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M. Fukumoto, D. G. Puro; Vulnerability of Retinal Capillaries to Oxidative Stress: Role of Endogenous Spermine. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1445.
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It is unclear why retinal capillaries are particularly vulnerable to diabetes. We postulate that the oxidized state of cells in the capillaries makes them particularly sensitive to oxidative stress, which is a hallmark of the diabetic retina. This idea is based on our recent report that the redox status of capillary cells is driven by spermine-dependent oxidation (J Physiol 587:2233, 2009); spermine is a polyamine that is abundant in retinal capillaries (J Physiol 573:483, 2006) and is catabolized to H2O2 and other potent oxidants. Here, we tested the hypothesis that spermine plays a key role in making retinal capillaries particularly prone to oxidant overload.
In microvascular complexes freshly isolated from non-diabetic and diabetic rat retinas, trypan blue dye exclusion was used to quantify H2O2-induced cell death in the capillary network and in the feeder vessels that link capillaries to myocyte-encircled arterioles. Diabetes was induced by streptozotocin. Standard statistical methods were used.
Dose-response relationships for H2O2-induced cell death demonstrated that capillaries are more vulnerable to oxidants than feeder vessels. Namely, in capillaries the EC50 for H2O2 was 9 µM, which was significantly less than the EC50 of 15 µM for feeder vessels. Indicative of spermine having an important role, the EC50 for H2O2-induced capillary cell death was significantly increased to ~40 µM when microvessels were pre-incubated in DFMO, a spermine synthesis inhibitor. Although DFMO also lessened the vulnerability to H2O2 of the feeder vessels, we found that at H2O2 concentrations between 8 and 15 µM, the DFMO-sensitive component of the H2O2-induced cell death was ~2-fold greater in capillaries than in feeder vessels. Here, we also assessed the effect of H2O2 on diabetic microvessels. Consistent with our reports that diabetes increases spermine in retinal feeder vessels, the sensitivity of diabetic feeder vessels to H2O2 was significantly increased via a spermine-dependent mechanism. However, despite the diabetes-induced increase in feeder vessel sensitivity to oxidative stress, cells in diabetic capillaries remained significantly more vulnerable to H2O2 than feeder vessel cells.
Endogenous spermine renders retinal capillaries particularly vulnerable to oxidant-induced cell death. Because oxidative stress is likely to play a role in diabetic retinopathy, our finding that the inhibition of spermine synthesis protects against oxidant toxicity suggests a new therapeutic strategy to minimize microvascular cell death in the diabetic retina.
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