Isoproterenol-induced maximal heart rate in normothermic and hyperthermic horses
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Summary
The heart rate (hr) induced by maximal β-adrenergic activation, which was elicited by infusion of isoproterenol, was studied in 8 healthy horses before (control) and after hyperthermia was induced by iv administration of 2,4-dinitrophenol (dnp). Isoproterenol was administered iv at 1.0 μg·kg-1·min-1 for 3 minutes, and the hr was determined during the final 30 seconds of the infusion. As the rectal temperature increased (P < 0.001) from 38.2 ± 0.1 C (mean ± SEM; normothermic control) to 40.1 ± 0.1 C at 60 minutes after dnp administration, the isoproterenol-induced hr also increased from 198 ± 4 beats/min (control) to 214 ± 4 beats/min (P < 0.001). It appeared that the values of hr achieved with maximal β-adrenergic activation were augmented by the hypermetabolic, hyperthermic state induced by dnp.
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