Loss of insulin-mediated vasoprotection: Early effect of diabetes on pericyte-containing microvessels of the retina

PURPOSE. Microvascular cell death is a prominent pathologic feature of the retinopathy associated with insulin-deficient diabetes. The aim of this study was to test the hypothesis that reduced insulin action may contribute,e to microvascular damage in the diabetic retina. METHODS. Microvascular complexes were isolated from retinas of healthy rats and those made insulin deficient by streptozotocin. As a model of ischemia, freshly isolated microvessels were maintained in a glucose-free, low-oxygen solution. Cell viability in pericyte-containing retinal microvessels was assayed by trypan blue dye exclusion. Cleaved caspase-3 immunoreactivity and nuclear morphology were used to detect apoptotic cells. RESULTS. Ischemia significantly increased apoptotic cell death in pericyte-containing microvessels of the normal rat retina. In a dose-dependent (IC50 = 600 pM) manner, physiological concentrations of insulin markedly decreased ischemic cell death in the retinal microvasculature. This insulin-mediated vasoprotection was prevented by inhibitors of phosphatidylinositol 3-OH kinase and extracellular signal-regulated kinase. Soon after the onset of diabetes, insulin failed to significantly diminish cell death in ischemic retinal microvessels, in contrast to the prosurvival effect of this hormone on nondiabetic microvessels. CONCLUSIONS. Results of this study support the hypothesis that a physiological concentration of insulin exerts a prosurvival effect on ischemic retinal microvessels and that diabetes impairs the ability of insulin to protect against ischemia-induced microvascular cell death. The authors propose that the loss of insulin-mediated vasoprotection may be a previously unappreciated mechanism by which diabetic retinopathy progresses.