High-intensity exercise stimulates glycolysis, subsequently leading to elevated lactate production within skeletal muscle. While lactate produced within the muscle is predominantly released into the circulation via the monocarboxylate transporter 4 (MCT4), recent research underscores lactate's function as an intercellular and intertissue signalling molecule. However, its specific intracellular roles within muscle cells remains less defined. In this study, our objective was to elucidate the effects of increased intramuscular lactate accumulation on skeletal muscle adaptation to training. To achieve this, we developed MCT4 knockout mice and confirmed that a lack of MCT4 indeed results in pronounced lactate accumulation in skeletal muscle during high-intensity exercise. A key finding was the significant enhancement in endurance exercise capacity at high intensities when MCT4 deficiency was paired with high-intensity interval training (HIIT). Furthermore, metabolic adaptations supportive of this enhanced exercise capacity were evident with the combination of MCT4 deficiency and HIIT. Specifically, we observed a substantial uptick in the activity of glycolytic enzymes, notably hexokinase, glycogen phosphorylase and pyruvate kinase. The mitochondria also exhibited heightened pyruvate oxidation capabilities, as evidenced by an increase in oxygen consumption when pyruvate served as the substrate. This mitochondrial adaptation was further substantiated by elevated pyruvate dehydrogenase activity, increased activity of isocitrate dehydrogenase – the rate-limiting enzyme in the TCA cycle – and enhanced function of cytochrome c oxidase, pivotal to the electron transport chain. Our findings provide new insights into the physiological consequences of lactate accumulation in skeletal muscle during high-intensity exercises, deepening our grasp of the molecular intricacies underpinning exercise adaptation.

高强度运动会刺激糖酵解，从而导致骨骼肌内乳酸的产生增加。虽然肌肉内产生的乳酸主要通过单羧酸转运蛋白 4 (MCT4) 释放到循环中，但最近的研究强调了乳酸作为细胞间和组织间信号分子的功能。然而，其在肌肉细胞内的特定细胞内作用仍不明确。在这项研究中，我们的目的是阐明增加肌内乳酸积累对骨骼肌适应训练的影响。为了实现这一目标，我们培育了 MCT4 敲除小鼠，并证实缺乏 MCT4 确实会导致高强度运动期间骨骼肌中乳酸明显积累。一项重要发现是，当 MCT4 缺乏与高强度间歇训练 (HIIT) 相结合时，高强度耐力运动能力显着增强。此外，MCT4 缺乏和 HIIT 相结合时，支持这种运动能力增强的代谢适应是显而易见的。具体来说，我们观察到糖酵解酶的活性显着上升，特别是己糖激酶、糖原磷酸化酶和丙酮酸激酶。线粒体还表现出增强的丙酮酸氧化能力，当丙酮酸作为底物时耗氧量增加就证明了这一点。这种线粒体适应通过丙酮酸脱氢酶活性的升高、异柠檬酸脱氢酶（TCA 循环中的限速酶）活性的增加以及对电子传递链至关重要的细胞色素C氧化酶的功能增强得到进一步证实。我们的研究结果为高强度运动期间骨骼肌中乳酸积累的生理后果提供了新的见解，加深了我们对支撑运动适应的分子复杂性的理解。