Key points: Dietary supplementation with inorganic nitrate has beneficial effects on skeletal muscle responses to exercise. Both mitochondrial and extra‐mitochondrial explanations have been proposed. Contractile force of fast‐twitch muscles was enhanced in mice supplemented with 1 mm NaNO₃ in drinking water for 7 days. Myoplasmic free [Ca²⁺] during tetanic stimulation was increased in fast‐twitch muscles of nitrate‐supplemented mice and this was accompanied by increased expression of calsequestrin 1 and the dihydropyridine receptor. These results provide a new mechanism by which nitrate exerts beneficial effects on muscle function with applications to sports performance and a potential therapeutic role in conditions with muscle weakness. Abstract  Dietary inorganic nitrate has profound effects on health and physiological responses to exercise. Here, we examined if nitrate, in doses readily achievable via a normal diet, could improve Ca²⁺ handling and contractile function using fast‐ and slow‐twitch skeletal muscles from C57bl/6 male mice given 1 mm sodium nitrate in water for 7 days. Age matched controls were provided water without added nitrate. In fast‐twitch muscle fibres dissected from nitrate treated mice, myoplasmic free [Ca²⁺] was significantly greater than in Control fibres at stimulation frequencies from 20 to 150 Hz, which resulted in a major increase in contractile force at ≤50 Hz. At 100 Hz stimulation, the rate of force development was ∼35% faster in the nitrate group. These changes in nitrate treated mice were accompanied by increased expression of the Ca²⁺ handling proteins calsequestrin 1 and the dihydropyridine receptor. No changes in force or calsequestrin 1 and dihydropyridine receptor expression were measured in slow‐twitch muscles. In conclusion, these results show a striking effect of nitrate supplementation on intracellular Ca²⁺ handling in fast‐twitch muscle resulting in increased force production. A new mechanism is revealed by which nitrate can exert effects on muscle function with applications to performance and a potential therapeutic role in conditions with muscle weakness.

Significance: High-intensity interval training (HIIT) has become popular because it is a time-efficient way to increase endurance. An intriguing and so-far-unanswered question is how a few minutes of HIIT can be that effective. We exposed recreationally active men to one session of three to six sets of 30-s high-intensity cycling exercise. Muscle biopsies taken 24 h later showed an extensive fragmentation of the sarcoplasmic reticulum (SR) Ca ²⁺ channels, the ryanodine receptor 1 (RyR1). In isolated mouse muscle fibers, this fragmentation was accompanied by increased SR Ca ²⁺ leak, which can trigger mitochondrial biogenesis. The HIIT-induced RyR1 fragmentation did not occur in muscles exposed to antioxidant, which offers an explanation for why antioxidants blunt effects of endurance training.