Nobuhiko Togashi

The contribution of skeletal muscle tumor necrosis factor-α to insulin resistance and hypertension in fructose-fed rats

Objective The aim of this study was to determine the role of tumor necrosis factor-α (TNF-α) in skeletal muscle tissue in insulin resistance and hypertension and the effect of anti-hypertensive medicine on skeletal muscle TNF-α in fructose-induced insulin-resistant and hypertensive rats(fructose-fed rats: FFR). Design and methods Six-week-old male Sprague-Dawley rats were fed either normal rat chow or fructose-rich chow. For the last 2 weeks of a 6-week period of either diet, the rats were treated with a vehicle (control or FFR); temocapril, an angiotensin converting enzyme inhibitor (ACEI); or CS-866, an angiotensin II type 1 receptor blocker (ARB). The euglycemic hyperinsulinemic glucose clamp technique was performed to evaluate insulin sensitivity (M value). TNF-α levels in soleus and extensor digitorum longus (EDL) muscles and epididymal fat pads were measured. We also measured the TNF-α concentration in an incubated medium secreted from soleus muscle strips with or without angiotensin II. Results TNF-α levels were significantly higher in the soleus and EDL muscles, but not in the epididymal fat, in the FFRs compared with the control rats. Temocapril and CS-866 lowered systolic blood pressure, improved insulin resistance, and reduced TNF-α in both skeletal muscles. There were significant negative correlations between M values and TNF-α levels in both soleus and EDL muscles. Also, the soleus muscle strip incubation with 10−7 mol/l angiotensin II increased TNF-α secreted into the incubation medium compared to the incubation without angiotensin II. These results suggest that skeletal muscle TNF-α is linked to insulin resistance and hypertension and that angiotensin II may be one of the factors that regulate skeletal muscle TNF-α.

The effects of an angiotensin-converting enzyme inhibitor and an angiotensin II receptor antagonist on insulin resistance in fructose-fed rats

The aim of this study was to compare the effects of an angiotensin-converting enzyme (ACE) inhibitor and an angiotensin II receptor (AT) antagonist on insulin resistance, especially on muscle fiber composition in fructose-induced insulin-resistant and hypertensive rats. Six-week-old male Sprague-Dawley rats were fed either normal rat chow (control) or a fructose-rich diet (FFR). For the last two weeks of a six-week period of either diet, the rats were treated with gum arabic solution as a vehicle (control or FFR), angiotensin-converting enzyme inhibitor (FFR+ACE), temocapril (1 mg/kg/ day) or an angiotensin II receptor antagonist (FFR+AT), CS-866 (0.3 mg/kg/day), by gavage, and then the euglycemic hyperinsulinemic glucose clamp technique was performed to evaluate insulin sensitivity. At the end of the glucose clamp, the soleus muscle was dissected for determination of the muscle fiber composition by ATPase methods. Blood pressure at the glucose clamp in the FFR group was significantly higher than that of the control group, and both temocapril and CS-866 significantly lowered the blood pressure of the FFR group. The average rate of glucose infusion during the glucose clamp, as a measure of insulin sensitivity (M value), was significantly lower in the FFR rats compared to the controls (15.4 +/- 0.4, 10.9 +/- 0.6 mg/kg/min, for control and FFR, respectively, P < .01). Both temocapril and CS-866 partially improved the M values compared to FFR (13.2 +/- 0.7, 12.8 +/- 0.5 mg/kg/min, for FFR+ACE, FFR+AT, respectively, P < .01 compared with FFR, P < .05 compared with control). The composite ratio of type I fibers of the soleus muscle was decreased significantly in the FFR rats compared with the controls (82% +/- 2%, 75% +/- 2%, for control and FFR, respectively, P < .01), and both temocapril and CS-866 restored a composite ratio of type I fibers to the same level as that of the controls (81% +/- 1%, 80% +/- 1% for FFR+ACE and FFR+AT, respectively). The M value was significantly correlated with the composition of type I and type II fibers. These results suggest that the fiber composition of skeletal muscle is correlated to insulin resistance, and that both ACE inhibitors and AT antagonists may modulate the muscle fiber composition in a hypertensive and insulin-resistant animal model, fructose-fed rats, to the same extent.

Correlations of adiponectin level with insulin resistance and atherosclerosis in Japanese male populations

objective  Adiponectin, which is secreted specifically by adipose tissue, has been shown to have an anti‐atherosclerotic effect and to improve insulin resistance. The aim of this study was to determine the correlations of plasma adiponectin concentration with insulin resistance and atherosclerosis.

Short Communication: Angiotensin II Type 1 Receptor–Mediated Upregulation of Calcineurin Activity Underlies Impairment of Cardioprotective Signaling in Diabetic Hearts

Rationale: The diabetic heart is resistant to ischemic preconditioning because of diabetes-associated impairment of phosphatidylinositol 3-kinase (PI3K)-Akt signaling. The mechanism by which PI3K-Akt signaling is impaired by diabetes remains unclear. Objective: Here, we examined the hypothesis that phosphorylation of Jak2 upstream of PI3K is impaired in diabetic hearts by an angiotensin II type 1 (AT1) receptor–mediated mechanism. Methods and Results: Infarct size (as percentage of risk area) after 20-minute ischemia/2-hour reperfusion was larger in a rat model of type 2 diabetes (Otsuka–Long–Evans–Tokushima fatty [OLETF] rat) than in its control (Long–Evans–Tokushima–Otsuka [LETO] rat) (60.4±1.6% versus 48.4±1.3%). Activation of Jak2-mediated signaling by erythropoietin or DADLE ([d-Ala2, d-Leu5]-enkephalin acetate), a &dgr;-opioid receptor agonist, limited infarct size in LETO rats (27.7±3.4% and 24.8±5.0%) but not in OLETF rats (53.9±5.3% and 55.0±2.2%). Blockade of the AT1 receptor by valsartan or losartan for 2 weeks restored the myocardial response of OLETF rats to erythropoietin-induced infarct size limitation (39.4±4.9% and 31.2±7.5). In OLETF rats, erythropoietin failed to phosphorylate both Jak2 and Akt, and calcineurin activity was significantly higher than in LETO rats. Two-week treatment with valsartan normalized calcineurin activity in OLETF rats and restored the response of Jak2 to erythropoietin. This effect of AT1 receptor blockade was mimicked by inhibition of calcineurin by FK506. Conclusions: These results suggest that the diabetic heart is refractory to protection by Jak2-activating ligands because of AT1 receptor–mediated upregulation of calcineurin activity.

GENDER DIFFERENCE IN THE RELATIONSHIPS AMONG HYPERLEPTINEMIA, HYPERINSULINEMIA, AND HYPERTENSION

Studies reveals that plasma leptin levels (LEP) in females are higher than those in males, and that LEP in hypertensive subjects are higher than those in BMI-matched normotensive subjects. To investigate the relationships among LEP, blood pressure (BP) and insulin sensitivity, we studied these relationships in 133 Japanese males and 263 females. LEP were positively correlated with BP, body mass index, body fat mass (FM) and homeostasis model assessment (HOMA). Regression analysis in which age and FM were adjusted showed LEP were associated with BP and HOMA. Even with adjustment by age, FM and HOMA, LEP were still positively correlated BP in males. LEP in insulin-resistant hypertensives was significantly higher than those in insulin-sensitive hypertensives, in insulin-sensitive normotensives and in insulin-resistant normotensives in males. However, in females, a significantly higher LEP was observed in insulin-resistant subjects than in insulin-sensitive subjects regardless of hypertension. These data suggest that it would be sexual difference in the relationships among hyperleptinemia, hyperinsulinemia and hypertension.

Olmesartan Ameliorates Insulin Sensitivity by Modulating Tumor Necrosis Factor- α and Cyclic AMP in Skeletal Muscle

We have reported that tumor necrosis factor (TNF)-α in skeletal muscle is one of the determinants of insulin resistance and that the renin-angiotensin system may be related to the regulation of TNF-α in skeletal muscle. Recent studies have suggested the involvement of cyclic adenosine monophosphate (cAMP) in the regulation of TNF-α in vascular smooth muscle cells or monocytes. The aim of this study was to determine the relationship between cAMP and TNF-α in skeletal muscle in connection with the renin-angiotensin system. Six-week-old male Sprague-Dawley rats were fed either normal rat chow or fructose-rich chow for 6 weeks. For the last 2 weeks of a 6-week period, the rats were treated with a vehicle or with an angiotensin II type 1 receptor antagonist (olmesartan medoxomil, 0.1 mg/kg/day). TNF-α levels in the soleus muscle were significantly higher and cAMP levels in the soleus muscle were significantly lower in fructose-fed rats than in control rats. Olmesartan increased cAMP and reduced TNF-α simultaneously in fructose-fed rats. There was a significant negative correlation between levels of cAMP and TNF-α. Moreover, a cAMP analogue reduced TNF-α levels in the soleus muscle. These results indicate that the increase in TNF-α via suppression of cAMP may affect the induction of insulin resistance. In addition, the facts that olmesartan increased cAMP and decreased TNF-α suggest that a part of the TNF-α regulation by angiotensin II might consist of modulation of cAMP through Gi protein activation in skeletal muscle.

Angiotensin II inhibits glucose uptake of skeletal muscle via the adenosine monophosphate-activated protein kinase pathway

Adenosine monophosphate-activated protein kinase (AMPK) mediates metabolic responses of muscle to exercise and is involved in improvement of insulin resistance by endurance exercise. Recent studies have suggested that the renin-angiotensin system might negatively modulate insulin-mediated actions, but there has been little investigation of the correlation between the renin-angiotensin system and AMPK. To determine this correlation, we performed studies with glucose clamp in vivo, and glucose uptake by skeletal muscle ex vivo using 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR). Six-week-old male Sprague-Dawley rats were fed standard chow (standard-diet rats; SD) or fructose-rich chow (fructose-fed rats; FFR) for 6 weeks. At the age of 12 weeks, SD and FFR were treated by oral gavage, either with angiotensin II (Ang II) receptor blockade (ARB; valsartan 30 mg/kg) or vehicle. Thirty minutes after the treatment, we performed glucose clamp studies to measure glucose infusion rates during infusion of insulin (GIR(I)) and of AICAR (GIR(A)), which stimulates AMPK, and studied the effect of ARB on either GIR(I) or GIR(A). In an ex vivo study, we used bilateral fresh soleus muscles from 3-week-old male Sprague-Dawley rats to examine the glucose uptake (measured by (3)H-2-deoxyglucose uptake) of one side of soleus muscle incubated with AICAR with or without Ang II, or with tumor necrosis factor-alpha, in comparison with that of the other (untreated control) side of the muscle. Blood pressure of FFR was significantly higher than that of SD rats. GIR(I) was significantly lower in FFR than in SD, and treatment with ARB did not change GIR(I). GIR(A) of FFR was significantly lower than that of SD, but GIR(A) of FFR treated with ARB was significantly increased compared with that of FFR treated with vehicle. In the ex vivo study, incubation with AICAR significantly increased glucose uptake of soleus muscles, Ang II significantly decreased AICAR-activated glucose uptake in a dose-dependent manner, and ARB canceled the effect of Ang II. The results suggest that acute inhibition of the angiotensin 1 receptor improves glucose metabolism via not insulin but AMPK pathway through the angiotensin 1 receptor in FFR.

The impact of elevation of serum uric acid level on the natural history of glomerular filtration rate (GFR) and its sex difference

BACKGROUND The impact of elevation of the serum uric acid level (SUA) on the natural history of glomerular filtration rate (GFR) remains controversial. METHODS If elevation of SUA is a result, rather than a cause, of a decline in GFR, the relationship between SUA and GFR should be the same in the same population over years except for shifts by age-dependent reduction of GFR. We tested this hypothesis using data from two cohorts and a group of allopurinol-treated patients. RESULTS In Cohort 1 consisting of urban residents aged 40.6 ± 9.0 years (n = 3 446), SUA was inversely correlated with estimated GFR (eGFR) in both men and women, and the slope of the SUA-eGFR relationship was steeper in women than in men. The slopes of the regression lines became significantly steeper after a 6-year interval in both sexes, and the change in the slope was larger in women. A similar sex difference in the SUA-eGFR relationship and 6-year change in the slope were observed in Cohort 2 consisting of rural town residents aged 61.7 ± 12.2 years (n = 404). Multiple regression analyses showed that explanatory factors of eGFR after a 6-year interval were age and SUA at baseline in both cohorts, and partial regression coefficients of SUA were more negative in women than in men. The SUA-eGFR relationship in allopurinol-treated patients (n = 346, 63.5 ± 13.3 years old) was similar to that in Cohort 2. CONCLUSIONS The results indicate that elevation of SUA accelerates the yearly decline in eGFR and that women are more susceptible to urate-induced decline in eGFR.

Angiotensin II receptor activation in youth triggers persistent insulin resistance and hypertension—a legacy effect?

Although the involvement of angiotensin II (Ang II) in insulin resistance and hypertension has been established, the temporal relationships between Ang II receptor activation and changes in insulin sensitivity and blood pressure are not clear. To better understand this issue, we infused rats with Ang II (200 ng kg−1 min−1) or vehicle for 4 weeks and assessed the residual effects after the discontinuation of the infusion on blood pressure, insulin sensitivity and tissue parameters of inflammation. Four weeks after the discontinuation of the Ang II infusion, the blood pressure was higher by 12.8 mm Hg, and insulin sensitivity as determined by a euglycemic hyperinsulinemic glucose clamp was reduced (glucose infusion rate: 11.1±0.7 vs. 17.6±0.5 mg kg−1 min−1) in the Ang II-treated group compared with controls. The persistent hypertension and insulin resistance were associated with greater than two-fold increases in macrophage chemoattractant protein-1, tumor necrosis factor-α and thiobarbituric acid-reactive substrates in the soleus muscle. Furthermore, total and activated forms of Rac-1, a regulatory subunit of the NADPH oxidase complex, were increased by 144±14% and 277±82%, respectively, in the skeletal muscle of Ang II-treated rats. These residual effects after Ang II infusion were all attenuated by the co-administration of tempol, a free radical scavenger, or candesartan with Ang II. The effects of candesartan were not mimicked by hydralazine at an equidepressant dose. These findings suggest that Ang II receptor activation in youth triggers the upregulation of inflammatory cytokines and the production of reactive oxygen species, thereby inducing later insulin resistance and hypertension.