EXPERIMENTAL PROCEDURES LEADING TO A RELATIVE ACCUMULATION OF NON-THYROGLOBULIN THYROIDAL IODOCOMPOUNDS IN RAT THYROID GLANDS

Abstract

ABSTRACT A small amount (3–4 per cent) of the homogenate of normal rat thyroid glands, labelled with radioiodine, resists enzymatic digestion by pancreatin and pronase and remains at the origin in alkaline and acid paper chromatography (»origin material«). Under certain conditions the pancreatin resistant compound can account for over 50 per cent of intrathyroidal iodine. The percentage of labelled origin material increases when the interval between single shot labelling and enzymatic digestion of the gland is extended to several days or weeks. The phenomenon is even more pronounced when the rats are kept on a low iodine diet (LID) during the whole experiment. Furthermore a relative increase of the labelled iodinated material occurs when rat thyroid glands are blocked with a thyrostatic drug or labelled to near isotopic equilibrium instead of short term labelling only. If the rats are kept on LID during the equilibration procedure, the relative concentration of origin material will again exceed that found in the thyroids of the control rats kept on HID. Autoradiography revealed that the centre of a PTU-blocked thyroid gland contains little radioactivity whereas large amounts of labelled colloid remain in the peripheral follicles. Mechanical separation of the peripheral layers from the thyroid core indicates that the enzyme resistant material is located mainly in hypcrplastic follicles. Ultracentrifugation studies have shown that the substance is heterogeneous. A small fraction is sedimented at low centrifugal speed, the bulk, however, remains in the supernatant and is of low molecular weight as shown by sucrose density centrifugation and sephadex gel filtration.