Persistence of the Steroidogenic Effect of Adenosine-3′,5′- Monophosphatein Vitro: Evidence for a Third Factor During the Steroidogenic Effect of ACTH

When added simultaneously with CAMP (adenosine-3′,5′-monophosphate), inhibitors of protein synthesis block the steroidogenic effect of CAMP in rat adrenal sections in vitro. However, after prior stimulation of adrenal sections by CAMP for 10–30 min, heightened rates of steroidogenesis persist despite removing CAMP from the medium and/or adding cycloheximide or chloramphenicol. This previously established steroidogenic effect continues undiminished for 30–40 min, and then decays with a half-life of about 25 min. Virtually identical results are obtained when ACTH is employed as the stimulator, and this is compatible with the hypothesis that CAMP is the intracellular mediator of ACTH action. Since aminoglutethimide, an inhibitor of cholesterol side chain cleavage, largely blocks the persistent steroidogenic effect of CAMP, the latter effect cannot be explained by an accumulation of steroid precursors beyond this important step in steroid biosynthesis. Thepresent findings indicate that the steroidogenic effect of CAMP, like that of ACTH, can be divided into 3 phases, viz., an early phase which is completely blocked by inhibitors of protein synthesis, a second phase which is largely insensitive to these inhibitors, and a third phase in which responsiveness to inhibitor returns, resulting in an exponential decay process. The explanation for the triphasic effect is not certain, but the findings are compatible with the hypothesis that ACTH, via CAMP, induces (? activates, stabilizes and /or stimulates the synthesis of) a third factor (? regulatory protein) whose synthesis, but not subsequent action, requires continued protein synthesis. This hypothesis is further supported by the observation that the ACTH-induced increase in steroidogenesis is more persistent than the ACTH-induced increase in tissue CAMP levels. (Endocrinology85: 867, 1969)