Oxidative stress occurs if oxygen and free electrons (energy) are available in excess. Because of the high blood flow and direct influence of light, oxidative stress is well known to occur in the choroid and retina.
24 Moreover, cycles of ischemia and reperfusion are also typical in such situations: During ischemia free electrons accumulate in the respiratory chain. After transition to reperfusion, oxygen availability increases over a short period of time, and free electrons can convert oxygen to a radical. Hydrogen peroxide is known to play a role in the regulation of coronary blood flow, dilating the vessels at low concentration levels, while acting as an antagonist to nitric oxide at higher levels.
25 The physiological role of ROS could, therefore, be a factor for vasodilation, which has been described in various previous articles. It has been hypothesized that ROS react as an endothelium-derived hyperpolarizing factor (EDHF),
26,27 although it is unclear whether the action is promoted by the ROS or H
2O
2.
26,28 The EDHF leads to a hyperpolarization that results in a reduced influx of Ca
2+ into the muscle cells. This may be promoted by an activation of the Na
+/H
+-exchanger (NHE),
29 which produces a rise in intracellular Na
+. The increased levels of Na
+ lead to at least two effects. The first is that Na
+/K
+-ATPase is activated to eliminate intracellular Na
+, leading to a hyperpolarization and subsequent relaxation of the smooth muscle cells. That ouabain blocks the relaxation in our experiments supports this theory, as it blocks the Na
+/K
+-ATPase and, therefore, eliminates a part of the downstream action. The second effect is that NCX is activated in reverse mode to lower intracellular Na
+ levels. This also results in increased intracellular Ca
2+ levels, which is normally overruled by the hyperpolarization but leads to a constriction under predepolarized conditions. The involvement of the NCX in reverse mode with resulting increased intracellular Ca
2+ is supported by the finding that blocking the NCX in reverse mode using KB-R7943 leads to reduced constriction on ROS treatment.