Cl- and membrane potential dependence of amino acid transport across the rat renal brush border membrane

The relative roles of the anion present and the membrane potential in the o peration of each of the seven amino acid transport systems in the renal tub ular brush border membrane were explored by manipulating transmembrane pote ntial and chemical gradients across the membrane. The effect of various ext ernal anions with different permeabilities of the membrane and of valinomyc in-generated K+ diffusion potential on Na+-coupled amino acid accumulation by rat renal brush border membrane vesicles was examined. Accumulation of a ll amino acids examined, except for cystine, was membrane potential depende nt. The highest voltage dependence was observed for taurine (equivalent to glucose) and L-methionine. Addition of taurine uptake values obtained under each electrical gradient (inside negative) and a chemical gradient (100 mM NaCl out) condition yielded markedly lower values than under conditions wh ere there was a combined electrochemical gradient. Cl- gradient rather than merely imposing a voltage gradient was a specific mediator of Na+-coupled transport of L-proline, taurine, L-glutamic acid, and glycine across the br ush border membrane. Cl- gradient alone under Na+-equilibrated conditions c ould energize an overshoot of taurine accumulation by vesicles providing ev idence that taurine is energetically activated by and coupled to Cl- transp ort. These data suggest that Na+-linked transport of most amino acids acros s the tubular luminal membrane is an electrogenic positive process and for proline, taurine, glutamic acid, and glycine, a Cl--requiring process. A ne gative intracellular potential combined with luminal chloride is required f or optimal Na+-coupled transport of these amino acids across the luminal me mbrane of the proximal tubule. The coupling of Cl- to the transport of thes e osmoprotective amino acids may enhance their volume regulatory effect in kidney cells and other mammalian cells. (C) 1999 Academic Press.