Impaired pressure natriuresis resulting in salt-sensitive hypertension is caused by tubulointerstitial immune cell infiltration in the kidney

Abstract

Immune cell infiltration of the kidney is a constant feature in salt-sensitive hypertension (SSHTN). We evaluated the relationship between the renal inflammation and pressure natriuresis in the model of SSHTN that results from transient oral administration of N(ω)-nitro-L-arginine methyl ester (L-NAME). Pressure natriuresis was determined in Wistar rats that received 4 wk of a high-salt (4% NaCl) diet, starting 1 wk after stopping L-NAME, which was administered alone (SSHTN group, n = 17) or in association with mycophenolate mofetil (MMF; MMF group, n = 15). The administration of MMF in association with L-NAME is known to prevent the subsequent development of SSHTN. Control groups received a high (n = 12)- and normal (0.4%)-salt diet (n = 20). Rats with SSHTN had increased expression of inflammatory cytokines and oxidative stress. The severity of hypertension correlated directly (P < 0.0001) with the number of tubulointerstitial immune cells and angiotensin II-expressing cells. Pressure natriuresis was studied at renal arterial pressures (RAPs) of 90, 110, 130, and 150 mmHg. Glomerular filtration rate was similar and stable in all groups, and renal blood flow was decreased in the SSHTN group. Significantly decreased natriuresis (P < 0.05) was found in the SSHTN group at RAPs of 130 and 150 mmHg, and there was an inverse correlation (P < 0.01) between the urinary sodium excretion and the number of tubulointerstitial inflammatory cells (lymphocytes and macrophages) and cells expressing angiotensin II. We conclude that tubulointerstitial inflammation plays a key role in the impairment of pressure natriuresis that results in salt-dependent hypertension in this experimental model.

Fig. 1.

Light microscopy and immunohistology of renal sections of rats from the control group, salt-sensitive hypertension (SSHTN) group, and mycophenolate mofetil (MMF) group. A: Light microscopy (periodic acid-Schiff staining), (B) macrophage (CD68-positive cells) infiltration, and (C) angiotensin II-positive cells. Tubulointerstitial inflammation and tubular cells and infiltrating cells staining positive for angiotensin II are evident in the SSHTN group. Scale marks correspond to the whole column. Staining details are in materials and methods.

Fig. 2.

Western blots of renal content of IL-2 (A), IL-6 (B), nitrotyrosine (C), and malondialdehyde (D) in experimental (SSHTN and MMF) and control [normal salt diet (C-NSD)] groups. Western blot data are expressed as optical density (O.D.) relative to β-actin. Gel pictures are samples for 1 run. Data correspond to n = 5 in each group. Molecular weight shown in nitrotyrosine corresponds to nitrosylated tyrosine-containing proteins. *P < 0.05; **P < 0.01; ***P < 0.001.

Fig. 3.

Renal blood flow (RBF) and glomerular filtration rate (GFR) at the studied renal arterial pressure (RAP) levels in the experimental (SSHTN and MMF) and control groups with C-NSD and a high-salt diet (C-HSD). A: no significant differences in RBF in C-NSD, C-HSD, and MMF groups. RBF in the SSHTN group was lower (*P < 0.01 vs. the rest). B: no significant differences among the GFRs at 90, 110, 130, and 150 mmHg RAPs in experimental and control groups. GFR in the SSHTN group increased progressively from 90 mmHg to 150 mmHg. C: filtration fraction in the SSHTN is significantly higher (*P < 0.05; **P < 0.01) than in the rest. Studies done with an induced (see text) RAP of 150 mmHg in normotensive rats from the C-NSD (n = 7), C-HSD (n = 5), and MMF (n = 6) groups, as indicated in materials and methods. Symbols and error bars are mean ± SE. LKW, left-kidney weight.

Fig. 4.

Variations in (A) urinary sodium-excretion (U_NaV) rate and (B) fractional sodium excretion (F_NaE) at the studied RAPs. The progressive increment in U_NaV and F_NaE observed in the control and MMF groups was not present in the SSHTN group. Studies done with RAP of 150 mmHg in normotensive rats from the C-NSD (n = 5), C-HSD (n = 5), and MMF (n = 6) groups, as indicated in materials and methods. *P < 0.05 vs. the rest.

Fig. 5.

Inverse relationship between interstitial immune cell (CD68- + CD3-positive cells) infiltration and (A) U_NaV rate and (B) F_NaE. Data correspond to values at RAP of 130 mmHg. Each symbol represents 1 animal of the corresponding groups.

Fig. 6.

Inverse relationship between the number of angiotensin II-positive cells and (A) U_NaV rate and (B) F_NaE. Data correspond to RAP of 130 mmHg. Symbols correspond to the experimental and control groups.

Fig. 7.

Direct relationship between the systolic blood pressure (SBP) and the number of infiltrating immune cells (CD68- + CD3-positive cells; A) and angiotensin II-positive cells (B). Symbols correspond to the experimental and control groups.