Steroidogenesis and cyclic adenosine 3', 5'-monophosphate accumulation in rat adrenal cells: divergent effects of adrenocorticotropin and its o-nitrophenyl sulfenyl derivative

Abstract

Both adrenocorticotropin (ACTH) and its o-nitrophenyl sulfenyl derivative (NPS-ACTH) in which the single tryptophan residue of the hormone is modified, were able to stimulate corticosterone synthesis to the same maximal rate in isolated rat adrenal cells. The concentration of NPS-ACTH required for half-maximal steroidogenesis was approximately 70 times that of ACTH. Although both ACTH and NPS-ACTH stimulated cyclic AMP accumulation, the effect of NPS-ACTH was marginal; the maximal stimulation of cyclic adenosine 3',5'-monophosphate (cyclic AMP) accumulation in response to ACTH was 30 to 100-fold greater than that due to NPS-ACTH. Apparently ACTH increased cyclic AMP accumulation well beyond that required for the stimulation of maximal steroidogenesis. NPS-ACTH appeared to inhibit in a competitive manner the effect of ACTH on cyclic AMP production but not steroid synthesis. The continued presence of ACTH or NPS-ACTH was necessary for the continued stimulation of steroidogenesis indicating that the factor (or factors) mediating the steroidogenic response must be present throughout the time of stimulation. The relationship between steroid synthesis and cyclic AMP accumulation was different for ACTH and NPS-ACTH. Much less cyclic AMP was produced when NPS-ACTH stimulated steroid synthesis to 75% of the maximal level than when ACTH enhanced steroidogenesis half-maximally. Even though cyclic AMP was found to leave the cells during the time of incubation, the same lack of correlation between cyclic AMP generation and rate of steroid synthesis mentioned above was found whether intracellular or total cyclic AMP was measured. These results suggest that there may be two receptors for ACTH in the adrenal cell population which may be in the same cell or in different cell types. NPS-ACTH stimulates one of these receptors but inhibits the other. Furthermore, these results imply that either very small amounts of cyclic AMP are required for the stimulation of steroidogenesis or factor (or factors) besides cyclic AMP may be involved in mediating this function of ACTH.