Abstract
Background. There are controversial results regarding the endothelial function in patients with white coat hypertension (WCH). The aim of this study was to assess endothelial function measuring nitric oxide (NO) and C-reactive protein (CRP) level in WCH and to compare those with essential hypertension (EH) and healthy subjects. Methods. The 40 newly diagnosed patients with EH, 40 patients with WCH and 40 healthy volunteers were included to study. Plasma CRP levels were measured by immunonephelometery method. Plasma NO level was also detected by using the Griess method. Results. Plasma CRP level was significantly higher in patients with EH when compared with those with WCH and healthy subjects (6.3 ± 2.1 mg/l, 2.1 ± 0.9 mg/l and 1.6 ± 1.3 mg/l, p < 0.05, respectively). However, there was no significant difference with respect to CRP level between those with WCH and healthy subjects. NO level was significantly lower in patients with EH when compared with those with WCH and healthy subjects (4.6 ± 1.1 µmol/l, 6.9 ± 1.2 µmol/l and 8.1 ± 1.5 µmol/l, p < 0.05, respectively). There was no significant difference with respect to NO level between those with WCH and healthy subjects. Plasma CRP level was positively correlated with office, daytime, night-time and 24-h blood pressure values, whereas NO level was inversely correlated with these parameters. Plasma CRP level was also inversely correlated with NO level. Conclusions. Our data suggest that CRP concentration is significantly higher and NO level is meaningfully lower in patients with essential hypertension when compared with those with WCH and controls. This may suggest that endothelial functions are preserved in patients with WCH in contrast to essential hypertension.
Introduction
White coat hypertension (WCH), which is defined as a persistently elevated blood pressure in the physician's office but normal blood pressure in other situations, is a well-known clinical situation and its prognostic significance remains controversial. It is unknown whether the patients with WCH have the risk of autonomic cardiac dysregulation, end-organ damage, cardiovascular event and progressive increase in blood pressure similar to essential hypertension (EH) as time elapsed (Citation1–3). Also, the exact mechanism behind WCH is still unclear and it is unknown whether WCH is a distinct entity, prehypertensive state or low-risk form of EH, although Pierdomenico et al. (Citation4) claimed that it was not a manifestation of generalized cardiovascular hyperactivity.
The endothelium has many different roles, including the regulation of blood-tissue permeability and vascular tonus, and control of vascular surface during inflammation (Citation5). It has been reported that nitric oxide (NO), synthesized by vascular endothelium, is implicated in the regulation of blood pressure (Citation6). Reduced levels of NO metabolites, nitrate plus nitrite, and impaired endothelium-dependent vasodilatation, which are mainly mediated by NO, have been reported in essential hypertension (Citation7–11).
The C-reactive protein (CRP) is a marker of systemic inflammation that has been associated with an increased risk of myocardial infarction and stroke (Citation12–16). It has been hypothesized that inflammation has a role in development of hypertension and accordingly cross-sectional studies have suggested higher CRP levels in subjects with elevated blood pressure (Citation17,Citation18). Elevated CRP level may increase blood pressure by reducing NO production in endothelial cells (Citation19,Citation20) and resulting in vasoconstriction and increase in endothelin-1 production (Citation21,Citation22).
Although there are a variety of studies investigating the endothelial function in WCH, conflicting results have been reported and therefore we attempted to assess endothelial function simultaneously measuring NO and CRP levels in WCH and to compare those with EH and healthy subjects.
Methods
Subjects
We consecutively selected 40 newly diagnosed, never-treated patients with EH, 40 patients with WCH and 40 healthy volunteers for the control group from our cardiology outpatient clinic. Exclusion criteria were the use of antihypertensive or other drugs, smoking, diabetes, obesity, secondary hypertension, renal failure, ischemic heart disease, peripheral vascular disease, gastrointestinal disease, systemic illness and recent history of infection (within the last month). The study was in accordance with the Second Declaration of Helsinki and was approved by Institutional Review Board and all subjects gave their informed consent.
Office blood pressure measurements
Blood pressure (BP) was measured with a mercury sphygmomanometer with an appropriate cuff for the arm circumference at the medical office. After a 5–10-min resting period, the measurements were taken from the patient's bare right arm, which was supported and maintained at the heart level. Three measurements was taken and averaged as the mean systolic and diastolic pressure values. Clinic hypertension was defined as systolic BP ≥ 140 mmHg or diastolic BP ≥ 90 mmHg, or both, at each of three measurements. Normal tension was defined as BP < 140/90 mmHg.
Ambulatory blood pressure monitoring
Ambulatory blood pressure monitoring (ABPM) was performed with a portable non-invasive, oscillometry device (SpaceLabs 92512, Redmond WA) on a daily activity. ABPM readings were obtained at 15-min intervals from 06:00 h to midnight, and at 30-min intervals from midnight to 06:00 h. The recording reliable for analysis had a minimum duration of 24 h and 80 valid readings, corresponding to at least 80% of all measurements. WCH was defined as systolic BP ≥ 140 mmHg or diastolic ≥ 90 mmHg, or both, in an office setting, and as an average daytime systolic BP < 135 mmHg and diastolic BP < 85 mmHg in ABPM.
Laboratory analyses
Blood samples were taken in the morning (between 08:00 and 10:00 h) from peripheral veins after a 12-h fasting period and collected in ice-cold vacuum glass tubes containing citric acid. All subjects rested for at least 5–10 min in a supine position before blood sampling. Plasma concentrations of total cholesterol, fasting triglycerides, high-density lipoprotein (HDL)-cholesterol and glucose were determined by the enzymatic dry chemistry method using a Behring apparatus. Low-density lipoprotein (LDL)-cholesterol values were computed according to the Friedewald formula.
NO measurement was performed using the Griess method for detection of nitrite levels (Citation23). NO is unstable and has a short lifetime; in the presence of oxygen, it reacts rapidly to form nitrite or nitrates. Since the direct determination of NO radicals is difficult, and since nitric oxide synthase (NOS) activity can only be determined in tissue or cell homogenates (which is not easily done), the determination of nitrite, the stable end product of NO radicals, is most often used as a measure of NO production (Citation24).
Plasma concentrations of CRP were measured by means of particle-enhanced immunonephelometry with the Behring nephelometer method using N Latex CRP mono reagent (Behring Werke, Marburg, Germany).
Statistical analysis
Data are expressed as mean ± SD. Continuous variables were tested for normal distribution with the the Kolmogorov–Smirnov test and compared by one-way analysis of variance (ANOVA). In case of non-normal distribution, the Kruskal–Wallis test was used. The Spearman correlation test was used to test possible associations. A value of p < 0.05 was considered significant. Data were analyzed with SPSS (SPSS Inc., Chicago, Illinois, USA).
Results
The general characteristics of study population and serum CRP and NO levels are shown in . There were no differences in age and sex between the three groups. Plasma glucose, total cholesterol, HDL-cholesterol, LDL-cholesterol and triglyceride levels were not significantly different in each group. The majority of patients with EH and WCH was overweight. Body mass index (BMI) was higher in EH and WCH patients than control groups but this difference was not significant statistically (p = 0.06).
Table I. Demographic and biochemical characteristics of the three groups.
Mean office BP values were significantly higher in EH patients and WCH patients than those of control subjects (p < 0.001). The mean ABPM asset value was significantly higher in EH patients than those in WCH patients and control subjects, but it was similar in WCH patients and control subjects (p < 0.001 and p > 0.05, respectively). Daytime and night-time BP values were higher in the WCH patients than control groups, but this difference was not significant statistically. Mean office BP and ABPM values of study population are shown in .
Table II. Office blood pressure and Ambulatory blood pressure monitoring data of study subjects.
Plasma CRP level was significantly higher in patients with EH when compared with those with WCH and healthy subjects (6.3 ± 2.1 mg/l, 2.1 ± 0.9 mg/l and 1.6 ± 1.3 mg/l, p < 0.05, respectively). Nevertheless, there was no significant difference with respect to CRP level between those with WCH and healthy subjects. NO level was significantly lower in patients with EH when compared with those with WCH and healthy subjects (4.6 ± 1.1 µmol/l, 6.9 ± 1.2 µmol/l and 8.1 ± 1.5 µmol/l, p < 0.05, respectively). In contrast, there was no significant difference with respect to NO level between those with WCH and healthy subjects.
There was a significant inverse correlation between NO and office systolic BP (r = −0.501, p = 0.02), office diastolic BP (r = −0.583, p = 0.01), daytime systolic BP (r = −0.677, p = 0.01), daytime diastolic BP (r = −0.723, p = 0.01), night-time systolic BP (r = −0.670, p = 0.01), night-time diastolic BP (r = −0.659, p = 0.02), 24-h systolic BP (r = −0.670, p = 0.01) and 24-h diastolic BP (r = −0.702, p = 0.01) in all hypertensive patients. Also, there was significant positive correlation between CRP and office systolic BP (r = 0.426, p = 0.02), office diastolic BP (r = 0.402, p = 0.02), daytime systolic BP (r = 0.625, p = 0.01), daytime diastolic BP (r = 0.699, p = 0.01), night-time systolic BP (r = 0.593, p = 0.02), night-time diastolic BP (r = 0.610, p = 0.01), 24-h systolic BP (r = 0.635, p = 0.01) and 24-h diastolic BP (r = 0.670, p = 0.01) in all hypertensive patients. Plasma CRP level was also inversely correlated with NO level (r = − 0.859, p = 0.001). CRP levels had a positive correlation with BMI in all patients (r = 780, p = 0.001).
Discussion
In the present study, we found significantly higher CRP and lower NO levels in patients with EH when compared with patients with WCH and controls. However, there was no significant difference with respect to CRP and NO levels between those with WCH and healthy subjects.
Many studies have demonstrated that endothelial dysfunction caused by elevated CRP levels have increased the cardiovascular mortality (Citation12–16). There are various mechanisms behind of the relationship between CRP and endothelial dysfunction. CRP is an exquisitely sensitive systemic marker of inflammation and tissue damage (Citation25) and its concentration is associated with future atherothrombotic events both in patients with known cardiovascular disease (Citation26–28) and in healthy individuals in the general population, albeit less strongly in the latter than previously suggested (Citation29).
CRP has been reported to decrease production of NO by endothelial cells (Citation19,Citation20) and thus might indirectly promote vasoconstriction, leukocyte adherence, platelet activation, oxidation and thrombosis (Citation21,Citation22,Citation30). In our study, there was a negative correlation between CRP and NO levels. In patients with higher CRP levels, NO levels were significantly lower.
CRP has also been reported to have proatherosclerotic properties by upregulating angiotensin type-1 receptor expression affecting the renin–angiotensin system and contributing to the pathogenesis of hypertension. These changes are all indicative of progressive atherosclerosis and endothelial dysfunction, with structural and functional changes in the endothelium ultimately leading to development of hypertension (Citation31–33).
Furthermore, CRP is also elevated in obesity. BMI and CRP have been correlated in young adults, middle-aged men, and elderly men and women from the Cardiovascular Health Study (Citation34). In our study, the majority of patients with EH and WCH was overweight. Similarly to previous studies, in our study there was positive correlation between BMI and CRP.
WCH has been widely studied in the past years, but it still remains a matter of controversy. The long-term outcome of patients with WCH is not well defined. In cross-sectional studies, some authors have found a clustering of cardiovascular risk factors or increased target organ damage in WCH compared with normotensives (Citation35,Citation36), whereas others have not (Citation37).
Previous studies investigating the NO and CRP levels in EH and WCH patients are conflicting. Pierdomenico et al. (Citation38) found significantly lower NO levels in hypertensive patients than the control group, but they found no significant difference between WCH patients and the control group. Their data suggest that middle-aged WCH subjects without other cardiovascular risk factors do not show endothelial dysfunction in contrast with EH patients. Our findings were in accordance of this study. On the contrary, Conen et al. (Citation39) found that CRP and BNP levels were similar in patients with untreated WCH patients and EH patients. Schillaci et al. (Citation40) reported that CRP levels were significantly higher in untreated hypertensive patients than the control group. In addition, a direct association between CRP levels and systolic blood pressure was found. In our study, we found higher CRP levels in EH patients than the WCH and the control group. However, we found no significant difference between the WCH and control group.
Study limitations
Our study has several limitations. First, only a small number of subjects were included to study. Second, we excluded subjects with smoking, diabetes, obesity, secondary hypertension, renal failure, ischemic heart disease and peripheral vascular disease, and therefore our findings could not be extrapolated to all WCH patients. Furthermore, we did not document endothelial dysfunction by non-invasive vascular study. Finally, patients were included from Turkey, white population, and our results may not be generalized to all world.
Conclusion
Our study suggests that an endothelial function is not impaired in patients with WCH as detected by similar CRP and NO concentration in patients with WCH and healthy control subjects. This may suggest that WCH could be regarded relatively a benign condition. However, our overall findings should be supported by further large-scale studies.
Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
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