Acute effects of propranolol on the circulation and on oxygen uptake in conscious rats.

1 The acute effect of orally administered propranolol (100 mg/kg) on blood pressure, heart rate, cardiac output, venous pressure, haemoglobin concentration and oxygen uptake was studied in conscious rats. 2 Oxygen uptake was measured in a closed circuit respirometer and cardiac output determined by the direct Fick method. Other variables were measured by means of chronically implanted cannulae. 3 Significant falls occurred in heart rate (8.9%), cardiac output (30.5%), derived stroke volume (21.3%) and oxygen uptake (12.2%). 4 There were significant rises in arterial blood pressure (12.9%), derived arterial resistance (69%), arterio-venous oxygen difference (31.5%) and arterial haemoglobin concentration (6.3%). 5 A rise in venous pressure occurred in lightly anaesthetized rats but was only of borderline significance. 6 Correlations between the different variables revealed only one significant relationship, between heart rate and oxygen uptake. 7 It is concluded that only one primary site of action of propranolol has to be postulated to account for these findings, that of the well known negative inotropic and chronotropic effect on the heart.