Experimental Study Endothelial Insulin Resistance Protects the Heart Against Prolonged Ischemia– Reperfusion Injury But Does Not Prevent Insulin Transport Across the Endothelium in a Mouse Langendorff Model

Aim: The endothelium plays an important role in the maintenance of cardiovascular homeostasis in healthy individuals. Insulin resistance can lead to the development of endothelial dysfunction, which is an important step in the pathogenesis of ather-osclerosis. We investigated specifically whether the presence of vascular insulin resistance and endothelial dysfunction has any influence on the myocardial tolerance to ischemia–reperfusion (IR) injury, using Endothelial Specific Mutant Insulin Receptor Over-expressing (ESMIRO) mice, which exhibit vascular insulin resistance and vascular dysfunction. Methods: ESMIRO or wild-type (WT) littermate mouse hearts were isolated and perfused on a Langendorff apparatus. These were subjected to either 35-minute or 45-minute ischemia followed by reperfusion, after which infarct size was determined. The ability of insulin to activate its target kinase pathway, that is, phosphoinositide 3 (PI3) kinase/protein kinase B (AKT) in ESMIRO hearts was also assessed by Western blot analysis. Results: Compared to 35-minute ischemia, the extended 45-minute ischemic protocol significantly exa-cerbated myocardial infarction in WT mice, (56%+ 4%, n 6 vs 32%+ 4%, n 9; P <.01) but not in ESMIRO littermates (34% + 7%, n 6 vs 32%+ 3%, n 9; not significant), suggesting some form of protective phenotype. Insulin treatment was associated with a significant increase in AKT phosphorylation in the myocardium in both the ESMIRO mice andWT littermates, and this was attenuated in both by inhibition of PI3 kinase using LY294002. Thus, insulin was able to directly activate PI3 kinase/AKT in the myocardium despite the absence of functional endothelial insulin receptors in the ESMIRO mice. Conclusion: (1) Insulin at pharmacologic concentrations can be transported across the endothelium independent of vascular insulin receptors and (2) vascular insulin resistance and/or endothelial dysfunction are protective against prolonged IR injury in the Langendorff model