Differential induction of gene expression of catecholamine biosynthetic enzymes and preferential increase in norepinephrine by forskolin.

We examined the effect of forskolin, an adenylate cyclase activator, on gene expression and the activities of the three enzymes specific for catecholamine biosynthesis [tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH) and phenylethanolamine N-methyltransferase (PNMT)] and on the amounts of available catecholamines in primary cultured bovine adrenomedullary chromaffin cells. The results showed that TH was increased by 4.7 +/- 0.7-fold and 69% in mRNA and activity levels, respectively, compared with the untreated control. DBH was elevated by 3.2 +/- 0.2-fold in mRNA and 45% in activity. The increase in PNMT, on the other hand, was smaller: 1.7 +/- 0.2-fold in mRNA and 13% in activity. This relatively small increase in PNMT was reflected in the catecholamine levels in that the total epinephrine (EPI) was elevated by only 16% while norepinephrine (NE) was elevated by 99%, which caused a shift in the molar ratio of EPI to NE from 7.0 in the untreated control to 4.1 after forskolin treatment. A large portion of the elevated catecholamines was found in the medium, which represented a 10.1-fold increase for NE and a 6.4-fold increase for EPI compared with the control. Interestingly, this caused the remaining intracellular NE and EPI to be only 117 and 66% of the control, respectively. Thus, forskolin caused coordinate up-regulation of gene expression and enzyme activities of the three catecholamine-synthesizing enzymes but to different degrees, resulting in a relatively larger increase in NE than in EPI, both of which were released dramatically. This large enhancement of catecholamine release, as well as the dramatic shift in their ratio, implicates an important physiological role for cAMP in the regulation of in vivo sympathetic activities.