Dobutamine-induced cardiac adaptations: Comparison with exercise-trained and sedentary rats

To investigate dobutamine-induced cardiac adaptations, we compared dobutamine-treated with swim-trained and sedentary control rats. After 14 wk of treatment, heart rate was lower in the dobutamine-treated (279 ± 6) and exercise-trained (287 ± 4 beats/min) groups than in the control animals (305 ± 3 beats/min; P < 0.05). The exercised rats gained less weight (164 ± 11 g; P < 0.05) than the dobutamine-treated (238 ± 16 g) and the control animals (231 ± 12 g). Also, compared with the two other groups, the exercise group had higher relative heart weights (3.47 ± 0.08 vs. 2.82 ± 0.06 and 290 ± 0.05 g/kg in the dobutamine and control groups, respectively; P < 0.05) and lower epididymal fat pad weights (6.6 ± 0.4 vs. 13.3 ± 1.0 and 11.4 ± 0.5 g/kg in the dobutamine and control groups, respectively; P < 0.05). However, both the maximum heart rate produced by isoproterenol and the isoproterenol dose producing 50% of the peak heart rate response were similar among the three groups. Myocardial norepinephrine content, β-adrenergic receptor number, and adenylate cyclase activation by isoproterenol, NaF, 5'-guanylyl imidodiphosphate, and forskolin also did not differ. Thus, although there were differences between the dobutamine-treated and the exercised rats, the two groups were similar in that they developed bradycardia that was not due to cardiac adrenergic desensitization.