Vulnerability of the retinal microvasculature to hypoxia: role of polyamine-regulated K(ATP) channels.

This study addressed the question of why the capillaries of the retina are particularly prone to hypoxia-induced cell damage and death, which occurs during the course of a variety of retinal vascular disorders. Here, we considered the idea that specialized physiological adaptations of the retinal capillaries boost their vulnerability to hypoxia. Evidence is accumulating that within the circulatory system of the retina, there is functional specialization.1–3 For example, most of the functional adenosine triphosphate-sensitive potassium (KATP) channels are located in the capillaries.2 In contrast, the activity of voltage-dependent calcium channels (VDCCs) is minimal in this microvascular region, but is robust in the precapillary tertiary arterioles.3 An important operational result of this topographical distribution of ion channels is that the hyperpolarizing KATP current activated by vasoactive signals, such as adenosine, is generated almost exclusively in the capillaries2 and must be transmitted proximally to microvascular sites where VDCCs are available to transduce the induced voltage change into a vasomotor response that alters blood flow.2,3 Although this functional specialization within the retinal microvasculature appears to be important for the effective regulation of local perfusion, we hypothesized that the abundance of KATP channels may boost the vulnerability of the capillaries to hypoxic damage. How could an abundance of KATP channels boost capillary vulnerability to hypoxia? We posited that a hypoxia-induced drop in the ATP concentration activates the capillary KATP channels, whose function is inhibited by intracellular ATP. Due to the hypoxia-induced activation of KATP channels, increased K+ efflux via these channels would cause hyperpolarization, which in turn, would increase the electrochemical gradient for the influx of calcium via nonspecific cation (NSC) channels, which are the predominant calcium-permeable ion channels expressed in retinal capillaries.4 Because increased intracellular calcium is known to exacerbate hypoxic damage in a variety of cell types,5,6 we proposed a working model in which the KATP channel-dependent increase in cell calcium boosts the vulnerability of retinal capillaries to hypoxia. In addition to KATP channels, we hypothesized that endogenous polyamines play a role in establishing the vulnerability of retinal capillaries to hypoxia. These ornithine-derived molecules were of interest because we found previously2 that the function of microvascular KATP channels, which are redox-sensitive,2 is dependent on endogenous polyamines, whose catabolism generates H2O2.7 Consistent with polyamines having a role in capillary cell death, these molecules are known to modulate death pathways in a variety of cell types,8 although its diversity of effects, which include enhancing and inhibiting cell death, remain confounding, and the mechanisms by which polyamines affect cell viability are incompletely understood. In this study, we tested the novel hypothesis that by regulating the function of KATP channels, endogenous polyamines may play a role in establishing the lethality of hypoxia in the capillaries of the retina. We report that in freshly isolated retinal microvascular complexes, the inhibitor of oxidative phosphorylation, antimycin A, causes substantially more cell death in the capillaries than in the precapillary arterioles. Experiments using the patch-clamp technique, calcium-imaging, time-lapse photography, and the trypan blue viability assay provided evidence that the greater vulnerability of the capillaries to hypoxia-induced cell death is due to the activation of a pathway involving endogenous polyamines, hyperpolarizing KATP channels, calcium influx, and calcium-induced calcium release (CICR). Our experimental results indicate that activation of the polyamine/KATP channel/Ca2+ influx/CICR pathway is a previously unappreciated mechanism by which the vulnerability of retinal capillaries to hypoxia is boosted.