Smooth Muscle PPARγ Mutation Causes Salt‐sensitive Hypertension

Abstract

Systemic loss of peroxisome proliferator‐activated receptor‐γ (PPARγ) function causes hypertension, whereas its activation lowers blood pressure largely via PPARγ activity in the vasculature. We have recently shown that loss of PPARγ in smooth muscle cells increases mesenteric myogenic tone, promotes vascular remodeling, and predisposes to deoxycorticosterone acetate (DOCA)‐salt‐induced hypertension in mice. However, it is unclear whether these effects were attributable to altered salt sensitivity. In this study, we examined the role of smooth muscle PPARγ in regulating renal function and salt‐sensitivity. Transgenic mice expressing dominant negative PPARγ in smooth muscle cells (S‐P467L) and non‐transgenic littermates (NT) were fed normal chow diet (containing 0.3% salt) or a 4% salt diet for a total of 8 weeks and metabolic cage studies were performed at baseline, 3 weeks, 6 weeks, and at the end of the protocol. S‐P467L and NT mice had similar food intake, feces weight, and weight gain on high salt diet throughout the study. While 24‐hour water intake tripled in both strains (6.3 ± 0.21 mL S‐P467L vs. 5.9 ± 0.40 mL NT, n=7) when they were switched to high salt diet from normal chow, S‐P467L mice excreted 32% less urine in the third week (24‐hour urine 1.96 ± 0.13 mL S‐ P467L vs. 2.90 ± 0.17 mL NT, two‐way ANOVA p<0.01). In order to assess renal function, mice were subjected to a bolus i.p. injection of normal saline equal to 10% of their body weight and the urine volume excreted in the subsequent 4 hours were plotted as a percentage of saline injected. We observed a marked decline in their capacity to excrete this volume challenge in S‐ P467L mice (28.2 ± 2.1% S‐P467L vs. 39.9 ± 5.1% NT, p<0.05) in the third week of high salt diet. Because nitric oxide (NO) is known to regulate sodium and water balance, we determined daily renal NO production as indicated by 24‐hour urinary nitrate/nitrite. Three‐week dietary salt induced a 4‐fold increase in urinary NO metabolites in NT mice, but this was blunted in S‐P467L mice (2.82 ± 0.19 μmoL/24 hours S‐P467L vs. 4.42 ± 0.27 μmoL/24 hours NT, p<0.01). In parallel with these renal phenotypes, S‐P467L mice developed marked elevation of blood pressure that peaked in the third week of high salt feeding (telemetry systolic pressure 136.4 ± 3.4 mmHg S‐P467L vs. 124.2 ± 2.4 mmHg NT, p<0.01) and remained elevated throughout the study. At the end of 8 weeks, vascular function was assessed in freshly isolated carotid and basilar arteries, and flow cytometry analysis was performed in single cell suspensions of aorta and kidney tissues. S‐P467L mice, but not NT littermates, exhibited severe impairment of acetylcholine‐ and sodium nitroprusside‐induced vascular relaxation in the carotid and basilar arteries in response to high salt. CD45+ total leukocytes, CD3+ T cells, CD4+/CD8+ T cell subsets, and F4/80+ monocytes/macrophages were not altered in the aortas but were paradoxically reduced in the kidneys of S‐P467L mice in response to high salt. These data indicate that loss of PPARγ in smooth muscle cells predispose to impaired vascular relaxation, renal dysfunction, and salt‐sensitive hypertension. This study highlights the significance of vascular PPARγ in regulating systemic physiological responses.Support or Funding InformationNIH and AHA Grants