Lactate transport by skeletal muscle sarcolemmal vesicles

Recent studies have indicated that lactate traversal of the sarcolemmal membrane of skeletal muscle could be a carrier mediated process. In the present study, the initial rates of L(+)-lactate flux (Jlact) were measured in highly purified rat hindlimb skeletal muscle sarcolemmal vesicles. Fluxes were determined by the vesicle uptake of L(+)-[U-14C]lactate from the extra-vesicular medium. Jlact was saturable with respect to increasing concentrations of L(+)-lactate. Regression of these data to the Michaelis-Menten equation yielded a Km of 12.5 mM. Jlact was inhibited 81% by 10 mM pyruvate and 83% by 5mM alpha-cyano 4 hydroxycinnamate (p < 0.05), but not by D-lactate, indicating the presence of a stereoselective monocarboxylate transporter in the sarcolemmal membrane. Preincubation of the vesicles with the protein modifier, N-ethylmaleimide (20mM), inhibited Jlact by 86% (p < 0.05). An inhibitor of the inorganic anion exchanger, SITS (1mM), had no effect on Jlact. However, Jlact was markedly sensitive to an inwardly directed proton gradient (p < 0.05), and the flux was more closely related to the concentration of external ionic L(+)-lactate than to the protonated (HLa) form. These studies suggest that skeletal muscle sarcolemmal membranes possess a specific transport system for L-lactate and other monocarboxylates, which has similar properties to the lactate carrier described for several other tissues.