Correlation of beta blockade with serum practolol levels after oral administration

This study relates practolol levels with the ability to block changes in heart rfite, blood pressure, and systolic time intervals (STI) during two levels of upright exercise and two levels of intravenous isoproterenol infusion. Normal men received placebo or oral practolol (1.5, 3.0, and 12 mg. per kilogram) on separate days in random fashion. Responses to exercise and isoproterenol were determined 2 and 4 hours later and those to exercise only at 7.5 hours. Serum pmctolol was measured in a serial manner after dosage. Practolol did not change oxygen uptake at rest or during exercise. Practolol produced a serum concentration‐dependent reduction in average heart rate and systolic blood pressure responses to exercise. Exercise shortened STI. Following practolol, electromechanical systole and left ventricular ejection time during exercise were markedly prolonged, whereas the duration of the prejection period did not change. Pmctolol produced a serum concentration‐dependent reduction in heart rate, systolic and diastolic blood pressure, and STI changes during isoproterenol infusion. Near maximal beta blockade occurred at serum practolollevels of 2 εg per milliliter. The half‐life of practolol was 6.9 hours. Practolol is a potent beta blocking drug with a relatively long duration of action when given orally. The serum concentration‐dependent reduction of the diastolic response to isoproterenol by practolol suggests blockade of peripheral beta receptors.