THE HAEMODYNAMIC EFFECTS OF PROLONGED ORAL ADMINISTRATION OF OXYFEDRINE, A PARTIAL AGONIST AT β‐ADRENOCEPTORS: COMPARISON WITH PROPRANOLOL

1 Haemodynamic changes have been studied in cats after the chronic oral administration of oxyfedrine (14 mg/kg for 3-4 weeks) or of placebo (lactose). The initial part of the study was carried out under double-blind conditions. The arterial blood pressure was between 19 mmHg (diastolic) and 27 mmHg (systolic) higher in the oxyfedrine treated animals, but there were no differences between the two groups with regard to cardiac output, left ventricular dP/dt max, heart rate or systolic ejection time.2 In cats similarly treated with propranolol (4 mg/kg) there was a slight (12%), but significant, reduction in cardiac output.3 Isoprenaline dose-response curves were shifted to the right in the cats administered oxyfedrine as well as in those administered propranolol. The degree of shift was five-fold (positive chronotropic response) and 20-fold (decrease in diastolic blood pressure) in the oxyfedrine group and 10- and 80-fold, respectively, in the propranolol group.4 In contrast to the partial blockade of the effects of isoprenaline, the haemodynamic response to oxyfedrine was largely unaltered, both in the cats pretreated with propranolol and in those pretreated with oxyfedrine. The pressor response to noradrenaline was potentiated in the cats pretreated with oxyfedrine.5 These results provide an explanation for the anti-anginal action of oxyfedrine. Some degree of beta-adrenoceptor blockade is achieved without a reduction in cardiac output or left ventricular dP/dt max. The relevance of these findings to the haemodynamic situation in angina is discussed.