The mechanism of Na+-l-lactate cotransport by brush border membrane vesicles from horse kidney: Analysis of rapid equilibrium kinetics in absence of membrane potential

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Abstract
Membrane transport of lactate was studied using vesicles prepared from horse kidney brush border. It is shown that the carrier-mediated transport ofl-lactate is Na dependent and thed-lactate Na dependence seems weaker than thel stereoisomer. Augmented transport rate is observed following imposition of an artificial chemical Na+ gradient of electrical potential difference. The effect of Na+ chemical gradient on thel-lactate uptake was analyzed using membrane vesicles incubated with 50mm KCl and valinomycin in order to short circuit any contribution of transmembrane electrical potential to the transport. Kinetics results and principally the absence of linearity between 1/v (lactate) versus 1/Na+ show that thel-lactate transport mechanism fit the properties of an ordered process with two Na+ ions cotransported with onel-lactate anion. Thel-lactate and sodium affinities (K m ) determined under Na+ chemical gradient were 1.05 and 48mm forl-lactate and Na, respectively. The sodium activation was shown to be highly cooperative with a Hill number of 2 although no “sigmoidal” activation effect was observed.