EFFECTS OF VASOPRESSIN AND ELIMINATION OF CORTICOTROPIN-RELEASING HORMONE-TARGET CELLS ON PROOPIOMELANOCORTIN MESSENGER-RNA LEVELS AND ADRENOCORTICOTROPIN SECRETION IN-OVINE ANTERIOR-PITUITARY-CELLS

Although arginine-vasopressin (AVP) is reported to produce greater ACT H biosynthetic and secretory responses than does corticotropin-releasi ng hormone (CRH) in sheep anterior pituitary cells, neither factor app ears to increase pro-opiomelanocortin (POMC) mRNA levels, as does CRH in the cells of some other species. Since only a fraction of cells tha t express POMC mRNA may be able to respond to AVP, the aim of this stu dy was to further delineate the regulation of POMC mRNA in ovine anter ior pituitary corticotrophs, as a whole and in functional subpopulatio ns of corticotrophs. We measured the effects of AVP, CRH or activation of protein kinase C by phorbol myristate acetate (PMA) in cultured ce lls. We compared responses in intact populations with those of culture s from which CRH-target cells were pharmacologically eliminated. Disso ciated adult ovine anterior pituitary cells were cultured overnight, t reated with either vehicle (intact) or a CRH-toxin conjugate that spec ifically eliminates CRH-target cells (CRH-target-depleted), washed, re turned to culture and subsequently challenged with vehicle, AVP (100 n M), CRH (10 nM) or PMA (1 mu M) for 5 h. The media were assayed for AC TH by RIA and the cells for POMC mRNA by Northern blot analysis. In in tact populations, AVP and CRH increased ACTH secretion from 6.5 +/- 1. 2 to 216 +/- 22 and 81 +/- 14 ng/well respectively, but only AVP cause d an increase in steady-state POMC mRNA levels (+ 48 +/- 10%). Direct activation of protein kinase C with PMA mimicked the effect of AVP on ACTH secretion (318 +/- 16 ng/well), but did not alter POMC mRNA level s. In CRH-target-depleted populations, control ACTH secretion (11 +/- 3 ng/well) and POMC mRNA (+ 69 +/- 7%) were elevated, compared with in tact populations. AVP (55 +/- 8 ng/well) and PMA (120 +/- 17 ng/well), but not CRH, increased ACTH secretion; POMC mRNA was not significantl y elevated by any oi the treatments. Taken together, these data provid e further support for the notion of dissociation between secretion of ACTH and expression of POMC mRNA, and demonstrate that AVP increases s teady-state POMC mRNA levels in ovine anterior pituitary cells. The da ta are also consistent with the concept that complex interactions, pos sibly including those between cells, influence ACTH secretion and stea dy-state POMC mRNA levels.