Apelin and Relaxin Plasma Levels in Young Healthy Offspring of Patients With Essential Hypertension

Abstract

Epidemiologic studies have shown that healthy offspring of hypertensive patients exhibit many features of the metabolic syndrome, such as hyperinsulinemia, insulin resistance, and lipid disorders, while hypoapelinemia and hyporelaxinemia may contribute to vascular damage that accelerates atherogenesis. The aim of this study was to determine apelin and relaxin plasma levels in the healthy offspring of hypertensive patients and to compare the findings with those of healthy offspring of healthy parents, matched for age, sex, and body mass index (BMI). Forty‐six (24 men and 22 women) healthy offspring of hypertensive patients, mean age 18±3 years and BMI 22.4±1.4 kg/m² (group A), and 50 healthy offspring of healthy parents (28 men and 22 women), mean age 18±3.2 years and BMI 22.6±1.7 kg/m² (group B), were studied. The apelin and relaxin plasma levels (enzyme‐linked immunosorbent assay method) were determined in the study population. The two groups were matched for age, sex, and BMI. Plasma apelin levels (6±3 vs 105 pg/mL, P<.001) and relaxin plasma levels (20±7 vs 29±8 pg/mL, P<.001) were significantly lower in group A compared with group B, respectively. Our findings suggest that healthy offspring of healthy parents have significantly lower plasma apelin and relaxin levels. This group of individuals needs closer follow‐up and further examination.

Healthy normotensive patients with one or more hypertensive parents exhibit various components of the metabolic syndrome. Metabolic syndrome is the combination of central abdominal obesity, glucose intolerance or noninsulin‐dependent diabetes mellitus, hyperinsulinemia, dyslipidemia (increased very low‐density lipoprotein and triglycerides levels, decreased high‐density lipoprotein cholesterol levels), and essential hypertensiοn (EH). All these metabolic disorders are considered to be consequences of insulin resistance.1, 2, 3 Numerous epidemiologic studies have confirmed that offspring of patients with EH are insulin‐resistant, and that this condition precedes the gain of redistribution of fat and the development of hypertension.4, 5, 6, 7 It is also worth mentioning that increased heart rate (HR) has been observed not only in hypertensive patients but in normotensive patients with a family history of hypertension.8

Adipocytes have recently been shown to secrete a variety of bioactive substances called adipocytokines, such as leptin, adiponectin, resistin, and visfatin, which have been recognized as endocrine cells. These adipocytokines contribute to the regulation of energy homeostasis by affecting insulin sensitivity, glucose and lipids metabolism, food intake, hemostatic/fibrinolytic balance, and inflammation.9, 10

Apelin, a recently described adipokine, although synthesized outside adipose tissue, exists in at least 3 forms, consisting of 13, 17, or 36 amino acids, all originating from a common 77‐amino‐acid precursor. In the cardiovascular system, apelin stimulates endothelium‐dependent nitric oxide–mediated vasorelaxation and reduces arterial blood pressure (BP). In addition, apelin demonstrates potent and long‐lasting positive inotropic activity, which is preserved even in injured myocardium and is not accompanied by myocardial hypertrophy. Apelin synthesis in adipocytes is stimulated by insulin, and apelin plasma levels are markedly increased in obesity associated with insulin resistance and hyperinsulinemia.11 In addition to regulating cardiovascular function, apelin inhibits water intake and vasopressin production.

Relaxin is a protein hormone that was first described in 1926 by Frederick Hisaw. In humans it consists of 7 members, relaxin‐1, ‐2, and ‐3, and insulin‐like peptides 3, 4, 5, and 6. It is an offshoot of the large insulin superfamily. Each member consists of two chains, commonly referred to as A and B, which are held together by two inter‐chain disulfide bonds and another intra‐chain disulfide bond present within the A chain. The cysteine residues present in each chain, together with the distinctive disulfide bonding pattern, are conserved across all members of the superfamily.12, 13, 14

Visceral fat accumulation causes dysregulation of adipocyte functions, which results in the development of a variety of metabolic and cardiovascular diseases.15 Recent studies have shown that apelin and relaxin plasma levels are correlated not only with EH but with masked hypertension as well.16, 17 The aim of our study was to determine the insulin, apelin, and relaxin plasma levels in young healthy offspring of patients with EH, and to compare the findings with those of young healthy offspring of normotensive patients matched for age, sex, and body mass index (BMI).

Methods

Study Population

Two groups of patients were studied: 46 (24 men and 22 women) healthy offspring of patients with essential hypertension with a positive family history, mean age 18±3 years and BMI 22.4±1.4 kg/m² made up group A. Fifty (28 men and 22 women) healthy offspring of normotensives with a negative family history, mean age 18±3.2 years and BMI 22.6±1.7 kg/m², made up group B.

Patients in group A were recruited by their hypertensive parents who attended the outpatient hypertension clinic of our hospital. Group B participants were offspring of individuals without a history of essential hypertension, cardiovascular disease, or diabetes mellitus. The demographic characteristics of the participants are presented in Table 1. The two groups had similar socioeconomic status and educational level. All participants were taking no medication and were nonsmokers. All patients adhered to a standardized diet before sampling and none of them had any metabolic (eg, diabetes) or functional (eg, thyroid) abnormalities.

Table 1.

Demographic Characteristics of the Study Population.

	Group A (n=46)	Group B (n=50)	P Value
Age, y	18±3	18±3.2	NS
BMI, kg/m2	22.4±1.4	22.6±1.7	NS
Men/women	24/22	28/22	NS
Serum glucose, mg	0.9±0.7	0.91±0.74	NS
Serum creatinine, mg	0.90±0.35	0.90±0.32	NS
Total cholesterol, mg	205±18	204±20	NS
HDL‐C, mg	48±3	49±4	NS
Triglycerides, mg	80±12	79±13	NS
LDL‐C, mg	89±7	93±6	NS
eGFR	109±11	104±7	NS

Abbreviations: BMI, body mass index; eGFR, estimated glomerular filtration rate; HDL‐C, high‐density lipoprotein cholesterol; LDL‐C, low‐density lipoprotein cholesterol; NS, not significant.

Alcohol consumption was expressed in grams per day, determined by a detailed questionnaire. Information concerning physical activity was obtained from a previously described questionnaire.18 Before the study, written informed consent was obtained from each participant, which was approved by the hospital's ethical committee. In addition, the whole study population underwent routine clinical examination.

The presence of hypertension in the hypertensive parents was documented by BP measurements according to 2013 European Society of Cardiology guidelines.19, 20 The mean systolic BP (SBP) of hypertensive parents was 142±8 mm Hg while the mean diastolic BP (DBP) was 95±4 mm Hg at baseline (before any medication). The mean age of the hypertensive and normotensive parents were 48±2.6 years and 46±3 years, respectively. In group A, 20 of 46 healthy offspring had both parents with hypertension and 26 of 46 had 1 parent with hypertension.

BP and Laboratory Investigations

SBP and DBP were measured at the time of the first and fifth Korotkoff sounds, respectively. Measurements were made on the right arm to the nearest millimeter of mercury with the use of a mercury sphygmomanometer. All measurements were made with the patients in the supine position after resting for 15 minutes. HR was measured by standard electrocardiography (ECG) under the same conditions (supine position, 15 minutes of rest) and calculated as the average of 9 R‐R intervals.21 Results regarding BP and HR were the average of measurements obtained on at least 3 separate occasions. BP measurements and ECGs were performed by the same trained nurse who was not aware of family history of the participants.

Blood sampling was performed after 12 hours of fasting at 8 am to 9 am without stasis after 10 minutes of supine rest. All participants were instructed to avoid strenuous physical activity during the hour preceding this examination. Serum cholesterol and triglyceride levels were determined by an enzymatic method and low‐density lipoprotein (LDL) was calculated according to the Friedwald formula, since no patients had a triglyceride level >400 mg/dL.

Plasma apelin levels were measured by a radioimmunoassay (Phoenix Pharmaceuticals Inc, Burlingame, CA) with intraassay and interassay coefficients of variation of 5.9% and 8.2%, respectively. A Quantikine Human Relaxin Immunoassay (DRL200; R&D Systems Inc., Minneapolis, MN) was used according to the manufacturer's protocol to determine the relaxin of the samples. All samples were run in triplicate, and serum relaxin concentration values were calculated based on the standard curve, including the zero dose standards. The intra‐assay and inter‐assay coefficients of variation for relaxin levels were 4.7% and 10.2%, respectively.

Statistical Analysis

All data are expressed as mean±1 standard deviation. An unpaired Student t test was used to assess differences between the two groups. A P value <.05 was considered statistically significant. All analyses were performed with the SPSS statistical package (SPSS Inc, Chicago, IL).

Results

As shown in Table 1, there were no statistical differences in age, sex, and BMI between the two groups. None of the participants were smokers. No differences were found concerning physical activity or alcohol consumption (data not shown). The results of our study are shown in Table 2. Mean SBP was significantly higher in group A than in group B (121±13 mm Hg vs 110±10 mm Hg, P<.01). Mean DBP was also significantly higher in group A than in group B (78±6 mm Hg vs 73±8 mm Hg, P<.05). Mean HR was significantly higher in group A than in group B (76±4 beats per minute vs 72±6 beats per minute, P<.01). As is also shown in Table 2, relaxin levels were significantly lower (6±3 pg/mL vs 10±5 pg/mL, P<.001) and apelin plasma levels were significantly lower (20±7 vs 29±8 pg/mL, P<.001) in group A compared with group B, respectively.

Table 2.

Results and Comparison Between Groups

	Group A (n=46)	Group B (n=50)	P Value
SBP, mm Hg	121±13	110±10	<.001
DBP, mm Hg	78±6	73±8	<.05
HR, beats per min	76±4	72±6	<.01
Apelin, pg/mL	20±7	29±8	<.001
Relaxin, pg/mL	6±3	10±5	<.001

Abbreviations: DBP, diastolic blood pressure; HR, heart rate; SBP, systolic blood pressure.

Discussion

The current study demonstrated that apelin and relaxin plasma levels were lower in young healthy offspring of hypertensive patients compared with young healthy offspring of normotensive patients, even when age, sex, BMI, and the other biomarkers of cardiovascular disease were not significantly different between the two groups. It also demonstrated that young healthy offspring of patients with EH tended to have significantly higher BP and HR compared with normotensive offspring, although these were within normal limits. Offspring of hypertensive patients had significantly higher HR compared with the young healthy offspring of normotensive patients, a finding which is in agreement with previous observations. Healthy offspring of patients with EH exhibit a variety of metabolic disorders, such as higher fasting insulin levels, lower insulin sensitivity, hyperleptinemia, and lower leptin receptor number.22, 23

Apelin is a novel endogenous peptide detected in a variety of tissues and organs, including heart and vessels, which has been shown to play an important role in the physiology and pathology of the cardiovascular system. By activating the G‐protein–coupled receptor APJ, apelin produces vasodilation with a subsequent BP reduction via a nitric oxide–dependent mechanism, enhances diuresis by antagonizing arginine vasopressin, and exerts positive inotropic action on the myocardium.24, 25, 26, 27 Apelin production is also regulated by factors such as fasting and refeeding, insulin, hypoxia, growth hormone, and tumor necrosis factor α.28 Hypertension, with its hemodynamic alterations and cardiac complications, might be an interesting field of exploration; however, the activity of the apelinergic system in this entity has not yet been extensively investigated. Indeed, apelin levels were significantly lower in the hypertensive patients compared with controls,29 while hypertensive patients without concomitant diseases affecting cardiovascular functions demonstrated lower plasma apelin than the controls.30 Our results have shown that offspring of hypertensive parents have significantly decreased apelin plasma levels compared with those of offspring of normotensive parents, a finding that is in agreement with the above‐mentioned results, indicating a strong relationship between hypertension and adipose tissue.

Relaxin is an ovarian hormone secreted by the corpus luteum during gestation in rodents and humans. Its role in the reproductive system has been well documented. Many recent studies have shown that it is a pleiotropic hormone capable of affecting numerous nonreproductive systems such as the cardiovascular, nervous, respiratory, tegumental, excretory, and digestive systems.31, 32 Moreover, relaxin levels are shown to be affected by factors such as fasting and obesity, pregnancy, heart failure, and stress.33, 34

Studies of relaxin levels in patients with cardiovascular disease are limited. On the contrary, in studies of heart failure treatment, the effects of relaxin include production of nitric oxide, inhibition of endothelin, inhibition of angiotensin II, production of vascular endothelial growth factor, and production of matrix metalloproteinases.34 It has also been demonstrated that relaxin is potent in mediating reversal of arterial remodeling and improving arterial structural compliance in aged hypertensive rats.35 In the study by Gedikli and colleagues, human relaxin levels were significantly lower in hypertensive patients compared with controls,16, 36 while in a recent study at our laboratory we found that patients with masked hypertension have significantly lower relaxin levels compared with controls, a finding that is in accordance with the results in the present study.17

Study Limitations

To our knowledge, this is the first study to investigate apelin and relaxin levels in young healthy offspring of hypertensive parents. The small number of patients (46 vs 50) is a limitation of the study. Another possible limitation is that we did not evaluate apelin receptor plasma levels in our study population. Further studies are necessary to establish the first observations and to elucidate the potential role of apelin and relaxin in young individuals with a positive family history of EH. This observation may have prognostic significance for future cardiovascular events in young healthy offspring of hypertensive parents.

Conclusions

Our findings suggest that in the absence of major cardiovascular risk factors, young healthy offspring of hypertensive patients have significantly lower apelin and relaxin plasma levels and significantly higher SBP, DBP, and HR compared with young healthy offspring of normotensive patients. These observations support the practice of examining children and young adults in an attempt to categorize them according to their cardiovascular risk as a basis for prevention of adult heart disorders.

J Clin Hypertens. 2014;16:198–201. ©2014 Wiley Periodicals, Inc.