The Endothelial Barrier Is Not Rate-limiting to Insulin Action in the Myocardium of Male Mice

To act on tissues, circulating insulin must perfuse the relevant organ and then leave the bloodstream by crossing the endothelium-a process known as insulin delivery. It has been postulated that the continuous endothelium is a rate-limiting barrier to insulin delivery but existing data are contradictory. This conflict is in part due to the limitations of current models, including the inability to maintain a constant blood pressure in animals and the absence of shear stress in cultured cells. We developed a murine cardiac ex vivo perfusion model that delivers insulin to the heart in situ at a constant flow. We hypothesized that if the endothelial barrier were rate-limiting to insulin delivery, increasing endothelial permeability would accelerate insulin action. The kinetics of myocardial insulin action were determined in the presence or absence of agents that increased endothelial permeability. Permeability was measured using Evans Blue, which binds with high affinity to albumin. During our experiments, the myocardium remained sensitive to insulin and the vasculature retained barrier integrity. Perfusion with insulin induced Akt phosphorylation in myocytes but not in the endothelium. Infusion of platelet-activating factor or vascular endothelial growth factor significantly increased permeability to albumin without altering insulin action. Amiloride, an inhibitor of fluid-phase uptake, also did not alter insulin action. These data suggest that the endothelial barrier is not rate limiting to insulin's action in the heart; its passage out of the coronary circulation is consistent with diffusion or convection. Modulation of transendothelial transport to overcome insulin resistance is unlikely to be a viable therapeutic strategy.