EXERCISE TRAINING IMPROVES LEFT-VENTRICULAR CONTRACTILE RESPONSE TO BETA-ADRENERGIC AGONIST

To determine whether endurance exercise training can improve left ventricular function in response to beta-adrenergic stimulation, young healthy sedentary subjects (10 women and 6 men) were studied before and after 12 wk of endurance exercise training. Training consisted of 3 days/wk of interval training (running and cycling) and 3 days/wk of continuous running for 40 min. The training resulted in an increase in maximal O2 uptake from 41.0 +/- 2 to 49.3 +/- 2 ml . kg-1 . min-1 (P < 0.01). Left ventricular function was evaluated by two-dimensional echocardiography under basal conditions and during beta-adrenergic stimulation induced by isoproterenol infusion. Fractional shortening (FS) under basal conditions was unchanged after training (36 +/- 1 vs. 36 +/- 2%). During the highest dose of isoproterenol, FS was 52 +/- 1% before and 56 +/- 1% after training (P < 0.05). At comparable changes in end-systolic wall stress (sigma(es)), the increase in FS induced by isoproterenol was significantly larger after training (13 +/- 1 vs. 17 +/- 2%, P < 0.01). Furthermore there was a greater decrease in end-systolic dimension at similar changes in sigma(es) in the trained state during isoproterenol infusion (-4.6 +/- 0.1 mm before vs. -7.0 +/- 0.1 mm after training, P < 0.01). There were no concurrent changes in end-diastolic dimension between the trained and untrained states during isoproterenol infusion, suggesting no significant changes in preload at comparable levels of sigma(es). Under basal conditions, end-diastolic dimension was increased (46 +/- 0.5 vs. 50 +/- 0.5 mm, P < 0.03) after training (P < 0.03), with no significant change in left ventricular wall thickness-to-radius ratio after training, consistent with volume-overload hypertrophy. The results indicate that endurance exercise training enhances the left ventricular contractile response to beta-adrenergic stimulation without affecting basal contractility in young subjects.