Lactate transport by cardiac sarcolemmal vesicles.

L-lactate is taken up by cardiac sarcolemmal vesicles in a process that is saturable with respect to L-lactate, stereospecific, associated specifically with the sarcolemmal membrane, and inhibited by other monocarboxylic acids and by the protein modifiers p-chloromercuriphenyl-sulfonate and N-ethylmaleimide. 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, an inhibitor of the inorganic anion transporter, is without effect. The L-lactate transport is very sensitive to pH. Uptake is stimulated by a proton gradient directed inward and decreased when internal pH is lower than external pH. Passive diffusion of nonionized lactic acid into the vesicles is negligible at physiological pH and appears to remain minor even when external pH is lowered by more than one unit. Also, the mechanism does not require specific Na+-L-lactate contransport. The properties of the L-lactate transporting system in cardiac sarcolemmal vesicles appear similar to those of the monocarboxylate transporter in erythrocytes, hepatocytes, and Ehrlich ascites cells. The present results do not allow a distinction to be made between stepwise interaction of lactate- and H+ or association of nonionized lactic acid with the carrier.