LACTATE METABOLISM IN INACTIVE SKELETAL-MUSCLE DURING LACTACIDOSIS

Contributions of carbohydrate and fat metabolism to the removal of a lactate (Lac-) load were quantified in inactive soleus (SL), plantaris (PL), and white gastrocnemius (WG) rat hindlimb muscle. Male Sprague-Dawley rats were perfused for 60 min with normal perfusate (NP, n = 8) or a high-lactate perfusate (LP, n = 8), simulating ionic conditions found in arterial blood and plasma after intense exercise: Lac- = 11.0 mM, K+ = 7.88 mM, and pH = 7.15. Metabolite fluxes across the hindlimb were calculated from blood flow and arteriovenous differences. In NP, Lac- was continuously released (2.9 +/- 0.2-mu-mol.min-1.100 g-1). However, in LP, a rapid and significant uptake of Lac- increased muscle Lac- fivefold to 39.6 +/- 1.1, 33.1 +/- 2.2, and 28.8 +/- 1.7-mu-mol/g dry wt in SL, PL, and WG, respectively. Glucose and O2 uptakes were similar during LP and NP perfusion. Glycerol release increased eightfold to 3.3 +/- 0.7-mol.min-1.100 g-1 in response to LP. Muscle ATP, creatine phosphate, glycogen, glycolytic intermediate, and triacylglycerol concentrations did not change. However, muscle lactate-to-pyruvate ratios were elevated in all muscles of the LP group postperfusion, indicating changes in the mass action ratio at the pyruvate dehydrogenase reaction. In LP, of 80-mu-mol of Lac- taken up, 11% was accounted for by increased muscle Lac-, 12-24% was oxidized, and 5% may have been involved in glycerol release. The remaining Lac- may have been involved in metabolic cycling along the glyconeogenic-glycolytic pathway and/or in triacylglycerol-free fatty acid substrate cycling.