Lactate transport activity in rat skeletal muscle sarcolemmal vesicles after acute exhaustive exercise

The effect of a single bout of exhaustive exercise on muscle lactate transp ort capacity was studied in rat skeletal muscle sarcolemmal (SL) vesicles. Rats were assigned to a control (C) group (n = 14) or an acutely exercised (E) group (n = 20). Exercise consisted of treadmill running (25 m/min, 10% grade) to exhaustion. SL vesicles purified from C and E rats were sealed be cause of sensitivity to osmotic forces. The time course of 1 mM lactate upt ake in zero-trans conditions showed that the equilibrium level in the E gro up was significantly lower than in the C group (P < 0.05). The initial rate of 1 mM lactate uptake decreased significantly from 2.44 +/- 0.22 to 1.03 +/- 0.08 nmol min-l mg protein-l (P < 0.05) after exercise, whereas that of 50 mM lactate uptake did not differ significantly between the two groups. For 100 mM external lactate concentration ([lactate]), exhaustive exercise increased initial rates of lactate uptake (219.6 +/- 36.3 to 465.4 +/- 80.2 nmol.lmin(-l).mg protein(-1), P < 0.05). Although saturation kinetics were observed in the C group with a maximal transport velocity of 233 nmol.min( -1).mg protein(-1) and a Michealis-Menten constant of 24.5 mM, saturation p roperties were not seen after exhaustive exercise in the E group, because i nitial rates of lactate uptake increased linearly with external [lactate]. We conclude that a single bout of exhaustive exercise significantly modifie d SL lactate transport activity, resulting in a decrease in 1 mM lactate up take and was associated with alterations in the saturable properties at [la ctate] above 50 mM. These results suggest that changes in sarcolemmal lacta te transport activity may alter lactate and proton exchanges after exhausti ve exercise.