High-salt Chow Research Articles

Abstract P456: Loss of Renal Resident Tissue Macrophages Reduces Nitric Oxide-Mediated Relaxation during Salt-Sensitive Hypertension

Salt-sensitive hypertension (SS-HTN) associates with increased renal sodium reabsorption, arterial dysfunction, and heightened pro-inflammatory cytokines such as interleukin 6 (IL-6). Renal resident tissue macrophages (rRTMs) function to both maintain tissue homeostasis and once activated, can secrete IL-6. We reported that loss of the rRTM population prevents SS-HTN but their influence on arterial reactivity has not been tested. We hypothesize that rRTM-induced IL-6 production impairs endothelial function. Wild-type (Wt) or rRTM-depleted (CX 3 CR 1 - EGFP+/+ ) male mice (12wks) were used. Mice were given the nitric oxide synthase (NOS) inhibitor, L-NAME (0.5mg/mL, in drinking water, 2wks) and allowed to washout (1-2wks) before fed normal chow (NC), high salt (HS) chow (4%) for 2 days (early salt-sensitive, eSS) or for 3wks HS (SS-HTN). eSS mice were salt-sensitive, but not yet hypertensive. Mice were also given HS chow alone (3wks). Serum was collected for systemic cytokines (Meso-Scale Discovery; n=4-15) and second-order mesenteric resistance arteries (MRAs) were obtained (n=3-5). Concentration response curves (CRCs) to phenylephrine (Phe) and acetylcholine (ACh) were performed in the presence or absence of potassium chloride (KCl) or L-NMMA. Serum IL-6 levels were not increased following HS diet alone, but concentrations trended higher during SS-HTN (1.46 ± 0.2 pg/mL vs. 1.07 ± 0.2 pg/mL NC). The nadir was observed in eSS mice, before the development of SS-HTN, and was prevented in rRTM-depleted mice (1.81 ± 0.3 pg/mL vs. 0.58 ± 0.17, **p<0.001, grouped ANOVA). No differences were observed between NC-fed Wt and rRTM-depleted MRAs, but the development of SS-HTN did impair relaxation responses. Overall, ACh-mediated relaxation responses were generally lower in MRAs from rRTM-depleted mice compared to Wt. Interestingly, rRTM-depleted MRAs from SS-HTN animals exhibited a near complete inhibition of all relaxation responses with NOS inhibition (R max 6.20% ± 8 rRTM SS-HTN vs. 63.0% ± 10 Wt SS-HTN, *p<0.05), suggesting greater dependence on NO-mediated relaxation. Together, these data suggest that rRTMs contribute to an early, hypertension-independent spike in IL-6 which may reduce endothelial function later during SS-HTN. We also show that rRTM-depletion affects the function of the resistance vasculature, including loss of non-NO-mediated relaxation during SS-HTN. These data suggest that renal-derived cytokines, via rRTMs, may influence endothelial function.

Abstract 05: Renal Resident Tissue Macrophages Contribute to Salt-Dependent Increases in IL-6

Salt-Sensitive hypertension is associated with an increase in pro-inflammatory cytokines and immune cells. We previously showed an acute increase in systolic blood pressure following IL-6 + high salt (HS) after 3 days, coupled with an immediate (1 day) reduction in sodium excretion. Renal resident tissue macrophages (rRTMs) form an intricate network within the kidney and function to both maintain homeostasis and secrete pro-inflammatory cytokines. We hypothesize that rRTMs are primary contributors to the immediate increases in IL-6 following high salt. To test this hypothesis, male wild-type mice lacking a functional CX 3 CR 1 chemokine receptor (CX 3 CR 1 - EGFP+/+ ) or wild-type (Wt, C57Bl6) were used. CX 3 CR 1 is required for RTM localization to the kidney; thus, these mice are rRTM-depleted. Mice were fed HS (1% saline or 4% HS chow) or normal chow (NC) for 6 days and kidneys harvested. Kidneys were digested and FACS analysis performed on total immune cell populations or used for RT-PCR (n=4-5 Wt & rRTM-depleted). Cells were gated on: Live + CD45 + + MHCII + Lyc6C - + B220 - + CD64 + + F4/F80 hi + CD11b lo. Or Lyc6C hi . Lyc6C hi and rRTMs were pre-stimulated (LPS, 1mg/mL, 3hrs) before staining with intracellular IL-6 antibody. FACS data were analyzed as a percent increase (mean fluorescence intensity, MFI) of IL-6 over control. RT-PCR data were analyzed as DCt/housekeeping. Our data show that rRTMs are the most abundant immune cell in the kidney (23.4% of total CD45 + ) and predominantly reside in the cortex and outer medulla (>99%). HS feeding (4%) significantly increased whole kidney IL-6 mRNA (12.0 ± 0.5 vs . 15.3 ± 0.6 DCt Wt NC, **p<0.01). This increase was completely prevented in rRTM-depleted mice. While intracellular IL-6 levels were generally increased in LPS-stimulated Lyc6C hi inflammatory monocytes, the rRTMs showed a preferential increase in intracellular IL-6 with both hyperosmolar saline (1%, >13% increase) and HS chow (4%, >50%). Here, we show that rRTM-depletion prevents the HS-induced increase in kidney IL-6. We additionally show that rRTMs are preferentially increasing intracellular levels of IL-6 following an acute HS load of hyperosmolar saline (1%) or HS chow (4%). We previously demonstrated that IL-6 contributes to increased blood pressure and sodium retention, and these data suggest that rRTMs preferentially contribute to the increased IL-6.

Azilsartan attenuates cardiac damage caused by high salt intake through the downregulation of the cardiac (pro)renin receptor and its downstream signals in spontaneously hypertensive rats.

We examined whether the stimulation of the angiotensin II AT1 receptor increases the expression of the cardiac (pro)renin receptor ((P)RR) and its downstream signals and whether the blockade of the angiotensin II AT1 receptor by azilsartan decreases the expression of the cardiac (P)RR and its signaling in spontaneously hypertensive rats (SHRs) with a high-salt intake. Rats received normal-salt (0.9%) chow, high-salt (8.9%) chow, normal-salt chow with 1 mg/day of azilsartan, and high-salt chow with 1 mg/day of azilsartan from 6 to 12 weeks of age. Rats with normal-salt chow were administered 100 ng/kg/min of angiotensin II by osmotic minipump from 6 to 12 weeks of age. A high-salt diet and angiotensin II significantly increased the systolic blood pressure; overexpressed cardiac (P)RR, phosphorylated (p)-ERK1/2, p-p38MAPK, p-HSP27, and TGF-ß1; enhanced cardiac interstitial and perivascular fibrosis, cardiomyocyte size, interventricular septum (IVS) thickness, and left ventricular (LV) end-diastolic dimension; and decreased LV fractional shortening. Azilsartan decreased systolic blood pressure, cardiac expressions of (P)RR, p-ERK1/2, p-p38MAPK, p-HSP27, and TGF-ß1, cardiac interstitial and perivascular fibrosis, cardiomyocyte size, and LV diastolic dimension, and improved LV fractional shortening. In conclusion, azilsartan attenuates cardiac damage caused by high salt intake through the downregulation of the cardiac (pro)renin receptor and its downstream signals in SHRs.

Maternal high-salt diet alters redox state and mitochondrial function in newborn rat offspring's brain.

Excessive salt intake is a common feature of Western dietary patterns, and has been associated with important metabolic changes including cerebral redox state imbalance. Considering that little is known about the effect on progeny of excessive salt intake during pregnancy, the present study investigated the effect of a high-salt diet during pregnancy and lactation on mitochondrial parameters and the redox state of the brains of resulting offspring. Adult female Wistar rats were divided into two dietary groups (n 20 rats/group): control standard chow (0·675 % NaCl) or high-salt chow (7·2 % NaCl), received throughout pregnancy and for 7 d after delivery. On postnatal day 7, the pups were euthanised and their cerebellum, hypothalamus, hippocampus, prefrontal and parietal cortices were dissected. Maternal high-salt diet reduced cerebellar mitochondrial mass and membrane potential, promoted an increase in reactive oxygen species allied to superoxide dismutase activation and decreased offspring cerebellar nitric oxide levels. A significant increase in hypothalamic nitric oxide levels and mitochondrial superoxide in the hippocampus and prefrontal cortex was observed in the maternal high-salt group. Antioxidant enzymes were differentially modulated by oxidant increases in each brain area studied. Taken together, our results suggest that a maternal high-salt diet during pregnancy and lactation programmes the brain metabolism of offspring, favouring impaired mitochondrial function and promoting an oxidative environment; this highlights the adverse effect of high-salt intake in the health state of the offspring.

Abstract P228: Chronically Enhanced Fructose Intake Induces Salt Sensitive Hypertension in Mice and Increases NKCC2 Phosphorylation

High fructose consumption is implicated in hypertension and renal damage in humans. The mechanisms for the renal effects of fructose are not clear. In normal rats, a high fructose diet (20%) does not increase blood pressure, but rapidly (one week) induces a salt sensitive increase in blood pressure. NaCl reabsorption by the thick ascending limb (TAL) is mediated by the NKCC2 cotransporter. Higher NKCC2 activity is involved in salt sensitive hypertension, and its phosphorylation is increased by fructose in rats. We hypothesized that high fructose (20% drinking water), would induce NKCC2 phosphorylation and induce salt sensitive hypertension in normal C57/Bl6J mice. To test this, mice were given 20% fructose in drinking water and after two weeks they were treated for 10 more weeks with either fructose alone or fructose plus 4% Na diet. Tail cuff systolic blood pressure (SBP) was measured after 3 weeks of training. Total NKCC2 expression, GAPDH and NKCC2 phosphorylation at Thre96/101 (P-NKCC2) were measured by Western blot in freshly isolated TALs. Baseline SBP was 112 ± 3 mmHg. A transient increase to 125± 4 mmHg ( p <0.05) was observed after 1 week on fructose which later decreased to 119 ± 3 mmHg ( p =0.14) by week 2. At the end of week 2, high salt chow was added. BP remained normal for 3 weeks and then increased to 137 ± 11 mmHg ( p <0.05) by week 4 on high salt, but not in fructose alone mice. After 10 weeks SBP remained elevated in the fructose plus high salt diet group (124 ± 3 vs 110 ± 5 mmHg; p <0.05). Water intake and urine volume was similar between the two high fructose groups. After 10 weeks, fasting glucose was higher in all mice with fructose (fructose + HS: 231± 8, fructose: 218 ± 7, normal diet: 122 ± 4 mg/dL, p <0.001). A fructose diet increased P-NKCC2 by 248 ± 82% (n=5, p <0.01) by one week and by 61 ± 21% ( p <0.05) at week 12. The combination of high-fructose high salt diet increased P-NKCC2 by 226 ± 75% ( p <0.05) at 12 weeks. Transcriptome profiling of isolated mouse TALs showed that 20% fructose (7 days) increased the expression of genes upstream of the SPAK and AMPK pathway. We conclude that 20% fructose in drinking water leads to a salt dependent increase in SBP, which appears to occur after mice develop abnormalities in glucose metabolism.

Reduced rat plasma lysophosphatidylglycerol or lysophosphatidic acid level as a biomarker of aristolochic acid-induced renal and adipose dysfunctions

AimsFood products and diet pills containing aristolochic acid (AA) are responsible for a rapid progression of nephropathy associated with reduced body weight in human beings. In this study, we investigated the relationship of dietary NaCl and lysophospholipid (LPL) plasma levels to body weight gain in AA-treated rats. Main methodsMale rats receiving a salt-deficient chow, normal salt chow or high salt chow were injected intraperitoneally daily with AA for 15days. Body weight, visceral fat mass, food intake, levels of LPL in plasma and its synthesized enzyme were investigated. Key findingsBody weight gain, visceral fat mass and daily food intake were smaller in AA-treated rats than those of control rats, regardless of dietary salt concentration. AA treatment decreased plasma levels of major lysophosphatidic acid (LPA) molecular species in rats fed the normal or high-salt chow but not the salt-deficient chow, whereas both the plasma lysophospholipase D activity and kidney mRNA level of autotaxin of AA-treated rats fed chow with defined salt concentrations were lower than those of control rats. Plasma levels of major molecular species of lysophosphatidylglycerol (LPG) in AA-treated rat groups fed chow with defined salt concentrations were lower than those of control rats. SignificancePlasma levels of LPG and LPA seem to be relevant to the reduced body weight gain and fat mass due to AA treatment.

Enhanced Dietary Fructose Rapidly Induces Salt‐Sensitive Hypertension in Rats

Fructose consumption is associated with increased sodium reabsorption, increased formation of reactive oxygen species, and the development of hypertension. In normal rats, 20% dietary fructose supplementation induces hypertension within 8–12 weeks. We hypothesized a diet combining 20% fructose supplemented with high salt (4% NaCl) would induce abnormal sodium retention, increased oxidative stress, and accelerate the development of elevated blood pressure. Rats weighing 200–225 g were pre‐trained for 2 weeks for non‐invasive tail cuff blood pressure measurements and then placed in metabolic caging. Rats were pair‐fed for 2 weeks with one of 4 different diets: 1) control group (0.4% NaCl), 2) high salt chow group (4% NaCl), 3) 20% fructose group (in the drinking water), and 4) 20% fructose plus high salt group (n's = 9,5,9,18). Blood pressure, urinary sodium excretion and cumulative sodium balance were measured over 2 weeks and urinary 8‐Isoprostane excretion, as a marker for reactive oxygen species, during the final two days. High salt chow replaced normal chow at the beginning of the second week. By the end of the protocol, systolic blood pressure was unchanged in control group (121±2 to 122±2 mmHg) as well as the high salt group (122±2 to 122±1 mmHg). 20% fructose alone had no effect on blood pressure over the 2 weeks (121±1 to 125±1 mmHg). However, blood pressure significantly increased in the 20% fructose plus high salt group (125 to 140 mmHg p<0.001). The increased blood pressure was concomitant with increased positive cumulative sodium balance. During the final week mean urinary sodium excretion in the high salt and 20% fructose plus high salt groups were 5–6 fold higher than normal salt controls (p<0.001): 1.13±0.07, 7.67±0.31, 1.20±0.10, 5.33±0.21 μmol/24 hrs, respectively. Despite similar consumption, urinary sodium excretion in the 20% fructose plus high salt group was significantly lower than the high salt group (p<0.0001). Urinary 8‐Isoprostane excretion was significantly higher in the high salt, 20% fructose, and 20% fructose plus high salt groups compared to control: 16.4±1.3, 29.5±2.5, 32.2±3.9, 31.2±2.5 ng/24 hrs, (p<0.001). Our results suggests enhanced fructose and salt consumption increases sodium retention and oxidative stress. High salt can induce elevations in blood pressure in 20% fructose‐fed rats within just one week. Increased blood pressure appears to associate with increased sodium retention and positive cumulative sodium balance. Overall, fructose combined with high salt in the diet contributes to the accelerated development of salt‐sensitive hypertension.Support or Funding InformationNational Institutes of Health Grant 5P01HL090550‐05

Abstract 343: EGF Deficiency in the Renal Cortex Contributes to Salt-sensitive Hypertension via Upregulation of ENaC Activity

Dahl salt-sensitive (SS) rats fed a high salt diet exhibit increased blood pressure and renal damage. Epithelial sodium channels (ENaC) in the aldosterone-sensitive distal nephron (ASDN) are responsible for the fine tuning of Na + absorption in the kidney and are regulated by various stimuli including hormones and growth factors. Members of the epidermal growth factor (EGF) family are important for maintaining transepithelial Na + transport. A combination of electrophysiological, immunohistochemical, biochemical in vitro and chronic in vivo studies were used to provide mechanistic insights into ENaC regulation by EGF and how changes in this pathway may contribute to salt-induced hypertension in SS rats. It was found that the ENaC inhibitor benzamil, administered through the drinking water (15 mg/L), significantly reduced the MAP from 173.3±15.0 to 127.0±10.1 mm Hg. ENaC expression (as measured by Western blotting and immunohistochemistry) and activity (single channel analysis in freshly isolated split open ASDN) were significantly enhanced in SS rats fed a high salt diet compared to SS rats fed a low salt diet and consomic SS.13 BN rats fed a high salt chow. Servo-controlled measurements in SS rats also revealed increased ENaC expression in the uncontrolled kidneys compared to controlled left kidneys. As assessed by ELISA assay, EGF level in the kidney cortex of SS rats fed a high salt diet was lower compared to rats on a low salt diet. To directly evaluate the role of EGF in the development of hypertension and its effect on ENaC activity, EGF (10 μg/h/kg) was intravenously infused and blood pressure was continuously monitored. Chronic infusion of EGF decreased ENaC activity ( NP o ; from 1.5±0.2 to 0.4±0.1), prevented the development of hypertension (from 184.0±8.9 to 149.0±9.6 mm Hg) and attenuated renal glomerular and tubular damage. Thus, in controversy to existing dogma, we provide an evidence of a critical role of ENaC in the ASDN in the development of salt-sensitive hypertension. Furthermore, these findings indicate that EGF deficiency in the renal cortex contributes to salt-sensitive hypertension and that in vivo EGF infusion prevents development of hypertension and kidney damage via downregulation of ENaC.

Aorta from angiotensin II hypertensive mice exhibit preserved nitroxyl anion mediated relaxation responses

Hypertension is a disorder affecting millions worldwide, and is a leading cause of death and debilitation in the United States. It is widely accepted that during hypertension and other cardiovascular diseases the vasculature exhibits endothelial dysfunction; a deficit in the relaxatory ability of the vessel, attributed to a lack of nitric oxide (NO) bioavailability. Recently, the one electron redox variant of NO, nitroxyl anion (NO −) has emerged as an endothelium-derived relaxing factor (EDRF) and a candidate for endothelium-derived hyperpolarizing factor (EDRF). NO − is thought to exist protonated (HNO) in vivo, which would make this species more resistant to scavenging. However, no studies have investigated the role of this redox species during hypertension, and whether the vasculature loses the ability to relax to HNO. Thus, we hypothesize that aorta from angiotensin II (AngII)-hypertensive mice will exhibit a preserved relaxation response to Angeli's Salt, an HNO donor. Male C57Bl6 mice, aged 12–14 weeks were implanted with mini-osmotic pumps containing AngII (90 ng/min, 14 days plus high salt chow) or sham surgery. Aorta were excised, cleaned and used to perform functional studies in a myograph. We found that aorta from AngII-hypertensive mice exhibited a significant endothelial dysfunction as demonstrated by a decrease in acetylcholine (ACh)-mediated relaxation. However, vessels from hypertensive mice exhibited a preserved response to Angeli's Salt (AS), the HNO donor. To confirm that relaxation responses to HNO were maintained, concentration response curves (CRCs) to ACh were performed in the presence of scavengers to both NO and HNO (carboxy-PTIO and L-cys, resp .). We found that ACh-mediated relaxation responses were significantly decreased in aorta from sham and almost completely abolished in aorta from AngII-treated mice. Vessels incubated with l-cys exhibited a modest decrease in ACh-mediated relaxations responses. These data demonstrate that aorta from AngII-treated hypertensive mice exhibit a preserved relaxation response to AS, an HNO donor, regardless of a significant endothelial dysfunction.

A critical role of sympathetic nerve regulation for the treatment of impaired daily rhythm in hypertensive Dahl rats

There is a deep relationship between impaired circadian rhythm and hypertension. However, the detailed mechanisms between the daily sleep-wake rhythm and cardiovascular disorders have not yet been elucidated. To clarify the mechanism, we examined salt-sensitive Dahl rats that were fed normal chow (n=10), high-salt chow (n=10) and high-salt chow with bisoprolol (n=10). Simultaneous electroencephalogram, electromyogram and locomotor activity were examined to analyze the sleep-wake state. We also examined heart rate, blood pressure and echocardiographic findings to verify the presence of hypertension. Hypertension with impaired ventricular contraction was observed in the rats with high-salt-chow consumption whereas normal-chow rats did not show these disorders. Although rats with the normal diet showed a standard daily rhythm with normal rapid eye movement (REM) sleep duration and locomotor activity, the high-salt-diet group exhibited an impaired daily rhythm with suppressed REM sleep and significant abnormal locomotor activity. Bisoprolol significantly improved the daily sleep-wake rhythm and locomotor activity. We showed that an impaired daily rhythm was closely related to the development of hypertension. Regulation of sympathetic nerve alterations may have a key role in the treatment of hypertension and circadian rhythm disorder.

Collectrin Is Involved in the Development of Salt-Sensitive Hypertension by Facilitating the Membrane Trafficking of Apical Membrane Proteins via Interaction With Soluble N -Ethylmaleiamide-Sensitive Factor Attachment Protein Receptor Complex

Collectrin, a homologue of angiotensin converting enzyme 2, is expressed in pancreatic beta cells and renal proximal tubular and collecting duct cells under the control of hepatocyte nuclear factors-1alpha and -1beta. Because collectrin interacts with the soluble N-ethylmaleiamide-sensitive factor attachment protein receptor (SNARE) complexes, we investigated whether collectrin is involved in sodium handling in hypertension by vesicle trafficking of apical membrane proteins. Collectrin physically interacts with the SNARE complex: snapin, synaptosomal-associated protein 23 kDa, syntaxin-4, and vesicle-associated membrane protein-2 in mIMCD-3 cells. siRNA knockdown of collectrin resulted in a reduction in membrane-associated aquaporin-2, alpha-epithelial Na+ channel, and H+-ATPase. Collectrin and SNARE proteins were abundantly expressed in collecting ducts of Wistar-Kyoto rats. Wistar-Kyoto rats and spontaneously hypertensive rats 7 weeks of age were subjected to normal-salt (1% NaCl) and high-salt (8% NaCl) chow for 10 weeks. High-salt chow prominently elevated blood pressure, oral intake, and urinary excretion of NaCl and water in both groups. Although urinary excretion of aldosterone was significantly suppressed in both groups, collectrin expression was upregulated and associated with the maintenance of aquaporin-2, alpha-epithelial Na+ channel, and H+-ATPase in membrane fractions. Collectrin promoter activities and mRNA and protein expressions were upregulated and ubiquitinated collectrin was reduced by high NaCl (175 to 225 mmol/L) and not altered by 1 micromol/L aldosterone in mIMCD-3 cells. Upregulation of collectrin by high NaCl independent of aldosterone functionally links to the trafficking of apical membrane proteins via the SNARE complex, and collectrin may be responsible for the sodium retention in salt-sensitive hypertension.

Hepatorenal reflex in the rat

The possible existence of a hepatorenal reflex was evaluated in male Sprague-Dawley rats. Sodium excretion was measured in two groups of six rats each, during the first 4 h following acute ingestion of a known amount of high salt chow (2.0-2.5 mequiv. NaCl). Hourly excretion rates for sodium before surgery were compared with results following 7 days of recovery from either hepatic denervation (n = 6) or sham denervation (n = 6). Before denervation, hourly sodium excretion between the groups was not different. Following surgery for hepatic denervation, sodium excretion was 91% lower than presurgery values for the 1st h (p < 0.02) and 44% lower in the 2nd h (p < 0.04). Sham denervation caused no significant change in sodium excretion when compared with presurgery results. A test for completeness of denervation showed that norepinephrine concentration in liver tissue taken from denervated rats was 5.1 +/- 8 ng/g and that taken from sham rats was 22.8 +/- 1 ng/g (p < 0.001). These data demonstrate that the liver is essential for the normal postprandial excretion of sodium following ingestion of a high salt meal in rats.

Glucose Metabolism and Insulin Receptor Binding and mRNA Levels in Tissues of Dahl Hypertensive Rats

Increased insulinemic response to an oral glucose load has been demonstrated in Dahl salt-sensitive hypertensive rats. To determine whether this abnormality is mediated at the level of the insulin receptor, we compared insulin receptor binding and mRNA levels in tissues of Dahl salt-sensitive rats (DS) and in their normotensive controls, Dahl salt-resistant rats (DR). To evaluate possible influences of dietary sodium intake, rats were fed either low (0.07% NaCl) or high salt (7.5% NaCl) chow until the DS became hypertensive, and then were killed by decapitation. Fasting plasma glucose and plasma insulin levels did not differ between DR and DS rats and were not affected by salt intake. In response to an oral glucose load, plasma glucose had a similar increase in DR and DS rats, but the increase in plasma insulin was significantly greater in DS rats. Scatchard analysis of binding was obtained from in situ autoradiographic studies performed in frozen skeletal muscle and kidney sections, and insulin receptor mRNA levels were measured by slot-blot hybridization. Number and affinity of insulin receptors were comparable in skeletal muscle and kidney of DR and DS rats and, in both groups, binding parameters were not affected by dietary sodium chloride. Hepatic and renal insulin receptor mRNA levels were also comparable in DR and DS rats fed either low or high salt chow. Thus, increased plasma insulin response to oral glucose load is associated with normal insulin receptor binding and gene expression in peripheral tissues in rats with Dahl hypertension. A postreceptor defect is likely responsible for the decreased sensitivity to insulin in this model of genetic hypertension.

Abnormalities of Insulin Receptors in Spontaneously Hypertensive Rats

Insulin resistance is present in some strains of rats with genetic hypertension. To determine whether this abnormality is present at the level of the insulin receptor, we compared insulin sensitivity, insulin receptor binding, and mRNA levels in tissues of 10-week-old spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto (WKY) controls. Because we have previously demonstrated an inverse relationship between dietary sodium intake and renal insulin receptor density and mRNA levels in normal Sprague-Dawley rats, the two rat strains in the current experiment were fed either low salt (0.07% NaCl) or high salt (7.5% NaCl) chow until the SHR became hypertensive. Fasting plasma glucose and plasma insulin levels did not differ between SHR and WKY and were not affected by salt intake. When the rats were maintained on the low salt diet, the rate of glucose infusion required to main euglycemia during a hyperinsulinemic clamp was significantly lower in SHR than WKY. High salt diet decreased the rate of glucose utilization during the hyperinsulinemic clamp in WKY but not SHR. During the low salt diet, insulin infusion decreased sodium excretion in both WKY and SHR. When the rats were maintained on the high salt diet, the antinatriuretic response to insulin was blunted in WKY but not SHR. Both the density and mRNA levels of insulin receptor were comparable in the kidney of WKY and SHR, but only WKY had the previously demonstrated decrease in receptor number and mRNA levels when fed the high salt chow. Hepatic insulin receptor mRNA levels were significantly lower in SHR than WKY fed the low salt diet. High salt diet decreased significantly insulin receptor mRNA levels in the liver of WKY but not of SHR. Thus, SHR appear to have lost the feedback mechanism that normally limits insulin-induced sodium retention when extracellular volume is expanded. A decreased expression of insulin receptor in the liver of SHR provides a possible explanation for the insulin resistance and decreased insulin clearance present in this strain.

Lovastatin but not enalapril reduces glomerular injury in Dahl salt-sensitive rats.

Dahl salt-sensitive (S) rats fed a high salt diet develop hypertension, hyperlipidemia, and progressive renal disease. Previous studies have suggested that lipids may be important in the pathogenesis of glomerulosclerosis in Dahl S rats. To investigate this possibility, Dahl S rats fed 4% NaCl chow were treated chronically with the cholesterol synthesis inhibitor lovastatin. After 22 weeks, lovastatin-treated rats had a 38% reduction in serum cholesterol, a 76% reduction in urine albumin excretion, and one-sixth the incidence of focal glomerulosclerosis compared with vehicle-treated control rats. Blood pressure in lovastatin-treated rats was significantly (p < 0.05) lower than that in vehicle-treated rats both early in the study (4 weeks of treatment) and at the end of the protocol. Lovastatin had no effect on glomerular filtration rate or glomerular ultrafiltration dynamics. The efficacy of angiotensin converting enzyme inhibitors in attenuating proteinuria and experimental glomerular disease may be dependent on sodium intake. Thus, we also investigated the effects of long-term enalapril treatment on glomerular injury in Dahl S rats fed high salt chow. Enalapril treatment (50 or 200 mg/l drinking water) significantly lowered blood pressure in Dahl S rats, but did not significantly affect albuminuria or glomerulosclerosis. Enalapril also had no effect on glomerular hemodynamics. These results suggest that lipids may be important in the development of both glomerular disease and hypertension in Dahl S rats and that angiotensin converting enzyme inhibition may not affect the course of renal disease in a setting of high salt intake.

L-arginine abrogates salt-sensitive hypertension in Dahl/Rapp rats.

This study examined the contribution of nitric oxide (NO) to the susceptibility or resistance to the hypertensive effects of high sodium chloride (8.0% NaCl) intake in young Dahl/Rapp salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) rats. Using NG-monomethyl-L-arginine (L-NMMA) as a probe for NO production in vivo, we found that increasing dietary sodium chloride increased NO activity in salt-resistant rats, but not in salt-sensitive rats. Exogenous L-arginine, the substrate for NO synthesis, decreased blood pressure to normotensive levels in salt-sensitive rats made hypertensive for 2 wk from 8.0% NaCl chow. D-arginine had no effect on blood pressure of these rats and L-arginine did not change blood pressure of salt-resistant rats. Intraperitoneal injections of L-arginine and its precursor, L-citrulline, and oral L-arginine, but not D-arginine, prevented the increase in blood pressure in salt-sensitive rats on the high salt chow over 2 wk of observation. In contrast, L-arginine did not alter the development of hypertension in spontaneously hypertensive rats. Mean urinary cGMP levels were higher in salt-sensitive rats on oral L-arginine than salt-sensitive rats on D-arginine. Infusion of L-NMMA acutely decreased, whereas intravenous L-arginine rapidly increased, urinary cGMP in both groups. L-arginine and L-citrulline increased production of NO and prevented salt-sensitive hypertension in Dahl/Rapp rats.

Determinants of renal vascular resistance in the Dahl strain of genetically hypertensive rat.

Hypertension-resistant (R) rats of the Dahl strain remain normotensive on diets containing excessive sodium chloride, while hypertension-sensitive (S) rats rapidly become hypertensive when eating the same diet. In the present study, renal hemodynamics were investigated in Dahl rats eating normal or high salt chow for 4 weeks. The R rats eating sodium-enriched chow were found to have significantly lower renal vascular resistance (RVR) than R rats on normal chow; RVR was identical in S rats on either diet. The reason for the failure of S kidneys to exhibit vasodilation during high salt ingestion was explored by examining renal vascular responses to interruption of the renal sympathetic nerves, renal sympathetic nerve stimulation (SNS), and to several vasoactive agents. Changes in RVR to nerve section and to SNS were similar in both strains regardless of diet. Renal vascular responses to intraarterial norepinephrine were suppressed significantly by high salt intake only in the R rats. Changes in RVR to intraarterial angiotensin II were uniformly greater in S than in R rats, but were not altered by salt intake in either strain. Renal vasodilation in response to intraarterial acetylcholine was reduced in R rats on high salt intake, but was not affected by salt intake in S rats. It is concluded that S rats exhibit inappropriately high renal vascular tone during ingestion of excessive salt, and that this alteration is not the result of increased neurogenic activity or increased vascular reactivity to angiotensin II or norepinephrine.