Purpose: Exercise training results in many health benefits, but few studies have focused on whether exercise training might attenuate the adverse effects of obesity on heart function. Therefore, the purpose of this study was to determine whether exercise training attenuated obesity-related decreases in systolic contractile function in response to beta-adrenergic stimulation, using the rabbit model of obesity.
Methods: Female New Zealand white rabbits were divided into four groups: lean sedentary, lean exercise-trained, obese sedentary, and obese exercise-trained. Obese rabbits were fed an ad libitum high-fat diet. Exercise-trained rabbits underwent a 12-wk progressive treadmill exercise training protocol. After 12 wk, the Langendorff isolated heart method was used to study developed pressure, +dP/dt, and -dP/dt responses to increasing concentrations of isoproterenol (10(-9)--3 x 10(-7) M). Log concentration-response data were fit to a sigmoidal function, using a four-parameter (minimum, maximum, EC(50), slope) logistic equation. Groups were compared using a 2 x 2 analysis of variance.
Results: Although obesity shifted the concentration-response curves for developed pressure, +dP/dt, and -dP/dt to the right as indicated by an increase in the EC(50) (P < or = 0.05), there was no effect of exercise training on any of the logistic regression parameters. EC(50) (log M) values for combined lean versus combined obese were -8.50 +/- 0.7 vs -8.20 +/- 0.09 (developed pressure), -8.04 +/- 0.06 vs -7.68 +/- 0.07 (+dP/dt), and -8.17 +/- 0.07 vs -7.91 +/- 0.09 (-dP/dt).
Conclusion: These results confirm the negative effect of obesity on responsiveness of the isolated heart to beta-adrenergic stimulation but indicate that exercise training does not significantly attenuate obesity-related changes.