The mechanisms involved in the development of nutrient oversupply-induced insulin resistance in skeletal muscle

Insulin resistance is a major metabolic defect associated with obesity and type 2 diabetes. The incidences of both are increasing at an alarming rate. Excessive consumption of nutrient rich foods have been implicated in the pathogenesis of insulin resistance. However, the mechanisms involved in the onset of insulin resistance in skeletal muscle caused by acute nutrient oversupply in vivo have not been fully elucidated. The broad aim of this thesis was to examine the mechanisms associated with the onset of skeletal muscle insulin resistance in models of acute nutrient oversupply.The effect of glucose oversupply was investigated in the first study, which resulted in insulin resistance at the whole body and skeletal muscle level following 5h of glucose infusion, but not after 3h. There was no change in markers of oxidative stress over the same time course during which insulin resistance developed. Furthermore, co-infusion of the antioxidant taurine had no effect on the decreased glucose uptake in skeletal muscle from glucose infused animals. There was no evidence of activation of inflammatory/stress signalling pathways or defects in the phosphorylation state of multiple insulin signalling intermediates over the same time course. In isolated soleus strips taken from control, 1h, or 5h glucose infused animals, insulin stimulated 2-deoxyglucose transport was similar. Although, insulin-stimulated glycogen synthesis was significantly reduced after 5h of glucose infusion, in the presence of significantly increased glycogen content. The reduced flux through the glycogen synthesis pathway and a reduced content of glucose-6-phosphate suggests in this model that the rate limiting step has shifted from glucose transport to glucose phosphorylation by hexokinase (HK).In an acute lipid and insulin infusion model, the onset of insulin resistance was similar to that observed in the glucose infusion model. The mechanisms for the insulin resistance in skeletal muscle in this model was not associated with defects in the phosphorylation of key insulin signalling intermediates or activation of inflammatory/stress signalling pathways. Furthermore, there was no change in markers of oxidative stress and the co-infusion of taurine had no effect on the onset of insulin resistance. There was an increased exposure of long chain acyl-CoA (LCACoA), although there was no change in the content of other lipid intermediates such as DAG or ceramides. Interestingly, muscle pyruvate dehydrogenase (PDH) kinase 4 (PDHK4) protein content was significantly decreased in hyperinsulinaemic glycerol infused rats after 3 and 5h, and this decrease was blunted in muscle from hyperinsulinaemic 3 and 5h lipid infused rats. These findings suggest that lipid infusion may reduce glucose metabolism by inhibition of the glucose phosphorylation due to LCACoA inhibition of HK and mitochondrial substrate competition regulated by increased PDHK4.In conclusion, the current studies demonstrate that the insulin resistance associated with nutrient oversupply was not associated with significant changes in phosphorylation of key insulin signalling intermediates, activation of inflammatory and stress signalling pathways, or a change in markers of oxidative stress. Overall, the studies in this thesis suggest that the initial onset of insulin resistance due to glucose and lipid oversupply (in the presence of high insulin) is associated with metabolic feedback regulation, which is likely to be a protective mechanism of the skeletal muscle to limit any further insult by the excess nutrients