Glucocorticoids are known to inhibit the ACTH response to a variety of stimuli. It has been suggested that vasopressin secretion is also inhibited by glucocorticoid negative feedback. The purpose of this study was to (1) determine the ACTH response to hypertonic saline and its sensitivity to glucocorticoid negative feedback and (2) to determine whether physiological elevations of plasma cortisol inhibit subsequent vasopressin responses to hypertonic saline. Five mongrel dogs (15–18 kg) were prepared with chronic arterial and venous catheters and studied while conscious. Ten experiments were performed on each dog in a randomized design separated by at least 5 days. Each experiment consisted of a pretreatment period (from −60 to −30 min except for dexamethasone administration) during which a glucocorticoid feedback signal was applied and a stimulus period (from 0 to 30 min) during which hypertonic saline was infused. The pretreatment and stimulus periods were separated by 30 min. Pretreatments were as follows: isotonic saline (control), half-maximal and maximal cortisol infusion (5·5 or 11 nmol/kg per min), ACTH(1–24) infusion (6·8 pmol/kg per min) which produces increases in endogenous cortisol, and dexamethasone (1·5 mg i.m.) given at 17.00 h the day before experimentation. Stimuli were as follows: hypertonic saline was infused at 0·2 or 0·4 mmol/kg per min which increased plasma sodium by about 6 or 12 mmol/l respectively. NaCl infusion at 0·2 mmol/kg per min had no effect on plasma ACTH or cortisol except when subsequent to ACTH(1–24) pretreatment when plasma ACTH actually increased to 41·4 ± 2·9 pmol/l in response to hypertonic saline. NaCl infusion at 0·4 mmol/kg per min resulted in a significant increase in plasma ACTH from 5·9 ± 0·9 to 11·7 ± 2·0 pmol/l in the control group. This ACTH response was blocked by pretreatment with either dose of cortisol and dexamethasone. ACTH pretreatment, however, did not completely block the ACTH subsequent response to infusion of 0·4 mmol NaCl/kg per min. The two doses of NaCl led to significant and dose-related increases in plasma vasopressin. None of the pretreatments significantly affected the vasopressin response to hypertonic saline except for significant inhibition after overnight dexamethasone. We conclude that (1) hypertonic saline can stimulate ACTH release if plasma sodium is increased sufficiently, (2) the ACTH response to hypertonic saline is potentiated by pretreatment with ACTH making it different from other stimuli studied previously, and (3) the vasopressin response to hypertonic saline is not inhibited by short-term elevations of plasma cortisol within the physiological range.
Journal of Endocrinology (1989) 122, 41–48
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