ADRENERGIC REGULATION OF THE FORCE-FREQUENCY EFFECT

An important interaction between beta-adrenergic receptor (AR) stimula tion and the myocardial contractile response to increased heart rate h as only recently been identified. The effect of beta-AR stimulation to amplify the force-frequency effect is responsible for a large compone nt of the positive inotropic effect of exercise in conscious dogs and also can be demonstrated at rest with dobutamine infusions over a rang e of paced heart rates. The interaction is apparent across species (do g, pig, mouse, rabbit, man). Amplification of the heart rate effect on myocardial contractility by dobutamine is lost in heart failure, whic h may play an important role in impaired exercise tolerance in that se tting. In syndromes of chronotropic incompetence when adrenergic contr ol is intact, it may be expected that a normal response of myocardial contractility to exercise will be achieved only when the interaction w ith heart rate is optimized by use of a rate-responsive cardiac pacema ker.