Influence of cell heterogeneity on skeletal muscle lactate kinetics

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Abstract
Lactate and [14C]lactate kinetics were studied in three rabbit skeletal muscle preparations with distinct fiber type profiles, 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 2 h in the presence of lactate (1 mM), glucose (5 mM), [6-3H]glucose, and [U-14C]lactate. All preparations displayed net lactate release, [14C]lactate removal, and 14CO2 release. Net lactate release was greatest in the glycolytic preparation, 9.7 +/- 0.5 mumol.100 g-1.min-1, and least in the oxidative preparation, 3.7 +/- 0.2 mumol.100 g-1.min-1. [14C]lactate arteriovenous difference was greatest in the mixed preparation, 1,688 +/- 58 (disintegrations/min)/ml (dpm/ml), and least in the glycolytic preparation, 505 +/- 10.3 dpm/ml. Steady-state incorporation of [14C]lactate was observed in CO2, amino acids, and pyruvate. Tissue lactate specific activity (LSA) in all preparations was significantly lower than arterial LSA, but not significantly different from venous LSA. Estimates of lactate removal based on venous LSA were not significantly different from net glycolytic flux. In conclusion, 1) under basal, resting conditions net lactate release and [14C]lactate removal are properties of all fiber types, and 2) tracer estimates of lactate turnover in skeletal muscle reflect net glycolytic flux through pyruvate.