Adrenal Sensitivity to Adrenocorticotropin Varies Diurnally*

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Abstract
To further characterize diurnal changes in the rhythm in adrenal responsiveness to ACTH, we have measured ACTH distribution volume, MCR, and t½. These do not change between morning and evening in groups of untreated, dexamethasone-pretreated, or hypophysectomized female rats. To characterize the nature of the change in adrenal responsiveness to ACTH, dexamethasone-pretreated rats were infused for 2 h with a variety of doses of ACTH in the morning and evening. The adrenal response to an infusion rate of ACTH that maximally stimulated the adrenals (200 pg/100 g BW-min) was the same in the morning and evening, showing that adrenal capacity does not change. However, infusion of ACTH at lower rates (50–100 pg/100 g BW-min) revealed that the slope of the steroid response curve increased between morning and evening, demonstrating a diurnal change in adrenal sensitivity to ACTH. These results together with previous data showing that the magnitude and time course of the adrenal cAMP response to ACTH changes diurnally strongly suggest that ACTH receptor affinity or coupling with adenylate cyclase changes diurnally. In other experiments, plasma ACTH and corticosterone levels were determined in groups of young and adult male and adult female untreated rats killed at 4-h intervals around the clock. Peak sensitivity to ACTH was found at lights-out, and trough sensitivity was found at lights-on, suggesting that the experimentally demonstrated rhythm occurs normally.