Dietary nitrate supplementation increases diaphragm peak power in old mice

Key points Respiratory muscle function declines with ageing, contributing to breathing complications in the elderly. Here we report greater in vitro respiratory muscle contractile function in old mice receiving supplemental NaNO3 for 14 days compared with age‐matched controls. Myofibrillar protein phosphorylation, which enhances contractile function, did not change in our study – a finding inconsistent with the hypothesis that this post‐translational modification is a mechanism for dietary nitrate to improve muscle contractile function. Nitrate supplementation did not change the abundance of calcium‐handling proteins in the diaphragm of old mice, in contrast with findings from the limb muscles of young mice in previous studies. Our findings suggest that nitrate supplementation enhances myofibrillar protein function without affecting the phosphorylation status of key myofibrillar proteins. Inspiratory muscle (diaphragm) function declines with age, contributing to ventilatory dysfunction, impaired airway clearance, and overall decreased quality of life. Diaphragm isotonic and isometric contractile properties are reduced with ageing, including maximal specific force, shortening velocity and peak power. Contractile properties of limb muscle in both humans and rodents can be improved by dietary nitrate supplementation, but effects on the diaphragm and mechanisms behind these improvements remain poorly understood. One potential explanation underlying the nitrate effects on contractile properties is increased phosphorylation of myofibrillar proteins, a downstream outcome of nitrate reduction to nitrite and nitric oxide. We hypothesized that dietary nitrate supplementation would improve diaphragm contractile properties in aged mice. To test our hypothesis, we measured the diaphragm function of old (24 months) mice allocated to 1 mm NaNO3 in drinking water for 14 days (n = 8) or untreated water (n = 6). The maximal rate of isometric force development (∼30%) and peak power (40%) increased with nitrate supplementation (P