MUSCLE LACTATE TRANSPORT STUDIED IN SARCOLEMMAL GIANT VESICLES

Lactate transport was studied in giant (median diameter 6.3-mu-m) sarcolemmal vesicles obtained by collagenase treatment of rat skeletal muscle. The lactate transport displayed stereospecificity, had a high temperature coefficient, and could be inhibited up to 90% with known transport inhibitors (PCMBS and cinnamate). In equilibrium exchange experiments, the L-lactate flux demonstrated saturation kinetics with K(m) = 23.7 mM and V(max) = 108 pmol cm-2 s-1. With lactate present on only one side of the membrane, (zero trans conditions), V(max) was reduced to 48 pmol cm-2 s-1. The flux rate displayed transacceleration. The lactate flux was coupled to a parallel H+ flux. Under equilibrium exchange conditions, the carrier-mediated lactate flux was not pH-dependent. In the zero trans experiments, H+ on the trans side acted as an inhibitor. The loaded form of the carrier reorients faster than the unloaded form, and the protonated form with no lactate bound reorients slowly or is immobile. When compared to intact muscles, the giant sarcolemmal vesicles retain their transport characteristics both qualitatively and quantitatively.