Human muscle response to sprint exercise and nutrient supply with focus on factors related to protein metabolism

Abstract: The overall aim of the present thesis was to study the muscular response to repeated bouts of 30-s sprint exercise. This type of exercise gives rise to a profound metabolic perturbation that might counteract an exercise-induced stimulation of muscle protein synthesis. Special focus was paid upon nutritional supply and how it can direct the muscular response towards activation of amino acid sensors, cell signalling pathway and muscle protein synthesis. Some questions were analysed by gender. It was hypothesised that: Amino acids decrease and ammonia increases following sprint exercise and more so in males than females. Activation of mTOR signalling increases more in females than males in skeletal muscle following sprint exercise. Oral ingestion of essential amino acids and carbohydrates increases the amino acid sensor Vps34 activity, the amino acid transporter and sensor SNAT2, Akt/mTOR signalling and consequently enhances the rate of muscle protein synthesis in skeletal muscle following sprint exercise. It is concluded that: Plasma leucine decreased and plasma and muscle ammonia increased following sprint exercise and more so in in males than females. Such changes might counteract a possible sprint exercise-induced stimulation of muscle protein synthesis. There was some activation of mTOR signalling in skeletal muscle following sprint exercise and more so in females than males. The larger increase in serum insulin and lower decrease in plasma leucine in females might have contributed to the enhanced signalling response in females. Oral ingestion of essential amino acids and carbohydrates, as compared to placebo, resulted in a markedly higher activation of Akt/mTOR signalling in skeletal muscle following sprint exercise. In contrast to the hypothesis, an oral ingestion of essential amino acids and carbohydrates did not result in an increased activation of hVps34 in skeletal muscle. However, this does not exclude the permissive role of hVps34 in mediating the amino acidinduced activation of Akt/mTOR signalling and skeletal muscle protein synthesis. The interaction between the condition (nutrients or placebo) and correlation between increase in Vps34 activity and increase plasma glucose may reflect the dual role of Vps34, i.e. sensing a nutrient-rich condition and activate mTOR or sensing a nutrient-poor condition and activate autophagy in order to counteract low levels of amino acids. Oral ingestion of essential amino acids and carbohydrates resulted in a higher SNAT2 expression, a higher activation of Akt/mTOR signalling and higher rate of muscle protein synthesis following sprint exercise. Thus, nutrients have a potential to stimulate muscle protein synthesis and related pathways after sprint exercise, in spite of the profound metabolic perturbation.