Effects of Thyrotropin, Acetazolamide, 4-Acetamido-4′-Isothiocyanostilbene-2,2′-Disulfonic Acid, Perchlorate, and Ouabain on the Distribution of Iodide Ion in Cells and Luminal Fluid of Turtle Thyroid*

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Abstract
The effects of TSH, acetazolamide, 4-acetamido-4′-isothiocyano-stilbene-2,2′-disulfonic acid (SITS), perchlorate, and ouabain on the distribution of 125I- in the cells and luminal fluid were measured in the follicles of turtle thyroid. The cell and luminal values were derived from the steady state 125I- values (24 h) in the whole gland and follicular luminal fluid, as measured in methimazole-blocked glands. The colloid in the lumen was obtained by use of micropipettes inserted into the luminal fluid (LF). In the thyroids removed from the control turtles, the cell 125I- concentration was 596 times the extracellular fluid (ECF) concentration, and the luminal fluid concentration was 151 times the ECF value. The cell to LF 125I- ratio was thus 3.95. TSH markedly increased the cell 125I- concentration (2-fold) by increasing the activity of the I- pump in the basal cell membrane. It also increased the LF 125I- concentration (3-fold) apparently by increasing the permeability of the apical membrane to anions, thus allowing more 125I- to leak along its electrochemical gradient into the LF. Acetazolamide, presumably by its inhibitory effect on thyroid carbonic anhydrase, decreased cell and increased LF 125I- concentrations; the cell to LF 125I- ratio was thus markedly decreased. SITS, an anionic channel blocker, increased the cell 125I- concentration, but had little effect on the LF 125I- concentration. The combination of SITS and acetazolamide produced the same effect as TSH. SITS is thought to increase cell 125I- by decreasing its exit from the cell via anionic channels in the basal cell membrane. Ouabain, a potent inhibitor of Na+,K+-ATPase, markedly decreased both cell and LF 125I- concentrations, an effect that was partially antagonized by TSH. The iodide pump is thus related to active Na transport and may be a Na+-anion cotransport system. Perchlorate, which blocks iodide transport by competing with it for the carrier system, markedly decreased both cell and LF 125I- concentrations, such that the ECF to cell 125I- ratio was 6:1, and the cell to LF 125I- ratio was 1:3.85, values indicating nearly passive distribution of I- across the basal and apical cell membranes sof the turtle thyroid.