Bioactive and immunoactive ACTH in the rat pituitary: influence of stress and adrenalectomy

Abstract

Tissue levels of bioactive and immunoactive ACTH were measured in both the anterior and neuro-intermediate lobes of the rat pituitary. Similar concentrations of bioactive (65 ng/mg) and immunoactive (83 ng/mg) ACTH were found in the anterior lobes control rats. A 2-min ether stress had no effect on either bioactive or immunoactive ACTH levels in the anterior lobe. Twenty-four h after adrenalectomy the anterior lobe content of both bioactive and immunoactive ACTH decreased only to return to supranormal levels 21 days after the operation. A 30-min neurogenic stress had no effect on anterior lobe bioactive ACTH content but reduced the immunoactive ACTH level to 50 ng/mg. Synthetic alphah-17-39 ACTH was used in our radio-immunoassay in order to measure the C-terminal ACTH activity of the neuro-intermediate lobe. The concentration of such C-terminal activity in control rats (890 ng alphah-17-39 ACTH/mg) considerably exceeded the amount of bioactive ACTH (15 ng/mg). This is presumably due primarily to the presence of the so-called corticotropin-like intermediate lobe-peptide (CLIP). The amounts of bioactive or C-terminal immunoactive ACTH in the neuro-intermediate lobe were not affected by ether stress nor short term (24-h) or long term (21-day) adrenalectomy. Neuro-intermediate lobe bioactive ACTH decreased (to 8 ng/mg) only with the introduction of a 30-min neurogenic stress. Neurogenic stress had no effect on the concentration of CLIP, but when the stress was imposed 24 h after adrenalectomy, a significant reduction was observed. The data support the presence of bioactive ACTH in the intermediate lobe of the rat pituitary and suggest that such ACTH is preferentially released by neurogenic stress and not appreciably regulated by circulating levels of glucocorticoids. Until the biological function and/or target organ of CLIP is identified, the significance of the changes in tissue levels of C-terminal immunoactive ACTH will remain unknown.