Plasma Hydroxylase Activity

Dopamine-β-hydroxylase (DBH), the enzyme that converts dopamine to norepinephrine is released with catecholamines from sympathetic nerves and adrenal medulla. The DBH activity has been reported in the serum of both man and rat and recent experiments in rats have suggested that the serum DBH activity arises mainly from sympathetic nerves and may be an index of sympathetic nerve activity. Plasma DBH activity was examined in six normal volunteers during three stresses that increase sympathetic nerve firing. Cold pressor test and exercise resulted in small but significant elevations of plasma DBH activity. No significant change occurred during tilting. Resting DBH activity was consistently highest in the first of three experimental days possibly as a result of apprehension. These data are consistent with the release of DBH and sympathetic neurotransmitters by exocytosis in man.