Effect of steroids on diaphragm of newborn, weanling adolescent, and adult rats.

The purpose of this study was to compare the effects of dexamethasone on the rat diaphragm during the postnatal period and into adulthood. Groups of 48 newborn, 60 weanling adolescent, and 60 adult rats were either (1) treated with DXM (ST, steroid-treated animals) or (2) untreated and pair-fed (C, control animals). After birth, 24 newborn rats were kept with their mother, which received a daily intramuscular injection of 0.5 mg/kg body weight of dexamethasone for 2 wk. Groups of thirty weanling adolescent and 30 adult rats were treated with 1 mg/kg/day of dexamethasone given intramuscularly for 2 wk. Diaphragm performance was compared between ST animals and age-matched C animals. Weights of the body, the diaphragm, the extensor digitorum longus (EDL), and the soleus were obtained. Diaphragm strips were studied in an in vitro preparation to assess contractile and endurance properties. In all the ST animals, body and total diaphragm weights were reduced compared with age-matched C animals (p less than 0.001). In newborn and weanling adolescent ST animals, loss in diaphragm weight was slightly less than in limb muscles, in contrast to adult animals (p less than 0.05). However, diaphragms from adult and weanling adolescent ST animals showed unaffected twitch characteristics, normalized force-frequency curves, and endurance capacity. In the meantime, in newborn ST animals, diaphragm atrophy was associated with significantly decreased force normalized for fiber cross-sectional area and muscle weight (p less than 0.01) and decreased endurance capacity (p less than 0.05). We conclude that the effects of DXM on the diaphragm depend on the developmental status of the muscle at the time of drug administration.