Short-term training increases human muscle MCT1 and femoral venous lactate in relation to muscle lactate

We examined the effects of increasing a known lactate transporter protein, monocarboxylate transporter 1 (MCT1), on lactate extrusion from human skeletal muscle during exercise. Before and after short-term bicycle ergometry training [2 h/day, 7 days at 65% maximal oxygen consumption (VO2max)], subjects (n = 7) completed a continuous bicycle ergometer ride at 30% VO2max (15 min), 60% VO2max (15 min), and 75% VO2max (15 min). Muscle biopsy samples (vastus lateralis) and arterial and femoral venous blood samples were obtained before exercise and at the end of each workload. After 7 days of training the MCT1 content in muscle was increased (+18%; P < 0.05). The concentrations of both muscle lactate and femoral venous lactate were reduced during exercise (P < 0.05) that was performed after training. High correlations were observed between muscle lactate and venous lactate before training (r = 0.92, P < 0.05) and after training (r = 0.85, P < 0.05), but the slopes of the regression lines between these variables differed markedly. Before training, the slope was 0.12 +/- 0.01 mM lactate.mmol lactate-1.kg muscle dry wt-1, and this was increased by 33% after training to 0.18 +/- 0.02 mM lactate.mmol lactate-1.kg muscle dry wt-1. This indicated that after training the femoral venous lactate concentrations were increased for a given amount of muscle lactate. These results suggest that lactate extrusion from exercising muscles is increased after training, and this may be associated with the increase in skeletal muscle MCT1.