PROTEOLYTIC CLEAVAGE OF ACTH IN CORTICOTROPES OF SEXUALLY MATURE AXOLOTLS (AMBYSTOMA-MEXICANUM)

Immunohistochemical analysis of the pituitary of sexually mature axolo tls revealed both ACTH(1-39)-related and alpha-MSH-related immunoreact ivity present in corticotropic cells located in the rostral anterior p ituitary. Gel filtration analysis indicated that the ACTH(1-39)-sized immunoreactivity and the alpha-MSH-sized immunoreactivity detected in acid extracts of the axolotl anterior pituitary were present in a rati o in a range between 1:1 and 1:0.6. Reversed-phase HPLC analyses indic ated that the alpha-MSH-sized immunoreactivity had the same retention time as synthetic ACTH(1-13)-NH2. The corticotropic activity of the AC TH(1-39)-sized immunoreactivity and the purified ACTH(1-13)-NH2 was te sted in a heterologous, larval bullfrog adrenal bioassay system. As ex pected, the ACFH(1-39)-sized immunoreactivity stimulated corticosteron e release; however, the purified ACTH(1-13)-NH2 lacked glucocorticoid activity. The proteolytic cleavage of ACTH in corticotropes of sexuall y mature axolotls was identical to the cleavage events observed in neo tenic Ambystoma tigrinum that had not reached sexual maturity. These s tudies indicate that the transient expression of ACTH cleavage activit y is not affected by the reproductive state of the animal. Since axolo tls do not undergo metamorphosis, it is possible that events associate d with metamorphosis may induce the decline in ACTH cleavage activity observed in amphibian corticotropes.