Action of KI and several iodocompounds on [3H]uridine incorporation into thyroid RNA

The present studies were performed in order to further clarify the action of iodine and iodocompounds on the incorporation of labelled uridine into thyroid RNA. KI decreased RNA labelling but did not alter total [3H]uridine uptake or [3H]inulin distribution space. KI also inhibited the increase in RNA labelling produced by 8 mM glucose. T4 was more potent on a molar basis than KI in impairing uridine incorporation. TETRAC, TRIAC and isopropyl-T3 also decreased RNA labelling, while T2 and isopropyl-T2 were ineffective. KI did not alter the distribution of the uridine derivatives, UMP, UDP and UTP, as determined by the distribution of [3H]uridine in these compounds by paper chromatography, suggesting that the action of KI does not take place at the step of uridine phosphorylation. Like its effect on TSH, KI also impairs the stimulatory effect of exogenous cAMP and cGMP on RNA labelling, suggesting that its action is exerted beyond the step of cyclic nucleotide formation. Iodine and iodocompounds may exert their inhibitory action on RNA labelling at the step of nucleotide polymerization.