A sympathetic component of baroreflex function in patients with essential hypertension.

1. The baroreflex-mediated changes in plasma norepinephrine (NE) and heart period (RR interval) to phenylephrine (PE)-evoked pressor and nitroprusside (NP)-evoked depressor stimulations were studied in 19 patients with chronic essential hypertension, 12 with borderline hypertension and in 11 age-matched normal controls. 2. Intravenous infusion of PE at a rate of 0.25-1.0 microgram/kg/min induced dose-related increases in mean arterial pressure (MAP) and in RR interval and a decrease in plasma NE. Similarly, NP infusion at a rate of 0.1-0.4 microgram/kg/min evoked the opposite changes in each variable. The reflex sensitivity was defined as the slope of linear regression between the changes in RR interval and MAP (RR/MAP) and between those in plasma NE (% of the baselines) and MAP (%NE/MAP). 3. Both RR/MAP and %NE/MAP for pressor and depressor stimulations were reduced below values found in normal subjects, in both chronic and borderline hypertensives. 4. The values of %NE/MAP was negatively related to the basal plasma NE during falls in blood pressure (r = -0.401, P less than 0.05). 5. The %NE/MAP may be a useful index of the sympathetic component of baroreflex sensitivity. A decrease in %NE/MAP in hypertensive and borderline hypertensive patients suggests a blunted sensitivity of the sympathetic constrictor reflexes.