Corticosteroid effects on isotonic contractile properties of rat diaphragm muscle

The effects of corticosteroids (CS) on diaphragm muscle (Dia(m)) fiber morphology and contractile properties were evaluated in three groups of rats: controls (Ctl), surgical sham and weight-matched controls (Sham), and CS- treated (6 mg · kg^-1 · day^-1 prednisolone at 2.5 ml/h for 3 wk). In the CS-treated Dia(m), there was a selective atrophy of type IIx and IIb fibers, compared with a generalized atrophy of all fibers in the Sham group. Maximum isometric force was reduced by 20% in the CS group compared with both Ctl and Sham. Maximum shortening velocity in the CS Dia(m) was slowed by ~20% compared with Ctl and Sham. Peak power output of the CS Dia(m) was only 60% of Ctl and 70% of Sham. Endurance to repeated isotonic contractions improved in the CS-treated Dia(m) compared with CtL. We conclude that the atrophy of type IIx and IIb fibers in the Dia(m) can only partially account for the CS- induced changes in isotonic contractile properties. Other factors such as reduced myofibrillar density or altered cross-bridge cycling kinetics are also likely to contribute to the effects of CS treatment.