Plasma adrenocorticotropin and cortisol responses to intravenous injection of corticotropin-releasing factor in the morning and evening.

Plasma ACTH and cortisol responses to corticotropin-releasing factor (CRF) were determined in the morning and evening in seven normal men. Either 100 micrograms synthetic ovine CRF or saline was given intravenously at 0900 h and at 2200 h. Blood samples were collected before and 15, 30, 45, 60, 90, and 120 min after CRF or saline injection. Plasma ACTH concentrations before and after CRF injection in the morning were significantly higher (P less than 0.05) than those in the evening at all times except 45 min after injection. Plasma cortisol concentrations before and at all times after CRF injection in the morning were also significantly higher (P less than 0.05) than those in the evening. However, neither the maximum increments in plasma ACTH and cortisol above the control levels nor increments at each time point following CRF injection in the morning differed significantly from those in the evening. Increments in the area under the ACTH and cortisol concentration curves after CRF injection in the morning also did not differ significantly from those in the evening. These results suggest that the responsiveness of the pituitary to CRF in the morning and in the evening does not differ significantly, although actual values of plasma ACTH and cortisol are higher in the morning.