Short term vs long term dexamethasone treatment: Effects on rat diaphragm structure and function

The effects of dexamethasone treatment duration (2.5 vs 10 weeks) on diaphragm myosin heavy chain isoforms, fiber types, and contractile characteristics were studied in male rats. Compared with ad libitum-fed and pair-fed controls, dexamethasone significantly decreased body weight, costal diaphragm weight, and the relative expression of myosin heavy chain isoform MHC-2B. Compared with pair-fed controls, the effect on MHC-2B expression was greater after l0 weeks than after 2.5 weeks. Type I and type II costal diaphragm fiber atrophy occurred, and type I1 fiber atrophy was greater after 10 weeks. Costal diaphragm-specific forces were not affected significantly by dexamethasone, regardless of the treatment duration or control group comparison. Fatigue resistance indexes were increased significantly after long term treatment compared with pair-fed controls and after both short term and long term treatment compared with ad libitum-fed controls. In conclusion, the effects of dexamethasone on MHC isoform phenotype expression, fiber type costal diaphragm atrophy, and fatigue resistance were dependent on treatment duration, with greater effects after long term (10 weeks) treatment.