Abstract:
Altered vascular responses to various vasopressors in animal models of insulin resistance (IR) and diabetes have been well documented. However, the precise mechanisms about vascular responses in IR with or without frank hyperglycemia (prediabetic state) are not available. Moreover, recently the role of peroxisome proliferators activated receptor-γ (PPARγ) has been linked to influence the vascular responses in hypertensive and diabetic state. Hence, the present study was conceived to determine the role of hyperglycemia on angiotensin II (Ang II) mediated vascular responses in the high fat diet (HFD) induced insulin resistance either with mild or frank hyperglycemia [induced by injection of low dose streptozotocin (STZ) to HFD fed rats (HFD + STZ)]. In addition, insulin-sensitizing agent such as rosiglitazone and pioglitazone were also studied on biochemical and vascular responses. Ang II-induced contractions were studied isometrically in thoracic aortic rings isolated from 4 weeks of normal pellet diet (NPD) fed control, HFD and HFD + STZ fed insulin resistant rats. Specific binding of Ang II receptors were carried out using radioligand ([3H]–Ang II) binding studies. After 4 weeks of HFD feeding, rats exhibited characteristics features of insulin resistance such as mild hyperglycemia, hyperinsulinemia, hypertriglyceridemia, hypercholesterolemia and hypertension; whereas HFD + STZ treated rats showed all above parameters along with frank hyperglycemia. Maximal contractile response (Emax) to Ang II is increased in HFD fed rats as compared to control rats. Moreover, Emax values are further elevated in HFD + STZ group where the frank hyperglycemia was induced by low dose of streptozotocin. Rosiglitazone (5 mg kg−1, p.o.) and pioglitazone (10 mg kg−1, p.o.) treatment significantly lowered the plasma glucose, triglycerides, insulin and cholesterol levels in insulin resistance rats. In addition, it also restored the elevated systolic, mean arterial, diastolic blood pressure and attenuated the enhanced contractile responses to Ang II in thoracic aortic rings obtained from both HFD and HFD + STZ treated rats. Specific binding of [3H]–Ang II is upregulated in HFD-fed and HFD + STZ treated rats. Treatment with pioglitazone and rosiglitazone significantly decreased the AT1R specific binding in HFD fed rats. Our results indicate the role of hyperglycemia in the elevation of Ang II induced vascular responses in thoracic aorta isolated from insulin resistant rats and PPARγ agonists can attenuate these responses.