The protective effect of propranolol on ischemic myocardium was studied experimentally and clinically by electron microscope. In an animal experiment, ischemic changes were produced in the posterior papillary muscle of the rabbit following 3, 15, and 30 minutes of occlusion of the circumflex coronary artery. Propranolol (0.25 mg/kg) was injected into the left atrial cavity before occlusion of the artery. The posterior papillary muscle was excised and examined by electron microscope. In clinical experience, propranolol (20 µg/kg) was given intravenously to 6 patients who underwent open heart surgery. Transmural left ventricular myocardial biopsy was performed after the anoxic cardiac arrest and the material, particularly the subendocardium, was examined by electron microscope. It was shown that propranolol was effective, both in the experiment and in the clinical experience, in preserving ischemic myocardium. The possible mechanisms through which propranolol might act were considered to be (1) indirect effect of altered oxygen supply vs. demand, effected by reducing heart rate and reducing cardiac output due to the drug’s function as a beta blocker, (2) direct cellular effect, i.e., reducing myocardial substrate metabolism along with stabilization of cellular structure, and (3) increase collateral circulation to the subendocardium
