Differential Effects of Nebivolol vs Metoprolol on Functional Sympatholysis in Hypertensive Humans

Abstract

In young healthy humans, sympathetic vasoconstriction is markedly blunted during exercise to optimize blood flow to the metabolically active muscle. This phenomenon known as functional sympatholysis is impaired in hypertensive humans and rats by angiotensin II-dependent mechanisms involving oxidative stress and inactivation of nitric oxide (NO). Nebivolol is a β1−adrenergic receptor blocker that has NO-dependent vasodilatory and antioxidant properties. We therefore asked if nebivolol would restore functional sympatholysis in hypertensive humans. In 21 subjects with stage I hypertension, we measured muscle oxygenation and forearm blood flow (FBF) responses to reflex increases in sympathetic nerve activity (SNA) evoked by lower body negative pressure (LBNP) at rest and during rhythmic handgrip exercise at baseline, after 12 weeks of nebivolol (5–20 mg/day), or metoprolol (100–300 mg/day), using a double-blind crossover design. We found that nebivolol had no effect on LBNP-induced decreases in oxygenation and FBF in resting forearm (from −29±5 to −30±5% and from −29±3 to −29±3%, respectively; p=NS). However, nebivolol attenuated the LBNP-induced reduction in oxygenation and FBF in exercising forearm (from −14±4% to −1±5% and from −15 ±2% to −6±2%, respectively, both p < 0.05). This effect of nebivolol on oxygenation and FBF in exercising forearm was not observed with metoprolol in the same subjects despite a similar reduction in BP. Nebivolol had no effect on SNA at rest or during handgrip, suggesting a direct effect on vascular function. Thus, our data demonstrate that nebivolol restored functional sympatholysis in hypertensive humans by a mechanism that does not involve β1-adrenergic receptors.

Introduction

Essential hypertension is a major public health problem in the U.S. and worldwide. Hypertensive patients display a blunted decline in systemic vascular resistance during exercise, which may contribute to impaired exercise tolerance even in the absence of heart failure ^1–4. Exercise triggers reflex activation of the sympathetic nervous system, resulting in elevated cardiac output while simultaneously producing vasoconstriction in many vascular beds, including the viscera and inactive skeletal muscles. In the working muscles, sympathetically-mediated vasoconstriction is greatly attenuated by vasoactive substances released during muscle contraction thereby optimizing muscle blood flow to meet metabolic demand ^5–9. We recently found that this protective mechanism, known as functional sympatholysis ⁹, is impaired in hypertensive humans ¹⁰.

The mechanisms responsible for impaired functional sympatholysis in hypertension are incompletely understood. Our previous studies in hypertensive rats indicated a deleterious role of angiotensin II (Ang II) to blunt functional sympatholysis due to excessive production of superoxide and inactivation of nitric oxide (NO) in the exercising muscles ¹¹. We recently extended these findings to hypertensive humans by showing that functional sympatholysis is restored by short-term treatment with the angiotensin receptor blocker (ARB) irbesartan, but not with the thiazide-type diuretic chlorthalidone despite an equivalent reduction in blood pressure. ¹⁰. These findings indicate that lowering blood pressure alone is not sufficient to restore functional sympatholysis. Therefore, based on our previous work, we anticipate that antihypertensive agents that inhibit the renin-angiotensin system, mitigate oxidative stress, or increase NO bioavailability would also normalize sympathetic regulation of muscle blood flow during exercise.

Nebivolol is a third-generation selective β1−adrenergic receptor (AR) blocker that also possesses vasodilator and antioxidant properties ¹². The vasodilator effect is attributed largely to activation of endothelial NO synthase (eNOS) and increased NO release, whereas the antioxidant effect is thought to be due to inhibition of NADPH oxidase and reduced superoxide formation ^12–14. Given the unique β1-AR-independent properties of nebivolol to improve NO signaling and reduce oxidative stress, we hypothesized that treatment with nebivolol would restore functional sympatholysis and improve muscle perfusion during exercise in hypertensive humans. In this study, we therefore sought to determine if short-term treatment with nebivolol would attenuate sympathetically-mediated vasoconstriction in the exercising muscles of subjects with primary hypertension. To determine if the effect of nebivolol could be explained by its distinct pharmacological profile independent of β1-AR antagonism, we compared nebivolol to metoprolol, a selective β1–AR blocker without vasodilator or antioxidant properties, using a randomized double-blind crossover design. Because our previous work implicated a major role for Ang II-induced oxidative stress to impair functional sympatholysis in hypertensive rats, we also asked if nebivolol treatment would mitigate the increases in oxidative stress and blood pressure evoked by Ang II infusion in hypertensive humans.

Methods

Twenty one subjects with untreated stage 1 hypertension participated in the study after providing written informed consent. The study was approved by the Institutional Review Board of the University of Texas Southwestern Medical Center at Dallas. All subjects had BP between 140–159/90–99 mmHg on 3 determinations by oscillometric technique in the seated position. The subjects had no history of heart disease, diabetes mellitus, or evidence of target organ damage such as left ventricular hypertrophy by electrocardiography or chronic kidney disease. The patients had not received antihypertensive drugs for at least 4 weeks before the study.

Experimental procedures

Subjects were studied in the supine position. Heart rate (HR) was recorded continuously by electrocardiography and systolic and diastolic blood pressures (BP) were measured by automated oscillometric sphygmomanometry (CE0050, Welch Allyn, Skaneateles Falls, NY). Respiration was monitored with a strain-gauge pneumograph and subjects were instructed to avoid sympathoexcitatory maneuvers including Valsalvas and prolonged expirations.

Skeletal muscle oxygenation

Near infrared (NIR) spectroscopy (NIRO-500, Hamamatsu Photonics, Hamamatsu, Japan) was used to measure changes in tissue concentrations of oxygenated haemoglobin and myoglobin (HbO₂+MbO₂) in the forearm, as previously described ^{5, 6, 15}. To monitor NIR light absorption, two fiber-optic bundles spaced 2 cm apart were placed over the flexor digitorum profundus muscle, which is the main muscle recruited during handgrip ¹⁶. NIR signals were sampled at a rate of 1 Hz, converted to chromophore concentrations using established algorithms, output to a computer, and digitally stored for later analysis. Changes in the NIR signals were quantified as a percentage of the total labile signal (TLS), which was defined in each experiment as the maximal decrease in HbO₂+MbO₂ achieved during inflation of a pneumatic cuff on the upper arm to 220 mmHg for 3 min. Because blood vessels larger than 1 mm in diameter maximally absorb NIR light, changes in HbO₂+MbO₂ reflect changes occurring mainly in the microvessels ¹⁷.

Forearm blood flow

Brachial artery diameter and mean blood velocity (MBV) were measured by Duplex Doppler ultrasonography (Philips ie33, Bothell, WA) using an 11-MHz probe in the non-dominant arm. The probe was placed in a holder and fixed to the skin over the brachial artery throughout the entire experiment. Diameter measurements were obtained at end-diastole. Blood velocity was acquired with a probe insonation angle of 60°. The output of the handgrip dynamometer was transferred into the auxiliary input of the ultrasound system and displayed simultaneously with the ultrasound images during handgrip exercise. Images were stored on DVD discs and were analyzed offline, using edge detection software (Brachial Analyzer, Medical Imaging Applications LLC, Coralville, IA). Forearm blood flow (FBF, ml min⁻¹) was calculated as MBV x π (brachial diameter/2)² x 60. Because motion artifact during handgrip produced distortion of the Doppler waveforms, images acquired during muscle contraction were excluded from analysis. Forearm vascular conductance (FVC, ml min⁻¹ (100 mmHg)⁻¹) was calculated as (FBF/MAP) x 100.

Reflex activation of sympathetic nerves

Lower body negative pressure (LBNP) was used to produce reflex sympathetic vasoconstriction in the forearm. The subject's lower body was enclosed to the level of the iliac crest in a negative pressure chamber. LBNP at −20 mmHg was used to unload mainly the cardiopulmonary baroreceptors and trigger increases in muscle sympathetic nerve activity (SNA) ⁵. Multiunit recordings of SNA were obtained with unipolar tungsten microelectrodes inserted into muscle fascicles of the peroneal nerve by microneurography ¹⁸. Neural signals were amplified, filtered (bandwidth 700–2000 Hz), rectified and integrated to obtain mean voltage neurograms. Recordings were considered acceptable based on well-defined criteria that discriminate muscle SNA from other neural signals including skin SNA and muscle spindle activity ⁵. Muscle SNA was expressed as burst frequency (bursts min⁻¹) and total activity (burst frequency x mean burst amplitude). Changes in SNA (% total activity) during the course of each experimental protocol were expressed as relative increases from the baseline activity at rest. Changes in SNA specifically in response to LBNP were expressed as the relative increases from the pre-LBNP baseline at rest or during handgrip.

Handgrip exercise

Maximal voluntary contraction (MVC) for each subject was designated as the greatest of at least three maximal squeezes of a handgrip dynamometer (Stoelting, Chicago, IL, USA). Subjects performed intermittent handgrip to the rhythm of a metronome (20 handgrips min⁻¹; 50% duty cycle) at 30% MVC for 6 min. Force production was displayed on an oscilloscope to provide subjects with visual feedback. This level of handgrip alone does not increase muscle SNA in healthy subjects ⁵.

Plasma F2-isoprostanes (F2-IsoPs)

F2-IsoP, a prostaglandin F2α-like compound was measured by gas chromatography-mass spectrometry (GC-MS) ¹⁹. Lower limit of detection of F2-IsoPs is 4 pg/ml. The precision of this assay in biological fluids is ± 6% and the accuracy 94% ¹⁹.

Experimental protocols

Protocol 1. Effects of nebivolol vs metoprolol on functional sympatholysis in hypertensive subjects (n = 21)

Blood pressure, HR, forearm muscle oxygenation, FBF, FVC, and SNA were measured in response to 2 min of LBNP at −20 mmHg applied at rest and during min 3–5 of handgrip in all hypertensive subjects at baseline. After the baseline study, subjects were randomized to receive 12 weeks of nebivolol (5–20 mg/day), or metoprolol succinate (100–300 mg/day), using a double-blind crossover design with 2-week washout between each treatment phase. Every 4 weeks clinic BP was measured using an oscillometric device (CE0050, Welch Allyn) after participants had been sitting quietly for at least 5 minutes. BP measurements were repeated twice after one minute between each reading and data were averaged with the first reading. The doses of nebivolol and metoprolol were titrated to achieve clinic BP of < 140/90 mmHg. After 12-week treatment with each study drug, measurement of BP, HR, forearm muscle oxygenation, FBF, FVC, and SNA during LBNP and handgrip were repeated and these variables were compared to those obtained at baseline.

Protocol 2. Assessment of vasoconstrictor response to Ang II during nebivolol vs metoprolol (n = 21)

To determine if nebivolol attenuates Ang II-induced vasoconstriction and BP elevation in hypertensive humans by preventing Ang II-induced increase in oxidative stress, we assessed changes in MAP, FBF, Forearm vascular resistance (FVR, which was calculated as MAP X 80 /FBF) at baseline and in response to intravenous infusion Ang II of at the dose of 1, 2, and 3 ng/kg/min each for 15 minutes in the same subjects participated in protocol 1 after completion of protocol 1 for at least 30 minutes. Plasma F2-IsoPs were also obtained at baseline and at the end of Ang II infusion protocol in the same subjects.

Statistical analysis

For protocol 1, drug treatment and experiment condition responses were compared with linear mixed model repeated measures analyses that included repeated factors to assess treatment phase (baseline, nebivolol, metoprolol), experimental condition (LBNP, rest, handgrip), and the treatment-by-condition interaction. In our analysis, the study subject is modeled as a random effect and, moreover, the mixed model approach permits flexibility in modeling the covariance structure of the repeated measures and can handle unbalanced data ²⁰. In the presence of a statistically significant main effect or interaction test, planned pairwise comparisons were made from the least squares means contrasts derived from the mixed models. For study in protocol 2, data were also analyzed with linear mixed models to test the treatment phase (baseline, nebivolol, metoprolol), angiotensin II dose, and the treatment-by-dose interaction effects. In all analyses, variables with skewed distributions were log or rank transformed prior to analysis. Statistical analyses were conducted using SAS version 9.2 (SAS Institute, Cary, NC, USA). All tests were two-sided and a p-value < 0.05 was considered statistically significant. Data are presented as means ± S.E.M.

Results

Baseline characteristics of our hypertensive subjects are shown in table 1. The average daily dose of nebivolol used in the study was 9 ± 1 mg and the average dose of metoprolol succinate was 174 ± 20 mg. Both nebivolol and metoprolol had no effect on fasting plasma glucose and there were no difference in fasting plasma insulin or HOMA-IR during treatment with nebivolol compared with metoprolol (p > 0.05, supplemental table S1).

Table 1.

Subject characteristics

Variables	Average ± SE
Age (yrs)	54 ± 3
Sex (M/F)	14 / 7
African Americans (%)	29 %
Body mass index (kg m−2)	29.7 ± 1
Systolic BP (mmHg)	141 ± 3
Diastolic BP (mmHg)	86 ± 2
Heart rate (bpm)	65 ± 2
Maximal voluntary contraction (kg)	31.8 ± 2
Serum creatinine (mg dl−1)	1.0 ± 0.04
Total cholesterol (mg dl−1)	171 ± 10
Triglycerides (mg dl−1)	113 ± 16
Fasting plasma glucose (mg dl−1)	101 ± 4

BP, blood pressure.

Protocol 1: Effects of Nebivolol vs Metoprolol on sympathetically-mediated vasoconstriction during exercise in hypertensive subjects

At baseline, LBNP at −20 mmHg evoked significant decreases in muscle oxygenation (−29±5%), FBF (−29±3%), and FVC (−28±4%) in the resting forearms of untreated hypertensive subjects that were accompanied by significant increases in muscle SNA (+55±15%) ( all p < 0.05 rest + LBNP vs rest; table 2, figure 1–2). When applied during rhythmic handgrip, this same level of LBNP caused reductions in muscle oxygenation, FBF, and FVC (−14±4%, −15±2%, and −18±2%, respectively, p < 0.05 handgrip + LBNP vs handgrip) that were only partially attenuated compared to the responses observed at rest (table 2, figure 1–2).

Table 2.

Hemodynamic and sympathetic responses to LBNP at rest and during handgrip in hypertensive subjects treated with nebivolol vs. metoprolol

Variables	Rest	Rest + LBNP	Handgrip	Handgrip + LBNP
Baseline (No Drug)
MAP, mmHg	104 ± 2	102 ± 2	110 ± 4*	110 ± 2*
HR, bpm	65 ± 2	65 ± 2	71 ± 1*	72 ± 2*
SNA, bursts min−1	40 ± 3	45 ± 3*	45 ± 4	50 ± 3*†
Δ SNA, % total activity	0 ± 0	52 ± 15*	40 ± 15*	100 ± 24 *†
FBF, ml min−1	114 ± 15	80 ± 11*	469 ± 50*	405 ± 49*†
FVC, units	110 ± 14	78 ± 10*	441 ± 45*	367 ± 39*†
Nebivolol
MAP, mmHg	97 ± 3‡	96 ± 3‡	103 ± 3‡	104 ± 3*‡
HR, bpm	56 ± 2‡	56 ± 3‡	62 ± 2*‡	63 ± 3*‡
SNA, bursts min−1	38 ± 3	44 ± 3*	47 ± 5*	52 ± 4*
Δ SNA, % total activity	0 ± 0	71 ± 19*	33 ± 10*	81 ± 22 *†
FBF, ml min−1	105 ± 13	75 ± 9*	427 ± 37*	404 ± 37*†
FVC, units	109 ± 13	78 ± 9*	413 ± 99*	389 ± 33*†
Metoprolol
MAP, mmHg	96 ± 2‡	95 ± 3‡	101 ± 3‡	100 ± 3*‡
HR, bpm	56 ± 2‡	55 ± 2‡	60 ± 2*‡	60 ± 2*‡
SNA, bursts min−1	39 ± 3	46 ± 3*	45 ± 4*	50 ± 3*
Δ SNA, % total activity	0 ± 0	77 ± 18*	30 ± 10*	99 ± 22*†
FBF, ml min−1	96 ± 1	75 ± 12*	398 ± 43*	340 ± 40*†║
FVC, units	99 ± 14	79 ± 12*	392 ± 39*	335 ± 35*†

LBNP, lower body negative pressure; MAP, mean arterial pressure; HR, heart rate; SNA, sympathetic nerve activity; FBF, forearm blood flow; FVC, forearm vascular conductance.

^*P < 0.05 vs Rest;

^†P < 0.05 vs Handgrip;

^‡P < 0.05 vs baseline,

^║p < 0.05 vs nebivolol.

Figure 1.

Original recordings of forearm muscle oxygenation (HbO₂+MbO₂) responses to lower body negative pressure (LBNP) applied at rest and during rhythmic handgrip in one hypertensive subject at baseline, after 12 weeks of nebivolol, and after 12 weeks of metoprolol. LBNP induced similar decrease in muscle oxygenation at rest when compared to during nebivolol and metoprolol treatment. During handgrip, LBNP evoked reduction in muscle oxygenation, which was attenuated by nebivolol but not by metoprolol in the same subject, despite similar reduction in BP. Complete forearm vascular occlusion after the exercise produced the maximal decrease in muscle oxygenation that was used to determine the total labile signal (TLS).

Figure 2.

Summary data showing changes in A. muscle oxygenation, B. FBF, C. FVC, and D. SNA in response to LBNP at rest and during handgrip in 21 hypertensive subjects. * P < 0.05 vs Rest; † P < 0.05 vs baseline (no drug) or metoprolol.

Treatment with nebivolol for 12 weeks had no effect on the LBNP-induced decreases in forearm muscle oxygenation (−30±5%), FBF (−29±3%), or FVC (−28±3%) or increase in muscle SNA (+71±19%) at rest (all p = NS nebivolol vs baseline). However, nebivolol markedly attenuated the LBNP-induced reductions in oxygenation (−1±5%), FBF (−6±2) and FVC (−6±1%) in the exercising forearms (all p < 0.05, figure 2). Although nebivolol attenuated the LBNP-induced vasoconstrictor responses during handgrip, it did not attenuate the increase in SNA during handgrip alone or handgrip plus LBNP (P = NS vs baseline (no drug period; Table 2). In contrast to nebivolol, metoprolol had no effect on LBNP-induced vasoconstriction or decrease in muscle oxygenation during handgrip in the same hypertensive subjects (Fig 1–2 and Table 2).

This beneficial effect of nebivolol on sympathetic vasoconstriction during exercise was not explained by the magnitude of BP reduction since a similar decrease in resting MAP was achieved by treatment with either nebivolol or metoprolol succinate (7 ± 3 vs 8 ± 2 mmHg, p = NS, table 2 and supplemental figure S1). Nebivolol also caused a similar reduction in MAP and HR during handgrip when compared to metoprolol succinate (p = NS, table 2 and figure S1).

Protocol 2: Effects of nebivolol vs metoprolol on Ang II-mediated vasoconstriction

In untreated hypertensive subjects at baseline, Ang II infusion caused a dose-dependent increase in MAP and FVR (p < 0.05, figure 3 and supplemental table S2) but had no effect on FBF or SNA (figure 3 and table S2). Although nebivolol and metoprolol cause similar reductions in resting MAP, neither affected the MAP or FVR responses to Ang II. Prior to Ang II infusion, plasma F2-IsoP levels were lower during nebivolol than metoprolol treatment (34.4 ± 2.7 vs 41.1 ± 3.9 pg/ml, respectively, p < 0.05), although not significantly different from baseline (38.8 ± 4.5 pg/ml, p > 0.05). Ang II infusion caused a significant increase in plasma F2-IsoP within 45 minutes of infusion during all phases of treatment (p < 0.01), but the increase in plasma F2-IsoP was unaffected by nebivolol or metoprolol (figure 3 and table S2).

Figure 3.

Summary data showing the changes in MAP, FVR, and plasma F2-IsoP in response to intravenous infusion of Ang II (3 ng/kg/min) in hypertensive subjects at baseline (no drug), after treatment with nebivolol for 12 weeks, and after metoprolol for 12 weeks. * P < 0.05 vs Rest; † P < 0.05 vs Baseline (no drug), ‡ P < 0.05 vs metoprolol.

Discussion

The major new findings of the present study are three-fold. First, nebivolol attenuated sympathetic vasoconstriction in the exercising forearm muscles of individuals with mild, uncomplicated hypertension which was not observed with metoprolol despite a similar reduction in resting BP. Second, this improvement in muscle blood flow regulation during exercise was not explained by a reduction in central sympathetic outflow, implicating a direct effect of nebivolol on the peripheral vasculature. Third, nebivolol did not ameliorate the increases in vascular resistance or plasma F2-isoprostanes evoked by short-term Ang II infusion, suggesting that the nebivolol-induced restoration of functional sympatholysis was likely not due to inhibition of Ang II-induced oxidative stress. Collectively, these findings indicate that the ability of nebivolol to improve functional sympatholysis in hypertensive individuals cannot be attributed to its conventional β-blocking and BP-lowering effects, but is likely due to its ancillary vasodilating properties.

In young and middle-aged healthy subjects, our group and others have shown that reflex sympathetic activation induced by mild orthostatic stress or infusion of sympathomimetic drugs evokes vasoconstriction in the resting forearm which is greatly attenuated during mild to moderate levels of rhythmic handgrip exercise ^{8, 10, 21}. In hypertensive subjects, however, we previously reported that sympathetic activation produces equivalent decreases in muscle blood flow and oxygenation in resting and exercising forearm, indicating impaired functional sympatholysis ¹⁰. We now confirm this finding of impaired sympatholysis in the present study in another group of mildly hypertensive, middle-aged subjects as evidenced by the persistent forearm vasoconstriction evoked by reflex sympathetic activation during handgrip exercise. Our new data show that this impairment is readily reversed by treatment with the third-generation vasodilating β1-AR blocker nebivolol, but not with the more traditional nonvasodilating β1-AR blocker metoprolol. Taken together with our previous work showing restoration of sympatholysis in hypertensive subjects treated with an ARB but not with a thiazide-type diuretic, these findings strengthen the conclusion that BP reduction alone is not sufficient to improve functional sympatholysis. Only those drugs with specific pharmacological properties that intersect with mechanistic pathways involved in sympatholysis are likely to normalize muscle blood flow regulation during exercise in hypertension.

Our findings provide some clues about the potential β1-AR independent mechanisms underlying nebivolol’s effect to attenuate sympathetically-mediated vasoconstriction during exercise. This could not be explained by a reduction in central sympathetic drive because LBNP evoked similar increases in SNA at rest and during handgrip in subjects at baseline (no treatment) and during treatment with either nebivolol or metoprolol. It is also unlikely that nebivolol reduced sympathetic neurotransmission or directly inhibited α-AR because LBNP caused similar decreases in FBF and FVC at rest during the baseline and treatment phases of our study. These data are consistent with studies in pithed rats showing that nebivolol did not attenuate the pressor responses to selective α1- or α2-AR agonists or to electrical stimulation of the spinal cord ²². Finding no evidence to suggest that nebivolol restored functional sympatholysis in hypertensive individuals by reducing sympathetic drive or inhibiting α-AR, we then turned our attention to its putative antioxidant properties.

One of the unique features of nebivolol that could explain its beneficial effect on functional sympatholysis is its ability to inhibit oxidative stress. Nebivolol may directly scavenge reactive oxygen species (ROS) as well as reduce ROS production by inhibiting the activity and expression of NADPH oxidase. We previously implicated a mechanistic role for oxidative stress to impair functional sympatholysis in rat models of Ang II-dependent hypertension due to upregulation of NADPH oxidase and excessive production of ROS in the contracting muscles. Infusion of the antioxidant tempol normalized sympatholysis in these hypertensive animals. Ang II also plays a role in the impaired functional sympatholysis in hypertensive humans, as treatment with an ARB readily reverses this abnormality. In the current study, we therefore asked if nebivolol would mitigate Ang II-induced oxidative stress in the hypertensive subjects. To address this question, we measured plasma levels of F2-isoprostanes in response to a brief systemic infusion of Ang II. Our finding that Ang II-induced increases in F2-isoprostanes were similar at baseline (without treatment) and during nebivolol treatment does not appear to support an antioxidant mechanism of action of nebivolol. However, our previous work in hypertensive rats suggests that functional sympatholysis is impaired by excessive muscle-derived ROS, which we were unable to measure in the present study. It remains to be determined if nebivolol can attenuate ROS production in skeletal muscle during exercise via inhibition of NADPH oxidase.

Contracting skeletal muscle also produces NO, which we have shown is a major mediator of functional sympatholysis in both rodents and humans ^{15, 23, 24}. More recently, adenosine triphosphate (ATP) has been identified as another potent sympatholytic factor in healthy humans ^{25, 26}. Thus, boosting NO production or ATP release are two attractive mechanisms to potentially restore functional sympatholysis in hypertension. A growing body of evidence indicates that nebivolol’s vasodilating effect is mediated by its ability to stimulate NO production in blood vessels ^{27 28}. Interestingly, in the renal microvasculature this effect of nebivolol appears to be mediated by increased ATP efflux resulting in activation of endothelial P2Y–receptors and subsequent calcium-dependent activation of eNOS²⁷. Nebivolol has also been shown to reduce circulating levels of the endogenous NOS inhibitor asymmetric dimethylarginine (ADMA) in hypertensive patients byincreasing the expression and activity of the ADMA-degrading enzyme dimethylarginine dimethylaminohydrolase (DDAH)²⁹. Whether nebivolol restores functional sympatholysis by stimulating NO and/or ATP release in the human skeletal muscle remains to be investigated.

Our study is limited by lack of a placebo arm for comparison because it is unethical to withhold antihypertensive treatment for an extended duration of 12 weeks. However, it is unlikely that the restoration of functional sympatholysis during nebivolol treatment is a random occurrence. In our previous study we showed that sympatholysis was normalized in hypertensive subjects during treatment with an ARB, but that sympatholysis was similarly impaired in the same subjects before treatment and 4 weeks after withdrawal from treatment. Our study is also limited by the lack of normotensive control subjects to verify that functional sympatholysis is impaired in the hypertensive cohort. However, in our previous study we showed that sympathetic vasoconstriction is significantly enhanced in the exercising but not the resting forearm muscles of hypertensive subjects compared with age-matched normotensive controls, thereby documenting impaired sympatholysis in the hypertensive group. Given this finding, we recruited hypertensive subjects with similar characteristics for the present study.

Perspectives

Use of older generation β-blockers without ancillary vasodilating properties is associated with exercise intolerance and fatigue ^30–32. This is thought to be due to attenuation of β-AR mediated increase in heart rate and cardiac output during exercise ³². These side effects appear to be much less common in patients treated with newer generation β-blockers like nebivolol due to its tendency to improve oxygen uptake or augment the reduction in systemic vascular resistance at peak exercise ^{31, 33}. While nebivolol has been shown to improve left ventricular function and functional capacity in hypertensive patients with systolic heart failure ³⁴, beneficial effects of nebivolol on exercise performance was also observed in heart failure patients with normal ejection fraction ³³, suggesting additional benefit beyond myocardial function. Impaired functional sympatholysis is increasingly recognized as a cause of skeletal muscle malperfusion not only in hypertension but also in the normotensive elderly population and individuals with physical inactivity ^{35, 36}. Thus, inhibition of sympathetic vasoconstriction may constitute an additional mechanism by which exercise tolerance is improved with nebivolol. Clinical trials with long-term follow-up are needed to determine if nebivolol can improve skeletal muscle perfusion and exercise capacity in hypertensive subjects without heart failure.

Novelty and Significance.

1) What Is New?

The first study which addresses effects of nebivolol on sympathetically-mediated vasoconstriction during exercise in hypertensive patients.

2) What Is Relevant?

During exercise, sympathetically-mediated vasoconstriction is greatly attenuated by vasoactive substances released during muscle contraction, thereby optimizing blood flow to the working muscle. This protective mechanism, known as functional sympatholysis, is shown to be impaired in hypertensive rats via Ang II-mediated production of superoxide and inactivation of nitric oxide (NO) in the exercising muscles. Thus, we postulate that nebivolol, a third-generation β1–AR blocker that also possesses vasodilator and antioxidant properties, can reverse this abnormality in hypertensive patients.

Summary

We identify a novel action of nebivolol in restoring functional sympatholysis and alleviate skeletal muscle ischemia during handgrip, independent of BP reduction. This beneficial effect of nebivolol was not due to reduction in oxidative stress nor inhibition of Ang II-induced superoxide formation.

Abbreviations

Ang II

angiotensin

AR

adrenergic receptor

ARB

AT₁ receptor blocker

BP

blood pressure

eNOS

endothelial nitric oxide synthase

FBF

forearm blood flow

FVC

forearm vascular conductance

HR

heart rate

LBNP

lower body negative pressure

MAP

mean arterial pressure

MBV

mean blood velocity

MVC

maximal voluntary contraction

NIR

near-infrared

NO

nitric oxide

RHG

rhythmic handgrip

SNA

sympathetic nerve activity

TLS

total labile signal

β1-AR

β1-adrenergic receptor