Effects of propranolol on cardiopulmonary function in the pony during submaximal exercise

Cardiopulmonary responses of four ponies were monitored during standard exercise tests (SET), before and after beta-adrenergic receptor blockade with propranolol. The SET consisted of four 5 min increments of increasing speed from 1.0 to 2.8 m/sec on a treadmill at a 7 degrees incline. Data were collected at rest, throughout the SET and recovery. Administration of propranolol to ponies at rest had no effect on cardiopulmonary function. During the SET, increases in heart rate, mean pulmonary artery flow velocity (an index of cardiac output) and right ventricular dP/dt (an index of myocardial contractility) were progressively attenuated as running speed increased. Body temperature and mean pulmonary artery and right ventricular pressures were significantly elevated over normal. Propranolol treatment had no effect on the responses of mean arterial pressure, haematocrit, haemoglobin, blood lactate and arterial blood gases and pH to the SET. These results suggest that in the pony there is no sympathetic activity to the heart at rest and that during exercise there is pulmonary vasodilation mediated by beta-adrenergic receptors.