A comparison of two exercise training programs on cardiac responsiveness to beta-stimulation in obesity

We demonstrated previously that exercise training did not restore normal cardiac beta-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 x 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 beta-adrenergic stimulation and caused reduced sensitivity to isoproterenol in both lean and obese animals.