Regional O₂ consumption and coronary flow during beta adrenoceptor stimulation in reperfused canine myocardium

This study was performed to assess the ability of a reperfused region to increase its flow and O₂ consumption upon inotropic stimulation. Isoproterenol (5 μg/kg/min) was infused i.v. during the reperfusion phase (4 hr) after 2 hr of the left anterior descending coronary artery occlusion in anesthetized open chest dogs (n = 7). The response was compared to a similar untreated reperfused group (n = 8). Coronary blood flow was obtained with labeled microspheres. Arterial and venous O₂ saturations were determined microspectrophotometrically. During reperfusion and isoproterenol infusion, significantly higher heart rate (226 ± 48 beats/min), maximum positive derivative of left ventricular pressure (3132 ± 838 mm Hg/sec) and systolic aortic blood pressure (162 ± 16 mm Hg) were recorded compared to reperfusion in the control group (133 ± 22, 1615 ± 184 and 124 ± 30, respectively). In the control group, coronary blood flow to the reperfused areas decreased from preocclusion values of 85 ± 22 and 95 ± 43 to 63 ± 45 and 47 ± 31 ml/min/100 g for the subepicardium and subendocardium, respectively. O₂ extraction was increased significantly in the reperfused region and O₂ consumption was similar to that in the nonoccluded area. The corresponding flow values in the treated group at the end of the reperfusion period were 272 ± 142 and 183 ± 150 ml/min/100 g, respectively. With isoproterenol, the O₂ extraction was not greater than in the nonoccluded area and the O₂ consumption increased about 3-fold in comparison to the control group, although the increase in the reperfused subendocardium was less. Thus, the reperfused ischemic myocardium responded to isoproterenol stimulation by increasing O₂ consumption. The response to isoproterenol in terms of flow and O₂ consumption appear greater in the reperfused subepicardium compared to the subendocardium.