SODIUM-TAURINE COTRANSPORT IN REPTILIAN RENAL BRUSH-BORDER MEMBRANE-VESICLES

The coupled transport of Na+ with taurine into snake renal brush-border membrane vesicles (BBMV) was studied using 5-s uptake conditions. Taurine transport into snake renal BBMV involved two parallel processes, one saturable (Na+-dependent) and one (Na+-independent) that behaved like passive diffusion. Below 1 mM taurine concentration, the Na+-dependent system accounted for 60% of total taurine uptake. Over both low (0.001-0.80 mM) and high (0.8-5.0 mM) taurine concentration ranges, the Na+-dependent taurine uptake within each range showed Michaelis-Menten kinetics, suggesting the presence of two independent saturable Na+-dependent transport systems for taurine. The high-affinity, low-capacity system saturated above 100-mu-M with a K(m) of 71.4 +/- 45.7-mu-M and a maximum velocity (V(max)) of 21.9 +/- 3.77 pmol (mg protein)-1 (5 s)-1. The low-affinity, high-capacity system saturated above 1 mM, with a K(m) of 1.11 +/- 0.63 mM and a V(max) of 252 +/- 47 pmol (mg protein)-1 (5 s)-1. The stoichiometric relationship between external Na+ concentration and taurine uptake (at 10-mu-M) by the high-affinity BBMV transport system was examined by the activation method under short-circuited conditions. The 5-s rate of taurine transport was a sigmoid function of increasing extravesicular Na+ concentration. Kinetic analysis of the interaction of Na+ with the high-affinity taurine transport system suggested that 3 Na+ ions (3.2 +/- 0.7) may be involved with 1 taurine molecule in the transport event. The data suggest the presence of a highly efficient and high-affinity reabsorptive taurine transport system on the luminal membrane of the kidney of the garter snake, a species that can secrete taurine in vivo.