Plasma ProANP1–30 Reflects Salt Sensitivity in Subjects With Heredity for Hypertension
The aim of the present study was to investigate whether plasma concentration of proANP1–30, the N-terminal fragment of the atrial natriuretic peptide prohormone, or 24-hour urinary excretion of urodilatin reflects the degree of salt sensitivity in hypertension-prone individuals. Plasma concentration of proANP1–30 and urinary urodilatin excretion were determined at baseline, after 1 week on a low-salt diet (10 mmol/d) and after another week on a high-salt diet (240 mmol/d) in 30 healthy subjects with heredity for hypertension. Salt sensitivity was defined as the difference between mean arterial blood pressure after the high-salt diet and the mean arterial blood pressure after the low-salt diet. High- versus low-salt intake increased proANP1–30 (668±330 versus 358±150 pmol/L; P<0.00001) and urodilatin (18.7±5.2 versus 16.0±8.3 pmol/24 h; P<0.05). ProANP1–30 correlated with salt sensitivity at baseline (r=0.76, P<0.000001), after the low- (r=0.80, P<0.0000001) and high-salt diets (r=0.85, P<0.00000001). The increase in proANP1–30 induced by changing from the low- to the high-salt diet was also directly related to salt sensitivity (r=0.78, P<0.000001). ProANP1–30 was not related to urinary sodium excretion. Neither urodilatin nor the sodium-induced change in urodilatin correlated with salt sensitivity. However, urodilatin was related to the urinary sodium excretion at baseline (r=0.58, P<0.01) and after the high-salt diet (r=0.62, P<0.001). In conclusion, the close correlations between proANP1–30 and salt sensitivity suggest that proANP1–30 may serve as a marker for salt sensitivity and could be useful in identifying subjects who would benefit from dietary salt restriction to prevent development of hypertension.
The etiology and pathophysiology of primary hypertension are largely unknown. Genetic factors are of great importance1 and a high intake of dietary salt has been suggested to be one of the most important environmental risk factors for development of the disease.2 However, the individual blood pressure response to a high- versus a low-salt intake varies among individuals and forms a Gaussian distribution in the population.3 Patients with primary hypertension are more salt sensitive, eg, they respond with a greater increase in blood pressure on a high- versus a low-salt intake compared with normotensive subjects.3 Subjects classified as “salt sensitive” display a greater increase in blood pressure over time when compared with subjects classified as “salt resistant,”4 and normotensive subjects with a family history of hypertension are more salt sensitive than normotensive subjects with no such family history.5 Salt sensitivity may therefore be a risk factor for development of primary hypertension and determination of salt sensitivity in normotensive subjects could be of help in identifying subjects who would benefit from dietary salt restriction to prevent the disease. However, measurement of salt sensitivity is quite cumbersome3,5⇓ and, therefore, hard to apply in a clinical setting. A surrogate marker for salt sensitivity would simplify clinical testing for the trait and could also contribute to the understanding of the mechanisms underlying salt sensitivity and hypertension.
The 28-amino-acid peptide hormone atrial natriuretic peptide (proANP99–126) is a potent mediator of natriuresis and vasodilatation.6 The 126-amino-acid atrial natriuretic peptide prohormone (proANP1–126) is synthesized and stored in the atrial myocytes.7 On distension of the cardiac atria, proANP1–126 is cleaved into the biologically active C-terminal fragment proANP99–126 and an inactive N-terminal fragment (proANP1–98). The 2 peptides are secreted in equimolar amounts into the circulation, and proANP1–98 is subsequently further cleaved to proANP1–30, proANP31–67, and proANP79–98.8
Alterations in synthesis, secretion or action of atrial natriuretic peptides could theoretically represent mechanisms regulating salt sensitivity. Several investigators have examined the relation between circulating levels of proANP99–126 and salt sensitivity. Taken together, these reports have not given conclusive results.9–14⇓⇓⇓⇓⇓ However, proANP99–126 is rapidly removed from the circulation,15 whereas the N-terminal peptides, such as proANP1–30, remain for a much longer time in the circulation at manifold higher concentrations than proANP99–126.16 Plasma concentration of proANP1–30 (P-proANP1–30) is, therefore, less prone to fluctuation and may thus be a more reliable measure of proANP99–126 secretion than plasma concentration of proANP99–126 itself.
Urodilatin is a 32-amino-acid peptide present in human urine but not in plasma. The amino acid sequence of urodilatin is identical to that of proANP99–126 except for an N-terminal extension of 4 amino acids (amino acids 95 to 98), and the 2 peptides are derived from the same gene.6 Urodilatin is believed to act in a paracrine fashion in the kidney, mainly mediating increased diuresis and natriuresis, and could therefore be of importance in the regulation of salt sensitivity.6 As far as we know, the relation between urodilatin and salt sensitivity in humans has not been studied.
In the present study, we examined whether P-proANP1–30 or 24-hour urinary excretion of urodilatin (tU-urodilatin) reflects the degree of salt sensitivity in genetically hypertension-prone individuals.
The protocol of the study was approved by the ethics committee of Lund University, and all participants gave written informed consent. The procedures were in accordance with institutional guidelines.
Thirty unrelated subjects (13 men and 17 women, aged 48.1±6.7 years, with a body mass index of 26.8±3.6 kg/m2) with at least 1 first-degree relative with primary hypertension, were recruited from an ongoing collection of families with a high frequency of primary hypertension in the Scania region in southern Sweden. The relation between salt sensitivity and insulin resistance has been reported earlier in 28 of these subjects.17 The baseline characteristics of the subjects are shown in the Table. None of the subjects received any medication or had ever been on antihypertensive treatment, neither did they have diabetes mellitus, kidney disease, or any other chronic disease. All but 3 women were postmenopausal. The premenopausal women were examined in the follicular phase of the menstrual cycle.
All subjects were investigated at baseline and after 1 and 2 weeks. After the baseline investigation, the study subjects were given a low-salt diet (10 mmol sodium and 70 mmol potassium per day) for 1 week. During the second week, sodium chloride capsules (230 mmol/d) were added to the diet to achieve a high-salt intake of 240 mmol/d. The diet was composed by a dietitian, and the daily energy intake was adjusted according to body weight and gender (8400 to 11 760 kJ). The study subjects received all meals and drinks from a metabolic ward. After 30 minutes of rest, blood pressure was measured in the supine position with an automatic oscillometric device (DINAMAP 1846 SX, Critikon) at 4-minute intervals during 40 minutes, and the mean value of the 10 measurements was used. Salt sensitivity was defined as the difference between the mean arterial blood pressure (the diastolic blood pressure plus one third of the pulse pressure) after the high-salt diet and that obtained after the low-salt diet. Because salt sensitivity is normally distributed in the population,3 we regarded it as a continuous variable. After the blood pressure measurements, fasting blood samples were drawn with the patients in the supine position. Urine samples (24 hours) were collected before the baseline investigation and at the end of the high- and low-salt diet weeks.
P-proANP1–30 and urine concentrations of urodilatin were measured by radioimmunoassay (RIA) using antiserum from Peninsula Laboratories (P-proANP1–30), and Immundiagnostik Gmbh (urodilatin). Calibrator materials were from Peninsula Laboratories, and tracers were prepared by in-house iodination. Plasma samples were diluted 1:15 with RIA buffer before assay, and urine samples were extracted using ethanol. Serum and urine concentrations of sodium and potassium were measured by standard biochemical methods. Plasma renin activity (PRA) and plasma aldosterone concentrations (PAC) were measured using diagnostic RIA kits (Abbot Laboratories).
NCSS Statistical Software (version 6.0.21, Statistical Solutions, Ltd) was used for the statistical analyses. Data are expressed as mean±SD. Differences between paired variables were compared with paired t test or Wilcoxon signed rank test, where appropriate. Correlations were determined using Pearson’s correlation coefficient (r value) if the residuals were normally distributed. Otherwise Spearman’s correlations (R value) were used. All probability values were calculated from 2-sided tests, and a level of <0.05 was considered statistically significant.
Endocrine, Circulatory, and Electrolytic Changes During High- and Low-Salt Intake
The 24-hour urinary sodium excretion (tU-Na) during the low- and high-salt diets indicated good dietary compliance (Table). Blood pressure, P-proANP1–30, tU-urodilatin, body weight, and serum sodium concentration increased, whereas PRA, PAC, heart rate, and serum potassium concentration decreased significantly on the high-salt diet compared with the low-salt diet (Table).
Relationship Between P-proANP1-30 and Salt Sensitivity
P-proANP1–30 at baseline and P-proANP1–30 after the low- and the high-salt diets were all positively correlated to salt sensitivity (Figures A through C). In addition, the salt-induced increase in P-proANP1–30 (Table) was positively correlated to salt sensitivity (Figure D). There was no correlation between P-proANP1–30 and tU-Na, whether at baseline (r=−0.05, NS), after the low- (r=0.12, NS), or after the high-salt diet (r=0.07, NS).
Relationship Between tU-Urodilatin and Salt Sensitivity
tU-urodilatin did not correlate with salt sensitivity at any of the 3 points of measurement (baseline, low-salt, and high-salt) (R=0.17, NS; R=0.30, NS; R=0.09, NS), nor did the sodium-induced change in tU-urodilatin correlate with salt sensitivity (R=−0.15, NS). tU-urodilatin was significantly correlated to tU-Na at baseline (r=0.58, P<0.01) and after the high-salt diet (r=0.62, P<0.001), but not after the low-salt diet (r=0.15, NS).
We found that the higher the P-proANP1–30 and the more P-proANP1–30 increased when changing from the low- to the high-salt diet, the greater was the degree of salt sensitivity in individuals with heredity for hypertension (Figures A through D).
Studies of the relationship between plasma concentrations of proANP99–126 and salt sensitivity have not given conclusive results.9–14⇓⇓⇓⇓⇓ However, the combination of a pulsatile secretion pattern and a short half-life (about 7 minutes) of proANP99–12615 is likely to limit the value of using plasma concentrations of proANP99–126 as a measure of its previous secretion rates. Because proANP1–30 is degraded much more slowly, it circulates at manifold higher concentration and is less prone to fluctuation.16 P-proANP1–30 may thus be a more reliable marker of proANP99–126 secretion than plasma concentration of proANP99–126 itself. This is supported by studies of congestive heart failure, where plasma concentrations of the stable N-terminal fragments of proANP1–126 show stronger correlation with the degree of congestive heart failure than does plasma concentration of proANP99–126.18–20⇓⇓ Furthermore, it has been reported that proANP1–30 possesses natriuretic effects by itself,8 suggesting that this peptide, apart from being a marker of proANP99–126 secretion, may have an active role in regulating salt sensitivity. However, the fact that we did not find any correlation between proANP1–30 and tU-Na suggests that the direct natriuretic effect of proANP1–30, if it exists, is small.
Disruption of the atrial natriuretic peptide gene in mouse leads to diminished production of the hormone and salt-sensitive hypertension,21 highlighting the importance of atrial natriuretic peptides in regulating salt sensitivity. However, our data do not indicate a defective proANP99–126 secretion in salt-sensitive subjects. Salt sensitivity was associated with increased P-proANP1–30, even at the baseline examination (Figure A), ie, before the salt-intake had been standardized. This suggests that salt sensitivity is a state of chronic volume expansion or of frequent episodes of volume expansion which, in turn, leads to increased secretion of atrial natriuretic peptides because of distension of the cardiac atria. Thus, P-proANP1–30 may be useful as a surrogate marker for salt sensitivity in healthy subjects with heredity for hypertension without the need for standardization of salt-intake (Figure A), although the correlation between P-proANP1–30 and salt sensitivity was even stronger when salt intake was standardized (Figures B and C). Furthermore, the correlation between the salt-induced increase in P-proANP1–30 and salt sensitivity (Figure D) implies that the atrial myocytes secrete more proANP99–126, in response to salt-induced plasma volume expansion, in salt-sensitive subjects than they do in salt-resistant subjects. This could reflect a defect in the action of proANP99–126 in salt-sensitive subjects at the receptor or postreceptor level leading to increased atrial secretion of the different proANP1–126 fragments in an attempt to overcome the defective action of proANP99–126. However, a defect in the proANP99–126 receptor (NPRA) as the cause of human salt sensitivity is not supported by studies on the NPRA knock-out mouse because these mice, although hypertensive, are salt resistant and have normal atrial natriuretic peptide concentrations in plasma.22 Alternatively, the elevated P-proANP1–30 and the greater increase in P-proANP1–30 in salt-sensitive subjects in response to salt loading may represent a compensatory mechanism in an attempt to counterbalance other forces, promoting an exaggerated salt-induced plasma volume expansion.
Our finding that tU-urodilatin, but not P-proANP1–30, correlated with tU-Na at baseline and after the high-salt diet is in line with earlier reports suggesting that a significant proportion of the blood pressure lowering effect of proANP99–126 results from a decrease in vascular tone, whereas the main action of urodilatin is to regulate renal sodium excretion.6 However, we did not find support for the view that tU-urodilatin would be a useful surrogate marker for salt sensitivity.
In conclusion, our data suggest that secretion of atrial natriuretic peptides is increased in subjects displaying a high degree of salt sensitivity. P-proANP1–30 may serve as a marker for salt sensitivity and could be useful in identifying subjects who would benefit from dietary salt restriction to prevent the development of hypertension.
This study was supported by grants from the Swedish Medical Research Council, the Swedish Heart and Lung Foundation, the Medical Faculty of Lund University, Malmö University Hospital, the Albert Påhlsson Research Foundation, the Crafoord Foundation, the Region Skane, the Ernhold Lundströms Research Foundation, and Diabetesföreningen i Malmö med omnejd. We are indebted to Philippe Burri, Gertrud Ahlqvist, and Marianne Lundberg for excellent technical assistance, to Gunilla Willsteen for dietary calculations and food preparations, to Peter Almgren for statistical advice, and to the study subjects for their participation.
- Received November 20, 2001.
- Revision received December 14, 2001.
- Accepted March 26, 2002.
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