Plasma dopamine-beta-hydroxylase activity in phaeochromocytoma.

Plasma dopamine-beta-hydroxylase (DbetaH) activity, a marker of catecholamine secretion that occurs via exocytosis, was determined preoperatively in eight patients with phaeochromocytoma. Despite very elevated urinary catecholamine concentrations, mean plasma DbetaH activity was 32-6 +/- 3 u/l and was not significantly different from apparently healthy subjects. Seven days after surgery, one subject exhibited a 60% postoperative decrease of enzyme activity. These results suggest that diffusion across plasma membranes is the predominant mechanism of catecholamine release from phaeochromocytomas. The postoperative change in one patient suggests tumour heterogeneity and that exocytosis may contribute to the process of catecholamine release in some patients.