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(Hypertension. 1996;28:652-655.)
© 1996 American Heart Association, Inc.
Articles |
the Department of Internal Medicine, Mercer University School of MedicineMedical Center of Central Georgia, Macon (S.E., D.C.P.), and Department of Biochemistry and Molecular Biology, University of Georgia, Athens (D.P., A.E.).
Correspondence to Dr Adviye Ergul, Department of Biochemistry and Molecular Biology, Life Sciences Building, University of Georgia, Athens, GA 30602-7229. E-mail aergul@uga.cc.uga.edu.
| Abstract |
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Key Words: endothelin blood pressure hypertension, essential race gender
| Introduction |
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On the basis of the broad spectrum of biological effects associated with ET-1, it has been suggested to be involved in a variety of diseases, such as hypertension, atherosclerosis, and asthma. Plasma ET-1 levels have been shown to be elevated in individuals with pulmonary hypertension, atherosclerosis, and ischemic heart disease.8 9 10 The role of ET-1 in the pathogenesis of hypertension is still unknown. Although a direct correlation between the concentration of circulating ET-1 and high BP has not been demonstrated, it has been shown that plasma ET-1 levels tend to increase with age and are higher in men than women.11 12 No studies have been reported on the relationship of race and circulating ET-1 concentrations.
It is well documented that a substantial difference exists in hypertension among people of African and European heritage.13 14 15 Hypertension among blacks is more prevalent, earlier in onset, and associated with more end-organ damage, including left ventricular hypertrophy, renal failure, stroke, and death.13 14 15 There are, however, few explanations for this difference despite substantial clinical and laboratory investigation. Further work is needed to identify factors that contribute to the problem.
This study reports the first investigation of racial variations in plasma ET-1 concentrations among hypertensive and normotensive individuals. Concentrations of irET-1 in the samples obtained from both male and female black hypertensive subjects were found to be significantly higher than those in normotensive black control subjects and white hypertensive subjects.
| Methods |
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Preparation and Analysis of Blood Samples
Blood samples (3 mL) were collected into EDTA-containing tubes, and plasma was aliquoted into plastic Eppendorf tubes (0.5 mL each) and stored at -80°C until the day of the assays. The amount of irET-1 in the plasma was determined with an enzyme-linked immunoassay (ELISA) kit specifically designed for direct measurement of irET-1 in plasma (American Research Products) that eliminates the extraction step of plasma with Sep-Pak C18 cartridges. To provide maximum sensitivity, an immunoaffinity purified polyclonal capture antibody and monoclonal detection antibody, both highly specific for ET-1, were used in the assay. The sensitivity of the assay was 0.1 to 15.6 nmol/L. The cross-reactivities of the antibodies used were reported to be less than 1% with big ET-1-(1-38) and big ET-1-(22-38), less than 5% with ET-3, and 100% with ET-2. The manufacturer reported the intra-assay and interassay variabilities of the kit to be 3.3% and 3.5%, respectively.
Each plasma sample was analyzed three times in separate ELISA runs. The correlation coefficient between each run ranged between .87 and .91. The samples that contained irET-1 above the maximum standard concentration were diluted with assay buffer provided in the kit, and diluted samples were also assayed in three individual experiments.
Statistical Analysis
Data were analyzed for multiple comparisons by one-way ANOVA written with the Statistical Analysis Program (SAS); a value of P<.05 was considered significant. The results are given as mean±SE of three independent measurements.
| Results |
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Among normotensive subjects, there were no significant differences between sex (P=.147) and race (P=.129). The difference between hypertensive blacks and whites was found to be extremely significant (P<.001). The means of irET-1 levels in black hypertensive women and men were fourfold and threefold higher than in white hypertensive women and men, respectively.
The Table
summarizes the statistical analysis of irET-1 concentrations in hypertensive subjects (n=50) versus normotensive subjects (n=50), including white and black women and men and effects of race and sex on ET-1 levels.
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| Discussion |
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Hypertension is characterized by an increase in peripheral vascular resistance that is associated with an elevation in vascular tone and structural changes in the blood vessel wall. Since ET-1 has potent contractile and proliferative properties, it has been suggested to be involved in the pathogenesis of hypertension.2 ET-1 has been proposed to modulate BP through several pathways. ET-1 exerts its contractile effects via endothelin-A receptors on vascular smooth muscle cells, and it also causes vasodilation via nitric oxide release mediated by endothelin-B receptors on endothelial cells. In addition, ET-1 interacts with nonendothelial pathways, eg, the sympathetic system, renin-angiotensin system, and central nervous system.16 Although in vitro studies have demonstrated the interaction of ET-1 with these systems, there is no direct evidence that ET-1 is involved in the pathogenesis of hypertension or in the in vivo regulation of vascular tonus. Several groups have reported that plasma irET-1 levels were elevated in moderate hypertension17 18 19 ; however, other studies found no significant increase.12 20 In an age-matched study, Miyauchi et al12 reported no difference in irET-1 levels between Japanese hypertensive and normotensive control individuals, but they found irET-1 concentrations to be higher in men than women and to increase with age. A sex-related difference in irET-1 levels in healthy people was found to be mediated by sex hormones.11 It was also suggested that this difference might contribute to the higher incidence of atherosclerotic cardiovascular disease in men. In our study, normotensive white men were found to have slightly higher levels of irET-1 than normotensive white women (twofold). However, we did not observe any significant sex-associated differences in irET-1 levels. We also analyzed the effect of age factors on plasma irET-1 concentrations and found them to have no significant effect. Thus, our results suggest that there might be a differential regulation of irET-1 levels in blacks and/or in hypertensive individuals.
The prevalence of essential hypertension in blacks is much higher compared with that in whites. Racial differences in renal physiology and socioeconomic factors have been suggested as possible causes of this difference.13 14 15 Recently, it was reported that the T235 allele of the angiotensinogen gene is linked to human hypertension and that T235 is the predominant angiotensinogen gene allele in blacks.21 It was also demonstrated that angiotensinogen concentrations are higher in hypertensive blacks than hypertensive whites.22 Interestingly, ET-1 has been shown to enhance the production of Ang II from Ang I in cultured endothelial cells by an angiotensin-converting enzymesensitive mechanism.5 Two groups have reported that BP elevation in ET-1infused rats can be prevented by angiotensin-converting enzyme inhibitors.23 24 Furthermore, Ang II stimulates ET-1 synthesis.25 On the basis of these observations, ET-1 and Ang II have been proposed to act synergistically to induce vasoconstriction and aldosterone secretion from the adrenal cortex.6 7 Our finding that irET-1 levels are elevated in hypertensive blacks compared with normotensive blacks and hypertensive whites suggests that interaction of Ang II and ET-1 might be an important factor in BP regulation in blacks. In addition, since ET-1 has diverse biological effects on cardiovascular and renal systems,7 elevated levels of ET-1 in hypertensive blacks may contribute to the higher incidence of complications in this population.14
In conclusion, we have demonstrated racial differences in plasma irET-1 concentrations in individuals with high BP. Further studies are required for determination of whether the elevated concentrations of plasma ET-1 in hypertensive blacks are involved in the development of complications of hypertension and to elucidate the possible interaction of ET-1 with Ang II.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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Received May 9, 1996;
first decision May 21, 1996; first decision June 13, 1996;
| References |
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