Activities of ACTH-potentiating substances isolated from rat serum on steroidogenesis in isolated rat adrenal cells

Action of partially purified ACTH-potentiating substance from rat serum on isolated rat adrenal cells

A serum extract possessing ACTH-potentiating activity was partially purified by Sephadex G-100 gel filtration. Fractions from 9,000 to 40,000 in molecular weight (APS-Fr) potentiated the corticosterone production induced by synthetic adrenocorticotropic hormone (ACTH1-24). Approximately 5 micrograms (as protein) of APS-Fr in 0.5 ml medium showed the maximum activity to potentiate the response of isolated rat adrenal cells to ACTH1-24. Puromycin, cycloheximide and actinomycin D did not abolish the potentiation by APS-Fr of ACTH1-24-induced steroidogenesis. The log-dose response curve for ACTH1-24 was shifted toward the side representing lower doses of ACTH1-24 by APS-Fr, but the maximum response was not changed. In a typical experiment, the median effective dose (ED50) for ACTH1-24 was decreased from 5.2 X 10(-11) M to 3.0 X 10(-12) M. ACTH1-24-induced production of cyclic AMP was also increased by APS-Fr. Time courses of both corticosterone and cyclic AMP production induced by ACTH1-24 did not differ qualitatively in either the presence or absence of APS-Fr, although they showed a marked difference quantitatively. When dibutyryl cyclic AMP was used in place of ACTH1-24, APS-Fr did not potentiate steroidogenesis. These results suggest that APS-Fr acts at a step between the binding of ACTH to its receptor and the formation of cyclic AMP. ACTH1-24-induced steroidogenesis which was increased by increasing concentration of calcium in the medium, was further increased by addition of APS-Fr. The calcium requirement for the same degree of response was decreased by addition of APS-Fr, in a similar way to when a higher concentration of ACTH1-24 was used.