Recent advances in acute stroke therapy have focused on reperfusion. Reperfusion however, is not always associated with good outcomes. Further improvements in outcome will require a better understanding of the physiology associated with reperfusing ischemic brain tissue. It has been noted that recanalization of an occluded cerebral artery does not guarantee reperfusion of the corresponding cerebral tissue due to microcirculatory constriction. This is known as the “no-reflow phenomenon.” Pericytes play a major role in regulating capillary blood flow and respond to multiple chemical signals originating from the surrounding astrocytes and neurons. Yemisci et al. (Nature Medicine 2009; 15:1031–1037) hypothesized that pericytes contract in response to oxygen and nitrogen radicals in the setting of ischemic brain injury, and do not relax even after recanalization, thus explaining the “no-reflow” phenomenon. They further hypothesize that reversing this contraction may impart a neuroprotective effect.
The authors visualized vessels injected with horseradish peroxidase (HRP) after subjecting the brain to 2 hours of ischemia followed by 6 hours of reperfusion. There were significantly more vessel constrictions in the ischemic hemisphere when compared to the contralateral nonischemic hemisphere. Pericytes were then marked with antibodies against α-smooth muscle actin (SMA). Optical sections revealed that the location of the pericytes matched the location of the constrictions.
Ischemic-like conditions were created in vitro by placing pericytes in an oxygen-glucose deprived solution, causing a decrease in the luminal diameter of capillaries. The capillaries did not dilate back to normal diameter after reverting to a physiologic solution.
The authors then tested the role of oxygen and nitrogen radicals in the ischemia-reperfusion induced pericyte contraction. For this purpose, they added a superoxide scavenger, N-tert-butyl-α-phenylnitrone (PBN), and a low dose of nitric oxide synthase (NOS) inhibitor, Nω-nitro-L-arginine (L-NA) just before reperfusion. Both agents relieved pericyte contraction.
Effects of ischemia on pericyte encapsulated microvessels. A, HRP-filed microvessels show discontinuities in ischemic hemispheres in contrast to the slender, thread-like structures in non ischemic hemispheres. B, interruptions in erythrocyte columns in the ischemic hemisphere. C, α-SMA pericytes colocalized with the constrictions. D, E, pericytes (arrows) seen in vitro before and after OGD in isolated mouse retina.
Effects of ischemia on pericyte encapsulated microvessels. A, HRP-filed microvessels show discontinuities in ischemic hemispheres in contrast to the slender, thread-like structures in non ischemic hemispheres. B, interruptions in erythrocyte columns in the ischemic hemisphere. C, α-SMA pericytes colocalized with the constrictions. D, E, pericytes (arrows) seen in vitro before and after OGD in isolated mouse retina.
As for the specific role of endothelial NOS (eNOS), the authors proved that its inhibition with L-N5-(1-iminoethyl)-ornithine (L-NIO), leads to a suppression of the NO surge in response to ischemia without affecting the basal NO secretion, therefore causing less peroxynitrite formation. L-NIO restored patency of microvessels in ischemic brain. In fact, in vivo injection of peroxynitrite caused pericyte contraction and capillary constriction and its in vitro addition to isolated retina had the same effect. Reoxygenation with the addition of PBN relieved both pericyte contraction and capillary constriction. In contrast, a selective neuronal NOS inhibitor was ineffective when administered in vivo shortly before reperfusion.
Interestingly, ischemic volume after ischemia-reperfusion was significantly lower in the groups treated with L-NIO, PBN or L-NA than in the group treated with a saline solution. All 3 agents were found to be equally neuroprotective even though L-NA does not cross the blood-brain barrier. The inhibition of the oxidative-nitrative stress by the aforementioned agents significantly decreased the number of vessels harboring entrapped erythrocytes.
Despite recent advances in recanalization techniques, stroke remains the leading cause of disability and the 3rd leading cause of mortality in the United States. Insights gained from this work may pave the way for clinical trials aimed at improving brain tissue perfusion after periods of ischemia. This has implications for acute endovascular stroke therapy, temporary arterial occlusion during aneurysm surgery as well as a plethora of other clinical scenarios in neurosurgery.
RUDY J. RAHME
BERNARD R. BENDOK
