High-salt diet blunts renal autoregulation by a reactive oxygen species-dependent mechanism

Abstract

High dietary salt is common in Western countries and is an important contributor to increased cardiovascular disease. Autoregulation of renal blood flow (RBF) and glomerular filtration rate (GFR) is an essential function of the renal microcirculation that could be affected by excessive dietary salt. High salt (HS) increases renal ROS generation partly by the enzyme NADPH oxidase. We hypothesized that a HS diet would impair autoregulation via NADPH oxidase-dependent ROS generation. The role of NADPH-dependent ROS production on the blunted autoregulatory response with a HS diet was assessed in vitro and in vivo using the blood-perfused juxtamedullary nephron preparation and anesthetized rats, respectively. The increase in renal lipid peroxidation and p67(phox) expression induced by HS was prevented by apocynin treatment. Control afferent arterioles exhibited normal autoregulatory behavior in response to acute increases in renal perfusion pressure, whereas arterioles from HS rats exhibited a blunted response. Autoregulatory behavior in HS rats was restored in vitro by acute exposure to the NADPH oxidase inhibitor apocynin. At the whole kidney level, in vivo experiments showed that both RBF and GFR declined in HS rats when left kidney renal perfusion pressure was reduced from ambient to 95 mmHg, whereas control rats maintained stable GFR and RBF consistent with efficient autoregulatory behavior. Apocynin treatment improved in vivo autoregulatory behavior in HS rats and had no detectable effect in normal salt diet-fed rats. These data support the hypothesis that impaired renal autoregulatory behavior in rats fed a HS diet is mediated by NADPH oxidase-derived ROS.

Keywords: 4-hydroxy-2-nonenal; afferent arterioles; apocynin; glomerular filtration rate; oxidative stress.

Fig. 1.

Baseline afferent arteriolar diameter in kidneys at a renal perfusion pressure (RPP) of 100 mmHg using the in vitro juxtamedullary nephron technique before and during topical administration of apocynin (Apo) in the superfusate. Rats were divided into the following groups. Rats were fed either a normal salt (NS) diet (0.8% NaCl) or a high-salt (HS) diet (8% NaCl) for 14 days and had ad libitum access to tap water. A subset of each group of rats received Apo in the drinking water at a concentration of 1.5 mM for HS rats (HS + Apo group) and 4.5 mM for NS rats (NS + Apo group). Data are expressed as means ± SE; n, number of afferent arterioles studied.

Fig. 2.

Afferent arteriolar diameter response to changing RPP in kidneys using the in vitro juxtamedullary nephron technique. A: responses of NS, HS, NS + Apo, and HS + Apo groups. B: data expressed as a percentage of the control diameter. The autoregulatory profile ranged from 65 to 170 mmHg. Data are expressed as means ± SE; n, number of afferent arterioles studied. *P < 0.05, HS vs. NS groups; +P < 0.05, HS vs. HS + Apo groups.

Fig. 3.

Renal blood flow (RBF) responses to decreasing RPP in kidneys of NS and HS rats with and without Apo in the drinking water (∼90 mg·kg⁻¹·day⁻¹). A: raw RBF. B: data normalized to starting blood flow. The autoregulatory profile ranged from ambient to 95 mmHg left kidney RPP. C: autoregulatory index for RBF. Data are expressed as means ± SE; n, number of rats studied. *P < 0.05, HS vs. NS groups; #P < 0.05, NS vs. HS + Apo groups; +P < 0.05, HS vs. HS + Apo groups.

Fig. 4.

Glomerular filtration rate (GFR) responses to decreasing RPP in kidneys of NS and HS rats with and without Apo in the drinking water (∼90 mg·kg⁻¹·day⁻¹). A: GFR normalized to body weight. The autoregulatory profile ranged from ambient to 95 mmHg left kidney RPP. BW, body weight. B: data normalized to starting GFR over the autoregulatory profile range from ambient to 95 mmHg left kidney RPP. Data are expressed as means ± SE; n, number of rats studied. *P < 0.05, HS vs. NS groups; +P < 0.05, HS vs. HS + Apo groups.

Fig. 5.

Effect of HS diet and chronic oral Apo treatment on 4-hydroxy-2-nonenal (4-HNE) levels in the renal cortex and medulla. 4-HNE staining intensity is presented as arbitrary units across groups. *P < 0.05 vs. kidneys from the NS group; +P < 0.05 vs. kidneys from the HS + Apo group; #P < 0.05 vs. kidneys from the NS + Apo group.

Fig. 6.

Western blot analysis to determine the effect of HS diet and chronic oral Apo treatment on renal p67^phox expression. Top: typical gel with three lanes each for each of the treatment groups. Bottom: densitometry data normalized against β-actin. *P < 0.05 vs. kidneys from the NS group; +P < 0.05 vs. kidneys from the HS + Apo group; #P < 0.05 vs. kidneys from the NS + Apo group.