Enhancement of Adrenal Cortisol Secretion after Intravenous High Dose Dexamethasone*

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Abstract
To assess the possible existence of a short loop feedback mechanism of direct glucocorticoid suppression on the adrenal glands, we performed a series of tests employing insulin hypoglycemia or ACTH infusions to obtain adrenal stimulation by ACTH levels that remained within the physiological range. Although the rapidity of glucocorticoid suppression of the pituitary thwarted efforts to use endogenous ACTH as a stimulus, we were able to mimic the stressed state by very low dose ACTH infusions (0.05 μg/kg BW). No inhibition of cortisol secretion in response to ACTH infusions was detected in tests done after administration of dexamethasone compared to placebo [mean integrated response, 29.37 ± 1.91 (±SE) vs. 29.12 ± 1.12 μg·h/ dl, respectively]. Furthermore, when high doses of dexamethasone were administered iv, a small paradoxical increase in cortisol secretion was found (33.82 ± 1.44 μg·h/dl) without a difference in ACTH levels. These data do not support the concept of significant direct glucocorticoid inhibition of adrenal secretion. Non-ACTH factors that may enhance cortisol secretion in the presence of ACTH may exist.