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Dissociation of ACTH‐Secretory Mechanisms in Rat Pituitary Cells: Evidence that Basal and Vasopressin‐Stimulated Secretion Act via a Mechanism Distinct from that of Corticotrophin‐Releasing Factor

To study the relationship between basal, corticotrophin‐releasing factor‐ (CRF) and vasopressin‐stimulated adrenocorticotrophic hormone (ACTH) secretion by rat anterior pituitary cells, dissociated anterior pituitary cells were seeded into tissue culture dishes and treated overnight with a cytotoxic conjugate specific for CRF‐target cells. Immediately after extensive washing, or 1, 3, 6, 9 or 12 days later, cellular ACTH content, basal secretion and secretion in response to CRF or vasopressin were measured. ACTH content and basal secretion rate increased over time in both cytotoxic conjugate‐pretreated and vehicle‐pretreated cell populations. Compared with vehicle‐pretreated cells, basal ACTH secretion was higher in cytotoxic conjugate‐pretreated populations by Day 3 and reached an apparent maximum by Day 6. In such cells, net ACTH secretion post‐vasopressin decreased as basal secretion increased; by Day 6 no vasopressin‐stimulated secretion was seen. In cytotoxic conjugate‐pretreated cells, the response to CRF was initially completely eliminated; however, as ACTH content and secretion increased with time, a small recovery of the response to CRF was observed on Days 3 and 6. In vehicle‐pretreated cells, ACTH secretion in response to vasopressin increased in parallel with basal secretion. The response to CRF increased progressively over Days 1 to 6 as well; this response was more closely related to the increases observed in ACTH content. The shift in responsiveness of the cytotoxic conjugate‐pretreated cells over time, from vasopressin‐responsive to CRF‐responsive, further demonstrates the dissociation of the mechanisms of the ACTH secretory responses to CRF and vasopressin. In addition, the increase in unstimulated secretion at the expense of the response to vasopressin in cytotoxic conjugate‐treated cells is consistent with a common pathway for vasopressin‐stimulated and basal release of ACTH.