Sympathetic nerve traffic responses to surgical removal of pheochromocytoma

Pheochromocytoma is usually characterized by a marked increase in periphera l catecholamine secretion. Whether this is accompanied by an alteration in central sympathetic drive has not been clarified. In 6 patients with adrena l pheochromocytoma (mean+/-SEM age, 49.3+/-7.2 years), we measured systolic and diastolic blood pressure (photoplethysmographic device), heart rate (E CG), venous plasma catecholamines (high-performance liquid chromatography), and postganglionic muscle sympathetic nerve activity (microneurography) be fore and 78.3+/-13 days after surgical removal of the tumor. In each experi mental session, measurements were performed during (1) a 60-minute resting period to compare several values of sympathetic nerve traffic at similar bl ood pressures before and after surgery and (2) voluntary end-expiratory apn ea, ie! a maneuver inducing sympathetic activation. Tumor removal significa ntly (P<0.05 at least) reduced plasma catecholamines, blood pressure, and h eart rate. In contrast, muscle sympathetic nerve activity was significantly (P<0.01) increased, both when quantified as bursts per minute (from 28.1+/ -5.7 to 54.3+/-7.5) and as bursts per 100 heartbeats (from 33.4+/-5.6 to 65 .1+/-6.5). This was also the case when data were evaluated in periods of 2 experimental sessions characterized by similar diastolic blood pressure val ues. The apnea maneuver induced sympathetic nerve traffic responses that we re significantly (P<0.05) greater after surgery than before surgery. These data provide the first direct evidence that in pheochromocytoma central sym pathetic outflow is markedly reduced and that this reduction cannot be ascr ibed to a reflex inhibitory response to elevated blood pressures. It is lik ely that this sympathoinhibition is rather due to a central depression of s ympathetic outflow induced by high circulating catecholamines.