A DEMONSTRATION OF CORTICOTROPIC ACTIVITY AND ITS DISTRIBUTION IN PARS DISTALIS OF REPTILE

Effects of hypophysectomy, mammalian ACTH, and reptilan pituitary extracts on in vivo adrenal corticosteroidogenesis were examined in the lizard Anolis carolinensis. Removal of the whole pars distalis resulted in the disappearance of corticosterone from the plasma after 9 days when animals were kept at 30°. Corticosterone levels were normal if the rostral half of the pars distalis remained but were greatly reduced if only the caudal half of the gland remained, indicating that corticotropin (ACTH) is secreted by the rostral half of the pars distalis. Corticosterone was also eliminated from the circulation within 3 days after dexamethasone treatment, indicating a feedback inhibition of ACTH secretion by circulating levels of corticosteroids. A graded increase in plasma corticosterone followed the subcutaneous injection of 1-100 mU mammalian ACTH in dexamethasone-blocked Anolis. Corticosterone concentrations reached a peak 30 min after injection of the ACTH and declined to zero within 60 min at 30°. At 21°, the response was much smaller but more prolonged. In vivo bioassay demonstrated corticotropic activity in the pars distalis from two lizards (Anolis carolinensis and Dipsosaurus dorsalis), a turtle (Pseudemys scripta), and a crocodilian (Caiman sp.). In all cases, ACTH activity was almost entirely restricted to the rostral half of the pars distalis. These findings suggest that the amphobilic gamma cells previously identified as luteotropes may represent the corticotropes in reptiles. © 1969.