Regional release and removal of catecholamines and extraneuronal metabolism to metanephrines

Norepinephrine (NE) and epinephrine (E) are metabolized extraneuronally by catechol-O-methyl-transferase to the metanephrines (MNs), normetanephrine (NMN) and metanephrine (MN). Subjects in this study received infusions of tritium-labeled NE and E. Concentrations of MNs and catecholamines were measured in plasma flowing into and out of the heart, forearm, lungs, kidneys, mesenteric organs (gastrointestinal tract, spleen, and pancreas), liver, and adrenals to examine the regional production of MNs from circulating and locally released catecholamines. NE spillover from mesenteric organs and kidneys accounted for 64% of the spillover from all tissues. There was detectable spillover of E from most extraadrenal tissues, but 91% was from the adrenals. The production of MNs from locally released and circulating catecholamines varied widely among tissues. The liver made the largest contribution to removal of circulating NE (57%) and E (32%) and the largest contribution to the production of NMN (54%) and MN (37%) from metabolism of circulating catecholamines. In all other tissues more NMN was produced from locally released than from circulating NE. Thus, the metabolism of circulating NE was responsible for only 19% of the total production of NMN. An even smaller portion (6%) of plasma MN was derived from metabolism of circulating E. Most plasma MN (91%) was produced within the adrenals, which also provided the largest single source (23%) of NMN. The regional variation in extraneuronal production of MNs indicates considerable heterogeneity in how circulating and locally released catecholamines are handled by different tissues. The substantial contribution of the adrenals to the production of MNs explains the extraordinary sensitivity of these metabolites for the diagnosis of pheochromocytoma.