Introduction

Dexamethasone is widely used therapeutic agent for Immunosuppressive and Anti-inflammatory action. Its use is associated with several adverse effects like Hypertension, Diabetes and Insulin Resistance due to over production of ROS (Bjelakovic et al., 2007). Insulin Resistance is a cardinal feature of type-2 diabetes and may be an Impetus for the development of Hypertension. Dexamethasone induced Insulin Resistance probably due to oxidative stress (Xi L et al., 2005). Although, ROS have been proposed to have a causal role in multiple forms of Insulin Resistance. It has been reported that Crocetin and fensuccinal prevents Dexamethasone induced Insulin Resistance probably due to Antioxidant properties. (Gorbenko et al., 2000). Hence, in the present study we aimed to evaluate the effect of Vitamin E (α-Tocopherol acetate, synthetic form) on Dexamethasone-Induced Experimental Insulin Resistance to explore the role of oxidative stress in Insulin Resistance.

Material and Methods

Male wistar rats weighing (240-300g) were used. They were housed in a group of six under environmentally controlled room with 12-h light/dark cycle and had free access to food and water. Dexamethasone injection was procured from Zydus cadila, 2-Thiobarbituric acid, α-Tocopherol Acetate and 1,1,3,3,-Tetramethoxy propane (Malonaldehyde bis) was procured from Himedia Laboratories Ltd, Mumbai, India,  Radio Immuno Assay Kit for Insulin- RIAK-1 was procured from Board of Radiation and Isotope Technology, BARC, Mumbai, India. Dosage of the Dexamethasone phosphate administered subcutaneously was prepared in the saline (0.9 % sodium chloride saline solution). Dose of the Dexamethasone phosphate (20µg/kg, once daily, subcutaneously) for 14 days to induce Insulin Resistance were chosen based on previous reports (Rhee MS et al., 2004). Dose of the Vitamin E (100mg/kg, once daily, mixed in sesame oil and food pallets were soaked) through diet were chosen based on previous reports. (Mehta J et al., 1999). The animals were divided into 4 groups containing six in each as follows.

(1)  Normal Control: Received normal saline (0.9% NaCl) subcutaneously for 14 days.

(2) Dexamethasone: Received Dexamethasone 20µg/kg, subcutaneously, once daily for 14 days.

(3) Dexamethasone and Vitamin E: Received dexamethasone 20µg/kg, subcutaneously, once daily for 14 days and fed with chow mixed with Vitamin E (100 mg/kg/day) from 1^stday and continued throughout study.

(4) Dexamethasone and Vitamin E: Received Dexamethasone 20µg/kg, subcutaneously, once daily for 14 days and fed with chow mixed with Vitamin E (100 mg/kg/day) from 8^th day and continued throughout study.

During the experimental period on 13^th day of experimental period Systolic blood pressure was measured by tail cuff method. At the end of treatment period, Blood samples were collected by retro-orbital plexus puncture under light ether anesthesia after 16 h fasting (on 14^th day) and then animals were sacrificed by excess ether anesthesia and thoracic aorta was isolated for estimation of lipid peroxides (TBARS). Systolic blood pressure was measured indirectly by the tail cuff method using noninvasive blood pressure apparatus. The Insulin estimation was done by using ImmuChem Radioimmunoassay method (Loraine JA et al., 1976). Using a standard kit obtained from BRIT, BARC, Mumbai, India. Triglyceride was estimated by method of (Buccolo G et al., 1976) using a standard kit obtained from ERBA diagnostics Manheim Ltd. Insulin resistance index (Henry RR et al., 2003) and Insulin sensitivity Index (Albareda M et al., 2000) were calculated. The TBARS estimation is based on the reaction between plasma malondialdehyde (MDA), a product of lipid peroxidation and thiobarbituric acid (TBA) and carried out as previously described (Yoshioka T et.al., 1979).

Results and Discussion

There was a significant elevation in fasting basal plasma Insulin, Triglycerides level and concentration of Malondialdehyde in plasma (P<0.01) as well in Aortic tissue (P<0.05) in rats injected with Dexamethasone for 14 days when compared with normal rats. There was a significant elevation of HOMA-IR value and decrease in MASTUDA INDEX in rats injected with Dexamethasone for 14 days (P<0.01) when compared with normal rats (Table no 2). There was a significant   increase in Systolic blood pressure in rats injected with Dexamethasone for 14 days (P<0.0001) when compared with normal rats. Vitamin E significantly (P<0.001) prevented the Dexamethasone induced increase in blood pressure.  There was no significant difference in fasting basal plasma Insulin level however Elevation of Triglycerides level induced by Dexamethasone was significantly (P<0.05) prevented by Vitamin E. It also prevented the elevation of malondialdehyde in plasma (P<0.01) as well as in Aortic tissue (P<0.05) when compared to Dexamethasone injected rats. Vitamin E significantly prevented the Insulin Resistance as indicated by reduction in HOMA-IR (P<0.05) index and there was a significant increase (P<0.01) in Insulin sensitivity (mastuda index) in Vitamin E treated rats when compared to Dexamethasone injected rats. Vitamin E significantly (P<0.001) reversed the Dexamethasone induced increase in systolic blood pressure.   There was no significant difference in fasting basal plasma Insulin level in therapeutic regimen of vitamin E. Elevation of Triglycerides, malondialdehyde level in plasma aswell in Aortic tissue  induced by Dexamethasone injected rats was  significantly reversed by  Vitamin E. It also significantly reversed the Insulin resistance as indicated by reduction in HOMA-IR (P<0.05) index and there was no significant difference in Insulin sensitivity (mastuda index) in Vitamin E treated rats when compared with dexamethasone injected rats.

Table 1: Systolic blood pressure, basal insulin level and plasma triglycerides.

Values are expressed in Mean ± SEM; n=6.

*P<0.05 when compared with normal.

**P<0.01 when compared with normal.

***P<0.001 when compared with normal.

 Table 2: Insulin Resistance index, Insulin sensitivity index and Concentration of malondialdehyde in plasma and in Aortic tissue.

Values are expressed in Mean ± SEM; n=6.

*P<0.05 when compared with normal.

**P<0.01 when compared with normal.

***P<0.001 when compared with normal.

Conclusion

In conclusion, Dexamethasone induced Insulin Resistance and Hypertension accompanied by increased oxidative stress. However, it appears that abnormality in Insulin action and elevation of systolic blood pressure occurs earlier than oxidative stress. Administration of Vitamin E found to be ineffective to prevent/or reverse the insulin resistance. However, it improved the Insulin sensitivity and prevented the Insulin Resistance probably due to its pleotropic effects other than Antioxidant property.

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