Studies on the Interactions of Thyroid Hormone and Catecholamines*

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Abstract
Studies have been carried out to assess the effects of adrenodemedul-lation and reserpine on the growth rate, metabolic rate, and heart rate of thyroidectomized rats given thyroid. Adrenodemedullation significantly reduced the growth rate, but was without effect on the heart rate or metabolic rate. The effects of reserpine are complex. It lowered the growth rate slightly of thyroid-treated rats, while raising slightly the growth rate of thyroidectomized rats. Reserpine lowered heart rate of thyroidectomized rats and prevented the tachycardia usually induced by triiodothyronine. This effect on heart rate was abolished by atropine and was not accompanied by any change in the in vitro oxygen consumption of myocardial tissue. Reserpine and guanethidine were lethal to thyroidectomized-adrenodemedullated rats but not to rats with intact adrenal medullas. The death of these rats was accompanied by a progressive decline in metabolic rate and heart rate and was prevented by triiodothyronine and epinephrine. These observations are discussed in relation to the thyroid hormones on the control of heart rate and metabolism.