Further Studies of the Effects of Inorganic Iodide on Thyroidal Intermediary Metabolismin Vitro

Beef thyroid slices have been employed for further study of metabolic changes induced by inorganic iodide. The prior observations in sheep thyroid slices that 0.1 mM iodide stimulates oxygen consumption and glucose oxidation, and that 50 mM iodide reverses this stimulation, have been confirmed in beef thyroids. Also, it has been shown that 0.1 mM iodide does not stimulate aerobic glycolysis (lactate-14C production from glucose-U-14C), but does stimulate oxidation of labeled pyruvate, acetate and palmitate. Therefore, it is postulated that 0.1 mM iodide leads to an increase in Krebs cycle oxidations. At 50 mM iodide, a number of inhibitory effects were shown: decreased oxygen consumption; decreased oxidation of glucose-U- 14C, -1-14C, and -6-uC; decreased aerobic glycolysis; and decreased oxidation of pyruvate-1-14C and acetate-1-14C when pyruvate or acetate was present in high concentration (10 and 25 mM, respectively). However, the proportionate oxidation of small amounts (concentration, less than 1.0 mM) of pyruvate-1-14C was not altered, and the proportionate oxidation of small amounts of acetate-1-14C was increased by 50 mM iodide, despite concomitant decreases in QO2. To explain the results with small amounts of labeled substrate, an isotope dilution effect is postulated. It is suggested that these several effects of very high iodide levels on thyroidal metabolism may be related to the inhibition of hormonogenesis caused by acute increases in extracellular iodide concentration in vivo. (Endocrinology79: 1, 1966)