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(Hypertension. 2001;37:433.)
© 2001 American Heart Association, Inc.

Scientific Contributions

Endothelial Dysfunction and Salt-Sensitive Hypertension in Spontaneously Diabetic Goto-Kakizaki Rats

Zhong Jian Cheng; Timo Vaskonen; Ilkka Tikkanen; Kaisa Nurminen; Heikki Ruskoaho; Heikki Vapaatalo; Dominik Muller; Joon-Keun Park; Friedrich C. Luft; Eero M. A. Mervaala

From the Institute of Biomedicine (Z.J.C., T.V., K.N., H.V., E.M.A.M.), Department of Pharmacology and Toxicology, University of Helsinki, Helsinki, Finland; the Department of Medicine (I.T.), Helsinki University Central Hospital and Minerva Institute for Medical Research, Helsinki, Finland; the Department of Pharmacology and Toxicology (H.R., D.M., J.-K.P., F.C.L.), University of Oulu, Oulu, Finland; and the Franz Volhard Clinic, Medical Faculty of the Charité, Humboldt University of Berlin, Berlin, Germany.

Correspondence to Eero Mervaala, MD, PhD, Assistant Professor, Institute of Biomedicine, Department of Pharmacology and Toxicology, University of Helsinki, PO Box 8, FIN-00014 Helsinki, Finland. E-mail eero.mervaala{at}helsinki.fi

	Abstract

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Endothelial dysfunction is associated with hypertension, hypercholesterolemia, and heart failure. We tested the hypothesis that spontaneously diabetic Goto-Kakizaki (GK) rats, a model for type 2 diabetes, exhibit endothelial dysfunction. Rats also received a high-sodium diet (6% NaCl [wt/wt]) and chronic angiotensin type 1 (AT₁) receptor blockade (10 mg/kg PO valsartan for 8 weeks). Compared with age-matched nondiabetic Wistar control rats, GK rats had higher blood glucose levels (9.3±0.5 versus 6.9±0.2 mmol/L for control rats), 2.7-fold higher serum insulin levels, and impaired glucose tolerance (all P<0.05). Telemetry-measured mean blood pressure was 15 mm Hg higher in GK rats (P<0.01) compared with control rats, whereas heart rates were not different. Heart weight– and kidney weight–to–body weight ratios were higher in GK rats (P<0.05), and 24-hour albuminuria was increased 50%. Endothelium-mediated relaxation of noradrenaline-precontracted mesenteric arterial rings by acetylcholine was impaired compared with the control condition (P<0.05), whereas the sodium nitroprusside–induced relaxation was similar. Preincubation of the arterial rings with the NO synthase inhibitor N^G-nitro-L-arginine methyl ester and the cyclooxygenase inhibitor diclofenac inhibited relaxations to acetylcholine almost completely in GK rats but not in Wistar rats, suggesting that endothelial dysfunction can be in part attributed to reduced relaxation via arterial K⁺ channels. Perivascular monocyte/macrophage infiltration and intercellular adhesion molecule-1 overexpression were observed in GK rat kidneys. A high-sodium diet increased blood pressure by 24 mm Hg and 24-hour albuminuria by 350%, induced cardiac hypertrophy, impaired endothelium-dependent relaxation further, and aggravated inflammation (all P<0.05). The serum level of 8-isoprostaglandin F₂, a vasoconstrictor and antinatriuretic arachidonic acid metabolite produced by oxidative stress, was increased 400% in GK rats on a high-sodium diet. Valsartan decreased blood pressure in rats fed a low-sodium diet and prevented the inflammatory response. In rats fed a high-sodium diet, valsartan did not decrease blood pressure or improve endothelial dysfunction but protected against albuminuria, inflammation, and oxidative stress. As measured by quantitative autoradiography, AT₁ receptor expression in the medulla was decreased in GK compared with Wistar rats, whereas cortical AT₁ receptor expression, medullary and cortical angiotensin type 2 (AT₂) receptor expressions, and adrenal ACE and neutral endopeptidase expressions were unchanged. A high-sodium diet did not influence renal AT₁, AT₂, ACE, or neutral endopeptidase expressions. In valsartan-treated GK rats, the cortical and medullary AT₁ receptor expressions were decreased in the presence and absence of a high-sodium diet. A high-sodium diet increased plasma brain natriuretic peptide concentrations in presence and absence of valsartan treatment. We conclude that hypertension in GK rats is salt sensitive and associated with endothelial dysfunction and perivascular inflammation. AT₁ receptor blockade ameliorates inflammation during a low-sodium diet and partially protects against salt-induced vascular damage by blood pressure–independent mechanisms.

Key Words: acetylcholine • nitroprusside • receptors, angiotensin • angiotensin-converting enzyme • peptides

	Introduction

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Non–insulin-dependent diabetes mellitus (NIDDM) in humans is a major risk factor for end-stage renal disease¹ and is commonly accompanied by hypertension, dyslipidemia, and prothrombotic factors.² In fact, frank hypertension is frequently found in subjects who have insulin resistance but not NIDDM, and 80% of patients with NIDDM are hypertensive at the time of the diagnosis.² At present, the mechanisms involved in the pathogenesis of hypertension and vascular complications in NIDDM are poorly understood.

Several lines of evidence suggest a crucial role of oxidative stress and angiotensin II (Ang II) in the pathogenesis of hypertension and endothelial dysfunction.^{3 4} Endothelial dysfunction in hypertension may be due to impaired NO synthesis and/or inactivation of endothelium-derived NO by reactive oxygen species, such as superoxide, hydrogen peroxide, and peroxynitrite.³ The balance between NO and superoxide might be even more important than the absolute levels of either alone.³ We have recently shown in double transgenic rats harboring human renin and human angiotensinogen genes that Ang II induces profound inflammation and activates redox-sensitive genes via nuclear factor-B activation, independent of blood pressure.^{5 6 7}

Endothelial dysfunction and oxidative stress have also been linked to the pathogenesis of diabetes.^{8 9} Impaired vascular relaxation in response to acetylcholine (ACh) has been reported in diabetic animals and humans.^{8 9} The mechanisms of endothelial dysfunction are incompletely understood; increased vascular production of superoxide⁸ and inactivation of endothelium-derived relaxing factors by glycosylated hemoglobin^{8 10} have been suggested. Interestingly, high glucose concentration and hyperglycemia promote leukocyte adhesion to the endothelium through upregulation of adhesion molecule expression in a nuclear factor-B–dependent fashion.¹¹ Whether increased adhesion of leukocytes to the endothelium causes endothelial dysfunction in NIDDM is not known. The pathogenesis of NIDDM in Goto-Kakizaki (GK) rats, a nonobese type II diabetic rat model originally derived by repeated inbreeding of glucose-intolerant Wistar rats,¹² includes impaired ontogenetic development of islet cells, abnormal insulin release after a glucose load, insulin resistance, a basal hyperinsulinemia, and abnormal glucose metabolism.^{13 14} Recent studies have demonstrated that GK rats showed several structural changes in the kidney similar to those observed in NIDDM patients without overt kidney disease. These changes consisted of thickening of the glomerular basement membrane and tubular basement membrane, glomerular hypertrophy, and early podocyte damage.^{13 14 15} However, the kidneys of GK rats do not typically show glomerulosclerosis or interstitial fibrosis.^{13 14 15} We used GK rats to test whether endothelial dysfunction and inflammation participate in the pathogenesis of hypertension and diabetic nephropathy in GK rats. We also evaluated the cardiovascular and renal effects of a high-sodium diet and chronic angiotensin type 1 (AT₁) receptor blockade in GK rats because of the potential importance of this system.¹⁶

	Methods

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We used forty 8-week-old male spontaneously diabetic GK rats (M&B, Ejby, Denmark) and 10 age-matched nondiabetic Wistar rats. The protocols were approved by the Animal Experimentation Committee of the Institute of Biomedicine, University of Helsinki, Helsinki, Finland, whose standards correspond to those of the American Physiological Society. In the beginning of the study, blood pressure– and body weight–matched GK rats were divided into 4 groups (n=10 in each group) that received different diet and drug regimens for 8 weeks: group 1, GK control rats on a low-sodium diet (NaCl 0.8% [wt/wt]); group 2, GK rats on a high-sodium diet (NaCl 6% [wt/wt]); group 3, GK rats treated with valsartan (10 mg/kg mixed in the food); and group 4, GK rats on a high-sodium diet plus valsartan treatment. Wistar rats served as controls. The rats had free access to chow and drinking water. Systolic blood pressure was measured by using a tail-cuff blood pressure analyzer (Apollo-2AB Blood Pressure Analyzer, model 179-2AB, IITC Life Science)¹⁷ or by radiotelemetry (Dataquest IV telemetry system, Data Sciences International).¹⁸ At 16 weeks, an oral glucose tolerance test was performed in some overnight-fasted GK and Wistar rats (1 g/kg glucose by gavage).¹⁹ At the end of the experimental period, urine samples were collected over 24 hours, and the animals were then decapitated, and blood samples were taken for blood glucose, plasma atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), serum insulin, and serum 8-isoprostaglandin F₂ measurements. The heart and kidneys were excised, washed with ice-cold saline, blotted dry, and weighed. Tissue samples for immunohistochemistry and autoradiography were snap-frozen in isopentane (-35°C). All samples were stored at -80°C until they were assayed. For measurements of vascular responses, superior mesenteric arteries were carefully excised and cleaned of the adherent connective tissue. Two successive sections (3 mm), 5 mm distal from the mesenteric artery–aorta junction, were used. For morphological analysis, tissue samples were fixed in 4% buffered paraformaldehyde at room temperature, dehydrated in graded alcohol, and embedded in paraffin.²⁰ Sections (2 to 3 µm) were cut with the use of a microtome (Leitz 1512). The sections were deparaffinized and rehydrated before being stained with hematoxylin-eosin and Masson’s trichrome. The tissues were examined without knowledge of the rat group from which they were taken.

Frozen kidneys were processed, and semiquantitative scoring was performed as described in detail previously.^{5 20} Briefly, ice-cold acetone-fixed cryosections (6 µm) were air-dried and immersed in TBS (0.05 mol/L Tris buffer and 0.15 mol/L NaCl, pH 7.6). All incubations were performed in a humid chamber at room temperature. At first, the sections were incubated in 10% normal donkey serum (Dianova) for 30 minutes to block any nonspecific binding. The sections were incubated for 60 minutes with the following monoclonal antibodies: anti–ED-1 (Serotec) and intercellular adhesion molecule-1 (ICAM-1, 1A29, R&D Systems). After they were washed with TBS, the sections were incubated with Cy3-conjugated secondary antibodies (donkey anti-mouse IgG-Cy3, Dianova) for 60 minutes. After a final washing with TBS, slides were mounted in Vectashield mounting medium (Vector Laboratories). In control sections, in which a primary antibody was substituted by isotype control antibody CBL 600 mouse IgG1-ve control (Cymbus Biotechnology) at the same final concentration, no specific immunolabeling was observed. The nonspecific binding of secondary antibodies was excluded by omitting the primary antibody. Preparations were analyzed under a Zeiss Axioplan-2 microscope (Carl Zeiss) and digitally photographed with use of the AxioVision 2 multichannel image processing system (Carl Zeiss Vison).

The relaxation curves to cumulative ACh and sodium nitroprusside (SNP) were determined in mesenteric artery rings precontracted with 1 µmol/L norepinephrine (FTO3C transducer, model 7 C8 polygraph, Grass Instrument Co) as described earlier.²¹ To analyze the components of endothelium-dependent vascular relaxation, cyclooxygenase and/or NO synthase (NOS) were inhibited in some experiments with diclofenac (3 µmol/L) and N^G-nitro-L-arginine methyl ester (L-NAME, 0.1 mmol/L), respectively. For autoradiographic studies, frozen kidney sections (20 µm thick) were cut on a cryostat at -17°C, thaw-mounted onto Super Frost Plus slides, dried in a desiccator under reduced pressure at 4°C overnight, and stored at -80°C with silica gel until further processing for autoradiographic studies. ACE, angiotensin receptors types 1 and 2 (AT₁ and AT₂ receptors, respectively), and neutral endopeptidase (NEP) were quantified by in vitro autoradiography as described earlier.^{22 23 24 25 26 27}

Urinary albumin was measured by ELISA with rat albumin used as a standard (Immun Diagnostik), and urinary nitrate+nitrite (NO_X) concentration was assessed with a commercially available NO_X colorimetric assay kit (Cayman). Serum 8-isoprostaglandin F₂ concentration was determined by ELISA (Cayman), and serum insulin was determined by radioimmunoassay (Linco Research) according the instructions of the manufacturer. Blood glucose was measured with an automatic analyzer (Reflotron, Boehringer-Mannheim). Plasma ANP and BNP levels were measured by radioimmunoassay as described in detail earlier.²⁸

Data are presented as mean±SEM. Statistically significant differences in mean values were tested by ANOVA and the Fisher protected least significant difference test. ANOVA for repeated measurements was applied for data consisting of repeated observations at successive time points. The differences were considered significant at P<0.05. The data were analyzed by use of SYSTAT statistical software (SYSTAT Inc).

	Results

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Radiotelemetry mean arterial blood pressure (MAP) was 15 mm Hg higher in 16-week-old GK rats than in control rats (Figure 1A); no difference in heart rate was observed (data not shown). MAP displayed a 24-hour rhythm characterized by several nighttime peaks, the first of which occurred immediately at the start of the dark period. GK rats showed impaired glucose tolerance in an oral glucose tolerance test (Figure 1B). Systolic blood pressure, heart weight– and kidney weight–to–body weight ratios, and 24-hour albuminuria were higher in GK rats compared with Wistar control rats (Figure 1D to 1F). The high-sodium diet for 8 weeks increased blood pressure by 24 mm Hg, induced cardiac hypertrophy, and increased albuminuria by 350% in GK rats (Figure 1C to 1F). Valsartan decreased blood pressure moderately during the low-sodium diet but did not significantly change heart and kidney weights or albuminuria (Figure 1C to 1F). During the high-sodium diet, valsartan did not decrease blood pressure or prevent the development of cardiac hypertrophy, but it significantly decreased albuminuria (Figure 1C to 1F).

View larger version (36K): [in this window] [in a new window]   Figure 1. Twenty-four–hour blood pressure profile (A), oral glucose tolerance (B-gluc) test (B), and bar graphs showing the influence of high-sodium diet and valsartan treatment on systolic blood pressure (SBP) (C), heart weight–to–body weight ratio (HW) (D), kidney weight–to–body weight ratio (KW) (E), and albuminuria (dU-Alb) (F) in GK rats (open squares) and Wistar rats (open circles). GK indicates GK rats on low-sodium diet; GK+Na, GK rats on high-sodium diet; GK+Val, GK rats on low-sodium diet+valsartan; GK+Na+Val, GK rats on high-sodium diet+valsartan; and Wis, Wistar rats on low-sodium diet. Mean±SEM values are given (n=10 in each group). *P<0.05 vs Wis; #P<0.05 vs GK; and ¤ P<0.05 vs GK+Na.

Endothelium-mediated vascular relaxation of norepinephrine-precontracted mesenteric arterial rings in response to ACh was markedly impaired in GK rats compared with Wistar rats, but the endothelium-independent relaxations to SNP were similar in both strains (Figure 2A and 2B). Preincubation of the arterial rings with the NOS inhibitor L-NAME and the cyclooxygenase inhibitor diclofenac inhibited relaxations to ACh almost completely in GK rats but not in Wistar rats (Figure 3). The high-sodium diet impaired vascular relaxation in response to ACh and also to SNP (Figure 2A and 2B). Valsartan treatment improved endothelium-dependent vascular relaxation during the low-sodium diet but had no effect on endothelium-independent vascular relaxation (Figure 2A and 2B). The improvement of endothelium-dependent vascular relaxation by valsartan was also found after preincubation of the arterial rings with L-NAME and diclofenac (Figure 3). Serum 8-isoprostaglandin F₂ levels tended (P<0.1>0.05) to be higher in GK rats than in Wistar rats (Figure 2C). The high-sodium diet increased serum 8-isoprostaglandin F₂ levels by 400% in GK rats; the increase was prevented by valsartan (Figure 2C). Twenty-four–hour urinary NO_X excretion, a marker of total body NO generation, was similar in both rat strains but was markedly increased in GK rats on the high-sodium diet (Figure 2D). NO_X excretion was normalized in valsartan-treated GK rats on the high-sodium diet (Figure 2D).

View larger version (30K): [in this window] [in a new window]   Figure 2. Relaxations to ACh (A) and to SNP (B) after precontraction with 1 µmol/L norepinephrine in isolated endothelium-intact mesenteric arterial rings. Open circles indicate Wis; open squares, GK; open triangles, GK+Val; solid squares, GK+Na; and solid triangles, GK+Na+Val. Endothelium-dependent vascular relaxation was impaired in GK (P<0.05 vs Wis), whereas endothelium-independent relaxation was unaltered. High-sodium diet impaired endothelium-dependent and -independent vascular relaxations (P<0.05 vs GK). Valsartan improved endothelium-dependent vascular relaxation during low-sodium diet (P<0.05 vs GK). Symbols indicate mean±SEM (n=6 to 10 in each group, 1 vascular ring per animal). High-sodium diet increased serum 8-isoprostaglandin F2 (s-8-isoprostaneF2) levels (C) and 24-hour urinary NOX excretion (dU-NOX) (D). Salt-induced changes were blocked by valsartan treatment. *P<0.05 vs Wis; #P<0.05 vs GK; ¤ P<0.05 vs GK+Na; and $P<0.05 vs all groups.

View larger version (16K): [in this window] [in a new window]   Figure 3. Relaxations of mesenteric arterial rings to ACh after preincubation with NOS inhibitor L-NAME and cyclooxygenase inhibitor diclofenac. Vessels were precontracted with 1 µmol/L norepinephrine. Open circles indicate Wistar control rats; open squares, GK rats on low-sodium diet; open triangles, GK rats on low-sodium diet+valsartan; solid squares, GK rats on high-sodium diet; and solid triangles, GK rats on high-sodium diet+valsartan. Endothelium-dependent vascular relaxation resistant to NOS and cyclooxygenase inhibition was impaired in GK rats (P<0.05 vs Wistar rats). High-sodium diet impaired endothelium-dependent vascular relaxation resistant to NOS and cyclooxygenase inhibition (P<0.05 vs GK rats on low-sodium diet). Valsartan improved vascular relaxation during low-sodium diet (P<0.05 vs GK rats on low-sodium diet). Symbols indicate mean±SEM (n=6 to 10 in each group, 1 vascular ring per animal).

The kidneys of GK rats on the high-sodium diet showed some mesangial thickening and slight thickening of the media of the afferent arterioles. There were no clear signs of glomerulosclerosis or interstitial fibrosis (Figure 4A). In GK rats on the low-sodium diet, there was already a significant monocyte/macrophage infiltration (ED-1–positive cells) in the renal perivascular space and increased expression of ICAM-1 in the interstitium, intima, and adventitia of the small renal vessels (Figure 4D and 4E). The high-sodium diet aggravated the inflammatory response in GK rats (Figure B to 4E). Valsartan treatment decreased the number of ED-1–positive cells and ICAM-1 expression during both low-sodium and high-sodium diets.

View larger version (63K): [in this window] [in a new window]   Figure 4. Representative photomicrograph from kidneys of GK rat on high-sodium diet (A) and immunohistochemical photomicrographs of monocyte/macrophage infiltration (ED-1–positive cells) (B) and ICAM-1 expression (C). Semiquantitative scoring of ED-1–positive cells in the kidney is shown in panel D. Fifteen different areas of each sample were analyzed by using a computerized cell count program. Semiquantitative scoring of ICAM-1 expression (from 0 to 5) in the kidney is shown in panel E. Each bar represents n=5. Mean±SEM values are given. *P<0.05 vs Wis; #P<0.05 vs GK; and ¤ P<0.05 vs GK+Na.

AT₁ receptor expression in the medulla was decreased in GK rats compared with Wistar rats, whereas cortical AT₁ receptor expression, medullary and cortical AT₂ receptor expressions, and renal ACE and NEP expressions were unchanged (Table). The high-sodium diet did not influence renal AT₁, AT₂, ACE, or NEP expressions. In valsartan-treated GK rats, the cortical and medullary AT₁ receptor expressions were decreased in the presence and absence of the high-sodium diet. There were no differences between the groups in plasma ANP concentrations (Table). Plasma BNP levels were increased by the high-sodium diet in the presence and absence of valsartan treatment (Table). Blood glucose and serum insulin levels were higher in GK rats compared with Wistar rats but were not markedly influenced by the high-sodium diet or valsartan treatment (Table).

View this table: [in this window] [in a new window]   Table 1. Renal AT1 and AT2 Receptor, ACE, and NEP Expression and ANP, BNP, Blood Glucose, and Serum Insulin Levels in Spontaneously Diabetic GK Rats and Nondiabetic Wistar Rats After 8 Weeks on Different Diet and Drug Regimens

	Discussion

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The main finding of the present study was that GK rats were already moderately hypertensive on the low-sodium diet and showed marked impairment of endothelium-mediated vascular relaxation. Furthermore, a pronounced perivascular monocyte/macrophage infiltration and adhesion molecule overexpression in the kidneys were observed. We were also able to demonstrate that hypertension in GK rats is strongly dependent on the level of dietary salt. The high-sodium diet also induced marked end-organ damage and aggravated endothelial dysfunction and the inflammatory response associated with increased oxidative stress. Finally, our findings of the beneficial cardiovascular and renal effects of valsartan indicate the involvement of increased renin-angiotensin system activity in the pathogenesis of diabetic nephropathy and vascular complications in GK rats.

Previous studies have revealed that unlike several other rodent models of NIDDM, GK rats are not hypertensive, hyperlipemic, or obese.¹³ Furthermore, the prolonged hyperglycemia and hyperinsulinemia in GK rats are not associated with any marked renal functional changes, although structural changes such as thickening of the glomerular and tubular basement membranes as well as glomerular hypertrophy are detected.^{13 14 15} In good accordance with these findings, we found only a 1.5-fold increase in 24-hour albuminuria in GK rats on the low-sodium diet. However, with radiotelemetry, we were able to detect a 15 mm Hg difference in MAP between GK and Wistar rats. Although hypertension is commonly associated with endothelial dysfunction, it is likely that factors other than hypertension are also involved in the pathogenesis of endothelial dysfunction in GK rats on a low-sodium diet. The endothelium-independent vascular relaxation in response to SNP was unchanged in GK rats, indicating that the sensitivity of arterial smooth muscle cells to NO is unaltered.

We also found a profound perivascular monocyte/macrophage infiltration and ICAM-1 overexpression in the kidneys of GK rats on a low-sodium diet. Our finding supports the previous notion that oxygen-derived free radicals generated by the inflammatory cells are able to reduce the bioavailability of endothelium-derived NO.^{3 4} There was no difference in the NO_X excretion, suggesting that endothelial NO production is unaltered in GK rats. The efficacy of valsartan to ameliorate inflammatory response as well as endothelial dysfunction indicates the involvement of increased renin-angiotensin system activity in the pathogenesis. Our findings agree with earlier studies demonstrating that Ang II stimulates superoxide generation by increasing the activity of NAD(P)H oxidase in vitro²⁹ and in vivo.³⁰ However, we would like to emphasize that the vascular responses and the degree were analyzed from different tissues, ie, from the mesenteric artery and the kidney, and in fact, the serum 8-isoprostaglandin level tended to be increased only in GK rats on a low-sodium diet. Therefore, we cannot completely conclude that the endothelial dysfunction found in the present study was due to increased oxidative stress.

The endothelium-mediated relaxation that remains resistant to NOS and cyclooxygenase inhibition is thought to be mediated by endothelium-derived hyperpolarizing factor, which appears to be a cytochrome P450–derived arachidonic acid epoxide. Current data support the concept that the vascular relaxation elicited by endothelium-derived hyperpolarizing factor is mediated by the opening of Ca²⁺-derived K⁺ channels in the vascular smooth muscle cells. Our findings suggest that endothelial dysfunction in GK rats could be attributed, at least in part, to reduced relaxation via arterial K⁺ channels. Furthermore, our finding that valsartan was able to improve the endothelium-dependent vascular relaxation also after preincubation of the arterial rings with the NOS inhibitor L-NAME and the cyclooxygenase inhibitor diclofenac suggests that valsartan is able to improve vascular relaxation via arterial K⁺ channels. However, the improvement of endothelium-dependent vascular relaxation by valsartan might also have been due, at least in part, to stimulation of AT₂ receptors, leading to increased production of bradykinin and NO as demonstrated by Siragy et al.³¹

High-sodium intake may increase blood pressure, induce cardiac hypertrophy, and deteriorate the therapeutic effects of most antihypertensive drugs.³² The effects are mediated primarily by volume overload as well as by increased sympathetic nervous system activity.³³ Previous studies have demonstrated that hypertension in patients with diabetes is often volume dependent and salt sensitive.¹ We found that a high-sodium diet causes a marked increase in blood pressure associated with the development of cardiac hypertrophy. A high-sodium diet also induces renal damage and aggravates inflammation and endothelial dysfunction. Furthermore, a high-sodium diet impairs the sensitivity of arterial smooth muscle cells to NO. The mechanism of the impaired sensitivity remains unsolved. The detrimental effects of a high-sodium diet are mediated partly by volume overload, inasmuch as plasma BNP concentration was markedly increased by the high-sodium diet. The 4-fold increase in serum 8-isoprostaglandin F₂ levels indicates that the detrimental effects of dietary sodium are associated with increased oxidative stress. In contrast, we were unable to detect any salt-induced changes in renal AT₁, AT₂, or ACE expressions. The renal expression of NEP, the major metabolizing enzyme for ANP and BNP, was also unaltered by the high-sodium diet. Interestingly, valsartan effectively protected against the development of salt-induced renal damage, inflammatory response, and oxidative stress. Our findings indicate that hypertension in diabetic GK rats is salt sensitive and associated with endothelial dysfunction and perivascular inflammation. AT₁ receptor blockade partially protects against salt-induced vascular damage by blood pressure–independent mechanisms.

	Acknowledgments

 
The study was supported by grants from the Finnish Foundation for Cardiovascular Research, the Academy of Finland, and the Sigrid Jusélius Foundation and by a Helsinki University Central Hospital research grant. The gift of valsartan by Novartis, Basel, Switzerland, is gratefully acknowledged. We are grateful to Dr Piet Finckenberg for histological photomicrographs and Anneli von Behr, Terhi Ilomäki, Tuula Riihimäki, Toini Siiskonen, and Sari Laakkonen for expert technical assistance.

Received October 26, 2000; first decision November 27, 2000; accepted December 14, 2000.

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