Effect of β-adrenoceptor stimulation or blockade on regional myocardial function and regional O₂ consumption during myocardial ischemia

The purpose of this study was to determine the effect of β-adrenoceptor activation and blockade on the relationship between regional myocardial function and regional O₂ consumption in ischemic and nonischemic myocardium. Myocardial regional segmental function was assessed in 28 open chest, anesthetized dogs using subepicardial dimension gauges. Ten min after LAD (left anterior descending coronary artery) occlusion, dogs were given i.v. saline, 2 mg/kg propranolol, 0.2 mg/kg pindolol, or 1 μg/kg/min isoproterenol. Coronary blood flow was determined using radioactive microspheres before LAD occlusion, 10 min after occlusion, and 2 hr after LAD occlusion. Regional O₂ consumption was determined using microspectrophotometry. LAD occlusion did not alter any index of myocardial function measured in the nonischemic region, but in the ischemic region, end systolic length (ESL) was increased 20% while shortening was converted to systolic bulging. No agent resulted in an improved ischemic regional function or an altered O₂ consumption during LAD occlusion. In the nonischemic region the per cent shortening was increased 60% with isoproterenol compared to control. Propranolol and pindolol both increased the nonschemic regional ratio of per cent shortening vs O₂ consumption significantly, suggesting an improved efficiency while isoproterenol lowered this ratio. When per cent shortening was plotted vs regional O₂ consumption for all treatments, a significant linear relationship was observed in the nonischemic region. Thus, no drug treatment used in this study significantly improved central ischemic regional function, or O₂ consumption, but both β-adrenoceptor blockers seemed to result in an improved relationship between segmental shortening and O₂ consumption in the nonischemic region.