Effect of hypoxia on the microvascular function in isolated rat hearts

The effect of hypoxia on the microvascular function in continuously perfused isolated rat hearts was studied. The development of hypoxic contracture was determined by measuring the left ventricular pressure or the decrease of the left ventricular volume. The myocardial perfusability was tested by infusing 0.1 percent sodium fluorescein in isotonic saline into the cannulated aortic root at 0, 15, 30, 60 and 90 minutes of hypoxic glucose-free buffer perfusion. The percentage perfused with the fluorescent tracer in horizontal frozen myocardial sections was estimated by point counting from colour photographs taken under ultraviolet light. The coronary flow was estimated by measuring the flow of the perfusate through the vasculature under constant perfusion pressure. The hypoxic contracture was completed in 25 minutes in which time the coronary flow decreased gradually by 50 percent and thereafter the flow remained constant. The myocardium was fully perfused in controls and at 0 minutes of hypoxic glucose-free perfusion. There was an abrupt decrease in the perfusability between 15 and 30 minutes from 99 percent to 58 percent, respectively. At 60 and 90 minutes 65 percent of the myocardium were still perfused. It was concluded that the myocardial perfusability decreases when the left ventricular contracture develops during hypoxic perfusion.