|Year : 2013 | Volume
| Issue : 1 | Page : 40-45
Is hyperleptinemia a risk factor for hypertension?
Nazish Rafique, Nasir Afzal
Department of Basic Health Science, Shifa College of Medicine, Shifa International Hospital, Islamabad, Pakistan
|Date of Web Publication||3-Jun-2013|
Department of Physiology, College of Medicine, PO Box 2114, University of Dammam, Dammam 31451, Kingdom of Saudi Arabia
Background: Major risk factors of hypertension (HTN) including obesity, insulin resistance, renal pathology, increased sympathetic nerve activity (SNA) and vascular inflammation are associated with hyperleptinemia. Therefore hyperleptinemia may act as a mediator of blood pressure, linking body fat mass with changes in insulin action, sympathetic neuronal outflow and renal pathology.
Objective: The objective of our study was to explore the relationship between hyperleptinemia and HTN in Pakistani population.
Materials and Methods: It was a case control study of 6 months duration, carried out at Shifa College of Medicine and Shifa International Hospital, Islamabad. One hundred and four male subjects, 52 with HTN and 52 without HTN were included in this study. Leptin levels were measured by enzyme-linked immunosorbent assay (ELISA) technique. The relationship of leptin (LEP), body mass index (BMI), fasting blood sugar (FBS) and cholesterol levels with blood pressure were assessed.
Results: Patients with HTN versus patients without HTN had a higher body mass index (28.0 ± 0.12 to 24.11 ± 0.27 kg/m 2 ), FBS (169.20 ± 7.34 to 127.50 ± 2.30 mg/dl), total cholesterol (160 ± 3.72 to 158 ± 2.71 mg/dl) and LEP (53.40 ± 5.32 to 21.65±3.50 ng/ml) levels. In univariate analysis, obesity, hyperglycemia and hyperleptinemia (with P values of 0.000, 0.001, 0.000, respectively), whereas in multivariate analysis, hyperglycemia and hyperleptinemia (with P values of 0.006 and 0.001, respectively) were risk factors for HTN. Our study also showed that hyperleptinemia is an independent risk factor for HTN.
Conclusions: We observed a strong positive relationship between hyperleptinemia and hypertension, thereby indicating that increased leptin levels could be considered as a risk factor for HTN and cardiovascular disease. Further studies are needed to probe the underlying pathophysiological mechanisms responsible for hyperleptinemia in HTN.
Keywords: BMI, hyperleptinemia, hypertension, leptin, obesity, sympathetic activity
|How to cite this article:|
Rafique N, Afzal N. Is hyperleptinemia a risk factor for hypertension?. Saudi J Med Med Sci 2013;1:40-5
| Introduction|| |
Leptin is a 16 KDa (167-amino acid) protein, expressed mainly by adipose tissues and relays a satiety signal to the hypothalamus.  Since the discovery of leptin, major advances have been made in the understanding of neuroendocrine mechanisms regulating appetite, metabolism, obesity, sympathetic tone and blood pressure.  Studies in animals and humans have shown associations between hyperleptinemia and HTN.  Leptin expression and plasma leptin concentrations are elevated in obese humans, and abnormalities in the generation or actions of leptin may be responsible for sympathetic, metabolic and renal changes associated with obesity. 
The effects of leptin on the sympathetic nervous system have provided possible links between obesity, sympathetic activation and HTN.  It has been shown that acute intracerebroventricular infusions of leptin increase sympathetic activity in the kidneys, adrenal glands and brown adipose tissue, and these effects have been suggested to be important in blood pressure regulation.  Interestingly, in several mouse models of obesity, the ability of leptin to increase SNA is preserved, despite resistance to leptin's effect on food intake, body weight, and thermogenic sympathetic tone (selective leptin resistance). ,
Leptin appears to be a potential pressure- and volume-regulating factor,  and can contribute to HTN via its effects on tubular sodium handling.  Leptin infusion increases renal sodium and water excretion. Intravenous bolus of leptin causes a six- to seven-fold increase in sodium excretion in normal rats, whereas spontaneously hypertensive rats (SHR) and obese rats are refractory to the natriuretic effects of leptin. These results indicate the existence of tubular leptin resistance, leading to decreased sodium excretion and therefore HTN.  Hyperleptinemia also results in higher baseline renal SNA, which leads to increased renal tubular sodium reabsorption, sodium retention; decreased renal blood flow and glomerular filtration rate; renal vasoconstriction and increased renin release, leading to angiotensin II production. All these alterations are known to promote increases in arterial pressure. 
Hyperleptinemia can also be a contributor in the generation of HTN via stimulating vascular inflammation and oxidative stresses.  Diabetic individuals are characterized by higher prevalence of hypertension.  Recent data indicates that hyperleptinemia is also associated with insulin resistance and HTN.  Multiple studies in different ethnic groups suggest a relationship between hypertension and hyperleptinemia. So far a very limited attention has been paid in Pakistani population, and to our knowledge this would be the first study to find out an association between hyperleptinemia and hypertension in a Pakistani population.
| Materials and Methods|| |
This was a case control study carried out at Shifa College of Medicine and Shifa International Hospital, Islamabad. Non-probability convenience sampling technique was used to recruit patients of hypertension. 104 subjects were included in this study, which were divided into two groups. Group A included 52 male patients of HTN and group B had 52 normotensive males. All the subjects underwent a detailed history, complete physical examination and relevant laboratory investigations. Hypertension was considered to be present when the patient was on antihypertensive medication, or blood pressure measured at three separate occasions was greater than 140/90 mm/Hg.
Blood pressure was measured either in a fasting condition or 2 hours after breakfast. A mercurial manometer was used to measure the blood pressure three times in both arms. Blood pressure was taken by medical technicians who had attended training sessions to standardize the procedures. The technicians were supervised to avoid possible biases in measurement.
Exclusion criteria included the subjects from whom the written consent could not be taken and the patients with acute inflammatory disease in the last 15 days (as hyperleptinemia is considered to be an acute phase reactant).
The protocol was approved by the Ethics Review Committee and Institutional Review Board of Shifa College of Medicine.
Five milliliters of venous blood was drawn from the HTN and control subjects in the morning after an overnight fast of at least 10 hours.
A drop of the blood was used instantly to measure the fasting blood glucose (FBS) levels with the help of a glucometer and glucose test strips. Rest of the blood was transferred to a gel containing plastic vacutainer tubes with sealed rubber caps (BD vacutainer Lot No. 4062731 Frankin Lakes). The gel tubes were placed at room temperature for 15-20 min and the blood was allowed to clot completely. The gel tubes were centrifuged in Hettich D-78532 centrifuge at a rate of 5000 revolutions/min for 3-5 min. Centrifugation resulted in the collection of serum at the top allowing easy removal of the serum. The serum was stored at -80°C refrigerator till the analysis of serum leptin and serum cholesterol levels.
Leptin levels were determined using sandwich ELISA (enzyme-linked immunosorbent assay) kit (EIA-2395 Germany). DRG Leptin (sandwich) ELISA EIA-2395 is a solid phase enzyme-linked immunosorbent assay (ELISA) based on the sandwich principle. The microtiter wells are coated with a monoclonal antibody directed towards a unique antigenic site on a leptin molecule. An aliquot of patient sample containing endogenous leptin is incubated in the coated wells with a specific rabbit anti-leptin antibody. A sandwich complex is formed. After incubation the unbound material is washed off and an anti-rabbit peroxidase conjugate is added for the detection of the bound leptin. Having added the substrate solution, the intensity of colour developed is proportional to the leptin in the person's sample. The human leptin concentrations of the controls and cases were determined from the standard curve by matching their mean absorbance readings with the corresponding human leptin concentration.
Blood glucose levels were measured with the help of glucose test strips (Lot No. 23202), which were inserted into the Medisense Optium Glucometer (Lot No. 12345). When apply blood signal was declared, a drop of blood was applied to the white target area at the tip of the test strip. Glucose in the blood reacts with chemicals (glucose dehydrogenase, NAD, phenanthroline quinine) on the test strip. This reaction produces a small electrical current that is measured. The result is then displayed on the monitor after a lapse of about 20 s.
The subject was considered of having normal blood glucose when his FBS ranged between 70 to 100 mg/dl. When the FBS was between 101 and 125 mg/dl, the subject was considered of having impaired fasting glucose and was labeled as hyperglycemic. The subject was considered diabetic if he was on anti-diabetic medicine or had fasting blood glucose > 126 mg/dl.
Serum cholesterol levels were determined by enzymatic method. Cholesterol esters were hydrolyzed to produce cholesterol. Hydrogen peroxide was produced from the oxidation of cholesterol by cholesterol oxidase. 4-aminoantipyrine, phenol and hydrogen peroxide were coupled to form quinoneimine red coloured dye in a dye catalyzed by peroxidase. The absorption of light at 505 nm of the solution of this dye was proportional to the concentration of cholesterol in the samples of cases and controls.
Hypercholesterolemia was considered to be present if the patient was on antihyperlipidemics or fasting cholesterol levels were greater than 200 mg/dl.
BMI of the subjects was calculated as weight (kg) divided by height (m 2 ). The subjects with BMI greater than 25 kg/m 2 were considered overweight, whereas the subjects with BMI greater than 30 kg/m 2 were considered obese. Weight was taken in a fasted state (after evacuation). A Torino scale (Tecno Lógica, Mexicana, Mexico) was used in obtaining the subject's weight, calibrated before each measurement. Height was measured with an aluminum cursor stadiometer, graduated in millimeters. The subject was barefoot with the back and head in contact with the stadiometer aligned in the Frankfurt horizontal plane.
Data and the results of the study were analyzed on computer software SPSS version 15.0. Descriptive statistics were presented as mean ± standard error of mean comparing the patients of hypertension (cases) and the healthy individuals (controls). Major risk factors of hypertension including FBS levels, BMI, cholesterol levels and serum leptin levels were compared between cases and controls, and univariate analysis was carried out by conditional logistic regression, by the application of t-test. Multivariate conditional regression model was then applied to the factors, which were proven significant in univariate analysis. The P value less than 0.05 was regarded as statistically significant.
| Results|| |
Baseline characteristics of cases and controls are compared in [Table 1]. Results of this comparison showed that BMI, FBS, serum cholesterol levels, serum leptin levels, systolic and diastolic blood pressure were higher in patients of hypertension as compared to the controls. Mean age of the patients with HTN was between 60 and 70 years, out of which 89% of the patients were older than 45 years showing higher prevalence of the disease in older age.
The primary objective of the study was to find out serum leptin levels in the patients of HTN in comparison with controls, and leptin levels were found to be significantly increased in the patients of HTN (53.40 ± 4.34 ng/ml) as compared to the controls (21.65 ± 3.50 ng/ml) [Table 1]. Moreover the FBS, Systolic and diastolic blood pressures also showed a statistically significant difference between cases and controls [Table 1].
Univariate analysis of the data revealed that increased BMI, FBS and leptin levels are the major risk factors for hypertension with a P value of 0.000, 0.001 and 0.000, respectively. Serum cholesterol levels were not found to be statistically significant (P value 0.867). The odds ratio (OR) for hyperleptinemia was 18, whereas the OR for increased BMI and FBS levels was 3.94 and 5.2. This indicates a stronger association of hyperleptinemia with HTN as compared to increased BMI and FBS [Table 2].
|Table 2: Univariate analysis of association between leptin, Traditional risk factors and Hypertension|
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Multivariate conditional logistic regression analysis model was applied to only those risk factors that were considered to be significant in the univariate analysis. In (Model A), odds ratio of hyperleptinemia was found to be 14.5, which was greater than the odds ratio of all the other risk factors included in the multivariate model. In the (Model A), BMI was found to be insignificant with a P value of 0.47, therefore multivariate analysis was reapplied excluding BMI from the multivariate model (variation to Model A). Leptin retained its significance even after this variation, with a slight fall in the odds ratio (11.41). This result indicated that hyperleptinemia is not only the strongest but also an independent risk factor for the development of hypertension. On the basis of our results we can recommend that hyperleptinemia can be used as a routine part of global risk assessment for HTN [Table 3].
| Discussion|| |
In our study, leptin levels were found to be significantly higher in the hypertensive males as compared to the control group thereby supporting the other studies. ,,, Makris et al. demonstrated higher leptin levels in healthy offspring of patients with hypertension compared to healthy offspring of normotensive patients, which suggests that hyperleptinemia have genetic basis.  Data from available animal studies clearly indicate an association between leptin and hypertension, ,,, whereas results of human studies are less consistent. ,,
Relationship between hyperleptinemia and hypertension have various explanations based on the results of ours, as well as of many other studies. As our data show a positive correlation between hyperleptinemia and increased RBS levels, this correlation may be responsible for insulin resistance, thereby leading to the development of hypertension. Observations by Stenvinkel P are also suggestive that the secretion of insulin by the pancreas is lower in hyperleptinemic patients leading to glucose intolerance and subsequently development of type II DM.  As the presence of diabetes always alerts the clinician to the possibility of HTN,  42% of the diabetics in our study were found to be hypertensive.
We and many others also strongly correlate hyperleptinemia with increased levels of BMI , and increased BMI is a risk factor for hypertension.  Obesity-related hypertension may be secondary to insulin resistance or increased sympathetic nerve activity. , Rahmouni K provided possible links between obesity, sympathetic activation and hypertension. He indicated that most obese humans are resistant to the metabolic actions of leptin (i.e., selective leptin resistance), but its sympatho-excitatory and pressor actions are preserved, leading to development of hypertension in obese individuals. 
Because leptin is an adipose tissue-derived hormone, so its association with BP can be explained by obesity and weight gain.  However, the multivariate model in the current study indicated that hyperleptinemia is also a risk factor for hypertension independent of BMI, suggesting the involvement of mechanisms other than obesity. Our findings are consistent with the findings by Galletti et al. who reported an association between plasma leptin levels and hypertension independent of BMI.  Moreover, Narkiewicz K also observed a direct relationship between plasma leptin levels and hypertension, independent of BMI, plasma insulin, smoking and physical activity. 
This independent association between leptin and HTN can be explained by some studies, which have shown that leptin up-regulates the synthesis and activity of angiotensin II, and leads to the development of hypertension.  It has also been shown that chronic leptin infusion into normal rats induces proteinuria and focal glomerulosclerosis resulting in glomerular pathology associated with HTN.  Moreover, some recent studies by Romero CA, Shamsuzzaman AS and Jamroz WA also correlate hyperleptinemia with vascular inflammation and oxidative stresses, thereby providing another link, which might be responsible for the independent relationship between hypertension and hyperleptinemia. ,,,,,,,,,,,,,,,,,, As most of the recent studies suggest a positive relationship between hyperleptinemia and essential hypertension, whether this association is independent of other hypertensive factors, such as insulin, BMI and sympathetic over activity, is not known and needs to be investigated.
The results of our study were opposed by Almeida PB, as his data do not support a role for leptin in the elevation of blood pressure.  Heymsfield SB reported no significant change in BP in obese and lean adults treated with leptin.  El-Gharbawy et al. was also not able to find an independent association of leptin with blood pressure when adjusted for general obesity. 
The fact that women are less predisposed to hypertension, although they usually have much higher plasma leptin levels than men, also appears to argue against an important role for leptin in the genesis of essential hypertension.  Ma D et al. reported a significant relation between systolic blood pressure and plasma leptin levels in hypertensive women but not in hypertensive men.  On the other hand, Galletti F Kennedy et al. demonstrated a relationship between elevated systolic and diastolic blood pressures and plasma leptin levels in hypertensive men only.  The reasons for the divergent findings in the literature are not obvious, but may be due to factors such as inclusion and exclusion criteria, limited numbers of patients, statistical methods, race and differences in anti-hypertensive treatment.
In conclusion our data indicates a strong positive relationship between hyperleptinemia and hypertension in Pakistani population, but the exact cause of this relationship cannot be explained from our study. Therefore suitable direct measure of the renin-angiotensin-aldosterone system and direct measures of sympathetic activity, such as resting arterial catecholamine levels, are ideally required to explore this association between leptin and hypertension on a greater population size including both males and females.
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[Table 1], [Table 2], [Table 3]