INCREASE OF THE PROPORTION OF CORTICOSTEROIDOGENIC ACTIVITY OF ACTH VERSUS IMMUNOREACTIVE ACTH IN RAT PLASMA EXTRACTS DURING STRESS

The relationship between bioactive and immunoreactive adrenocorticotrophin (ACTH) was studied under various conditions. Rats were stressed by ether and the immunoreactive (I-ACTH) and bioactive (B-ACTH) ACTH levels were compared to the normal values obtained from ''handled'' rats. The bioactivity was determined by measuring the corticosteroid production in an isolated rat adrenal cell system. Stress exposure enhanced both the I-ACTH and B-ACTH levels. I-ACTH rose from 86.9 +/- 37.9 to 363 +/- 148.7 pg/ml, whereas the B-ACTH levels rose from 34.2 +/- 11.9 to 424.2 +/- 170.9 pg/ml. The bioactive to immunoreactive ratio (B/I ratio) was calculated as 0.47 +/- 0.13 in the control group and rose to the significantly higher value of 1.10 +/- 0.22 in the stressed rats. The conclusion is that under normal circumstances only about 50% of plasma immunoreactive ACTH shows corticosteroidogenic activity and that the B/I ACTH ratio might be a useful tool to estimate the physiological phase of the pituitary releasing activity.