Opposing Effects of Dietary Nitrate on Muscle Contractile Function in Fast and Slow Human Muscles.

Abstract

Increasing nitric oxide bioavailability via nitrate (NO 3- ) ingestion enhances muscle contractility, which may be due to phosphorylation of the regulatory light chain of myosin. If so, there should be an interaction between NO 3- supplementation and post activation twitch potentiation, which acts via the same pathway. A double-blind, placebo-controlled, randomized crossover study was conducted to determine the influence of dietary NO 3- on the contractile properties of the triceps brachii (60%-70% fast-twitch, or type II) and triceps surae (60%-70% slow-twitch, or type I) muscles of healthy young men ( n = 14). Participants were studied after acute ingestion of 2.2 mL/kg of beetroot juice either containing or lacking 200 µmol/kg of NO 3- , with neuromuscular electrical stimulation used to determine muscle function. NO 3- supplementation did not alter unpotentiated or potentiated peak twitch torque or the maximal rates of torque development or relaxation in either muscle group. On the other hand, NO 3- ingestion resulted in significant changes in the torque-frequency relationship of both the triceps brachii ( P = 0.019) and the triceps surae ( P < 0.001). This was due to a leftward shift of this sigmoidal relationship in the triceps brachii, but a rightward shift in the triceps surae. We conclude that dietary NO 3- has opposing effects on the contractile properties of fast and slow human muscles, which could be due to differential changes in Ca 2+ sensitivity. However, potentiation was unaltered, suggesting that this occurs independently of changes in regulatory light chain phosphorylation. Additional research will be required to determine the underlying biochemical mechanisms.