Further Observations Concerning the EflFect of Actinomycin D on Thyroidal Iodide Transport in Rats

Abstract

Previous findings from this laboratory concerning the ability of actinomycin D (act D) to enhance the thyroid/serum radioiodide concentration ratio (T/S) (3) were confirmed. This effect was not due to altered food intake, since it could be elicited in fasted animals. Act D did not interfere with the blocking influence of propylthiouracil on organic binding of 131I in the thyroid. A raised thyroid/medium radioiodide concentration ratio (T/M) could be observed when glands from intact rats injected with act D were incubated in a medium containing methimazole and 131I, The elevating influence of TSH injected into hypophysectomized rats on the T/M established by their thyroids was not affected by act D given concurrently with the TSH. In contrast to the previous study (3), we found that act D consistently raised the T/S of untreated by hypophysectomized rats. When it was given before the injection of a single dose of TSH into such animals, it did not prevent the initial drop in T/S. At or near the height of the T/S-raising effect of TSH, further elevation of the T/S by act D was erratic, but it became clear-cut again by the time the influence of TSH had waned. 5-Fluorouracil injected into intact rats in doses from 40 to 300 mg did not elevate the T/S.