Mineralocorticoid and glucocorticoid effects on 31,000- and 29,000-dalton proopiomelanocortin in rat anterior pituitary and neurointermediate lobe

Abstract

The effects of adrenal steroids on proopiomelanocortin (POMC) levels in rat pituitary have been studied by two-dimensional gel electrophoresis. In intact rats the relative abundance of POMC was much higher in the neurointermediate lobe (N-IL) than in anterior pituitary (AP); in both tissues the predominant species appeared to be of 29,000-dalton (29K) molecular mass, with lesser amounts of a 31K form. In both tissues, the 31K and 29K forms showed multiple spots, consistent with different degrees of sialoglycosylation. Adrenalectomy was followed by a marked increase in AP levels of POMC, and a marked decrease in N-IL levels. In adrenalectomized rats, dexamethasone administration did not affect N-IL levels of POMC, but suppressed 35S incorporation into POMC in AP in a dose-related manner; deoxycorticosterone showed minimal effects on AP levels of POMC, but progressively elevated N-IL levels; 9 alpha fluorocortisol (9 alpha fF) progressively both suppressed AP levels, and raised N-IL levels of POMC. Estimation of immunoreactive (ir) ACTH and ir-beta-endorphin in parallel samples showed an elevation of N-IL levels in response to mineralocorticoids (deoxycorticosterone, 9 alpha fF), and a paradoxical elevation of AP levels in response to glucocorticoids (dexamethasone, 9 alpha fF) compared with oil-injected adrenalectomized controls. We conclude (a) that glucocorticoids suppress the secretion of ir-ACTH and ir-beta-endorphin to a greater extent than they inhibit the synthesis of POMC; (b) that mineralocorticoids specifically elevate the N-IL levels of both POMC and its immunoreactive product (beta-endorphin).