Increases in muscle MCT are associated with reductions in muscle lactate after a single exercise session in humans

To investigate the effects of a single session of prolonged cycle exercise [60% peak O-2 uptake ((V) over dot O-2peak) for 5-6 h] on metabolic adaptations in working vastus lateralis muscle, nine untrained males (peak O2 uptake = 47.2 +/- 1.1 ml.kg(-1).min(-1), means +/- SE) were examined before (Pre) and at 2 (Post-2), 4 (Post-4), and 6 (Post-6) days after the training session. On the basis of 15 min of cycle exercise at 59% (V) over dot O-2peak, it was found that training reduced (P< 0.05) exercise muscle lactate (mM) at Post-2 (6.65 +/- 0.69), Post-4 (7.74 +/- 0.63), and Post-6 (7.78 +/- 1.2) compared with Pre (10.9 +/- 1.3). No effect of training was observed on exercise ATP, phosphocreatine, and glycogen levels. After the single session of training, plasma volumes were elevated (P< 0.05) at Post-2 (6.7 +/- 1.7%), Post-4 (5.86 +/- 1.9), and Post-6 (5.13 +/- 2.5). The single exercise session also resulted in elevations (P< 0.05) in the monocarboxylate transporters MCT1 and MCT4 throughout the 6 days after exercise. Although epinephrine and norepinephrine both increased with exercise, only norepinephrine was reduced (P< 0.05) with training and only at Post-4. These results indicate that regulation of cellular lactate levels occurs rapidly and independently of other metabolic adaptations. It is proposed that increases in MCT and plasma volume are at least partly involved in the lower muscle lactate content observed after the training session by increasing lactate membrane transport and removal, respectively.