Effect of exogenous and endogenous ACTH on adrenocortical cyclic GMP in the rat

Abstract

In normal intact rats exhibiting circadian variations of adreno-cortical cyclic nucleotides, injection of ACTH at any time of the nycthemere increased cyclic AMP and decreased cyclic GMP levels in the adrenal cortex (fasciculata-reticularis zona). While the cyclic AMP rise was very transient, culminating at 3 min, the drop in cyclic GMP was maximal between 8 and 15 min with a return towards initial values 30 min after ACTH injection. To check the effect of endogenous ACTH, the levels of which increase during the diurnal phase in that species, plasma and adrenal corticosterone and, simultaneously, adreno-cortical cyclic GMP were measured at close intervals thoughout the day. A progressive rise in plasma and adrenal corticosterone and a concomitant decrease in adrenocortical cyclic GMP were observed. These studies demonstrate that ACTH is directly responsible for a drop in cyclic GMP levels in the adrenal cortex of normal intact rats. Although these in vivo results cannot support the hypothesis that cyclic GMP rather than cyclic AMP plays the role of a second messenger in the ACTH-activated adrenal steroidogenesis, they do not exclude cyclic GMP acting as a modulatory factor in that process.