Published Online
on February 28, 2005

A more recent version of this article appeared on April 1, 2005

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Submitted on December 7, 2004
Revised on January 2, 2005

Persistent Sympathetic Activation During Chronic Antihypertensive Therapy. A Potential Mechanism for Long Term Morbidity?

Qi Fu; Rong Zhang; Sarah Witkowski; Armin Arbab-Zadeh; Anand Prasad; Kazunobu Okazaki; and Benjamin D. Levine^*

From the Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas; and the University of Texas Southwestern Medical Center at Dallas.

^* To whom correspondence should be addressed. E-mail: BenjaminLevine{at}TexasHealth.org.

Abstract--Previous studies have demonstrated that antihypertensive treatment resets baroreflex control of heart rate (HR) and increases cardiac vagal baroreflex sensitivity. However, it is uncertain whether baroreflex control of muscle sympathetic nerve activity (MSNA) also resets after treatment. We tested the hypothesis that chronic antihypertensive therapy alters baroreflex regulation of MSNA in patients with untreated moderate hypertension. Seven newly diagnosed patients with systolic blood pressure (BP) of 159±5 mm Hg (mean±SE) and diastolic BP of 103±4 mm Hg were studied before and after 1 to 2 weeks and 3 months (chronic) of antihypertensive treatment with losartan-hydrochlorothiazide (Hyzaar). MSNA and hemodynamics were measured supine, during a Valsalva maneuver (VM), and at 70° head-up tilt (HUT) for 10 minutes. Data were compared with those obtained in 7 age-matched healthy controls. We found that Hyzaar lowered mean BP acutely and chronically by 20±4 and 23±3 mm Hg (both P<0.01) but did not change HR. Supine MSNA increased by 43±11% and 34±11% after acute and chronic treatment (both P<0.01). However, MSNA responses to VM and HUT did not differ after treatment compared with before treatment, indicating unchanged reflex control. These data indicate that sympathetic neural activity was augmented substantially by antihypertensive treatment with Hyzaar, consistent with an ongoing baroreflex unloading, and did not return to baseline or "reset" after 3 months of therapy. We speculate that persistent and marked sympathetic activation by the baroreflex may be a potential mechanism for hypertension that is refractory to antihypertensive therapy and may provide a target mechanism for persistent morbidity despite adequate BP control.

Key words: baroreceptors • autonomic nervous system • renin-angiotensin system

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