Neural regulation of phenyethanolamine N-methyltransferase in vivo: transcriptional and translational changes

Abstract

The hypothesis that neural regulation of rat adrenal medullary phenylethanolamine N-methyltransferase (PNMT, EC 2.1.1.28) occurs through transcriptional control is examined by following temporal changes in PNMT mRNA expression using paradigms of acute and chronic reserpine treatment. Concommitant changes in PNMT activity and protein were also measured to determine if reserpine induced changes depend solely on gene activation. Further, changes in adrenal corticosterone were measured to examine whether mRNA and enzyme responses might be mediated via reserpine induced changes in ACTH, and hence, corticosterone. Steady-state levels of PNMT mRNA peaked at ∼ 8-fold normal by 6 h after a single reserpine injection (10 mg/kg i.p.), and then declined to control values. With continued treatment, a second, slower rise occurred after three alternate day injections (∼ 3-fold basal levels). Enzyme activity and protein rose simultaneously but were attenuated in magnitude and time course by comparison to message. With both acute and chronic treatment, activity increased 2.0-fold, peaking at 12 h after a single dose of reserpine and again after four doses of the drug. Protein, as measured by immunotitration, was elevated 1.2-and 1.4-fold, respectively. Adrenal corticosterone rose ∼ 8-fold at 6 h, declined slightly at 12 h, rose again, and remained elevated thereafter. Comparison of the time courses for the various indices demonstrated that the early parallel bursts in PNMT mRNA and corticosterone are consistent with an increase in transcriptional activity. However, subsequent discordant expression in these two indices and concomittant magnitudinal differences in enzyme activity and protein suggest that other regulatory controls may limit the transcriptional activation of PNMT and the expression of active PNMT enzyme. Moreover, these latter controls appear to determine the ultimate limitation on PNMT expression and hence, adrenergic function.