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

PURPOSE: It is uncertain why retinal capillaries are particularly vulnerable to hypoxia. In this study, it was hypothesized that their specialized physiology, which includes being the predominant microvascular location of functional adenosine triphosphate-sensitive potassium (K(ATP)) channels, boosts their susceptibility to hypoxia-induced cell death.

METHODS

Cell viability, ionic currents, intracellular calcium, and pericyte contractility in microvascular complexes freshly isolated from the rat retina were assessed using trypan blue dye exclusion, perforated-patch recordings, fura-2 fluorescence, and time-lapse videos. Chemical hypoxia was induced by antimycin, an oxidative phosphorylation inhibitor.

RESULTS

In freshly isolated retinal microvascular complexes, chemical hypoxia caused more cell death in capillaries than in arterioles. Indicative of the role of polyamine-dependent K(ATP) channels, antimycin-induced capillary cell death was markedly decreased in microvessels treated with the polyamine synthesis inhibitor, difluoromethylornithine, or the K(ATP) channel inhibitor, glibenclamide. These inhibitors also diminished the antimycin-induced hyperpolarization, as well as the antimycin-induced intracellular calcium increase, which was significantly dependent on extracellular calcium and was diminished by the inhibitor of calcium-induced calcium release (CICR), dantrolene. Consistent with the importance of the CICR-dependent increase in capillary cell calcium, dantrolene significantly decreased hypoxia-induced capillary cell death. We also found that activation of the polyamine/K(ATP) channel/Ca(2+) influx/CICR pathway not only boosted the vulnerability of retinal capillaries to hypoxia, but also caused the contraction of capillary pericytes, whose vasoconstrictive effect may exacerbate hypoxia.

CONCLUSIONS

The vulnerability of retinal capillaries to hypoxia is boosted by a mechanism involving the polyamine/K(ATP) channel/Ca(2+) influx/CICR pathway. Discovery of this pathway should provide new targets for pharmacological interventions to minimize hypoxia-induced damage in retinal capillaries.