Cardiac Beta 2-adrenoceptors and the Inotropic Response to Exercise in Man

To find out what role, if any, beta 2-adrenoceptors play in cardiac contractility, the heart rate, stroke volume and cardiac output of twelve healthy male and female volunteers (aged 18-28 years) were studied at rest (standing) and during two stages of treadmill exercise, 2 h after ingestion of propranolol (100 mg) or atenolol (100 mg) or a placebo, on different occasions, in a double-blind crossover manner. Cardiac output was measured by a carbon dioxide-rebreathing method. Atenolol and propranolol caused equal reductions in heart rate at rest, and in heart rate and cardiac output during exercise (p less than 0.001, two-way analysis of variance). Neither atenolol nor propranolol had any significant effect on resting cardiac output, resting stroke volume or stroke volume during exercise. Since atenolol (100 mg) has been shown to be beta 1-adrenoceptor-selective, we conclude that cardiac inotropic function during exercise is largely beta 1-adrenoceptor-mediated with little or no beta 2-adrenoceptor involvement.