Effects of chronic hypoxia on diaphragm muscle contractile and endurance properties

Chronic hypoxia is encountered in patients suffering from respiratory disease and also in healthy subjects exposed to altitude and it is known to affect skeletal muscle structure and oxidative capacity. Few studies have examined the effects of chronic hypoxia on isolated skeletal muscle function. The aim of this study was to examine the effects of chronic hypoxia on diaphragm muscle contractile and endurance properties. Male Wistar rats were exposed to normoxia (n=6) or hypobaric hypoxia (n=6; barometric pressure 380mmHg) for 6 weeks. At the end of the treatment periods, isometric contractile and endurance properties of isolated strips of diaphragm muscle were measured in tissue baths under hyperoxic (95%O-2/5%CO2) or hypoxic (95%N-2/5%CO2) conditions. Force-frequency relationship and fatigue tolerance was examined. Chronic hypoxia decreased isometric specific force but increased muscle endurance and greatly improved hypoxic tolerance. These changes may be potentially adaptive in circumstances of prolonged hypoxaemia. The mechanism of hypoxia-induced plasticity is unknown but warrants further investigation.