A COMPARATIVE-STUDY OF THE ROLE OF ADENYLATE-CYCLASE IN THE RELEASE OF ADRENOCORTICOTROPIN FROM THE OVINE AND RAT ANTERIOR-PITUITARY

The interaction between corticotropin-releasing factor (CRF) and argin ine vasopressin (AVP) is important in the regulation of adrenocorticot ropin (ACTH) release from the anterior pituitary (AP). CRF exerts its effect on the AP by activating the adenylate cyclase (AC) complex wher eas 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 rev ersal of this order of potency, these studies were undertaken to test the hypothesis that a functional alteration of the AC in the ovine cor ticotrope might limit the ability of CRF to release ACTH from these ce lls. When rat AP cells were incubated with CRF, a dose-dependent incre ase in AC activity was observed. This effect was potentiated either by AVP or PMA, although neither agent alone altered AC activity. In cont rast, CRF alone, or in combination with AVP or PMA, did not increase A C activity in ovine AP cells. Both cholera toxin (CT) and pertussis to xin (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 bot h agents was relatively reduced. In the ovine cells, however, AVP acte d synergistically with CT or PT to markedly increase the release of AC TH to levels which approached those obtained when the rat AP cells wer e exposed to CT or PT alone. Forskolin increased AC activity in AP cel ls 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 th e rat cells when incubated with forskolin alone. Forskolin also releas ed 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 simi lar 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 o vine AP reflects a net decrease in AC activity and cannot be ascribed to an ovine corticotropic resistance to cAMP; (2) the decreased activi ty of the ovine corticotropic AC complex may in turn reflect functiona l alterations at the level of both the G proteins and the catalytic su bunit; (3) since AVP causes protein kinase C substrate phosphorylation in the ovine AP, AVP may increase AC activity in this tissue by phosp horylating the G proteins and/or the catalytic subunit.