RENAL TRANSPORT OF TAURINE IN LUMINAL MEMBRANE-VESICLES FROM RABBIT PROXIMAL TUBULE

The uptake of taurine by luminal membrane vesicles from pars convoluta and pars recta of rabbit proximal tubule was examined. In pars convoluta, the transport of taurine was characterized by two Na+-dependent (K(m1) = 0.086 mM, K(m2) = 5.41 mM) systems, and one Na+-independent (K(m) = 2.87 mM) system, which in the presence of an inwardly directed H+-gradient was able to drive the transport of taurine into these vesicles. By contrast, in luminal membrane vesicles from pars recta, the transport of taurine occurred via a dual transport system (K(m1) = 0.012 mM, K(m2) = 5.62 mM), which was strictly dependent on Na+. At acidic pH with or without a H+-gradient, the Na+-dependent flux of taurine was drastically reduced. In both kind of vesicles, competition experiments only showed inhibition of the Na+-dependent high-affinity taurine transporter in the presence of beta-alanine, whereas there was no significant inhibition with alpha-amino acids, indicating a beta-amino acid specific transport system. Addition of beta-alanine, L-alanine, L-proline and glycine, but not L-serine reduced the H+-dependent uptake of taurine to approx. 50%. Moreover, only the Na+-dependent high-affinity transport systems in both segments specifically required Cl-. Investigation of the stoichiometry indicated 1.8 Na+:1 Cl-:1 taurine (high affinity), 1 Na+:1 taurine (low affinity) and 1 H+:1 taurine in pars convoluta. In pars recta, the data showed 1.8 Na+:1 Cl-:1 taurine (high affinity) and 1 Na+:1 taurine (low affinity).