Endurance training, expression, and physiology of LDH, MCT1, and MCT4 in human skeletal muscle

To evaluate the effects of endurance training on the expression of monocarb oxylate transporters (MCT) in human vastus lateralis muscle, we compared th e amounts of MCT1 and MCT4 in total muscle preparations (MU) and sarcolemma -enriched (SL) and mitochondria-enriched (MI) fractions before and after tr aining. To determine if changes in muscle lactate release and oxidation wer e associated with training-induced changes in MCT expression, we correlated band densities in Western blots to lactate kinetics determined in vivo. Ni ne weeks of leg cycle endurance training [75% peak oxygen consumption ((V) over dot o(2peak))] increased muscle citrate synthase activity (+75%, P < 0 .05) and percentage of type I myosin heavy chain (+50%, P < 0.05); percenta ge of MU lactate dehydrogenase-5 (M4) isozyme decreased (-12%, P < 0.05). M CT1 was detected in SL and MI fractions, and MCT4 was localized to the SL. Muscle MCT1 contents were consistent among subjects both before and after t raining; in contrast, MCT4 contents showed large interindividual variations . MCT1 amounts significantly increased in MU, SL, and MI after training (+9 0%, +60%, and +78%, respectively), whereas SL but not MU MCT4 content incre ased after training (+47%, P < 0.05). Mitochondrial MCT1 content was negati vely correlated to net leg lactate release at rest (r = -0.85, P < 0.02). S arcolemmal MCT1 and MCT4 contents correlated positively to net leg lactate release at 5 min of exercise at 65% (V) over dot o(2peak) (r = 0.76, P < 0. 03 and r = 0.86, P < 0.01, respectively). Results support the conclusions t hat 1) endurance training increases expression of MCT1 in muscle because of insertion of MCT1 into both sarcolemmal and mitochondrial membranes, 2) tr aining has variable effects on sarcolemmal MCT4, and 3) both MCT1 and MCT4 participate in the cell-cell lactate shuttle, whereas MCT1 facilitates oper ation of the intracellular lactate shuttle.