Vesicular monoamine transporters (VMATs) in adrenal chromaffin cells: stress-triggered induction of VMAT2 and expression in epinephrine synthesizing cells.

Vesicular monoamine transporters (VMATs) mediate transmitter uptake into neurosecretory vesicles. There are two VMAT isoforms, VMAT1 and VMAT2, encoded by separate genes and displaying different cellular distributions and pharmacological properties. We examined the effect of immobilization stress (IMO) on expression of VMATs in the rat adrenal medulla. Under basal conditions, VMAT1 is widely expressed in all adrenal chromaffin cells, while VMAT2 is co-localized with tyrosine hydroxylase (TH) but not phenylethanolamine N-methyltransferase (PNMT), indicating its expression in norepinephrine (NE)-, but not epinephrine (Epi)-synthesizing chromaffin cells. After exposure to IMO, there was no change in levels of VMAT1 mRNA. However, VMAT2 mRNA was elevated after exposure of rats to 2 h IMO once (1× IMO) or daily for 6 days (6× IMO). The changes in VMAT2 mRNA were reflected by increased VMAT2 protein after the repeated IMO. Immunofluorescence revealed an increased number of cells expressing VMAT2 following repeated IMO and its colocalization with PNMT in many chromaffin cells. The findings suggest an adaptive mechanism in chromaffin cells whereby enhanced catecholamine storage capacity facilitates more efficient utilization of the well-characterized heightened catecholamine biosynthesis with repeated IMO stress.