Symmetry and pH dependency of the lactate proton carrier in skeletal muscle studied with rat sarcolemmal giant vesicles

Muscle lactate transport was studied in rat sarcolemmal giant vesicles, in which the membrane orientation was similar to that of intact cells. The lactate concentration dependency was studied using influx and efflux experiments under equilibrium exchange, zero-trans, and infinite-cis conditions. At constant external and internal pH the carrier behaved symmetrically with regard to lactate transport from the two sides of the membrane. Lactate at the trans side stimulated the lactate tracer flux (trans-acceleration). Both in zero-trans influx and efflux experiments the lactate flux was stimulated in a symmetric way by H+ at the cis side, consistent with protons being the substrate for the carrier. In contrast, the lactate flux was symmetrically inhibited by H+ at the trans side (trans-inhibition), the underlying mechanism is likely that the protonated carrier with no lactate bound is unable to reorient in the membrane. These findings have implications both for the kinetic model and for understanding the physiological function of the lactate/proton transporter.