IODIDE TRAPPING WITHOUT ORGANIFICATION IN A TRANSPLANTABLE RAT THYROID TUMOR1

Abstract

The present report describes a transplantable rat thyroid tumor that concentrates 40-70% of an injected dose of I131-iodide and loses it rapidly with a half life of 4-8 hours. The I131 in the tumor is 98-99% iodide as judged by trichloracetic acid solubility, chromatography and dialysis. The remainder of the I131 is organically bound and exists in the "particulate" fraction. Iodide accumulation in vivo appeared to be independent of the level of thyrotropic hormone. Slices of the tumor concentrate iodide with a gradient (T/M ratio) ranging from 8 to 44. The gradient is largely abolished by various anions such as ClO4-, Se CN-, SCN- and by certain agents which uncouple oxidative phosphoryla-tion, such as dinitrophenol and dicumarol. Thyroxine and triiodothyronine have no significant effect on the T/M ratio at 1.10-4 M. The defect in hormone synthesis seems to lie with the iodinating system since unlabeled thyroglobulin appears to be present. The ability to deiodinate L-diiodotyrosine has also been lost. The iodide concentrating mechanism of the tumor differs from that of normal sheep thyroid tissue by its markedly decreased sensitivity to certain cardiac glycosides which were shown to inhibit thyroidal iodide concentration.