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 d iaphragm myosin heavy chain isoforms, fiber types, and contractile cha racteristics were studied in male rats. Compared with ad libitum-fed a nd pair-fed controls, dexamethasone significantly decreased body weigh t, costal diaphragm weight, and the relative expression of myosin heav y chain isoform MHC-2B. Compared with pair-fed controls, the effect on MHC-2B expression was greater after 10 weeks than after 2.5 weeks. Ty pe I and type II costal diaphragm fiber atrophy occurred, and type II fiber atrophy was greater after 10 weeks. Costal diaphragm-specific fo rces were not affected significantly by dexamethasone, regardless of t he treatment duration or control group comparison. Fatigue resistance indexes were increased significantly after long term treatment compare d with pair-fed controls and after both short term and long term treat ment compared with ad libitum-fed controls. In conclusion, the effects of dexamethasone on MHC isoform phenotype expression, fiber type cost al diaphragm atrophy, and fatigue resistance were dependent on treatme nt duration, with greater effects after long term (10 weeks) treatment .