A comparative study of the role of adenylate cyclase in the release of adrenocorticotropin from the ovine and rat anterior pituitary

Abstract

The interaction between corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) is important in the regulation of adrenocorticotropin (ACTH) release from the anterior pituitary (AP). CRF exerts its effect on the AP by activating the adenylate cyclase (AC) complex whereas AVP increases the turnover of phosphatidylinositol. In the rat and in man, CRF is the most potent ACTH secretagogue whereas AVP alone is only a weak agonist. Since recent studies in the sheep indicate a reversal of this order of potency, these studies were undertaken to test the hypothesis that a functional alteration of the AC in the ovine corticotrope might limit the ability of CRF to release ACTH from these cells. When rat AP cells were incubated with CRF, a dose-dependent increase in AC activity was observed. This effect was potentiated either by AVP or PMA, although neither agent alone altered AC activity. In contrast, CRF alone, or in combination with AVP or PMA, did not increase AC activity in ovine AP cells. Both cholera toxin (CT) and pertussis toxin (PT) caused a dose-dependent release of ACTH from rat and ovine AP cells, but the amount of ACTH released from the ovine AP cells by both agents was relatively reduced. In the ovine cells, however, AVP acted synergistically with CT or PT to markedly increase the release of ACTH to levels which approached those obtained when the rat AP cells were exposed to CT or PT alone. Forskolin increased AC activity in AP cells of both species, but to a much lower extent in ovine cells than in the rat cells. However, when the ovine cells were exposed to AVP, the AC response to forskolin became similar to the response observed in the rat cells when incubated with forskolin alone. Forskolin also released significantly less ACTH from the ovine AP cells, but AVP also acted synergistically with forskolin to greatly enhance the amount of ACTH released from these cells. Finally, 8-bromo-cyclic AMP produced a similar release of ACTH from both ovine and rat AP cells. We conclude that: (1) the decreased ability of CRF to increase ACTH release from the ovine AP reflects a net decrease in AC activity and cannot be ascribed to an ovine corticotropic resistance to cAMP; (2) the decreased activity of the ovine corticotropic AC complex may in turn reflect functional alterations at the level of both the G proteins and the catalytic subunit; (3) since AVP causes protein kinase C substrate phosphorylation in the ovine AP, AVP may increase AC activity in this tissue by phosphorylating the G proteins and/or the catalytic subunit.