A Comparison of Two Exercise Training Programs on Cardiac Responsiveness to β-Stimulation in Obesity

Abstract

We demonstrated previously that exercise training did not restore normal cardiac β-adrenergic responsiveness in obese rabbits. This study tested the hypothesis that an increased training volume was required to attenuate obesity-related reductions in isolated heart responsiveness to isoproterenol. Female New Zealand White rabbits were divided into lean control, lean exercise-trained, obese control, and obese exercise-trained groups. For the exercise-trained groups, total treadmill work over 12 weeks was increased 27% when compared with lean and obese animals trained with lower total training volume. After 12 weeks, Langendorff isolated hearts were used to study developed pressure, +dP/dt_max, and –dP/dt_max responses to isoproterenol (10⁻⁹ – 3 × 10⁻⁷ M). Concentration-response data were fit to a sigmoidal function using a four-parameter logistic equation. Controls were compared with animals trained under the low– and high–training volume programs using one-way analysis of variance and Tukey's post-hoc test; separate analyses were conducted for lean and obese rabbits. In both lean and obese groups trained under the high–training volume program, EC₅₀ values for +dP/day_tmax and –dP/dt_max were higher compared with same-weight controls and animals trained under the low–training volume program, indicating that contractility and relaxation responsiveness to isoproterenol was reduced by the higher training volume. Therefore, these data indicate that increased training volume failed to attenuate obesity-related decrements in isolated heart responsiveness to β-adrenergic stimulation and caused reduced sensitivity to isoproterenol in both lean and obese animals.