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J Am Coll Cardiol, 1998; 32:1207-1213
© 1998 by the American College of Cardiology Foundation
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CLINICAL STUDIES

Impairment of the nitric oxide–mediated vasodilator response to mental stress in hypertensive but not in hypercholesterolemic patients

Carmine Cardillo, MDa, Crescence M. Kilcoyne, RN, MSa, Richard O. Cannon, III, MDa and Julio A. Panza, MDa

a Cardiology Branch, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland, USA

Manuscript received June 16, 1998; accepted July 2, 1998.

Address for correspondence: Dr. Julio A. Panza, Cardiology Branch, NHLBI, NIH, Building 10, Room 7B-15, 10 Center Drive, Bethesda, Maryland 20892-1650
panzaj{at}gwgate.nhlbi.nih.gov


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
Objectives. This study investigated whether mental stress–induced vasodilation mediated by endothelium-derived nitric oxide (NO) is defective in conditions with endothelial dysfunction, such as hypertension and hypercholesterolemia.

Background. Vascular release of NO modulates the vasodilator response to mental stress in healthy subjects. Previous studies have shown that hypertensive and hypercholesterolemic patients have impaired endothelium-dependent vasodilation to pharmacologic agents due to decreased NO activity. However, whether this abnormality also operates in response to physiologic stimuli such as mental stress has not been defined.

Methods. Forearm blood flow responses (plethysmography) to mental stress were compared in 12 normal subjects, 12 hypertensive patients and 10 hypercholesterolemic patients before and during NO synthesis inhibition with NG-monomethyl-L-arginine (4 µmol/min). Vascular responses to acetylcholine (7.5, 15 and 30 µg/min), an endothelium-dependent vasodilator, and sodium nitroprusside (0.8, 1.6 and 3.2 µg/min), an exogenous NO donor, were also assessed in each group.

Results. During saline the vasodilator response to mental stress was significantly blunted in hypertensive (37 ± 11%; p = 0.01) but not in hypercholesterolemic (85 ± 21%; p = 0.78) patients compared with controls (93 ± 15%). NG-Monomethyl-L-arginine administration significantly blunted mental stress–induced vasodilation in healthy subjects (p = 0.004 vs. saline) and hypercholesterolemic patients (p = 0.03 vs. saline), but not in hypertensive patients (p = 0.69 vs. saline). The vasodilator effect of the highest dose of acetylcholine was similarly blunted in hypertensive (215 ± 44%; p = 0.02) and hypercholesterolemic (172 ± 71%; p = 0.02) patients compared with controls (364 ± 34), whereas the vasorelaxing response to sodium nitroprusside was similar in the three groups.

Conclusions. Hypertensive but not hypercholesterolemic patients have impaired NO-dependent vasodilation during mental stress. These findings may be accounted for by different mechanisms underlying endothelial dysfunction in these two conditions and might explain an increased susceptibility of hypertensive patients to vascular damage over repeated exposure to stressful situations.

Endothelial synthesis and release of nitric oxide (NO) play an important role in the regulation of vascular tone as well as in the modulation of several processes, such as leukocyte adhesion and platelet aggregation, involved in the pathophysiology of atherosclerosis (1). In recent years a number of studies have shown that NO-dependent vasodilator function is impaired in patients with risk factors for atherosclerosis, such as arterial hypertension (2–6) and hypercholesterolemia (7–9), suggesting that a decreased bioavailability of NO might play a causal role in the genesis and progression of the atherosclerotic disease. However, most of the previous studies investigating NO-dependent vasodilator activity in humans have used pharmacologic stimuli as a tool to investigate the ability of the endothelium to produce NO. It has not been clearly defined, therefore, whether a decreased production of NO in the resistance vessels of patients with hypertension and hypercholesterolemia also occurs in response to other physiologic stimuli.

We and other investigators have recently shown that NO synthesis inhibition by NG-monomethyl-L-arginine (L-NMMA) markedly blunts the vasodilator response to mental stress in the forearm circulation of healthy subjects, thereby suggesting that this response is partly mediated by local release of NO (10,11). On the basis of these observations, we hypothesized that conditions with a demonstrated decrease in NO activity, such as hypertension and hypercholesterolemia, may also be associated with reduced vasodilator response to mental stress. Further, because activities involving mental stress are commonly performed several times a day, assessment of the ability of the vascular endothelium to synthesize and release NO during stressful situations in patients with hypertension or hypercholesterolemia might provide a more relevant estimate of the pathophysiologic significance of endothelial dysfunction in these conditions. For this reason, in the present study we compared the NO-dependent vasodilator response to mental stress in normal subjects, hypertensive patients and hypercholesterolemic patients.


   Methods
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 Methods
Results
 Discussion
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Study population.   The clinical characteristics and lipid profile of the subjects who took part in the study are reported in Table 1.


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  		Table 1 Clinical Characteristics of the Study Population

 
The hypercholesterolemic group included 10 patients selected for the study because their plasma cholesterol levels after a 12-h fasting period were greater than 250 mg/dl. Patients had not taken any cholesterol-lowering agent within the previous 2 months or any antioxidant vitamin supplement in the preceding 6 months. No effort was made to change the patients’ diet before the performance of the study. All hypercholesterolemic patients had normal blood pressure values.

The hypertensive group included 12 patients with a history of chronically elevated blood pressure (>145/95 mm Hg) without any apparent underlying cause, who were followed in the outpatient clinic of the National Heart, Lung and Blood Institute. Each patient had been previously treated with one or more antihypertensive agents for more than 3 years. Patients were asked to discontinue their current antihypertensive therapy 2 weeks before the study day; during that period they were closely monitored for any evidence of accelerated or malignant hypertension. Patients in whom the withdrawal of antihypertensive therapy was considered hazardous, mostly because of severely elevated blood pressure despite medications, were excluded from the study. All hypertensive patients had plasma cholesterol levels <200 mg/dl.

Twelve normal volunteers with no evidence of present or past hypertension or hypercholesterolemia (total cholesterol <200 mg/dl) were selected as a control group. They were matched with the patients of both groups for approximate age and sex.

Before admission, subjects of each group were screened by clinical history, physical examination, routine chemical analyses, electrocardiography and chest radiography. Exclusion criteria were history or evidence of present or past diabetes mellitus, cardiac disease, peripheral vascular disease, coagulopathy or any other disease predisposing them to vasculitis or Raynaud’s phenomenon.

The study protocol was approved by the National Heart, Lung and Blood Institute Investigations Review Board, and all participants gave written informed consent for all procedures.

Protocol.   All studies were performed in the morning in a quiet room with a temperature of approximately 22°C. Participants were asked to refrain from drinking alcohol or beverages containing caffeine and from smoking for at least 24 h before studies.

Each study consisted of the measurement of the response of the forearm vasculature by means of strain-gauge venous occlusion plethysmography under different experimental conditions. All drugs used in this study were approved for human use by the Food and Drug Administration in the form of Investigational New Drug and were prepared by the Pharmaceutical Development Service of the National Institutes of Health following specific procedures to ensure accurate bioavailability and sterility of the solutions.

While the participants were supine, a 20-gauge Teflon catheter (Arrow Inc., Reading, Pennsylvania) was inserted into the brachial artery of the left arm. This arm was slightly elevated above the level of the right atrium, and a mercury-filled Silastic strain gauge was placed in the widest part of the forearm (12). The strain gauge was connected to a plethysmograph (model EC-4; D.E. Hokanson, Bellevue, Washington) calibrated to measure the percent change in volume and connected in turn to a chart recorder to record the flow measurements. For each measurement a cuff placed around the upper arm was inflated to 40 mm Hg with a rapid cuff inflator (model E-10; Hokanson) to occlude venous outflow from the extremity. A wrist cuff was inflated to suprasystolic pressures 1 min before each measurement to exclude the hand circulation (13). Flow measurements were recorded for approximately 7 s every 15 s; seven readings were obtained for each mean value.

Basal measurements were obtained after a 3-min infusion of saline solution at 1 ml/min. Forearm blood flow was then measured during mental stress and during the infusion of sodium nitroprusside. These measurements were separated by a 30-min resting period. The mental stress testing required each subject to subtract continuously the number 7 from a three-digit number as quickly and accurately as possible for 3 min. During the test the participants were intentionally frustrated by being asked to perform faster and by being immediately corrected in case of wrong answers. The test procedure was explained to the subjects upon their arrival in the laboratory room, and they were asked to practice to get familiar with the task.

A 30-min rest period was allowed and another forearm blood flow measurement was obtained to corroborate the return to basal values. NG-Monomethyl-L-arginine (Calbiochem, La Jolla, California) was then infused at 4 µmol/min (infusion rate, 1 ml/min) for 15 min, and baseline flow measurements were obtained during the last 2 min of infusion. NG-Monomethyl-L-arginine is an arginine analog that competitively antagonizes the synthesis of NO from L-arginine and thus provides a tool for investigating the rate of vascular NO production (14). The mental arithmetic task was subsequently repeated using the same procedure reported above.

Assessments of vascular responsiveness to acetylcholine and sodium nitroprusside were performed either on the same day of mental stress testing or on a different occasion. Acetylcholine induces vasodilation by stimulating the release of relaxing factors from the vascular endothelium (15). Sodium nitroprusside was used as an endothelium-independent vasodilator because its vasodilator effect is largely due to its direct action on smooth muscle cells (16). Acetylcholine chloride (Sigma Chemical Co., St. Louis, Missouri) was infused at 7.5, 15 and 30 µg/min, and sodium nitroprusside was infused at 0.8, 1.6 and 3.2 µg/min (the infusion rates were 0.25, 0.5 and 1 ml/min, respectively, for each drug). Each dose was infused for 5 min and forearm flow was measured during the last 2 min. A 30-min rest period was allowed and another basal measurement was obtained between the infusion of the two drugs. The sequence of acetylcholine and sodium nitroprusside infusion was randomized to avoid any bias related to the order of these procedures.

During the study all blood pressures were recorded directly from the intraarterial catheter immediately after each flow measurement, and heart rate was recorded from an electrocardiographic lead.

Statistical analysis.   Group differences were analyzed by one-way analysis of variance and by chi-square test as appropriate. The effects of L-NMMA on vascular responses to mental stress within each group were analyzed by paired Student t test. Group differences in the dose–response curves to acetylcholine and sodium nitroprusside were analyzed by analysis of variance for repeated measures. All calculated p values are two-tailed, and a p value less than 0.05 was considered to indicate statistical significance. All group data are reported as means ± SEM.


   Results
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 Methods
 Results
Discussion
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Clinical characteristics and baseline measurements.   The clinical characteristics of the subjects in the three study groups are indicated in Table 1. There was no significant difference among the three groups in sex, age, weight, height and smoking habit. As expected, mean arterial pressure was higher in hypertensive patients than in the other groups, and total cholesterol and low-density lipoprotein cholesterol were higher in hypercholesterolemic patients than in the other groups. Baseline forearm blood flow was not significantly different (p = 0.63) among the three group (Table 2).


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  		Table 2 Hemodynamic Variables at Baseline and During Mental Stress Testing Before and After Nitric Oxide Synthesis Inhibition

 
Hemodynamic responses to mental stress.   During infusion of saline, mental stress testing induced an increase in forearm blood flow from baseline in each group (Table 2). The vasodilator response to mental stress testing was significantly reduced in hypertensive but not in hypercholesterolemic patients compared with controls (Fig. 1).



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  		Figure 1 Bars show the percent changes in forearm blood flow from baseline in response to mental stress in normotensive, hypertensive and hypercholesterolemic subjects. Values represent mean ± SEM.

 
Mental stress testing caused an increase in mean arterial pressure from baseline in each group (Table 2) without any significant difference among them (p = 0.69). Similarly, the performance of mental arithmetic caused an increase in heart rate over baseline in each group (Table 2) without any significant difference among them (p = 0.66).

Effects of L-NMMA on hemodynamic responses to mental stress.   During L-NMMA administration, the performance of mental stress testing resulted in an increase in forearm blood flow from baseline in all groups (Table 2). In both controls and hypercholesterolemic patients, however, the vasodilator response to the performance of mental arithmetic test was significantly lower during NO synthase inhibition than during saline (Fig. 2). In contrast, in hypertensive patients the increase in forearm blood flow induced by mental stress was not significantly different during saline or L-NMMA administration (Fig. 2). As a result, the vasodilator effect of mental stress testing during NO synthesis inhibition was not significantly different among the three groups. (p = 0.49).



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  		Figure 2 Bars show the effect of nitric oxide synthesis inhibition by NG-monomethyl-L-arginine (L-NMMA, 4 µmol/min) on the percent changes in forearm blood flow from baseline in normotensive, hypertensive and hypercholesterolemic subjects. Values represent mean ± SEM. Open box = saline; solid box = L-NMMA.

 
During L-NMMA administration, mean arterial pressure and heart rate values during mental stress testing (Table 2) were not significantly different among the three groups (p = 0.70 and p = 0.95 for mean arterial pressure and heart rate, respectively), and within each group they were similar to those observed during saline infusion (all p > 0.05).

Forearm blood flow responses to acetylcholine and sodium nitroprusside.   Infusion of increasing doses of acetylcholine resulted in a progressive increase in forearm blood flow from baseline in the three groups (Table 3); the vasodilator response to acetylcholine, however, was significantly blunted in hypertensive and hypercholesterolemic patients compared with controls (Fig. 3). Sodium nitroprusside administration induced a dose-dependent vasodilator response in each group (Table 3); in contrast with acetylcholine results, however, the vasodilator response to sodium nitroprusside was not significantly different among the three groups (Fig. 3).


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  		Table 3 Forearm Blood Flow Responses to Intraarterial Infusion of Increasing Doses of Acetylcholine and Sodium Nitroprusside

 


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  		Figure 3 Bars show the percent changes in forearm blood flow from baseline in response to intraarterial infusion of acetylcholine (30 µg/min) and sodium nitroprusside (3.2 µg/min) in normotensive (NV), hypertensive (HT) and hypercholesterolemic (HC) subjects. Values represent mean ± SEM.

 

   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
References
 
The results of this study demonstrate that, compared with healthy subjects, hypertensive but not hypercholesterolemic patients have a blunted vasodilator response to mental stress. Nitric oxide synthase inhibition by L-NMMA significantly reduced mental stress–induced vasodilation in healthy subjects and hypercholesterolemic patients, but did not induce any significant change in hypertensive patients; as a result, during NO synthesis inhibition the vasomotor response to mental stress was similar in all groups. These findings indicate that the defect in the vasodilator responsiveness to mental stress testing observed in hypertensive patients is related to decreased NO activity, whereas the bioavailability of NO during mental stress is normal in hypercholesterolemic patients. In contrast with these results, endothelium-dependent vasodilator responses to acetylcholine were similarly impaired in hypertensive and hypercholesterolemic patients. On the other hand, endothelium-independent vasodilator responsiveness to sodium nitroprusside was similar in patients and controls, thereby indicating that the generalized impairment of endothelium-dependent vasodilation observed in hypertensive patients is specifically accounted for by a defect in endogenous NO activity, because the ability of vascular smooth muscle to respond to exogenous NO is preserved. Of note, blood pressure and heart rate responses to mental stress were not different among the three groups and were not significantly affected by L-NMMA. These observations indicate that the systemic hemodynamic response elicited by mental stress was similar in all groups and was not affected by NO inhibition, thus enhancing the specificity of group differences in mental stress–induced vasodilation and in the effects of L-NMMA on this response.

Pathophysiologic mechanisms.   Previous studies in our laboratory have suggested the possibility of different abnormalities underlying the defect in NO activity in patients with hypertension and hypercholesterolemia. Thus, although both hypertensive and hypercholesterolemic patients have impaired vasodilator response to acetylcholine, the vasorelaxing effect of bradykinin, another endothelium-dependent vasodilator, is selectively impaired in hypertensive (17) but not in hypercholesterolemic (18) patients. Moreover, an increased production of superoxide anions through the xanthine oxidase pathway seems to be partially responsible for the decreased NO bioavailability in hypercholesterolemic but not in hypertensive vessels (19). The results of this study confirm and expand these previous observations by demonstrating that hypercholesterolemic and hypertensive patients have a different NO-mediated vasodilator response to a physiologic stimulus such as mental stress.

To shed light on the nature of the selective abnormality in NO-mediated vasodilator responsiveness to mental stress observed in hypertensive compared with hypercholesterolemic patients, it is necessary to analyze the potential mechanisms involved in NO release during this task. It has been proposed that acetylcholine released from cholinergic nerves could be involved in the vasodilator effect of mental stress because vasodilation is significantly blunted by atropine administration (10). Although in our study both hypertensive and hypercholesterolemic patients had blunted vasodilator response to exogenous acetylcholine, it cannot be excluded that the release of acetylcholine from cholinergic nerve endings during mental stress was reduced in hypertensive patients. In addition, other mechanisms independent of cholinergic nerve stimulation seem to contribute to NO release in response to mental stress, as suggested by previous evidence showing that the combined infusion of atropine and L-NMMA results in a further decrease of mental stress–induced vasodilation compared with atropine alone (10). One possible explanation of this phenomenon is the activation by mental stress of autonomic neurons releasing or coreleasing neurotransmitters different from the classic adrenergic and cholinergic mediators. Among the variety of nonadrenergic, noncholinergic neurons identified in recent years, peptidergic nerves (releasing substance P) and nitrergic nerves (releasing NO) are present in several perivascular beds of different species, where they contribute to the regulation of vasomotor tone (20). It is therefore possible that NO released from vascular endothelium in response to stimulation of tachykinin receptors released by nitrergic nerves, or both, might participate in the physiologic adaptation to mental stress, and such a mechanism could be impaired in patients with essential hypertension.

Another mechanism that might contribute to NO-mediated vasodilation during mental stress is the stimulation of alpha2- (21,22) or beta2-adrenoceptors (23,24), or both, on endothelial cells after the increase in catecholamine levels (25). Although a recent study in our laboratory has shown that NO production in response to beta-adrenoceptor stimulation is similar in hypertensive patients and healthy controls (26), a possible impairment of alpha2-adrenoceptor–mediated release of NO in hypertensive patients cannot be excluded. Also, potential differences in local or systemic catecholamine release in hypertensive patients leading to a defect in catecholamine-stimulated NO release or alternatively an increased vascular smooth muscle constrictor responsiveness to catecholamines could also explain our results.

Finally, a phenomenon that is possibly involved in NO release during mental stress is flow-mediated vasodilation as a consequence of mental stress–induced increase in cardiac output. This mechanism is suggested by the findings of a previous study, showing that mental stress–induced vasodilation is inhibited by systemic administration of selective beta-adrenergic blocking agents (27), which do not affect beat2-adrenoceptor–mediated peripheral vasodilation, thus implying that preventing cardiac output increase might blunt the rise in limb blood flow in response to mental stress. Flow and shear stress are known to stimulate constitutive endothelial NO synthase (eNOS) by inducing a series of mechanochemical signal transduction events involving G protein activation and increase in intracellular levels of inositoltriphosphate (28–31). Previous studies have shown that flow-mediated vasodilation is impaired in the conductance arteries of hypercholesterolemic patients (32,33) and relatively preserved in those of hypertensive patients (34,35). Therefore, it is unlikely that an impairment of this mechanism in resistance arteries of hypertensive patients explains their blunted NO-mediated vasodilator response to mental stress observed in our study. However, because those reports (32–35) are related to the response of large conductance arteries, whereas our study addresses the behavior of the microvasculature, this possibility cannot be completely ruled out.

Despite the presence of reduced NO-dependent vasodilator responsiveness to mental stress in our hypertensive study patients, the pressor response to psychologic challenge was not different among the three groups. Because the blood pressor response to mental stress may be achieved by variable combinations of changes in cardiac output and peripheral resistance (36), our findings are probably explained by a different pattern of hemodynamic reactivity in hypertensive patients, with higher peripheral resistance and lower cardiac output increases, as previously reported by other investigators (36).

The findings of this investigation point out a potential pathophysiologic mechanism through which stressful situations may lead to untoward acute and chronic cardiovascular effects in patients with arterial hypertension. Thus, defective endothelial release of NO in hypertensive vessels may amplify the deleterious effect of hemodynamic and hemostatic changes induced by mental stress and lead to those plaque complications that underlie the occurrence of acute cardiovascular events. Under these conditions, repeated exposure to environmental stress may present a stronger stimulus for vascular injury, with consequent acceleration in the development of atherosclerosis and its complications.

It must be noted, however, that because of the heterogeneity of essential hypertension, the reduced vasodilator response to mental stress might not be a generalized finding, but may be limited only to a subset of hypertensive patients. This possibility is suggested by the results of a previous study showing that normotensive subjects and hypertensive patients have a similar vasodilator response to mental stress induced by the Stroop color–word test (37). Although the precise reason for this discrepancy is unknown, it might be hypothesized that the different mental stress protocol used in the two studies could have influenced the results, as suggested by a recent study showing a lower degree of sympathetic activation during the Stroop test as compared with another active coping task (38).

Our study participants could have experienced a lower level of stress during the second exposure to the psychologic challenge (during L-NMMA administration) as they became familiar with the demands of the experiment (39). Care was taken to avoid this phenomenon by allowing all subjects and patients to practice before the performance of the tasks, thus minimizing the possibility of adaptation of the cardiovascular response to stress testing over repeated exposure. Because plasma catecholamine levels were not measured in our study, we do not have direct evidence that the two mental stress experiments evoked the same magnitude of sympathetic response. However, it must be noted that blood pressure and heart rate responses were similar between the two tasks in all groups, thus suggesting that the degree of systemic hemodynamic responsiveness to mental stress did not decrease over time.

Conclusions.   Our study indicates that hypertensive but not hypercholesterolemic patients have impaired NO-dependent vasodilation during mental stress. These findings may be accounted for by different mechanisms underlying endothelial dysfunction in these two conditions and might explain an increased susceptibility of hypertensive patients to vascular damage over repeated exposure to stressful situations.


   References
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 Abstract
 Methods
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 Discussion
 References
 

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