Yoshimi Yoneki

Methylglyoxal contributes to the development of insulin resistance and salt sensitivity in Sprague-Dawley rats.

Objectives Methylglyoxal, a metabolite of the glycolysis pathway, may play an important role in the development of diabetes and hypertension, but the exact mechanism has not been fully elucidated. The present study was designed to investigate whether methylglyoxal could directly induce insulin resistance and salt sensitivity in Sprague–Dawley rats. Methods Rats were allocated to four groups: control (normal drinking water), 1% methylglyoxal in drinking water, 1% methylglyoxal plus N-acetyl cysteine (NAC) (800 mg/kg per day), a methylglyoxal scavenger, or TM2002 (100 mg/kg per day), an advanced glycation endproducts (AGEs) inhibitor. After 4-week treatment insulin resistance was evaluated by an euglycemic hyperinsulinemic glucose clamp technique. In another set of rats, either a high-salt diet (4%) alone, standard rat chow with 1% methylglyoxal in drinking water or high-salt diet plus methylglyoxal was given for 4 weeks. Immunohistochemistry was performed to measure nitrotyrosine and methylglyoxal-induced AGEs, Nϵ-carboxyethyl-lysine (CEL) in the kidney. Results Four-week treatment with NAC or TM2002 completely improved methylglyoxal-induced insulin resistance. Co-administration of methylglyoxal and high-salt diet significantly increased systolic blood pressure, urinary albumin excretion, urinary thiobarbituric acid-reactive substances excretion and the renal nitrotyrosine expression in the kidney (markers of oxidative stress) compared with methylglyoxal or high-salt diet alone. Renal CEL was significantly increased in methylglyoxal-treated rats compared with nonmethylglyoxal-treated rats. Conclusion These results indicate that methylglyoxal-induced insulin resistance and salt sensitivity at least in part by increasing oxidative stress and/or AGEs formation in Sprague–Dawley rats. The present study provides further evidence for methylglyoxal as one of the causative factors in the pathogenesis of insulin resistance and salt-sensitive hypertension.

Carbonyl stress induces hypertension and cardio-renal vascular injury in Dahl salt-sensitive rats.

One major precursor of carbonyl stress, methylglyoxal (MG), is elevated in the plasma of chronic kidney disease (CKD) patients, and this precursor contributes to the progression of vascular injury, hypertension and renal injury in diabetic nephropathy patients. This molecule induces salt-sensitive hypertension via a reactive oxygen species-mediated pathway. We examined the role of MG in the pathogenesis of hypertension and cardio–renal injury in Dahl salt-sensitive (Dahl S) rats, which is a rat model of CKD. Nine-week-old Dahl S rats were fed a 1% NaCl diet, and 1% MG was added to their drinking water for up to 12 weeks. Blood pressure and cardio–renal injuries were compared with rats treated with tap water alone. The angiotensin II receptor blocker (ARB), candesartan (10 mg kg−1 day−1), was administered to MG Dahl S rats to determine the impact of this drug on the pathogenesis of MG-induced CKD. A progressive increase in systolic blood pressure was observed (123±1–148±5 mm Hg) after 12 weeks of MG administration. MG administration significantly increased urinary albumin excretion, glomerular sclerosis, tubular injury, myocardial collagen content and cardiac perivascular fibrosis. MG also enhanced the renal expression of Nɛ-carboxyethyl-lysine (an advanced glycation end product), 8-hydroxydeoxyguanosine (a marker of oxidative stress), macrophage (ED-1) positive cells (a marker of inflammation) and nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase activity. Candesartan treatment for 4 weeks significantly reduced these parameters. These results suggest that MG-induced hypertension and cardio–renal injury and increased inflammation and carbonyl and oxidative stress, which were partially preventable by an ARB.

Losartan modulates muscular capillary density and reverses thiazide diuretic-exacerbated insulin resistance in fructose-fed rats

The renin–angiotensin system (RAS) is involved in the pathogenesis of insulin sensitivity (IS). The role of RAS in insulin resistance and muscular circulation has yet to be elucidated. Therefore, this study sought to determine the mechanisms of angiotensin II receptor blockers (ARBs) and/or diuretics on IS and capillary density (CD) in fructose-fed rats (FFRs). Sprague-Dawley rats were fed either normal chow (control group) or fructose-rich chow for 8 weeks. For the last 4 weeks, FFRs were allocated to four groups: an FFR group and groups treated with the thiazide diuretic hydrochlorothiazide (HCTZ), with the ARB losartan, or both. IS was evaluated by the euglycemic hyperinsulinemic glucose clamp technique at week 8. In addition, CD in the extensor digitorum longus muscle was evaluated. Blood pressure was significantly higher in the FFRs than in the controls. HCTZ, losartan and their combination significantly lowered blood pressure. IS was significantly lower in the FFR group than in the controls and was even lower in the HCTZ group. Losartan alone or combined with HCTZ significantly increased IS. In all cases, IS was associated with muscular CD, but not with plasma adiponectin or lipids. These results indicate that losartan reverses HCTZ-exacerbated insulin resistance, which can be mediated through the modulation of muscular circulation in rats with impaired glucose metabolism.

Blockade of angiotensin II type-1 receptor increases salt sensitivity in Sprague―Dawley rats

This study determined the role of angiotensin II type-1 (AT1) receptor in the salt sensitivity of blood pressure. The mean arterial blood pressure (MAP) of Sprague–Dawley rats was monitored by radio telemetry and, after baseline measurements, rats were treated either with (1) vehicle, (2) AT1 receptor blocker (ARB) olmesartan (OLM, 100 nmol kg−1 h−1, subcutaneously), (3) OLM with hydrochlorothiazide (HCTZ, 40 mg kg−1 day−1, orally), (4) angiotensin II (AngII, 100 ng kg−1 min−1, subcutaneously) or with (5) AngII with OLM. Rats were fed a 0.5% salt diet during the baseline and first 7 days of treatment period, and the diet was then switched to one containing 8% salt for another 7 days. Urinary samples were collected in a metabolic cage at the end of each period. MAP of the vehicle group did not change throughout the study. In AngII-infused rats, BP increased only when rats were fed an 8% salt diet. OLM and OLM with AngII significantly reduced MAP when rats were on a 0.5% salt diet, but not on an 8% salt diet, indicating an enhanced salt sensitivity by OLM. Co-treatment with HCTZ reduced the salt sensitivity of OLM. The urinary level of the oxidative stress marker was increased by an 8% salt diet and was not altered by either OLM alone or in combination with HCTZ. However, OLM attenuated the salt-induced renal NAD(P)H (nicotinamide adenine dinucleotide phosphate) oxidase activity. These results indicate that AT1 receptor blockade increases salt sensitivity, which is reversed by diuretics. We conclude that OLM and HCTZ could be a useful combination for reduction of blood pressure even under high salt intake without changes in urinary oxidative stress levels.

Albuminuria indicates the pressure-associated injury of juxtamedullary nephrons and cerebral strain vessels in spontaneously hypertensive stroke-prone rats.

Albuminuria is an indicator of renal injury and is closely linked with cardiovascular disease (CVD). However, the mechanism by which albumin is excreted in the urine remains unclear. As the juxtamedullary region of the kidney is highly susceptible to pressure increase, juxtamedullary injury is observed from an early phase in hypertensive rat models. Anatomical similarities are observed between the pre-glomerular vessels of the juxtamedullary nephron and the cerebral vasculature. We previously named these ‘strain vessels’ for their high vascular tone and exposure to higher pressures. The current studies were designed to determine whether albuminuria is the result of juxtamedullary nephron injury, indicating the presence of pressure injury to the strain vessels in spontaneously hypertensive stroke-prone rats (SHR-SP) fed a high-salt diet. Albuminuria was associated with juxtamedullary nephron injury, and the enhanced expression of monocyte chemotactic protein-1 (MCP-1) and tumor growth factor-beta (TGF-β) in 12-week-old SHR-SP rats fed a 4% high-salt diet from the age of 6 weeks. The wall thickness of the pre-glomerular vessels of the juxtamedullary nephron was also associated with that of the perforating artery of the middle cerebral artery. Reducing the blood pressure with nifedipine reduced the degree of albuminuria and juxtamedullary nephron injury as well as MCP-1 and TGF-β expression in the SHR-SP rats fed an 8% high-salt diet from the age of 9 weeks. Nifedipine inhibited stroke events in these animals until they were 14 weeks old. These results indicate that albuminuria is a result of juxtamedullary nephron injury and a marker of pressure-induced injury of the strain vessels.

Role of specific T-type calcium channel blocker R(-) efonidipine in the regulation of renal medullary circulation.

Objectives: Blockade of the T-type calcium channel (TCC), which is expressed in the renal efferent arterioles of the juxtamedullary nephron and vasa recta, has been shown to protect against renal injury. Studies were designed to determine the effects of a specific TCC blocker, R(−) efonidipine [R(−)EFO], on the regulation of renal circulation. Methods and results: Renal medullary blood flux (MBF) and cortical blood flux (CBF) were simultaneously monitored using laser-Doppler flowmetry in Sprague-Dawley rats. Responses were also determined in rats with angiotensin II (AngII) induced renal ischemia. Intravenous (i.v.) or renal interstitial (r.i.) infusion of R(−)EFO (0.25 mg/h, i.v. or r.i.) significantly increased MBF by 24.0 ± 7.0 and 21.0 ± 4.4%, respectively, but without changing CBF or mean arterial pressure. The nitric oxide (NO) synthase inhibitor NG-nitro-L-argininemethylester (L-NAME, 1 &mgr;g/kg per min, i.v. or r.i.) significantly attenuated R(−)EFO-induced increase in MBF. R(−)EFO inhibited the AngII-mediated (50 ng/kg per min, i.v.) reduction of MBF (28.4 ± 1.7%), which was associated with increased urinary NO2− + NO3− excretion and decreased urinary hydrogen peroxide (H2O2) excretion. Intracellular H2O2 fluorescence (real-time fluorescence imaging) in the epithelial cells of isolated medullary thick ascending limb (mTAL) significantly increased following AngII stimulation (1 &mgr;mol/L, 235 ± 52 units), which was significantly inhibited by pre and coincubation with R(−)EFO. R(−)EFO stimulation also increased the intracellular NO concentration in the epithelial cells of mTAL (220 ± 62 units). Conclusion: These results suggest that TCC blockade with R(−)EFO selectively increases MBF, an effect that appears to be mediated by changes in renal NO and oxidative stress balance, which may protect against ischemic renal injury in the renal medullary region.