Baroreflex control of sympathetic nerve activity in essential and secondary hypertension

Studies performed in experimental animals and in humans have documented that high blood pressure markedly impairs baroreceptors control of heart rate. Whether a similar impairment also characterizes baroceptor control of sympathetic activity modulating peripheral vasomotor tone is still unknown. In 28 untreated essential hypertensive subjects [14 of moderate and 14 of more severe degree, age 51.6±2.4 and 52.6±2.1 (mean±SEM)] and in 13 untreated secondary hypertensives (renovascular or pheochromocytoma, age 50.1±4.6 years), we measured beat-to-beat arterial blood pressure (finger photoplethysmographic device), heart rate (electrodiogram), and efferent postganglionic muscle sympathetic nerve activity (microneurography) at rest and during baroreceptor stimulation and deactivation induced by stepwise intravenous infusions of phenylephrine and nitroprusside, respectively. Data were compared with those obtained in 15 age-matched normotensive control subjects. Muscle sympathetic nerve activity (bursts per 100 heart beats) showed a progressive and significant (P