The effects of aerobic fitness and β₁-adrenergic receptor blockade on cardiac work during dynamic exercise

Abstract

The purpose of this investigation was to determine whether cardiovascular adaptations characteristic of long-term endurance exercise compensate more effectively during cardioselective beta(1)-adrenergic receptor blockade-induced reductions in sympathoadrenergic-stimulated contractility. Endurance-trained (ET) athletes (n = 8) and average-trained (AT; n = 8) subjects performed submaximal cycling exercise at moderate [45% maximum oxygen uptake (Vo(2max))] and heavy (70% Vo(2max)) workloads, with and without metoprolol. Cardiac output (Qc), heart rate (HR), and systolic blood pressure were recorded at rest and during exercise. Cardiac work was calculated from the triple product of HR, stroke volume, and systolic blood pressure, and myocardial efficiency is represented as cardiac work for a given total body oxygen consumption. Metoprolol reduced Qc at 45% Vo(2max) (P = 0.004) and 70% Vo(2max) (P = 0.022) in ET subjects, but did not alter Qc in the AT subjects. In ET subjects at 45% Vo(2max), metoprolol-induced reductions in Qc were a result of decreases in HR (P < 0.05) and the absence of a compensatory increase in stroke volume (P > 0.05). The cardiac work and calculated cardiac efficiency were reduced with metoprolol in ET subjects at both exercise intensities and in the AT subjects during the high-intensity workload (P < 0.01). The cardiac work and the calculated cardiac efficiency were not affected by metoprolol in the AT subjects during the 45% Vo(2max) exercise. Therefore, in AT subjects, beta-blockade reduced the amount of pressure generation necessary to produce the same amount of work during moderate-intensity exercise. In patients with heart disease receiving metoprolol, a decrease in the generation of cardiac pressure necessary to perform a given amount of work during mild-to-moderate exercise would prove to be beneficial.