Inotropic effects of thyroxine and related compounds on guinea-pig left atria in vitro

Triiodothyronine, diiodothyronine and diiodotyrosine have positive inotropic activity on normal guinea-pig left atria in vitro. The increases produced by triiodothyronine and diiodothyronine are small but their detection shows that the inotropic responses to these agents can be studied in vitro. Thyroxine, thyronine and monoiodotyrosine are inactive. Reserpine pre-treatment reduces the inotropic effect of diiodotyrosine but does not reduce those of triiodothyronine and diiodothyronine.