Plasma catecholamines, heart rate, and cardiac sympathetic activity in exercising dogs

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Abstract
The purposes of the study were, 1) to assess the respective roles of locally released norepinephrine in the sinus node and of plasma catecholamines in the control of heart rate during exercise and 2) to verify whether the heart is a source of plasma catecholamines during exercise. Plasma catecholamines (radio-enzymatic assay) and heart rate were measured in the last minute of a 5-min exercise period (3.2 km·h−1, 39% slope) in six normal dogs, in six dogs treated with 5 mg·kg−1 sotalol, and in six dogs sympathectomized with 50 mg·kg−1 6-hydroxydopamine. Compared to the exercise heart rate values of the normal dogs (228 ± 8 beats·min−1) and the sympathectomized dogs (226 ± 9 beats·min−1), the sotalol-treated dogs had significantly lower rates (148 ± 6 beats·min−1). However, plasma catecholamine response was higher in the sotalol-treated dogs (7380 ± 1350 pg·ml−1) and in the sympathectomized dogs (4280 ± 680 pg·ml−1) than in the normal dogs (1890 ± 360 pg·ml−1). Since the action of plasma catecholamines on the sinus node is potentiated by denervation hypersensitivity, it is suggested that in exercising normal dogs, heart rate control could be ensured by locally released norepinephrine rather than by plasma catecholamines. Plasma catecholamines were assessed in six normal dogs at rest and at various exercise levels (HR = 90–200 beats·min−1) and in blood sampled simultaneously in the aorta and the coronary sinus. Plasma catecholamines in the coronary sinus and aorta were similar at rest (490 ± 90 vs 580 ± 80 pg·ml−1, respectively) and at the low-intensity exercise (710 ± 140 vs 880 ± 120 pg·ml−1, respectively). For moderate and severe work loads, plasma catecholamine concentrations in the coronary sinus (960 ± 160 and 1570 ± 340 pg·ml−1, respectively) were lower than in the aorta (1380 ± 260 and 2950 ± 100 pg·ml−1, respectively). These results suggest that in exercising dogs the heart is not a source of plasma catecholamines.