Observations on the Relation Between the Action and the Degradation of Thyroid Hormones as Indicated by Studies in the Tadpole and the Frog

Abstract

Studies have been performed to confirm or to refute previous reports that indicate that the adult frog may be unresponsive to the thyroid hormones. The oxygen consumption of the whole frog or of tissue slices was not altered by the prior administration of l-thyroxine, 3-5-3'-L-triiodothyronine, their acetic acid analogues, or by methimazole. Oxygen consumption also was unaffected by injection of adrenalin and noradrenalin, but was markedly increased by 2,4-dinitrophenol. The activity of hepatic glucose-6- phosphatase was not altered by administration of the 4 iodinated compounds, but a marked increase was observed in thyroxinetreated rats. Similarly, although the activity of hepatic ATPase was increased in tadpoles treated with thyroxine, no comparable change could be induced in the frog. No change was observed in the histologic appearance of the anterior pituitary glands of frogs treated with thyroxine derivatives or with methimazole. The apparent failure of the adult frog to respond to thyroactive materials has been correlated with marked changes in the ability of the animal to deiodinate these compounds. Preparations of tadpole liver, which possess a system for deiodinating thyroxine and its derivatives, were employed as standards in these experiments. No deiodination of thyroxine, triidothyronine or the acetic acid derivatives occurred in preparations of liver, kidney, heart or spleen (slices or homogenates) from the frog. Deiodination was observed in preparations of muscle and intestine. This was variable and sometimes heat-resistant. The effect of certain metabolic cofactors, reported to enhance deiodination in other biologic systems, was tested in preparations of frog tissue. No evidence of any stimulation of enzymically mediated deiodination was observed.