Trans-stimulation of lactate transport from rat sarcolemmal membrane vesicles

We examined the trans-stimulation characteristics of L(+)-lactate efflux from rat skeletal muscle sarcolemmal membrane vesicles isolated by sucrose density gradient centrifugation. Vesicles were preloaded with 1 mM L(+)-lactate containing 14C tracer lactate. Initial efflux of intravesicular lactate was stimulated above baseline efflux by the presence of 30, 50, or 100 mM L(+)-lactate in the external medium; resulting efflux rates were 120, 138, and 141% of baseline values, respectively. Extravesicular pyruvate (100 mM) stimulated efflux to only 110% of baseline, while unlabeled D(-)-lactate in the external medium did not stimulate lactate efflux. Trans-stimulation of L(+)-lactate efflux in response to extravesicular lactate exhibited saturation kinetics. We conclude that the sarcolemmal lactate transporter is stereospecific for L(+)-lactate; pyruvate can share the transporter, although the affinity of the transporter for pyruvate is less than that for L(+)-lactate. Furthermore, because the presence of lactate on one side of the sarcolemma stimulates transport from the opposite side, the transporter is not likely to be a pore or channel, but rather this carrier probably undergoes a conformational change as it translocates lactate from one membrane side to the other.