Synthèses et activités biologiques des dérivés thyroxiniens et adrénergiques XIV. Croissance et ARN polymérase hépatique du rat thyroïdectomisé traité chroniquement aux hormones thyroïdiennes et effets de l'adrénaline sur le métabolisme basal

Abstract

Experiments were performed in order to study if calorigenesis and protein synthesis, both stimulated by thyroid hormones, are influenced independently or if one phenomenon proceeds from the other. The effects of l-thyroxine (T 4), 3,5,3′-triiodo- l-thyronine (T 3), 3,5,3′-thyroacetic acid (TA 3) on growth; BMR and hepatic DNA-dependent RNA polymerase Mg ++ activated were compared. 1. 1. Chronic administration of TA 3 to thyroidectomized rats at a dose promoting almost normal growth and higher BMR induced a partial RNA polymerase activity. 2. 2. In defined experimental conditions and at same physiological doses T 4 or T 3 affected similarly BMR, growth and enzyme activity but it must be considered that half-life of the two hormones is different. 3. 3. Hypothyroidism seemed unable to suppress completely the effect of somatotropine secretion on RNA polymerase for the enzymic response was much more reduced in hypophysectomized than in thyroidectomized rats. Therefore it appeared that the decrease of RNA polymerase activity was essentially due to the absence of thyroid hormone. 4. 4. Calorigenic action of epinephrine abolished after thyroidectomy was not re-established to a normal level when small doses of thyroid derivatives were injected chronically, meanwhile isolated rat liver mitochondria respiration was, in same experimental conditions, relatively independent of the thyroid status.