SECRETION OF ADRENOCORTICOTROPIN (ACTH) AND ACTH PRECURSORS IN OVINE ANTERIOR-PITUITARY-CELLS - ACTIONS OF CORTICOTROPIN-RELEASING HORMONE,ARGININE-VASOPRESSIN AND GLUCOCORTICOIDS

Although corticotrophin-releasing hormone (CRH) and arginine vasopress in (AVP) have been extensively characterized as stimulators, and gluco corticoids as inhibitors of ACTH secretion, far less is known about th e control of the secretion of ACTH precursors from the anterior pituit ary or about the types of corticotrophs involved. The present study wa s designed to systematically evaluate the actions of stimulatory and i nhibitory factors on the secretion of ACTH and ACTH precursors (pro-op iomelanocortin, M(r) 31 000; pro-ACTH, M(r) 22000) from dissociated ov ine anterior pituitary cells. The cells were stimulated for 3 h with C RH (10 nmol/l) and AVP (100 nmol/l), alone or in combination with the synthetic glucocorticoid dexamethasone. In designated wells, cells wer e treated with dexamethasone, (100 nmol/l), beginning 16-18 h before a nd continuing through the 3-h secretion experiments in the presence of CRH and AVP. Secretion of ACTH-like peptides from intact cultures was compared with that from cultures which had been pretreated with a cyt otoxic CRH conjugate (cytotoxin) to eliminate CRH-target cells specifi cally. Immunoreactive (ir)-ACTH was measured by radioimmunoassay (RIA) ; ACTH(1-39) and ACTH precursors were specifically measured by two-sit e immunoradiometric assays that discriminate between the two. In intac t populations of cells, dexamethasone had no effect on basal ACTH(1-39 ) secretion, but decreased the secretion of ACTH(1-39) in response to CRH or AVP. Pretreatment of cells in the same experiments with cytotox in (for 18 h, beginning 3.5 days before secretion studies) also had no significant effect on basal ACTH(1-39) secretion, but eliminated the response to CRH and decreased the response to AVP. In contrast to the situation in intact populations, dexamethasone had no effect on the re sidual secretion of ACTH(1-39) in response to AVP. These results mirro red those for secretion of ir-ACTH, measured by RIA. Secretion of ACTH precursors followed a different pattern from that for ir-ACTH and ACT H(1-39). In intact populations, dexamethasone decreased the secretion of ACTH precursors in response to CRH, but had no effect on basal secr etion or the precursor response to AVP. Elimination of CRH-target cell s also had no effect on basal precursor secretion and eliminated the s ecretion of precursors in response to CRH. Loss of CRH-target cells wa s accompanied by a smaller decrease in the secretion of ACTH precursor s than ir-ACTH and ACTH(1-39) in response to AVP. Interestingly, dexam ethasone significantly increased the secretion of ACTH precursors in r esponse to AVP after cytotoxin. These results suggest either that the inhibition by glucocorticoids of the ACTH(1-39) secretory response to AVP is confined to those AVP-responsive cells that are sensitive to th e CRH-target-specific cytotoxin, or that glucocorticoid-induced inhibi tion of the response to AVP depends on the functional presence of CRH- responsive cells. The results further suggest that the secretion of AC TH precursors in response to AVP is resistant to inhibition by glucoco rticoids, regardless of the presence of CRH-target cells and is, gener ally, much less influenced by, or dependent upon, CRH-target cells. Ta ken together, the data suggest that those corticotrophs which are resi stant to cytotoxin are the source of ACTH precursors secreted in respo nse to AVP, and resist inhibition by glucocorticoids.