Role of cell type in net lactate removal by skeletal muscle.

Net lactate uptake and subsequent pathways for removal were studied in three rabbit skeletal muscle preparations of distinct fiber type composition, i.e., glycolytic (99.1 +/- 0.2% type IIb fibers), oxidative (97.5 +/- 0.6% type I fibers), and mixed (type I, IIa, and IIb fibers). Single-pass perfusions were carried out for 3 h in the presence of glucose, lactate, and [U-14C]lactate. Lactate levels, initially set at either 1 mM (n = 4/prep) or 2 mM (n = 4/prep), were elevated twice during the perfusion at 60 and 120 min. Net lactate uptake (mumol.100 g-1.min-1) was first observed in the oxidative preparation, 1.4 +/- 0.2, at an arterial lactate concentration of approximately 2.5 mM, whereas net lactate uptake in the glycolytic, 0.7 +/- 0.2, and mixed preparations, 7.0 +/- 0.5, was first observed at 4 mM. Net lactate balance, [14C]lactate removal, and 14CO2 release demonstrated strong linear correlations (r = 0.94-0.98) with arterial lactate concentration. To quantify the fate of [14C]lactate, preparations were perfused at a single elevated lactate concentration (approximately 8 mM) for 2 h. Oxidation was the primary means of disposal in the oxidative and mixed preparations, whereas glyconeogenesis dominated removal in the glycolytic preparation. The arterial lactate concentration at which a given muscle switches from net production to net removal, the rate of removal, and subsequent pathway(s) for disposal are a function of that muscle's fiber type composition.