Potentiating activity of rat serum extract on ACTH-induced corticosteroidogenesis in isolated rat adrenal cells

ACTH potentiating activity was found in rat serum. The extract, obtained from ACTH-free rat serum by the QUSO G 32 adsorption method, potentiated ACTH1-24-induced corticosterone production in isolated rat adrenal cells. In our assay system, the maximal potentiation was observed with the extract of 0.5 ml of rat serum. With the extract, the log dose response curve for ACTH1-24 shifted to the side of lower doses of ACTH1-24. The potentiating substance was stable in the serum: the activity was hardly decreased even after leaving the serum stand for six days at room temperature. On Sephadex G-100 gel filtration of the extract, the most of activity was found between about 40,000 to 9,000 in molecular weight and a small portion of the activity was in the range of lower molecular weight. After hypophysectomy, the potentiating activity found in the fractions was markedly decreased, but a part of the activity still remained 30 days after the operation. This result suggests that the potentiating substance is produced mainly by the pituitary, but also produced by the other organ(s). SDS polyacrylamide gel electrophoresis of the active fractions revealed five peptides which were decreased quantitatively by hypophysectomy.