Hormonal control of rat adrenal phenylethanolamine N-methyltransferase. Enzyme activity, the final critical pathway

To examine whether glucocorticoids control rat adrenal phenylethanolamine N-methyltransferase (PNMT) through gene transcription, the effects of hypophysectomy and acute and chronic glucocorticoid replacement on PNMT mRNA and enzymatic activity were determined. Glucocorticoid depletion through hypophysectomy did not alter PNMT mRNA, whereas PNMT activity declined to approximately 25% of normal. A single dose of ACTH (4 IU SC) rapidly induced PNMT mRNA, with a six-fold peak at 6 hours postinjection. The short-term rise in PNMT mRNA was accompanied by an increase in corticosterone and elevated levels of glucocorticoid receptor mRNA. Ribosomal loading experiments suggested that available PNMT mRNA was fully utilized for protein synthesis. However, PNMT activity did not increase commensurately. Chronic ACTH treatment (4 IU SC daily for 7 days) sustained elevated levels of glucocorticoid receptor mRNA but returned corticosterone to hypophysectomized levels and decreased PNMT mRNA to 50% of normal. Despite the decline in PNMT mRNA and its partial utilization for protein synthesis, PNMT enzymatic activity was fully restored. These findings indicate that glucocorticoids exert marked but complex influences on PNMT gene transcription. In addition, corticosteroids appear to posttranscriptionally regulate PNMT protein expression, underscoring the uncoupling between the expression of PNMT mRNA and active enzyme. Thus, glucocorticoid control of gene transcription and protein synthesis do not fully account for changes in PNMT expression, consistent with the previous observation that glucocorticoid control of PNMT proteolysis is also important in PNMT regulation and the potential for epinephrine biosynthesis.