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Dexamethasone was given to female rats by intraperitoneal injection (400 μg/100 g) or in the overnight drinking water (20 μg/ml). ACTH release was studied by measuring the plasma corticosterone response 20 min after the application of various stimuli. Four hr after the injection of dexamethasone the response to a tourniquet about the calf was inhibited; the response to urethane, gut stretch, hemorrhage or 50 mU/100 g vasopressin was partially inhibited but there was no inhibition of the response to 500 mU/100 g vasopressin. Overnight dexamethasone markedly reduced the response to all stimuli except 500 mil vasopressin. The relationship between the time of injection of dexamethasone and the application of the stimulus was tested by stimulating ACTH release with ip urethane. Ten hr after injection of dexamethasone there was complete suppression of ACTH release in response to urethane, but this suppression largely disappeared at 24 hr. It is concluded that the inhibition of ACTH release by large doses of dexamethasone is a function of the strength of the ACTH-releasing stimulus and the time interval between the administration of the dexamethasone and the application of the stimulus. (Endocrinology85: 355, 1969)
Sirett et al. (Fri,) studied this question.