Inflammation is not a contributing factor in the development of insulin resistance in diet induced obesity in rats

Obesity-related insulin resistance is accompanied by impaired microvascular recruitment within skeletal muscle. Loss of recruitment within the microvasculature of the muscle has been shown to be an early defect leading to impaired insulin sensitivity and in time skeletal muscle insulin resistance, although the mechanism of impairment is unknown. Both obesity and insulin resistance have been associated with chronic low-grade inflammation. A number of inflammatory factors have been shown to directly influence vascular and myocyte responses to insulin and may therefore be a contributing factor towards the development of insulin resistance. The aim of this thesis was to investigate whether impaired vascular and metabolic responses to insulin are attributed to the presence of inflammation. Dietary models of obesity-induced insulin resistance were studied in Sprague Dawley rats. Rats were placed on diets of different fat content (41% and 58% calories derived from fat) in addition to a cafeteria-style diet. All diets were given for a 4 week duration, with both high fat (58%) and cafeteria-style diets also extended to 12 weeks. Hyperinsulinemic euglycaemic clamps and 1-methyl xanthine (1-MX) techniques were used to measure whole body insulin sensitivity and microvascular recruitment, with inflammatory gene expression in the skeletal muscle and epididymal adipose tissue measured by quantitative real-time PCR (q-PCR). Four weeks of 41% high fat diet caused obesity, insulin resistance, and impaired capillary recruitment in rats; however inflammatory markers in epididymal fat were not altered. Raising dietary fat content to 58% resulted in increased adiposity of the epididymal fat pad but did not cause insulin resistance or inflammation of adipose tissue. Long term feeding with this diet attenuated obesity and these animals did not differ from paired control diet rats in insulin sensitivity or adipose tissue inflammation. A more palatable and varied cafeteria diet resulted in even greater obesity and insulin resistance than the 41% high fat diet and this was sustained after 12 weeks of feeding. However this diet did not cause inflammation after either 4 or 12 weeks of dietary intervention. Finally, restoration of muscle insulin sensitivity by metformin in 41% high fat fed rats was tested to see if adipose tissue inflammation could be reduced. Metformin was found to significantly improve insulin sensitivity in insulin resistant rats after 4 weeks of high fat feeding with the 41% diet, and significantly improve skeletal muscle microvascular recruitment compared to that of control animals. Therefore in addition to its known glucoregulatory actions, this study has shown metformin to have significant actions directly on the vasculature and can restore microvascular blood flow within insulin resistant skeletal muscle. However, these improvements were not accompanied by attenuation of adipose inflammatory gene expression. The lack of up-regulated inflammatory responses in insulin-sensitive tissues of obese and insulin resistant rats suggests that inflammation may not be a driving factor for the development of metabolic or vascular dysfunction present in insulin resistance in rats. The lack of inflammatory response despite significant obesity suggests that rats may possess mechanisms protecting against obesity-induced inflammation. Adipose tissue microenvironment and expansion have been identified as potential mechanisms regulating the induction of inflammation with evidence suggesting that adipose tissue can undergo both healthy and unhealthy expansion in response to lipid accumulation. Data presented in this thesis suggest that in rats adipose tissue expansion occurs in a manner that protects against the development of inflammation, although further investigation is required to identify such potentially protective mechanisms involved in obesity-induced adipose tissue expansion.