Kinetic impairment of nitrogen and muscle glutamine metabolisms in old glucocorticoid-treated rats

Aged rats are more sensitive to injury, possibly through an impairment of n itrogen and glutamine (Gln) metabolisms mediated by glucocorticoids. We stu died the metabolic kinetic response of adult and old rats during glucocorti coid treatment. The male Sprague-Dawley rats were 24 or 3 mo old. Both adul t and old rats were divided into 7 groups. Groups labeled G3, G5, and G7 re ceived, by intraperitoneal injection, 1.50 mg/kg of dexamethasone (Dex) for 3, 5, and 7 days, respectively. Groups labeled G3PF, G5PF, and G7PF mere p air fed to the G3, G5, or G7 groups and were injected with an isovolumic so lution of NaCl. One control group comprised healthy rats fed ad libitum. Th e response to aggression induced specifically by Dex (i.e., allowing for va riations in pair-fed controls) appeared later in the aged rats (decrease in nitrogen balance from day 1 in adults but only from day 4 in old rats). Th e adult rats rapidly adapted to Dex treatment, whereas the catabolic state worsened until the end of treatment in the old rats. Gin homeostasis was no t maintained in the aged rats; despite an early increase in muscular Gin sy nthetase activity, the Gin pool was depleted. These results suggest a kinet ic impairment of both nitrogen and muscle Gin metabolisms in response to De x with aging.