Angiotensin receptor and tumor necrosis factor-α activation contributes to glucose intolerance independent of systolic blood pressure in obese rats

Abstract

Pathological activation of the renin-angiotensin system and inflammation are associated with hypertension and the development of metabolic syndrome (MetS). The contributions of angiotensin receptor type 1 (AT1) activation, independent of blood pressure, and inflammation to glucose intolerance and renal damage are not well defined. Using a rat model of MetS, we hypothesized that the onset of glucose intolerance is primarily mediated by AT1 activation and inflammation independent of elevated systolic blood pressure (SBP). To address this hypothesis, we measured changes in SBP, adiposity, plasma glucose and triglyceride levels, and glucose tolerance in six groups of rats: 1) lean, strain control Long-Evans Tokushima Otsuka (LETO; n = 5), 2) obese Otsuka Long-Evans Tokushima Fatty (OLETF; n = 8), 3) OLETF + angiotensin receptor blocker (ARB; 10 mg olmesartan/kg; n = 8), 4) OLETF + tumor necrosis factor-α (TNF-α) inhibitor (ETAN; 1.25 mg etanercept/kg; n = 6), 5) OLETF + TNF-α inhibitor + angiotensin receptor blocker (ETAN+ARB; 1.25 mg etanercept/kg + 10 mg olmesartan/kg; n = 6), and 6) OLETF + calcium channel blocker (CCB; 5 mg amlodipine/kg; n = 7). ARB and ETAN+ARB were most effective at decreasing SBP in OLETF, and ETAN did not offer any additional reduction. Glucose tolerance improved in ARB, ETAN, and ETAN+ARB compared with OLETF, whereas CCB had no detectable effect. Furthermore, all treatments reduced adiposity, whereas ETAN alone normalized urinary albumin excretion. These results suggest that AT1 activation and inflammation are primary factors in the development of glucose intolerance in a setting of MetS and that the associated increase in SBP is primarily mediated by AT1 activation.

Keywords: hypertension; inflammation; insulin resistance; metabolic syndrome; renin-angiotensin system.

Fig. 1.

Inhibition of the biological activity of TNF-α decreases systolic blood pressure. Mean (± SE) weekly systolic blood pressure of Long-Evans Tokushima Otsuka (LETO; n = 5), Otsuka Long-Evans Tokushima Fatty (OLETF; n = 8), OLETF + angiotensin receptor blocker (ARB; n = 8), OLETF + TNF-α inhibitor (ETAN; n = 4), OLETF + TNF-α inhibitor + angiotensin receptor blocker (ETAN+ARB; n = 4), and OLETF + calcium channel blocker (CCB; n = 7). ^aP < 0.05, LETO vs. OLETF; ^bP < 0.05 vs. OLEFT; ^cP < 0.05, ARB vs. ETAN; ^dP < 0.05, ARB vs. ETAN+ARB; ^eP < 0.05, ARB vs. CCB; ^fP < 0.05, ETAN vs. ETAN+ARB; ^gP < 0.05, ETAN vs. CCB; ^hP < 0.05, ETAN+ARB vs. CCB. ↓, Etanercept removed and reintroduced.

Fig. 2.

Inhibition of the biological activity of TNF-α improves glucose intolerance. The response of blood glucose (A) and plasma insulin (B) to an oral glucose tolerance test and the mean (± SE) area under the curve (AUC) for glucose (C) and insulin (D) of fasted Long-Evans Tokushima Otsuka (LETO; n = 5), Otsuka Long-Evans Tokushima Fatty (OLETF; n = 8), OLETF + angiotensin receptor blocker (ARB; n = 8), OLETF + TNF-α inhibitor (ETAN; n = 4), OLETF + TNF-α inhibitor + angiotensin receptor blocker (ETAN+ARB; n = 4), and OLETF + calcium channel blocker (CCB; n = 6–7). BL, baseline; ND, not determined; r.u., relative units. ^aP < 0.05 vs. LETO; ^bP < 0.05 vs. OLETF; ^cP < 0.05 vs. ARB; ^dP < 0.05 vs. ETAN; ^eP < 0.05 vs. ETAN+ARB.

Fig. 3.

Renal TNF-α protein is not elevated in obese Otsuka Long-Evans Tokushima Fatty (OLETF) rats. Mean (± SE) renal TNF-α protein [% change from Long-Evans Tokushima Otsuka (LETO)], and the representative Western blot bands (A), TNF-α receptor 1 (TNFR1, B), and TNF-α receptor 2 (TNFR2, C) mRNA expressions of LETO (n = 4–8), OLETF (n = 5–8), OLETF + angiotensin receptor blocker (ARB; n = 5–6), OLETF + TNF-α inhibitor (ETAN; n = 3–5), OLETF + TNF-α inhibitor + angiotensin receptor blocker (ETAN+ARB; n = 4–5), and OLETF + calcium channel blocker (CCB; n = 5–7). CT, threshold cycle. ^aP < 0.05 vs. LETO; ^bP < 0.05 vs. OLETF; ^cP < 0.05 vs. ARB; ^dP < 0.05 vs. ETAN; ^eP < 0.05 vs. ETAN+ARB.

Fig. 4.

Mean (± SE) glomerulosclerosis index of Long-Evans Tokushima Otsuka (LETO; n = 5), Otsuka Long-Evans Tokushima Fatty (OLETF; n = 5), OLETF + angiotensin receptor blocker (ARB; n = 5), OLETF + TNF-α inhibitor (ETAN; n = 4), and OLETF + TNF-α inhibitor + angiotensin receptor blocker (ETAN+ARB; n = 3). Mean glomerulosclerosis index of OLETF + calcium channel blocker (CCB) was not determined (ND).

Fig. 5.

Calcium channel blockade enhances renal antioxidant activities. Mean (± SE) activity of kidney superoxide dismutase (A), catalase (B), and glutathione peroxidase (C) of Long-Evans Tokushima Otsuka (LETO; n = 4–5), Otsuka Long-Evans Tokushima Fatty (OLETF; n = 5–7), OLETF + angiotensin receptor blocker (ARB; n = 6–7), OLETF + TNF-α inhibitor (ETAN; n = 5–6), OLETF + TNF-α inhibitor + angiotensin receptor blocker (ETAN+ARB; n = 5–6), and OLETF + calcium channel blocker (CCB; n = 6–7). ^aP < 0.05 vs. LETO; ^bP < 0.05 vs. OLETF; ^cP < 0.05 vs. ARB; ^dP < 0.05 vs. ETAN.

Fig. 6.

Contributions of angiotensin receptor type 1 (AT1) activation, inflammation, and impaired endothelial Ca²⁺-mediated signaling to the manifestation of metabolic syndrome. SOD, superoxide dismutase; TG, triglycerides.