Effect of cardioselective beta-blockade by metoprolol on the phasic myocardial blood content and epi/endocardial ratio.

Using I131-albumin labelling of plasma and a special biopsy cutting device, the blood content of the subendocardium and subepicardium was determined in guinea pig hearts. Measurements were done both in untreated animals and after intravenous administration of metoprolol, a new cardioselective beta-blocking compound. The data were compared with results obtained with propranolol and practolol. Metoprolol decreased the blood content less than propranolol. Practolol in a small dose even increased the blood content in the subendocardium in end-systole. The heart rate decreased by about the same magnitude after all the compounds tested, suggesting that the cardioselectivity of metoprolol was the likely explanation for its differing response from propranolol.