L-Arginine reduces downstream vascular contractility after flow-diverting device deployment: A preliminary study in a rabbit model.

BACKGROUND: Flow diverters (FDs) are an effective treatment for intracranial aneurysms, though not free from hemorrhagic complications. A previous study demonstrated increased vascular contractility after FD-implantation as a potential mechanism of distal complications. Our study aimed to investigate whether L-arginine medication affects vascular contractility following FD deployment in a rabbit model. METHODS: FDs were implanted in the aorta of normal rabbits (+FD, n = 10), with sham-operated aorta as controls (n = 5). L-Arginine was given in the drinking water (2.25% L-arginine hydrochloride) of half of the +FD animals (+FD/+Arg). Force contraction vascular contractility studies were performed on the aortic rings proximal and distal to the FD using an organ bath. Total eNOS, eNOS(pS1177), eNOS(pT495), COX-2, and S100A4 were quantified by western analysis on total protein lysates from aortic segments, normalizing to GAPDH. RESULTS: Mean vascular contractility was 53% higher in distal relative to proximal aortic segments (P = 0.0038) in +FD animals, but were not significantly different in +FD/+Arg animals, or in sham-operated controls. The +FD animals expressed significantly reduced levels of eNOS(pS1177) than sham-operated controls (P = 0.0335), while both the +FD and +FD/+Arg groups had reduced levels of eNOS(pT495) relative to sham-operated controls (P = 0.0331 and P = 0.0311, respectively). CONCLUSION: These results suggest that L-arginine medication reduces distal vascular contractility after FD treatment via nitric oxide production and thus might mitigate risk for downstream complications.

Downstream vascular changes after flow-diverting device deployment in a rabbit model.

BACKGROUND: Flow diverters (FDs) are increasingly used in the treatment of intracranial aneurysms, and carry the risk of thromboembolic complications, even in patients treated with dual antiplatelet therapy. The effect of FDs on the downstream vascular is unknown. The aim of the study was to investigate vascular wall pulse wave velocity (PWV) and contractility changes following FD treatment in a rabbit model. METHODS: FDs (Pipeline Embolic Device, Medtronic Inc., Irvine, California, USA) were implanted in the aorta of normal rabbits and sham-operated aorta were used as controls (n=6 per group). Pulse wave imaging with ultra-fast ultrasound at 1600 frames per second (Vantage, Verasonics, Inc., Kirkland, WA) was performed in the vessel wall distal to FD prior to device implantation and at 8- week follow-up to measure the PWV. Force contraction vascular reactivity studies were conducted in the aortic rings using an organ bath. RESULTS: The difference in mean PWV in the follow-up compared with pre-implantation was significantly higher in the distal vessels compared with sham controls (1.18 m/s [SD=0.54] vs. 0.37 m/s [SD=1.09], P=0.03). Conversely, the aortic segments distal to the FD exhibited a 55% increase in vascular contractility compared with proximal segments (P=0.002). We observed a significant positive correlation between mean PWV and mean vascular contractility. CONCLUSION: Implantation of FD was associated with increased PWV and vascular contractility, suggesting that FD implantation causes changes to the vascular wall. Further studies are needed to understand the clinical implication of changes in vascular PWV and contractility.

Rabbit aneurysm models mimic histologic wall types identified in human intracranial aneurysms.

BACKGROUND: Semiquantitative scales correlate histopathologic findings in the walls of human aneurysms with rupture status. OBJECTIVE: To apply a semiquantitative scale to the rabbit elastase-induced aneurysm model to determine whether rabbit histologic types mimic the full range of histologic subtypes of humans. MATERIALS AND METHODS: Twenty-seven elastase-induced female rabbit aneurysms were studied, harvested at 2 weeks (n=5) and 12 weeks (n=22). Paraffin-embedded sections received hematoxylin-eosin and Verhoeff-Van Gieson staining. Immunohistochemistry was performed for α-smooth muscle actin and CD31 for endothelial cells. A semiquantitative scale was used for scoring based on human aneurysm tissue, divided into four subtypes according to cellular and extracellular matrix findings: type A, linear organized smooth muscle cells (SMCs) and intact endothelium; type B, thickened wall with disorganized, proliferating SMCs; type C, thick, collagenized and hypocellular wall with or without organizing thrombosis, and type D, extremely thin, hypocellular wall. Separate scoring was performed of the aneurysm neck and proximal and distal zones. RESULTS: Findings compatible with all subtypes of human aneurysm tissue were identified. Types A and C were found in 13 (48%) and 11 (41%) of 27 aneurysms and in the proximal and distal wall at both time points. Type B was found in 16 aneurysms (59%), exclusively at the neck at both time points; type D, in 14 aneurysms (52%), exclusively at proximal and distal zones of 12-week aneurysms. CONCLUSIONS: The wall of elastase-induced rabbit aneurysm demonstrates histologic findings similar to the four categories of human cerebral aneurysms based on cellular and extracellular wall content.

Assessment of endothelialization of aneurysm wall over time in a rabbit model through CD31 scoring.

BACKGROUND: Intracranial aneurysms represent a significant health concern and are poorly understood despite decades of research. Our study focused on understanding temporal patterns of endothelial cell distribution in different spatial locations within the aneurysm early after creation in a rabbit model. METHODS: Elastase induced saccular aneurysms were created in rabbits and harvested on day 1 (n=3) and after 2 (n=5), 4 (n=4), 8 (n=5), and 12 (n=6) weeks. Sham operated controls (n=3) were harvested on the same day. Aneurysm and control tissue samples were subjected to en face whole mount CD31 staining for endothelial cells. Semiquantitative scoring was performed on the basis of endothelial coverage of the vessel wall (proximal, middle, and distal portions of the aneurysm dome). Mixed effects models were used to assess the effect of time and aneurysm section on endothelial coverage. RESULTS: Aneurysmal segments were near completely de-endothelialized at 4 and 8 weeks but had re-endothelialized by 12 weeks. Compared with controls, aneurysms at all time points showed decreased endothelialization, but the difference was only significant compared with the 4 and 8 week groups. Both time (P=0.03) and aneurysm section (P=0.07) were significantly associated with the degree of endothelialization. Proximal locations showed increased endothelialization compared with distal locations (P=0.03). CONCLUSION: In experimental aneurysms of rabbits, endothelial cells regress during the first month after creation, followed by ascending re-endothelialization that stays incomplete. These findings suggest that re-population of endothelial cells comes from resident cells in the adjacent parent artery and that deranged hemodynamics may affect full reconstitution of endothelial cells long term.