Chronic hypoxia increases rat diaphragm muscle endurance and sodium-potassium ATPase pump content

The effects of chronic hypoxia (CH) on respiratory muscle are poorly understood. The aim of the present study was to examine the effects of CH on respiratory muscle structure and function, and to determine whether nitric oxide is implicated in respiratory muscle adaptation to CH. Male Wistar rats were exposed to CH for 1-6 weeks. Sternohyoid and diaphragm muscle contractile properties, muscle fibre type and size, the density of fibres expressing sarco/endoplasmic reticulum calcium-ATPase (SERCA) 2 and sodium-potassium ATPase (Na+,K+-ATPase) pump content were determined. Muscle succinate dehydrogenase (SDH) and reduced nicotinamide adenine dinucleotide phosphate (NADPH) dehydrogenase activities were also assessed. Acute and chronic blockade of nitric oxide synthase (NOS) was employed to determine whether or not NO is critically involved in functional remodelling in CH muscles. CH improved diaphragm, but not sternohyoid, fatigue tolerance in a time-dependent fashion. This adaptation was not attributable to increased SDH or NADPH dehydrogenase activities. The areal density of muscle fibres and relative area of fibres expressing SERCA2 were unchanged. Na+,K+-ATPase pump content was significantly increased in CH diaphragm. Chronic NOS inhibition decreased diaphragm Na+,K+-ATPase pump content and prevented CH-induced increase in muscle endurance. This study provides novel insight into the mechanisms involved in CH-induced muscle plasticity. The results may be of relevance to respiratory disorders characterised by CH, such as chronic obstructive pulmonary disease.