Proteolytic cleavage of ACTH in corticotropes of sexually mature axolotls ( Ambystoma mexicanum)

Abstract

Immunohistochemical analysis of the pituitary of sexually mature axolotls revealed both ACTH(1–39)-related and α-MSH-related immunoreactivity present in corticotropic cells located in the rostral anterior pituitary. Gel filtration analysis indicated that the ACTH(1–39)-sized immunoreactivity and the α-MSH-sized immunoreactivity detected in acid extracts of the axolotl anterior pituitary were present in a ratio in a range between 1:1 and 1:0.6. Reversed-phase HPLC analyses indicated that the α-MSH-sized immunoreactivity had the same retention time as synthetic ACTH(1–13)-NH 2. The corticotropic activity of the ACTH(1–39)-sized immunoreactivity and the purified ACTH(1–13)-NH 2 was tested in a heterologous, larval bullfrog adrenal bioassay system. As expected, the ACTH(1–39)-sized immunoreactivity stimulated corticosterone release; however, the purified ACTH(1–13)-NH 2 lacked glucocorticoid activity. The proteolytic cleavage of ACTH in corticotropes of sexually mature axolotls was identical to the cleavage events observed in neotenic Ambystoma tigrinum that had not reached sexual maturity. These studies indicate that the transient expression of ACTH cleavage activity is not affected by the reproductive state of the animal. Since axolotls do not undergo metamorphosis, it is possible that events associated with metamorphosis may induce the decline in ACTH cleavage activity observed in amphibian corticotropes.