Cardiac Antiischemic Effect of Metoprolol: Role of β-Blockade within the Ischemic Region

The distribution of metoprolol and atenolol into ischemic and nonischemic myocardium was studied in anesthetized dogs, pigs, and cats. The β-blockers were administered intravenously after coronary artery occlusion. Metoprolol was found to be significantly more efficiently distributed to the ischemic myocardium than atenolol in all three species. To investigate the functional implications of this difference in tissue distribution, the antiischemic effects of the two β-blockers were studied in the 2-h period following coronary artery occlusion in anesthetized cats, in which heart rate was kept at a constant level. In this model, metoprolol (0.3 mg ± kg + 0.15 mg ± kg ± h) was found to attenuate or delay the developing ischemic process. This is shown by its significant reduction of (a) the decline of CK activity in ischemic myocardium, (b) the ST elevation in a precordial ECG lead, and (c) the decrease of arterial pressure and cardiac output. In contrast to metoprolol, atenolol (0.3 mg ± kg + 0.15 mg ± kg ± h) caused no significant antiischemic effect in this cat model. The difference in the effectiveness of the two drugs can most probably be explained by their differential distribution in the ischemic heart. Furthermore, the anti-ischemic effect of metoprolol shows that the presence of a β-blocker in ischemic left ventricular myocardium can favorably affect the early phase of developing infarction.