Nitric oxide modulates rat pharyngeal dilator muscle force and endurance during hypoxia

Abstract

Nitric Oxide (NO), a highly reactive second messenger, plays an important role in skeletal muscle physiology. The aim of this study was to investigate the effects of the NO synthase inhibitor, N‐nitro‐L‐arginine (L‐NNA) on the contractile and endurance properties of the sternohyoid (upper airway dilator) muscle under hyperoxic and hypoxic conditions in vitro.Adult male Wistar rats were killed humanely and sternohyoid muscle strips were mounted isometrically in an organ bath containing aerated physiological salt solution maintained at 30degC. Four groups were assessed: control muscle strips exposed to hyperoxic (95%O2, 5% CO2; n=9) or hypoxic (95%N2, 5%CO2; n=8) conditions and L‐NNA‐treated strips (1mM) studied under hyperoxic (n=9) or hypoxic conditions (n=8). Force‐frequency relationship was assessed.L‐NNA had no effect on sub‐maximal forces but significantly decreased peak tetanic force at 100Hz under both hyperoxic (17+/−1.5 vs. 13.8+/−1.2 N/cm2; mean+/−SEM, control vs. L‐NNA, P<0.05 ANOVA) and hypoxic conditions (9.3+/−0.9 vs. 6.6+/−0.7, control vs. L‐NNA, P<0.05).We conclude that NO facilitates rat sternohyoid muscle contractile function especially under hypoxic conditions. Although NO is reported to inhibit diaphragm contractile function, a positive inotropic effect of NO donors has been observed in limb muscle.