TRANS-STIMULATION OF LACTATE TRANSPORT FROM RAT SARCOLEMMAL MEMBRANE-VESICLES

We examined the trans-stimulation characteristics of L(+)-lactate effl ux from rat skeletal muscle sarcolemmal membrane vesicles isolated by sucrose density gradient centrifugation. Vesicles were preloaded with 1 mML(+)-lactate containing C-14 tracer lactate. Initial efflux of int ravesicular lactate was stimulated above baseline efflux by the presen ce 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 b aseline, while unlabeled D(-)-lactate in the external medium did not s timulate lactate efflux. Trans-stimulation of L(+)-lactate efflux in r esponse to extravesicular lactate exhibited saturation kinetics. We co nclude that the sarcolemmal lactate transporter is stereospecific for L(+)-lactate; pyruvate can share the transporter, although the affinit y of the transporter for pyruvate is less than that for L(+)-lactate. Furthermore, because the presence of lactate on one side of the sarcol emma stimulates transport from the opposite side, the transporter is n ot likely to be a pore or channel, but rather this carrier probably un dergoes a conformational change as it translocates lactate from one me mbrane side to the other. (C) 1994 Academic Press, Inc.