Kinetics of intramuscular triglyceride fatty acids in exercising humans

A pulse ([C-14]palmitate)-chase ([H-3]palmitate) approach was used to study intramuscular triglyceride (imTG) fatty acid and plasma free fatty acid (FFA) kinetics during exercise at similar to 45% peak O-2 consumption in 12 adults. Vastus lateralis muscle was biopsied before and after 90 min of bicycle exercise; (H2O)-H-3 production, breath (CO2)-C-14 excretion and lipid oxidation (indirect calorimetry) rates were measured during exercise. Results: during exercise, 8.2 +/- 1.2 and 8.4 +/- 0.7 mu mol.kg(-1).min(-1) of imTG fatty acids and plasma FFA, respectively, were oxidized according to isotopic measurements. The sum of these two values was not different (P = 0.6) from lipid oxidation by indirect calorimetry (15.4 +/- 1.6 mu mol.kg(-1).min(-1)); the isotopic and indirect calorimetry values were correlated (r = 0.79, P < 0.005). During exercise, imTG turnover rate was 0.32 +/- 0.07%/min (6.0 +/- 2.0 mol of imTG kg wet muscle(-1).min(-1)) and plasma FFA were incorporated into imTG at a rate of 0.7 +/- 0.1 mu mol.kg wet muscle(-1).min(-1). The imTG pool size did not change during exercise. This pulse-chase, dual tracer appears to be a reasonable approach to measure oxidation and synthesis kinetics of imTG.