(Hypertension. 2004;44:746.)
© 2004 American Heart Association, Inc.
Scientific Contributions |
From the Vascular Biology and Hypertension Program (V.F., Y.-F.C., S.O., J.A.F., D.W., F.H., G.P.), Division of Cardiovascular Disease, University of Alabama at Birmingham, and the Cardiology Section (G.P.), Birmingham VA Medical Center, Birmingham, Ala.
Correspondence to Veronica Franco, MD, University of Alabama at Birmingham, ZRB 1024, 703 19th St S, Birmingham, AL 35294. E-mail vfranco{at}uab.edu
| Abstract |
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Key Words: atrial natriuretic factor natriuretic peptides receptors, atrial natriuretic factor hypertrophy, cardiac remodeling extracellular matrix collagen
| Introduction |
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ANP-heterozygous mice (Nppa+/) have normal blood pressure on either a normal- (0.5% NaCl) or an intermediate-salt (2% NaCl) diet,6,14 in contrast to the hypertension observed in Nppa/ mice under these conditions.11 On a very-high-salt (8% NaCl) diet, however, Nppa+/ mice develop hypertension.6 The cardiac phenotype of the heterozygous ANP-knockout has not been rigorously studied under either basal or stress conditions. However, in their original report of this model, John et al6 did not find a significant difference in cardiac weight between Nppa+/+ and Nppa+/ mice. The relatively normal cardiac phenotype in the heterozygous ANP-knockout might indicate that the quantity of ANP is not critical under nonstressful conditions. In a different murine model, we demonstrated that variation in angiotensin-converting enzyme levels does not importantly affect either angiotensin II levels or the hypertrophic response to volume overload.15 Thus, effects of the complete absence or pharmacological blockade of a gene product does not predict whether or not a modest variation in that gene product will have important effects on phenotype.
The effect of varying levels of endogenous ANP expression on cardiac structure and the development of heart failure is unknown and difficult to study in humans, owing to a confounding effect of increased ANP expression with worsening severity of failure. The Nppa+/ mouse provides an ideal model to study the effect of a modest reduction in ANP expression on cardiac remodeling and function in response to hemodynamic stress. We hypothesized that ANP-heterozygous mice would demonstrate a normal cardiac phenotype under nonstressful conditions but an exaggerated cardiac hypertrophy and remodeling in response to pressure-overload stress.
| Methods |
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Surgical Procedure
Male Nppa+/+, Nppa+/, and Nppa/ mice, 9 to 12 weeks of age, were anesthetized with an intraperitoneally administered mixture of ketamine (8 mg/100 g) and xylazine (1.2 mg/100 g), and transverse aortic constriction (TAC) was performed as described previously.13 Pressure gradients across the TAC were similar among genotypes (53±6.5 mm Hg in Nppa+/+ and 55±6.8 mm Hg in Nppa/ mice).13 Sham-operated mice served as controls.
Echocardiographic Study
One week after TAC or sham surgery, echocardiography was performed as previously described11,12 with a 6- to 15-MHz transducer (Philips) and a commercially available ultrasound system (Phillips Sonos 5500). Left ventricular (LV) end-diastolic dimension (EDD), LV end-systolic dimension (ESD), and septal (interventricular septal [IVS[) and posterior wall (PW) diastolic thicknesses were measured by 2D-guided M-mode echocardiography from the parasternal long-axis view. Wall thickness was calculated as the average of IVS and PW. Fractional shortening (FS) was calculated by the formula (LVEDDLVESD)/LVEDD. A single examiner, blinded to genotype and treatment, performed all studies.
Plasma ANP
Blood (0.5 mL) was collected via retro-ocular approach from conscious mice the day after the echocardiographic study. ANP measurement was performed as previously described with the use of radioimmunoassay kits (Peninsula Laboratories).17
Tissue Collection
Mice were humanely killed after blood collection by cervical dislocation. Hearts were removed, and the LV, right ventricle (RV), and atria were weighed. The LV was divided into 2 portions: the apical portion was fixed with 4% paraformaldehyde, embedded in paraffin, and sectioned for histologic analysis; the basal portion was immediately frozen in LN2 for RNA isolation. Lung, kidney, brain, liver, and spleen were also weighed.
Collagen Volume
LV myocardial collagen (interstitial and perivascular) volume, at the level below the mitral valve, was measured in picrosirius red (0.1%)-stained cross sections on a microscopic system with a green (540-nm) filter to enhance contrast for computer imaging analysis (Image-1 Software). Only collagen fibers appear green; green regions are counted by the aforementioned software and are given as a percentage of the total area per field. A minimum of 8 randomly selected images were counted from each LV. A single examiner, blinded to the experimental group, performed all histologic analyses.
Northern Blot Analysis
Northern blot analysis was performed as described previously13 with 32P-labeled selective cDNA probes for ANP (generated in our laboratory by reverse transcription-polymerase chain reaction with mouse heart RNA as the template18). 18S rRNA was used as a control to account for variation in RNA loading.
Statistical Analysis
Results are expressed as mean±SEM. Tissue weights and echocardiographic measurements were normalized by ANCOVA with body weight as the covariate.19,20 Our primary statistical test was ANOVA, 1-way ANOVA to evaluate the differences in mean values due to main effects (genotype or TAC), and 2-way ANOVA to test their interactions. P<0.05 was considered significant.
| Results |
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Tissue Weight
Both genotype and TAC affected whole-heart, LV, and RV weight, and there was an interaction between these 2 variables (Table 1). Whole-heart and LV weight followed ANP gene expression in Nppa/ versus both Nppa± and Nppa+/+ mice, as in Nppa+/ and Nppa+/+ mice, after sham surgery. Whole-heart, LV, and RV weight differed among all 3 genotypes in a graded fashion (Nppa/>Nppa+/>Nppa+/+) after TAC.
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Echocardiography
Normalized echocardiographic measurements are shown in Table 2. LVEDD was increased by TAC and by decreasing the number of ANP genes. Pairwise multiple comparison revealed that LVEDD did not differ among the 3 genotypes after sham surgery. After TAC, LVEDD was greater in Nppa/ versus Nppa+/ and in Nppa/ versus Nppa+/+ mice but did not differ between Nppa+/ and Nppa+/+ mice. LVFS did not differ among the 3 genotypes after sham surgery and was reduced only in Nppa/ mice subjected to TAC.
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Collagen Volume
Collagen volume differed significantly among the 3 genotypes after sham surgery (Nppa+/+, 0.2±0.0%; Nppa+/, 1.1±0.2%; and Nppa/, 3.5±0.2%; P<0.05, 1-way ANOVA) and after TAC (Nppa+/+, 0.1±0.0%; Nppa+/, 6.4±1.3%; and Nppa/, 24.3±5.9%; P<0.01, 1-way ANOVA). Collagen deposition occurred predominantly in the interstitium and perivascular area after TAC and was inversely related to ANP gene load (Figure 2 A and 2B). There was robust interstitial and perivascular fibrosis in TAC-Nppa/ mice and a significant but less severe response in TAC-Nppa+/ mice (2-way ANOVA, P<0.01, interaction of genotypexTAC). Collagen deposition increased by
6-fold in TAC-Nppa/ and TAC-Nppa+/ mice when compared with their sham controls but did not differ between sham and TAC-Nppa+/+ mice.
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| Discussion |
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Previous studies have documented that the complete absence of ANP or its receptor results in the development of salt-sensitive hypertension, cardiac hypertrophy, and remodeling.3,5,6,21,22 Partial ANP deficiency does not affect blood pressure on a normal-salt diet6 but has not been rigorously studied with regard to cardiac phenotype. John et al6 reported cardiac weight only for Nppa+/+ and Nppa+/ mice fed 2% NaCl and found a statistically nonsignificant increase in the Nppa+/ genotype, similar in magnitude to that observed in sham-operated animals in the current study. We also observed a modest increase in both collagen volume and heart weight in the heterozygotes versus wild types under basal conditions, despite apparently similar ANP levels. Because of limitations in the sensitivity (10 to 20 pg/mL of plasma) of the radioimmunoassay for ANP and Northern blot analysis of ANP mRNA, it is possible that there are small (<20%) differences in these parameters between Nppa+/ and Nppa+/+ mice that were undetectable by our methods. Clearly, major differences in ANP levels between Nppa+/ and Nppa+/+ were manifested only under stress conditions. It is unlikely that higher blood pressure explains these increases in heart weight and collagen volume in heterozygous versus wild-type mice, because previous reports have documented no difference in blood pressure between Nppa+/+ and Nppa+/ mice on either a normal- (0.5%) or an intermediate- (2.0% NaCl) salt diet.6,14 In addition, correction of hypertension in Nppa/ mice by a very-low-salt diet (0.05% NaCl) does not prevent cardiac hypertrophy, either at baseline or in response to hemodynamic stress, suggesting that alternative mechanisms are operative in ANP-induced hypertrophy and fibrosis.11
In the current study, partial ANP deficiency clearly resulted in excess cardiac hypertrophy and remodeling in response to hemodynamic stress compared with wild-type mice. This was accompanied by a 6-fold increase in collagen in TAC-Nppa+/ and TAC-Nppa/ versus the respective sham-operated mice compared with no change in collagen volume in Nppa+/+ mice after TAC. Importantly, whereas collagen volume increased after TAC in Nppa+/ relative to sham and to TAC-Nppa+/+ mice, it was still substantially less than that seen in TAC-Nppa/ mice. Consistent with those findings, the presence of 1 copy of the ANP gene was sufficient to protect against the development of LV dysfunction compared with mice with a complete absence of ANP. In contrast to ANP, B-type natriuretic peptide (BNP) appears not to modulate cardiac enlargement. BNP/ mice do not develop cardiac hypertrophy23; circulating BNP is not increased in Nppa/ mice24; and BNP is unable to compensate for the lack of ANP in Nppa/ mice under stress conditions.13,16,24 These findings suggest that ANP deficiency alone is sufficient to generate cardiac hypertrophy/remodeling, particularly under conditions of hemodynamic stress. Our findings are consistent with a growing body of evidence that remodeling of the cardiac interstitium is a major determinant of pathologic hypertrophy, leading to cardiac dysfunction and failure,2529 and underscore the importance of ANP as a modulator of that interstitial remodeling.12,13
Perspectives
The observation in the current study that cardiac weight and collagen content vary with ANP gene load both at baseline and after 1 week of pressure-overload hemodynamic stress underscores the critical role of ANP in modulating cardiac growth and structure under both physiologic and pathophysiologic conditions. We chose heterozygous ANP-knockout mice for the current study because humans are more likely to have partial rather than an absolute deficiency of ANP. Both environmental and genetic variations in ANP have been described in humans. Blunted secretion of ANP has been observed in black salt-sensitive hypertensives in response to high salt intake,30 and a polymorphism of the ANP gene has been observed more frequently in black salt-sensitive hypertensives compared with normotensives or white hypertensives.3133 Obese individuals demonstrate decreased levels of both ANP and BNP, possibly related to more rapid clearance by adipocyte NPR-C receptors.34 Our findings suggest that these observed variations in ANP levels maybe a fruitful area for study with regard to the development of cardiac hypertrophy, remodeling, and failure in response to hemodynamic stress in humans.
| Acknowledgments |
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Received July 10, 2004; first decision August 1, 2004; accepted September 1, 2004.
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