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J. Biol. Chem., Vol. 263, Issue 25, 12190-12193, 09, 1988

Stoichiometry of the renal sodium-L-lactate cotransporter

B Barbarat and RA Podevin
Department of Physiology, Faculte de Medecine Xavier Bichat, Paris, France.

We re-examined the electrical and stoichiometric properties of the Na+- L-lactate cotransporter using highly purified brush-border membrane vesicles prepared from the whole cortex of rabbit kidney. A valinomycin- induced K+ diffusion potential (interior-negative) stimulated Na+ gradient-dependent L-lactate uptake. However, this stimulation reflected catalytic rather than energetic activation as an inside- negative membrane potential did not induce net uphill lactate accumulation in the presence of Na+ but in the absence of a Na+ concentration gradient. Additional evidence for electroneutrality of the cotransporter was the finding that, under voltage-clamped conditions, L-lactate flux was a hyperbolic function of extravesicular Na+ concentration with a Hill coefficient (n) of 1.0. Moreover, the plot of V/[Na+]n versus V was linear for n = 1, indicating that one Na+ ion is co-transported with an anionic lactate1- molecule. Finally, addition of L-lactate to vesicles under Na+ equilibrium conditions failed to generate an inside-positive membrane potential as monitored by 3,3'-dipropylthiodicarbocyanine iodide fluorescence quenching, arguing against Na+-L-lactate cotransport by an electrogenic process. Taken together, these data indicate that the luminal Na+-L-lactate co- transporter is electroneutral with a stoichiometry of 1.
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