BAROREFLEX CONTROL OF SYMPATHETIC-NERVE ACTIVITY IN ESSENTIAL AND SECONDARY HYPERTENSION

Studies performed in experimental animals and in humans have documente d that high blood pressure markedly impairs baroreceptor control of he art rate. Whether a similar impairment also characterizes baroreceptor 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 years (mean+/-SEM)] and in 13 untreated secondary hypertensives (ren ovascular or pheochromocytoma, age 50.1+/-4.6 years), we measured beat -to-beat arterial blood pressure (finger photoplethysmographic device) , heart rate (electrocardiogram, and efferent postganglionic muscle sy mpathetic nerve activity (microneurography) at rest and during barorec eptor stimulation and deactivation induced by stepwise intravenous inf usions of phenylephrine and nitroprusside, respectively. Data were com pared with those obtained in 15 age-matched normotensive control subje cts. Muscle sympathetic nerve activity (bursts per 100 heart beats) sh owed a progressive and significant (P<.01) increase from normotension (40.3+/-3.3) to moderate (55.6+/-4.1) and more severe essential hypert ension (68.2+/-4.1), paralleling the progressive increase in blood pre ssure values. In contrast, muscle sympathetic nerve activity was not i ncreased in secondary hypertensives (40.5+/-6.7) despite blood pressur e values similar to or even greater than those of severe essential hyp ertensives. In both essential and secondary hypertensives, barorecepto r-heart rate control was displaced toward elevated blood pressure valu es and markedly impaired compared with normotensive subjects (average reduction, 38.5%). In contrast, the sympathoinhibitory and sympathoexc itatory responses to baroreceptor stimulation and deactivation were di splaced toward elevated blood pressure values but similar in all group s. Thus, sympathetic activation characterizes essential but not second ary hypertension. Regardless of its nature, however, hypertension is n ot accompanied by an impairment of baroreceptor modulation of sympathe tic activity.