Increased dietary NaCl potentiates the effects of elevated prorenin levels on blood pressure and organ disease

Abstract

Background: Rats with several 100-fold elevation of plasma prorenin levels due to liver-specific expression of a rat prorenin transgene have cardiac and aortic hypertrophy, renal lesions, and myocardial fibrosis. The effect of increased dietary NaCl on the phenotype of prorenin transgenic rats has not been examined.

Methods and results: We compared the effects of 0.3 and 2% dietary NaCl in wild-type and transgenic rats from 3 to 12 months of age. In comparison with wild-type rats, transgenic rats receiving 0.3% dietary NaCl had approximately 1000-fold elevation of prorenin, 1.5-fold to 2.5-fold elevation of renin concentration and activity, wild-type levels of angiotensin II, and were hypertensive with cardiac and aortic hypertrophy, and increased renal glomerular and tubulo-interstitial injury score. In wild-type rats, 2% dietary NaCl reduced angiotensin levels, produced a delayed increase in blood pressure, and caused cardiac hypertrophy and tubulo-interstitial injury. By contrast, 2% NaCl did not reduce angiotensin levels in transgenic rats, potentiated their hypertension, cardiac and aortic hypertrophy, and increased myocardial interstitial and perivascular fibrosis, without effect on glomerular or tubulo-interstitial injury score.

Conclusion: Increased dietary NaCl had a greater impact on the phenotype of transgenic than wild-type rats that may have been due, in part, to their hypertension and their failure to suppress angiotensin levels, consequent to their elevated prorenin levels.

Figure 1

Effect of 0.3% (circles) and 2% (squares) dietary NaCl on body weight (BW) and systolic blood pressure (SBP) in male wildtype (WT, open symbols) and (hAT-rpR) rats (closed symbols). Data shown as means ± SEM, n=7-8. For BW, 2-way repeated measures ANOVA for 3-12 months of age showed no effect of genotype, dietary NaCl, or interaction between genotype and dietary NaCl. However, when BW at 12 months was analysed separately, there was a significant effect of genotype (P=0.03) but not dietary NaCl, and (hAT-rpR) rats receiving 2% dietary NaCl had lower BW than WT rats receiving 2% dietary NaCl (P=0.006). For SBP, 2-way ANOVA for 3-12 months of age showed higher SBP in (hAT-rpR) than WT rats (P<0.0001), and in comparison with 0.3% dietary NaCl, 2% NaCl increased SBP in both WT (P=0.02) and (hAT-rpR) rats (P=0.004). When SBP at 12 months was analysed separately, (hAT-rpR) rats receiving 2% dietary NaCl had higher SBP than WT rats receiving either 0.3% (P=0.0001) or 2% dietary NaCl (P=0.01), and WT rats receiving 2% dietary NaCl had higher SBP than WT rats receiving 0.3% NaCl (P=0.02). *P<0.05, †P<0.01 for comparison of indicated groups at 12 months of age.

Figure 2

Effect of 0.3% (circles) and 2% (squares) dietary NaCl on plasma prorenin, renin concentration (PRC), and renin activity (PRA) in male wildtype (WT, open symbols) and (hAT-rpR) rats (closed symbols). Data shown as means ± SEM, n=7-8; where error bars are not shown they are contained with the symbol. For prorenin, 2-way ANOVA for 6-12 months of age showed significant effects of genotype (P<0.0001) and dietary NaCl (P=0.0003), without interaction between genotype and dietary NaCl, and 2% NaCl reduced prorenin in both WT (P=0.01) and (hAT-rpR) rats (P=0.0002). When prorenin levels at 12 months were analysed separately, (hAT-rpR) rats had higher prorenin than WT rats for either diet (P<0.0001), and 2% dietary NaCl reduced prorenin levels for both (hAT-rpR) (P=0.001) and WT rats (P=0.04). For PRC, 2-way ANOVA for 6-12 months of age showed significant effects of genotype (P<0.0001) and dietary NaCl (P=0.003), without interaction between genotype and dietary NaCl, and 2% NaCl reduced PRC in WT (P=0.005) but not in (hAT-rpR) rats (P=0.06). When PRC at 12 months was analysed separately, (hAT-rpR) rats receiving either 0.3% (P=0.0002) or 2% dietary NaCl (P=0.001) had higher PRC than WT rats receiving 2% NaCl, and (hAT-rpR) rats receiving 0.3% NaCl had higher PRC than WT rats receiving 0.3% NaCl (P=0.03). However, the effect of 2% NaCl on PRC was not statistically significant for either (hAT-rpR) or WT rats at 12 months of age. For PRA, 2-way ANOVA for 6-12 months of age showed significant effects of genotype (P<0.0001) and dietary NaCl (P<0.0001), without interaction between genotype and dietary NaCl, and 2% NaCl reduced prorenin in both WT (P<0.0001) and (hAT-rpR) rats (P=0.0003). When PRA at 12 months was analysed separately, (hAT-rpR) rats receiving 0.3% dietary NaCl had higher PRA than WT rats receiving either 0.3% (P=0.046) or 2% dietary NaCl (P=0.0002), whereas PRA of (hAT-rpR) rats receiving 2% NaCl was not different from that of WT rats receiving either diet. Increased dietary NaCl reduced PRA in both (hAT-rpR) (P=0.01) and WT rats (P=0.047) at 12 months of age. *P<0.05, †P<0.01 for comparison of indicated groups at 12 months of age.

Figure 3

Effect of 0.3% NaCl diet (open columns) and 2% NaCl diet (closed columns) on renal glomerular, tubulo-interstitial, and vascular injury scores in male wildtype (WT) and (hAT-rpR) rats at 12 months of age. Data shown as means ± SEM, n = 7-8. For glomerular injury score, 2-way ANOVA showed a significant effect of genotype (P=0.002), but not of dietary NaCl. For tubulo-interstitial injury score, 2-way ANOVA showed a significant effect of both genotype (P=0.0002) and dietary NaCl (P=0.02); the effect of dietary NaCl was significant for WT rats (P=0.0004), but not for (hAT-rpR) rats, when analyzed separately. The vascular injury score was zero for WT rats on either diet and for (hAT-rpR) rats on a 0.3% NaCl diet, and 2-way ANOVA showed no effect of either genotype or dietary NaCl.

Figure 4

Representative kidney sections of wildtype (WT) and transgenic (hAT-rpR) rats administered either 0.3% or 2% dietary NaCl from 3 to 12 months of age. At 12 months of age, (hAT-rpR) rats receiving on 0.3% dietary NaCl showed glomerulosclerosis, tubulo-interstitial inflammation and fibrosis, tubular atrophy and casts, in comparison with WT rats. In comparison with 0.3% dietary NaCl, 2% dietary NaCl had no affect on glomerular or tubulo-interstitial injury score of (hAT-rpR) rats, whereas WT rats showed increased tubulo-interstitial, but not glomerular, injury score in response to 2% dietary NaCl. Masson's trichrome stain; bar = 50μm.

Figure 5

Effect of 0.3% NaCl diet (open columns) and 2% NaCl diet (closed columns) on myocardial interstitial and perivascular fibrosis, and myocardial vessel wall/lumen ratio in male wildtype (WT ) and (hAT-rpR) rats at 12 months of age. Data shown as means ± SEM, n = 7-8. For interstitial fibrosis, 2-way ANOVA showed a significant effect of dietary NaCl (P=0.003), but not of genotype; the effect of dietary NaCl was significant for (hAT-rpR) rats (P=0.008), but not for WT rats, when analyzed separately. For perivascular fibrosis, 2-way ANOVA showed a significant effect of dietary NaCl (P=0.03), but not of genotype; the effect of dietary NaCl was significant for (hAT-rpR) rats (P=0.03), but not for WT rats, when analyzed separately. For vascular wall/lumen ratio, 2-way ANOVA showed no significant effect of either genotype or dietary NaCl.

Figure 6

Representative heart sections of wild type (WT) and transgenic (hAT-rpR) rats administered either 0.3% or 2% dietary NaCl from 3 to 12 months of age. At 12 months of age, there was no difference in interstitial or perivascular fibrosis, or myocardial vessel wall/lumen ratio between WT and (hAT-rpR) rats administered 0.3% dietary NaCl. In comparison with 0.3% dietary NaCl, 2% dietary NaCl did not modify myocardial interstitial or perivascular fibrosis in WT rats. However, 2% dietary NaCl increased myocardial interstitial and perivascular fibrosis in (hAT-rpR) rats. Dietary NaCl did not influence myocardial vessel wall/lumen ratio in either WT or (hAT-rpR) rats. Sirius red staining collagen; bar = 200μm.