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We investigated the effects of 3 wk of moderate- (21 m/min, 8% grade) and highintensity treadmill training (31 m/min, 15% grade) on 1) monocarboxylate transporter 1 (MCT-1) content in rat hindlimb muscles and the heart and 2) lactate uptake in isolated soleus (Sol) muscles and perfused hearts. In the moderately trained group MCT-1 was not increased in any of the muscles Sol, extensor digitorum longus (EDL), and red (RG) and white gastrocnemius (WG) (P > 0.05). Similarly, lactate uptake in Sol strips was also not increased (P > 0.05). In contrast, in the heart, MCT-1 (+36%, P 0.05). In the highly trained group, heart MCT-1 (+44%, P < 0.05) and lactate uptake (+173%, P < 0.05) were increased. In conclusion, it has been shown that 1) in both heart and skeletal muscle lactate uptake is increased only when MCT-1 is increased; 2) training-induced increases in MCT-1 occurred at a lower training intensity in the heart than in skeletal muscle; 3) in the heart, lactate uptake was increased much more after high-intensity training than after moderate-intensity training, despite similar increases in heart MCT-1 with these two training intensities; and 4) the increases in MCT-1 occurred independently of any changes in the heart's oxidative capacity (as measured by citrate synthase activity).
Baker et al. (Sun,) studied this question.