Vascular and metabolic actions of insulin and AMPK activation in muscle

It has been well established that both insulin and muscle contraction increase total blood flow to skeletal muscle. A number of studies have also demonstrated that insulin and muscle contraction increase muscle microvascular perfusion in vivo in both humans and animals. The haemodynamic responses to insulin have been thought to promote delivery of glucose and hormones to the myocyte to enhance glucose disposal. In models of insulin resistance, insulin mediated increases in blood flow (total and microvascular) and glucose uptake are impaired. In contrast contraction stimulated increases in blood flow and glucose uptake are generally not impaired in insulin resistant rodents and humans. Although increases in total blood flow, microvascular perfusion and glucose uptake in skeletal muscle can be stimulated by either insulin or muscle contraction these processes use different signalling pathways. The main focus of this thesis was to explore mechanisms involved in insulin- and contraction-stimulated haemodynamics in skeletal muscle and their impact on insulin-mediated glucose uptake. The in vivo techniques employed in this thesis include hyperinsulinemic euglycaemic clamps performed in anesthetised rats, arterial blood flow measurements by Transonic¬¨vÜflow probes, microvascular perfusion measurements by either metabolism of exogenously infused 1- methylxanthine (1-MX) or contrast enhanced ultrasound (CEU). Test agents were administered either systemically or locally into one hindlimb via the epigastric artery. Isolated resistance arteries were used in one study to compliment the in vivo studies. The hypothesis that a PI3K dependant signalling pathway, leading to activation of eNOS and the production of NO, is involved in the hemodynamic actions of insulin in muscle was examined. Wortmannin, a PI3K inhibitor, was infused systemically during an insulin clamp in vivo. Wortmannin administration resulted in inhibition of the haemodynamic effects of insulin including total flow and microvascular perfusion as well as glucose uptake. The relationship between insulin mediated NOS activation and microvascular perfusion was also studied. A NOS inhibitor, L-NAME, was infused locally in one leg and attenuated insulin action in skeletal muscle by inhibiting microvascular perfusion and blunting glucose uptake. AMPK is a potential mediator of metabolic changes in muscle during contraction and can be artificially activated by the compound AICAR. Acute activation of AMPK in vivo by low dose AICAR resulted in increased microvascular blood volume without effects on blood pressure, femoral blood flow or hindleg glucose uptake. In addition, in isolated resistance arteries, AICAR induced vasodilatation, which was abolished by the NO synthase inhibitor L-NA or the AMPK inhibitor compound C. Activation of AMPK by AICAR also sensitised smooth muscle cells to sodium nitroprusside (SNP) mediated vasodilatation and induced vasodilatation of resistance arteries. AMPK activation by AICAR resulted in marked enhancement of insulinmediated microvascular perfusion and glucose uptake. Collectively the findings in this thesis highlight the important role of microvascular perfusion for facilitating skeletal muscle glucose metabolism and explore some of the mechanisms which are involved. Insulin mediated microvascular perfusion occurs via a PI3K dependant pathway and is strongly associated with NOS activation. Activation of AMPK increases microvascular perfusion which is also associated with NOS activation. The combination of AMPK activation and insulin resulted in marked enhancement of insulinmediated microvascular perfusion and glucose uptake. Improvement of microvascular perfusion through targeting AMPK activation may provide a potential therapeutic avenue for enhancing glucose metabolism.