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Interactions of β‐adrenoceptor antagonists and thyroid hormones in the control of heart rate in the dog

1 Propranolol, sotalol and nadolol have been infused into conscious dogs, and doses at which the three drugs are equipotent as β‐adrenoceptor antagonists determined. 2 In euthyroid dogs, sotalol was more effective at lowering heart‐rate than an equivalent dose of propranolol, while an equivalent dose of nadolol was without effect. 3 Hyperthyroidism potentiated the lowering of heart‐rate by sotalol, but inhibited that by propranolol. 4 The effect of sotalol on heart‐rate was correlated with its prolongation of the Q‐T interval of the ECG. That of propranolol was correlated with its prolongation of the P‐R interval. Nadolol did not affect P‐R interval or Q‐T interval except at relatively high dosage. 5 We conclude that the tachycardia of hyperthyroidism is not affected by blockade of β‐adrenoceptors and therefore that it is not mediated by adrenergic mechanisms. The effectiveness of propranolol and sotalol in lowering heart‐rate must be due to actions peculiar to those drugs, and not to β‐adrenoceptor antagonism.