Maximum heart rate responses to exercise and isoproterenol in the trained rat

Male rats were assigned to light (C) or strenuous (T) running programs. Both groups ran at 30 m/min, 8% elevation. Over 16 wk, T and C completed 2,939 +/- 72 and 507 +/- 7 min (mean +/- SE). In a graded running test, maximum exercise heart rates for T and C were 542 +/- 7 and 554 +/- 6 beats/min (P greater than 0.05). Heart rates elicited by maximum effective concentrations of isoproterenol (ISO) in vivo and in vitro were 483 +/- 8 and 489 +/- 11 beats/min for T and 499 +/- 5 and 502 +/- 5 beats/min for C (no difference between groups or treatments). A lower heart rate was recorded in T for both resting (353 +/- 7 vs. 373 +/- 4 beats/min) and in vitro intrinsic states (231 +/- 22 vs. 299 +/- 22 beats/min) (P less than 0.05 for both conditions). The difference between maximum ISO-stimulated and maximum exercise heart rates was attributed to a temperature difference. In a separate group of lightly trained rats, ISO was administered intravenously during hard exercise when heart rate approached exercise maximum. Heart rate after ISO did not increase beyond the maximum heart rate observed in a control run. It was concluded that the maximum chronotropic response to sympathetic stimulation can be elicited during hard exercise and that maximum exercise heart rate reflects this limit rather than a saturation of cardiac sympathetic activity.