Abstract
The present studies were performed to determine the arrhythmogcnic dose of ouabain in euthyroid and hyperthyroid guinea pigs and rabbits. The animals were made hyperthyroid by daily injection of thyroxine 100 mcg./kg. i.p. for 10–14 days. Cardiovascular responses, including heart rate from limb lead II electrocardiogram and arterial blood pressure, were monitored. By the method of analysis of variance, no significant difference was shown between the arrhythmogenic dose of ouabain in euthyroid and hyperthyroid guinea pigs. However, this difference was statistically significant in rabbits. An increase in extracellular serum calcium-ion concentration did not change the responsiveness of the myocardium to ouabain toxicity in either the euthyroid or the hyperthyroid animal. In addition, the β-adrenergic receptor blocking drug practolol (10 mg./kg.) was ineffective in completely abolishing the resting tachycardia in hyperthyroid rabbits. These studies do not support the idea of decreased responsiveness to cardiac glycosides due to the hyperthyroid state.
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