NA+-I- SYMPORT ACTIVITY IS PRESENT IN MEMBRANE-VESICLES FROM THYROTROPIN-DEPRIVED NON-I(-)-TRANSPORTING CULTURED THYROID-CELLS

The active accumulation of I- in the thyroid gland is mediated by the Na+-I- symporter and driven by the Na+ gradient generated by the Na+/K+-ATPase. Thyrotropin (TSH) stimulates thyroidal I- accumulation. Rat thyroid-derived FRTL-5 cells require TSH to accumulate I-. TSH withdrawal for over 7 days results in complete loss of Na+-I- symport activity in these Cells [Weiss, S. J., Philp, N. J. and Grollman, E. F. (1984) Endocrinology 114, 1090-1098]. Surprisingly, membrane vesicles prepared from FRTL-5 cells maintained in TSH-free medium [TSH(-) cells] accumulate I-, suggesting that the absence of Na+-I- symport activity in TSH(-) cells cannot be due solely to a decrease in the biosynthesis of either the symporter or a putative activating factor. This finding indicates that the Na+-I- symporter is present, probably in an inactive state, in TSH(-) cells despite their lack of Na+-I- symport activity. Na+-I- symport activity in thyroid membrane vesicles is enhanced when conditions for vesicle preparation favor proteolysis. Subcellular fractionation studies in both TSH(+) and TSH(-) cells show that Na+-I- symport activity is mostly associated with fractions enriched in plasma membrane rather than in intracellular membranes, suggesting that the Na+-I- symporter may constitutively reside in the plasma membrane and may be activated by TSH.