Integral as well as proportional adrenal responses to ACTH

responses to ACTH, conscious dogs were infused for 40 min with normal saline or 204,500 ng/min CW~-‘~-ACTH. Plasma ACTH and corticosteroid levels were measured during the infusion and for 80 min after the end of the infusion. Half-maximal plateau corticosteroid responses were achieved with ACTH levels of 105 pg/ml (23 PM); a maximal plateau corti-costeroid response (10.2 pg/lOO ml) resulted from ACTH levels greater than 335 pg/ml(74 PM). When levels of ACTH between 335 and 3,000 pg/ml were achieved during the 40-min infusion, the duration of maximal corticosteroid levels was prolonged beyond the duration of the ACTH infusion. Plasma ACTH concentration decreased after the end of the infusion with a half-disappearance time of 3.6 t 0.2 min, but plasma cortico-steroid concentrations and cortisol and corticosteroid secretory rates remained at or above the plateau secretory rates durin