The role of pericytes in microcirculatory dysfunction after subarachnoid hemorrhage

Subarachnoid hemorrhage is a subtype of stroke that is caused by a bleeding into the subarachnoid space. Cerebral ischemia develops early after the bleeding and has a negative influence on outcome in patients. The underlying pathophysiology triggering early ischemia has not been characterized in detail. Suggested pathomechnisms are pial microvasospasm, endothelial dysfunction and microthrombosis. The aim of the current thesis was to characterize and reveal the pathophysiology of microcirculatory perfusion deficits early after subarachnoid hemorrhage. The main hypothesis was that pericytes constrict upon subarachnoid hemorrhage and thereby induce capillary spasm and hamper parenchymal blood flow dynamics. We found that pial arterioles constrict in three different characteristic patterns and that spastic vessel segments were continuously covered with vascular smooth muscle cells. Superficial microvasospasm was associated with reduced blood flow velocity and significant reduction of endothelial intracellular Ca2+ concentration which may be a trigger for endothelial dysfunction. Reduced blood flow velocity in combination with reduced vessel diameter diminished total blood volume that reached the parenchymal microcirculation via penetrating arterioles. This led to a severe reduction of perfused capillary volume in the cortex. Leukocyte numbers that were sticking in capillaries and venules were increased after subarachnoid hemorrhage but their numbers were too low to explain severe perfusion deficits. Capillaries revealed a significantly reduced vessel diameter after subarachnoid hemorrhage, however vessel narrowings were not co-localizing with sites where pericytes were associated to capillaries. Furthermore pericytes neither underwent cell death nor migrated away from capillaries within 24 hours after the bleeding. In conclusion we showed that microvasospasm on the brain surface lead to severe perfusion deficits in the parenchyma. Microvasospasm are probably induced by vascular smooth muscle cells and are accompanied by reduced intracellular Ca2+ concentration in endothelial cells. Pericytes do not play a major role in the pathophysiology of early ischemia after subarachnoid hemorrhage: they neither migrate, nor die or induce capillary spasm

The role of pericytes in microcirculatory dysfunction after subarachnoid hemorrhage

Subarachnoid hemorrhage is a subtype of stroke that is caused by a bleeding into the subarachnoid space. Cerebral ischemia develops early after the bleeding and has a negative influence on outcome in patients. The underlying pathophysiology triggering early ischemia has not been characterized in detail. Suggested pathomechnisms are pial microvasospasm, endothelial dysfunction and microthrombosis. The aim of the current thesis was to characterize and reveal the pathophysiology of microcirculatory perfusion deficits early after subarachnoid hemorrhage. The main hypothesis was that pericytes constrict upon subarachnoid hemorrhage and thereby induce capillary spasm and hamper parenchymal blood flow dynamics. We found that pial arterioles constrict in three different characteristic patterns and that spastic vessel segments were continuously covered with vascular smooth muscle cells. Superficial microvasospasm was associated with reduced blood flow velocity and significant reduction of endothelial intracellular Ca2+ concentration which may be a trigger for endothelial dysfunction. Reduced blood flow velocity in combination with reduced vessel diameter diminished total blood volume that reached the parenchymal microcirculation via penetrating arterioles. This led to a severe reduction of perfused capillary volume in the cortex. Leukocyte numbers that were sticking in capillaries and venules were increased after subarachnoid hemorrhage but their numbers were too low to explain severe perfusion deficits. Capillaries revealed a significantly reduced vessel diameter after subarachnoid hemorrhage, however vessel narrowings were not co-localizing with sites where pericytes were associated to capillaries. Furthermore pericytes neither underwent cell death nor migrated away from capillaries within 24 hours after the bleeding. In conclusion we showed that microvasospasm on the brain surface lead to severe perfusion deficits in the parenchyma. Microvasospasm are probably induced by vascular smooth muscle cells and are accompanied by reduced intracellular Ca2+ concentration in endothelial cells. Pericytes do not play a major role in the pathophysiology of early ischemia after subarachnoid hemorrhage: they neither migrate, nor die or induce capillary spasm

Microvasospasms After Experimental Subarachnoid Hemorrhage Do Not Depend on Endothelin A Receptors

Background and Purpose-Perturbations in cerebral microcirculation (eg, microvasospasms) and reduced neurovascular communication determine outcome after subarachnoid hemorrhage (SAH). ET-1 (endothelin-1) and its receptors have been implicated in the pathophysiology of large artery spasms after SAH;however, their role in the development of microvascular dysfunction is currently unknown. Here, we investigated whether inhibiting ETA (endothelin A) receptors can reduce microvasospasms after experimentally induced SAH. Methods-SAH was induced in male C57BL/6 mice by filament perforation of the middle cerebral artery. Three hours after SAH, a cranial window was prepared and the pial and parenchymal cerebral microcirculation was measured in vivo using two-photon microscopy before, during, and after administration of the ETA receptor inhibitor clazosentan. In separate experiments, the effect of clazosentan treatment on neurological outcome was measured 3 days after SAH. Results: Clazosentan treatment had no effect on the number or severity of SAH-induced cerebral microvasospasms nor did it affect neurological outcome. Conclusions: Our results indicate that ETA receptors, which mediate large artery spasms after SAH, do not seem to play a role in the development of microarterial spasms, suggesting that posthemorrhagic spasms are mediated by distinct mechanisms in large and small cerebral vessels. Given that cerebral microvessel dysfunction is a key factor for outcome after SAH, further research into the mechanisms that underlie posthemorrhagic microvasospasms is urgently needed

Role of Pial Microvasospasms and Leukocyte Plugging for Parenchymal Perfusion after Subarachnoid Hemorrhage Assessed by In Vivo Multi-Photon Microscopy

Subarachnoid hemorrhage (SAH) is associated with acute and delayed cerebral ischemia. We suggested spasms of pial arterioles as a possible mechanism; however, it remained unclear whether and how pial microvasospasms (MVSs) induce cerebral ischemia. Therefore, we used in vivo deep tissue imaging by two-photon microscopy to investigate MVSs together with the intraparenchymal microcirculation in a clinically relevant murine SAH model. Male C57BL/6 mice received a cranial window. Cerebral vessels and leukocytes were labelled with fluorescent dyes and imaged by in vivo two-photon microscopy before and three hours after SAH induced by filament perforation. After SAH, a large clot formed around the perforation site at the skull base, and blood distributed along the perivascular space of the middle cerebral artery up to the cerebral cortex. Comparing the cerebral microvasculature before and after SAH, we identified three different patterns of constrictions: pearl string, global, and bottleneck. At the same time, the volume of perfused intraparenchymal vessels and blood flow velocity in individual arterioles were significantly reduced by more than 60%. Plugging of capillaries by leukocytes was observed but infrequent. The current study demonstrates that perivascular blood is associated with spasms of pial arterioles and that these spasms result in a significant reduction in cortical perfusion after SAH. Thus, the pial microvasospasm seems to be an important mechanism by which blood in the subarachnoid space triggers cerebral ischemia after SAH. Identifying the mechanisms of pial vasospasm may therefore result in novel therapeutic options for SAH patients