1. The mechanisms that control resting heart rate in hyperthyroidism were evaluated in six patients before and after treatment with propylthiouracil.

2. The patients were subjected to pharmacological blockade under resting conditions in two experimental sessions: first session, propranolol (0.2 mg/kg body weight); second session, atropine (0.04 mg/kg body weight) followed by propranolol (0.2 mg/kg body weight). All drugs were administered intravenously.

3. Resting heart rate was significantly reduced from 100 ± 6.5 beats/min to 72 ± 2.5 beats/min (P < 0.005) after clinical and laboratory control of the disease. After double blockade, intrinsic heart rate was reduced from 105 ± 6.8 beats/min before treatment to 98 ± 6.0 beats/min after treatment (P < 0.025). The reduction in heart rate caused by propranolol was not significantly different before (−13 ± 1.4 beats/min) and after (−9 ± 1.0 beats/min) propylthiouracil. In contrast, atropine induced a higher elevation of heart rate after treatment (45 ± 8.6 beats/min) than before treatment (26 ± 4.0 beats/min).

4. The present results suggest no appreciable participation of the sympathetic component of the autonomic nervous system in the tachycardia of hyperthyroidism, at least under the conditions of the present study. The small change observed in intrinsic heart rate, although significant, seems to indicate that this is not the most important mechanism involved in this tachycardia.

5. Our results suggest that an important reduction in the efferent activity of the parasympathetic component participates in the mechanisms that modify resting heart rate in hyperthyroidism.

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