Threshold Duration of Ischemia for Myogenic Tone in Middle Cerebral Arteries

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Abstract
We investigated the effect of different periods of ischemia on the myogenic tone of middle cerebral arteries (MCAs) and tested the hypothesis that ischemia disrupts the actin cytoskeleton in vascular smooth muscle. The MCA occlusion model was used in male Wistar rats (n=27) to induce different periods of ischemia (15, 30, and 120 minutes) with 24 hours of reperfusion. Successful occlusion was determined by laser-Doppler flowmetry. MCAs were then studied in vitro with a specialized arteriograph system that allowed control of transmural pressure and measurement of lumen diameter. After equilibration for 1 hour at transmural pressure of 75 mm Hg, lumen diameter was measured, and the amount of spontaneous myogenic tone was determined. Arteries were then fixed with 10% formalin while still pressurized in the arteriograph bath and stained for filamentous (F-) actin with fluorescently labeled phalloidin, a specific probe for F-actin. The amount of F-actin was quantified by confocal microscopy. The amount of tone was similar between control and 15 minutes of ischemia (27.0+/-2.0% and 25.3+/-1.7%, respectively; P>0.05) but was significantly diminished after 30 and 120 minutes (11.7+/-2.0% and 8.5+/-2.0%, respectively; P0.05), 2.32+/-0.15 (P<0.01), and 2.22+/-0.16 (P<0.01), respectively. These results demonstrate that ischemia disrupts the actin cytoskeleton in smooth muscle and diminishes vascular tone of MCAs in a threshold-dependent manner. This effect likely exacerbates brain tissue damage during stroke, including infarction and edema formation.