The Effects Of Exercise On Capillary Supply To The 'transition Zone' Of Infarcted Rat Hearts
Within nine hours following coronary artery occlusion, muscle fibers adjacent to the developing necrosis (those in the border or 'transition' zone) are thought to be viable but potentially ischemic. The long-term (> 24 hour) fate of these fibers is not known. My objective was to determine whether a transition zone is present in an animal model of chronic myocardial infarction. As ischemia implies reduced oxygenation, possible due to reduced capillary supply, I quantified the capillary supply to the region adjacent to the necrosis in infarcted hearts, and compared this to values in healthy myocardium.;Myocardial infarcts were produced in rats by ligating the left coronary artery (LCA) midway along its course. Five weeks later, the capillary supply was found to be significantly less than that measured in healthy rat hearts. These results confirm the chronic presence of a transition zone, defined as a region of viable muscle fibers with a subnormal microvascular supply, in the rat model of myocardial infarction.;Evidence from the literature indicates that exercise can stimulate capillary growth in skeletal muscle and healthy myocardium. My second objective was to determine whether voluntary exercise (running) also promotes revascularization in the transition zone.;One week following LCA occlusion, rats were allotted into one of four exercise protocols; these differed in the frequency of running and duration of the protocol. Exercise was associated with significant improvements in capillary supply in the transition zone. Important factors in this revascularization process included: (1) an intermediate total distance run. Rats exercised 6 days/week for 4 weeks, that ran a total of 5 to 10 km, restored a normal capillary supply in the transition zone; no significant increases were demonstrated in animals running 10 km. (2) a balance between exercise frequency and mean running speed. Animals exercised half as often had to run at twice the mean speed as rats exercised 6 days/week to obtain the same degree of improvement in capillary supply. (3) regular reinforcement of the exercise periods during the protocol.;Thus, moderate voluntary exercise, if reinforced regularly, stimulates revascularization in the transition zone of infarcted rat hearts.
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