Relation Between Steroidogenesis and 3', 5'-Cyclic AMP Production in Isolated Adrenal Cells

Octadecapeptide β-Ala1, Orn15-(1-18) ACTH-NH2 (I) or β-Ala1, D-Phe7, Orn15-(1-18) ACTH-NH2 (II) was incubated with isolated rat adrenal cells at 38°C for 1 hr. The quantities of corticosterone and 3', 5'-cyclic AMP produced were determined simultaneously. In the isolated adrenal cell system, the maximal steroidogenic responses induced by these two peptides were elicited with only a slight increase of 3', 5'-cyclic AMP level and higher concentrations of ACTHs were needed to evoke the maximal 3', 5'-cyclic AMP levels. I-induced increases in corticosterone production and in 3', 5'-cyclic AMP levels were not altered by the presence of an equimolar amount of II. This ffnding is contrary to that obtained with a cell-free system of rat adrenals, in which the activation of adenylate cyclase induced by I or natural ACTH was inhibited by the simultaneous addition of II. The concentration-response curves for I with respect to steroidogenesis and increase in 3', 5'-cyclic AMP level were compared with those for II. The curves for II were respectively parallel to those for I but were shifted to higher concentration along the logconcentration axis. A theoretical consideration on the kinetics suggests that the isolated adrenal cells may have spare receptors for ACTH, and that the slight difference in affinity between I and II would be emphasized and the inhibitory effect of II reduced.