Threshold duration of ischemia for myogenic tone in middle cerebral arteries - Effect on vascular smooth muscle actin

Background and Purpose - We investigated the effect of different periods of ischemia on the myogenic tone of middle cerebral arteries (MCAs) and teste d the hypothesis that ischemia disrupts the actin cytoskeleton in vascular smooth muscle. Methods The MCA occlusion model was used in male Wistar rats (n=27) to indu ce different periods of ischemia (15, 30, and 120 minutes) with 24 hours of reperfusion. Successful occlusion was determined by laser-Doppler flowmetr y. MCAs were then studied in vitro with a specialized arteriograph system t hat allowed control of transmural pressure and measurement of lumen diamete r. After equilibration for 1 hour at transmural pressure of 75 mm Hg, lumen diameter was measured, and the amount of spontaneous myogenic tone was det ermined. Arteries were then fixed with 10% formalin while still pressurized in the arteriograph bath and stained for filamentous (F-) actin with fluor escently labeled phalloidin, a specific probe for F-actin. The amount of F- actin was quantified by confocal microscopy. Results - 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 sig nificantly diminished after 30 and 120 minutes (11.7 +/-2.0% and 8.5 +/-2.0 %, respectively; P <0.01 versus control). F-actin content also decreased at the longer ischemic periods and correlated significantly with vascular ton e (P = 0.04) such that the lesser the tone, the lesser was the F-actin cont ent. Fluorescence intensity for control and 15, 30, and 120 minutes of isch emia was (X10(7)) 3.21 +/-0.25, 2.54 +/-0.32 (P >0.05), 2.32 +/-0.15 (P <0. 01), and 2.22 +/-0.16 (P <0.01), respectively. Conclusions - These results demonstrate that ischemia disrupts the actin cy toskeleton in smooth muscle and diminishes vascular tone of MCAs in a thres hold-dependent manner. This effect likely exacerbates brain tissue damage d uring stroke, including infarction and edema formation.