration. On the assumption that the Na+-de pendent influx was mediated by two separate systems, the maximal veloc ity of the low capacity system was 2.55 nmol . g dry weight-1.min-1 at 145 mmol.l-1 extracellular Na+. This capacity was about 50% lower tha n that of the high capacity system. The Michaelis constants were 0.013 and 1.34 mmol.l-1, respectively. Reduction of the extracellular Na+ c oncentration reduced maximal velocity and the affinity to taurine of b oth transport systems. At 10 mmol.l-1 Na+ or lower concentrations the high capacity system did not seem to operate. The activation method su ggested that each taurine molecule transported by the high capacity sy stem was accompanied by two Na+. The stoichiometry of the low capacity system was 1 taurine: 1 Na+. The Hill-coefficient for both transport systems was 1.00.