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Catecholamines in the Cardiovascular System: Proceedings of the 21st Symposium on Catecholamines in the Cardiovascular System; Tokyo, Japan; November 20, 1999

Effect of a Hypocaloric Diet on Adrenomedullin and Natriuretic Peptides in Obese Patients with Essential Hypertension

Minami, Junichi; Nishikimi, Toshio; Ishimitsu, Toshihiko; Makino, Yuriko*; Kawano, Yuhei*; Takishita, Shuichi*; Kangawa, Kenji; Matsuoka, Hiroaki

Section Editor(s): Motomura, Shigeru; Toyo-oka, Teruhiko; Hirata, Yasunobu

Author Information
Journal of Cardiovascular Pharmacology: 2000 - Volume 36 - Issue - p S83-S86
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Abstract

A number of studies have demonstrated a decrease in blood pressure occurring concomitantly with a body-weight reduction by calorie restriction in obese subjects (1), although the mechanism involved in the hypotensive effect of caloric restriction has not been fully elucidated. So far, only a few investigations have been carried out regarding the possible involvement of natriuretic and vasodilating properties of atrial natriuretic peptide (ANP) and brain natriutretic peptide (BNP) in the mechanism of the hypotensive response to calorie restriction in obese subjects (2,3). Adrenomedullin (AM) is a potent vasodilator peptide discovered in the human pheochromocytoma (4). The possible involvement of AM during calorie restriction has never been demonstrated.

The present study was undertaken to investigate the effect of a 3-week nutritionally balanced, moderately hypocaloric diet on plasma AM, ANP, and BNP concentrations in mildly-to-moderately obese patients with essential hypertension.

METHODS

Subjects

Twelve subjects with essential hypertension and a body mass index greater than 26 kg/m2 were included in the study. The clinical characteristics of the patients are shown in Table 1. The patients had systolic blood pressure greater than 160 mmHg, or diastolic blood pressure greater than 95 mmHg or both, on at least three occasions at an outpatient clinic. Secondary causes of hypertension were ruled out through a comprehensive check-up including medical history, physical examination, urinalysis, blood chemistry, and endocrinological and radiological examinations when needed. All the patients had normal renal function as judged by the endogenous creatinine clearance. According to the World Health Organization criteria for organ damage, all patients were classified as having stage I or II hypertension. Any antihypertensive agents being used were withheld for at least 2 weeks prior to the study. Informed consent was obtained from each patient after a detailed explanation of the study protocol. The study protocol was in accordance with the Declaration of Helsinki (1989) of the World Medical Association and was approved by the institutional ethical committee of the National Cardiovascular Center.

TABLE 1
TABLE 1:
The clinical characteristics of the study patients

Study protocol

During the study period, the patients were fully ambulatory in the hospital environment. For the initial week, a standard diet of approximately 2000 kcal/day (80 g protein, 290 g carbohydrates, and 55 g fat) was given, followed by a 3-week hypocaloric diet of 850 kcal/day (57 g protein, 85 g carbohydrates, and 33 g fat). Salt intake was maintained at approximately 120 mmol/day throughout the study protocol. Nursing staff members measured blood pressure on the same arm using a standard mercury sphygmomanometer every morning after the patients had rested for at least 5 min in the supine position. The diastolic blood pressure was taken as the level at which the Korotkoff sounds disappeared. At 08.00 h in the last days of the standard diet and the hypocaloric diet, antecubital venous blood was taken after an overnight fast and 30 min of supine rest. Twenty-four hour urine was collected on the last 2 days of the standard diet and the hypocaloric diet.

Analysis of blood and urine samples

Blood glucose was determined with the glucose oxidase method using an auto&Stat GA-1160 Analyzer (KDK Co., Kyoto, Japan). Serum lipid profiles were assayed enzymatically with a TBA-80M Analyzer (Toshiba Co., Tokyo, Japan). Plasma and urinary electrolytes were measured by flame photometry. Aprotinin-supplemented plasma was used to assay AM, ANP, and BNP. Plasma AM concentration was measured by specific radioimmunoassy after extraction and purification as described previously (5). In the evaluation of plasma AM concentration, 24 age-matched non-obese healthy subjects (11 men and 13 women; mean age, 60.5 ± 1.0 years) served as the control group. Plasma ANP and BNP concentrations were measured with Shiono RIA ANP and BNP assay kits (Shionogi & Co., Ltd., Osaka, Japan) (6). Plasma renin activity and aldosterone concentration were determined by radioimmunoassay. Urinary noradrenaline and adrenaline excretion was measured by high-performance liquid chromatography based on radioimmunoassy (7).

Statistical analysis

Values are expressed as means ± SEM. Significance of differences in variables between the last periods of the standard diet and the hypocaloric diet was determined by Student's test or Wilcoxon's signed-rank test when required. p < 0.05 was considered significant.

RESULTS

All 12 of the subjects completed the study protocol. As shown in Table 2, the patients lost 3.7 ± 0.2 kg body weight during a 3-week hypocaloric diet (p < 0.0001). Systolic blood pressure was significantly decreased by a 3-week hypocaloric diet (p = 0.017), although the change in diastolic blood pressure was not significant. The decrease in blood pressure during the study period was 10.3 ± 3.6 mmHg systole and 4.2 ± 3.2 mmHg diastole.

TABLE 2
TABLE 2:
Body weight, blood pressure, blood glucose, and serum, plasma, and urinary variables in the last periods of the standard diet and the hypocaloric diet

As shown in Table 2, fasting blood glucose and serum triglyceride concentration were significantly decreased and serum high-density lipoprotein cholesterol concentration was significantly increased by the hypocaloric diet, although total cholesterol concentration did not differ significantly between the two periods. Serum electrolytes were unchanged by a 3-week hypocaloric diet. Plasma renin activity and aldosterone concentration did not differ significantly between the two periods. Plasma ANP and BNP concentrations were also significantly decreased by the hypocaloric diet (p = 0.042 for each). The daily urinary volume and excretion of sodium and potassium were unchanged between the two periods. The daily urinary excretion of noradrenaline and adrenaline did not differ significantly between the two periods.

As shown in Figure 1, plasma AM concentration in the last period of the standard diet was significantly raised compared with that of the control group (4.88 ± 0.46 vs. 2.43 ± 0.11 pmol/l; p < 0.0001). Plasma AM concentration was significantly decreased from 4.88 ± 0.46 to 3.97 ± 0.38 pmol/l by the hypocaloric diet (p = 0.004).

FIG. 1
FIG. 1:
Plasma adrenomedullin concentrations of age-matched non-obese healthy control subjects (n = 24) and obese hypertensive patients (n = 12) in the last periods of standard diet (SD) and the hypocaloric diet (HD).

DISCUSSION

We have so far reported that plasma AM concentration is increased in various cardiovascular disorders (8,9). In particular, plasma AM concentration is markedly increased in patients with heart or renal failure. These findings appear to be the case in obese patients with essential hypertension. In fact, as shown in Fig. 1, the plasma AM level of the subjects in the current study in the last period of the standard diet was considerably higher than that of our age-matched non-obese healthy control subjects.

No studies have so far investigated the possible involvement of AM during calorie restriction. In the present study, plasma AM concentration was significantly decreased by a 3-week hypocaloric diet in obese patients with essential hypertension. We previously reported that successful antihypertensive therapy reduced plasma AM concentration in malignant hypertension (10). It has also been shown that plasma AM concentration is significantly correlated with blood pressure (11). Taken together, it is possible that AM is involved in the defense mechanism against further elevations in blood pressure in hypertensive patients. Moreover, it has been shown that plasma AM concentration increases in response to body fluid volume expansion, as in the case of ANP (12). Since obese subjects present an increased body fluid volume (13), it is also possible that plasma AM decreased in accordance with a reduction in body fluid volume during the hypocaloric diet in these patients.

To the best of our knowledge, there have been two studies that investigated changes in plasma ANP concentration during calorie restriction. McMurray and Vesely (2) reported that there was a dramatic fall in plasma ANP concentration after 12 weeks of weight reduction in both obese normotensive and hypertensive subjects, although the intake of sodium was not constant in their study. Recently, Messaoudi et al. (3) showed that a very-low-calorie diet decreased plasma ANP concentration in 12 obese subjects during 8 days on a very low-calorie diet with a constant intake of sodium. The findings of the present study are almost in line with these earlier findings. It is well known that ANP is responsive to changes in body fluid volume (14) and in sodium intake (15). In the present study, salt intake was constant throughout the study period. Taken together, it is thought that a significant reduction in plasma ANP concentration largely reflected a decrease in body fluid volume by a hypocaloric diet in the current study.

In the present study, plasma BNP concentration was also significantly decreased by a 3-week hypocaloric diet. There has been only one study so far that investigated the effect of calorie restriction on plasma BNP concentration in obese subjects, although the study protocol and subjects were quite different from those in the current study. Messaoudi et al. (3) reported that no significant variation of plasma BNP was noted during the 8-day-period of semi-starvation. They speculated that BNP was not involved in the hypotensive effect of calorie restriction. However, their study protocol differed from the present study; they investigated the effects of a very-low-calorie diet on plasma BNP only during 8 days in obese subjects. Moreover, blood pressure of the subjects enrolled ranged from a normotensive to hypertensive level. These factors may limit the interpretation of the data obtained. It is well known that plasma BNP is elevated in hypertensive subjects, especially in patients with left ventricular hypertrophy (16). BNP appears to act against further elevation in blood pressure even in obese patients with essential hypertension, as in the case of AM.

There is a limitation in the current study. The mechanisms underlying reductions in these hypotensive peptides are not necessarily clarified by the current findings. There might be a possibility that the changes in these peptides were passive responses to falls in blood pressure independent of body weight reduction. In fact, the elevated plasma concentrations of AM, ANP, and BNP in patients with malignant hypertension have been reported to decrease after antihypertensive treatment (10,11), although the finding is not consistent (17). Therefore, ideally, further studies are needed to evaluate changes in these peptide by a hypocaloric diet in obese normotensive subjects.

In summary, a 3-week hypocaloric diet decreased plasma AM concentration significantly in mildly-to-moderately obese patients with essential hypertension. Plasma ANP and BNP concentrations were also decreased significantly by the calorie restriction. These vasodilator peptides may act against further elevation in blood pressure in obese patients with essential hypertension.

Acknowledgement: This study was supported in part by the Vehicle Racing Commemorative Foundation, the Comprehensive Research Project on Aging and Health from the Ministry of Health and Welfare of Japan. This study was also supported in part by the Grant-in-Aid for Scientific Research (09670776 and 10218209) from the Ministry of Education, Science and Culture of Japan, and by the Research Grant for Cardiovascular Disease (9A-5) from the Ministry of Health and Warfare of Japan. We also thank Ms Yoko Saito for her technical assistance.

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Section Description

The symposium and the publication of this supplement were supported by an educational grant from Novartis Pharma K.K. Tokyo, Japan.

Keywords:

Obesity; Hypertension; Adrenomedullin; Natriuretic peptides; Hypocaloric diet

© 2000 Lippincott Williams & Wilkins, Inc.