Adrenocorticotropin secretion by fetal sheep anterior and intermediate lobe pituitary cells in vitro: effects of gestation and adrenalectomy.

Immunoreactive (ir) ACTH is present in the fetal sheep intermediate lobe (IL) as well as the anterior pituitary (AP). It is not clear whether fetal IL cells can secrete irACTH and if gestational age and glucocorticoids influence the secretion of ACTH from these tissues in a similar fashion. Therefore, we examined the control of irACTH secretion by IL cells, whether the responsiveness of AP and IL cells to arginine vasopressin (AVP) and CRH changes during gestation, and whether withdrawal of adrenal steroids by adrenalectomy influences AP and IL responses. Cultured pituitary cells were studied from intact fetuses at an immature (n = 5; 108 +/- 5 days) and a mature (n = 8; 139 +/- 0 days) stage, from mature fetuses 3 weeks after bilateral adrenalectomy (n = 6), and from neonatal lambs within 16 h of birth (n = 6). Secretion of irACTH was determined by RIA of incubation medium obtained during 3-h exposure of cells to vehicle, AVP, CRH, or both. In all cases, IL cells secreted measurable irACTH. The IL cells of immature fetuses responded to CRH (133 +/- 8% increase over basal secretion), AVP (52 +/- 6%), and CRH plus AVP (244 +/- 8%). In contrast, IL cells from mature fetuses responded only to CRH (160 +/- 20%) or CRH plus AVP (259 +/- 44%), as did cells from mature adrenalectomized fetuses (CRH, 356 +/- 70%; CRH plus AVP, 627 +/- 100%). Secretion from neonatal IL cells was not significantly increased above basal rates by CRH and/or AVP. The AP cells from immature fetuses responded significantly to CRH (406 +/- 16%), AVP (114 +/- 8%), and CRH plus AVP (559 +/- 38%), whereas cells from mature fetuses responded only to AVP (249 +/- 40%) or to CRH plus AVP (570 +/- 146%). In AP cells from mature adrenalectomized fetuses, the response pattern resembled that of immature intact fetal sheep (CRH, 429 +/- 76%; AVP, 146 +/- 15%; CRH plus AVP, 541 +/- 94%). Neonatal AP cells responded to CRH (196 +/- 25%), AVP (442 +/- 71%), and CRH plus AVP (646 +/- 93%). Further characterization of IL cells (n = 6 fetal and 2 neonatal) indicated that they were inhibited by dopamine (basal ACTH secretion decreased by 25 +/- 4%; ACTH secretory response to CRH decreased by 32 +/- 10%). These results show that fetal neurointermediate lobe cells secrete irACTH under basal and stimulated conditions. Moreover, the pattern of response of AP and neurointermediate lobe cells to secretagogues is influenced by gestational age and, possibly, cortisol.