Comparative Effects of DL-Thyronine, L-Triiodothyronine and L-Thyroxine on the Isolated Perfused Frog Heart

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Abstract
Thyronine in dosages between 0.2 and 0.8 mg (7.4 x 10–4m – 3.0 x 10–3m) produced no significant changes in cardiac output, heart rate or electrical potential. Thyroxine in low concentrations (0.2 mg) usually produced a slight increase in heart rate with no change in cardiac output. Triiodothyronine at the same dose increased both parameters. At higher concentrations 0.6 mg (approx. 7.7 x 10–4m) and above, both of the latter agents decreased the heart rate and cardiac output, and produced variable degrees of heart block, arrhythmias and occasionally electrical alternans. There were minimal effects on the P wave. The R wave was usually decreased, the S-T segment was depressed and the T wave became increasingly inverted. The changes caused by triiodothyronine occurred earlier and were also more pronounced. In contrast the absence of any changes following administration of thyronine suggests that different effects on myocardial function can be attributed to the introduction of iodine atoms into the thyronine nucleus.

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