Electrophoretic analysis of contractile proteins of the diaphragm in chronically hypoxic rats

We questioned whether the relative composition of the diaphragm proteins [myosin heavy chain (MHC), actin, tropomyosin (TM)-alpha and TM-beta, and MHC isoforms] was altered by chronic hypoxia. Rats were exposed to hypobaric hypoxia (inspired O2 pressure approximately 95 mmHg) from birth for 60 days or 9-11 mo. From the muscle homogenate, relative protein content was computed by densitometric analysis after protein separation with polyacrylamide gel electrophoresis and identification by immunoblotting. The relative concentrations of MHC, actin, TM-alpha and TM-beta did not differ from those of control rats for either duration of exposure. The main MHC isoforms (I, IIa, IIb, IIx) remained unaltered after 60 days but changed significantly after 9 mo of hypoxia, mostly because of a decrease in the slow (I) component and an increase of II complex. We conclude that chronic hypoxia of long duration modifies the expression of MHC isoforms in the rat diaphragm. Qualitative similar changes also occurred in a muscle of the thigh, the rectus femoris; hence, the changes in diaphragm MHC probably reflected the systemic effects of hypoxia rather than the increased mechanical load imposed by the chronic hyperventilation.