Compartmentation in the Turtle Thyroid: Water and Iodide Distribution*

Abstract

The penetration of ³H₂O, [¹⁴C]antipyrine, [¹⁴C]-inulin, and ¹²⁵I^- into the whole gland and follicular luminal fluid of the turtle thyroid was studied by measuring the whole thyroid tissue and sampling the colloid in the lumen by micropipettes. ³H₂O equilibrated in the luminal water within 0.5 h. Linear regression analysis of the long half-life component of the ³H₂O uptake curve of whole thyroid gland showed that this component had a t_½ of 10.7 min and a y-intercept of 51.9%; it is a measure of the luminal volume. The [¹⁴C]inulin interstitial space was 13.8%. The difference (27.8%) between the slow component intercept value of the ³H₂O uptake curve and the total water content is a measure of the combined interstitial and cellular water volumes; the cellular volume is thus 14% (27.8–13.8%). Analysis of the uptake curves for ¹²⁵I^- penetration into the luminal fluid of methimazole-treated turtles demonstrated that there was only one component, with a t_½ of 9.2 h for control and 10.5 h for TSH-treated animals. The asymptotic luminal fluid to plasma water ratio was 120 for the controls and 350 for the TSH-treated turtles. The asymptotic values for the thyroid to plasma water ratio of ¹²⁵I^- uptake in the same group of turtles were 160 for the controls and 440 for the TSH-treated group. Calculation of the intracellular and luminal concentrations of ¹²⁵I^- from these data and the compartmental volumes derived from the ³H₂O and [¹⁴C]inulin data in the two groups of turtles showed that cellular iodide concentration was 3.57 times higher than luminal iodide in controls and 2.41 times higher in TSH-treated animals. The data thus indicate that I^- is actively transported out of the lumen and into the cells and that there is a passive leak of I^- from cells to lumen across the apical cell membrane generated by the high concentration of I^- in the cell resulting from its active transport across the basal cell membrane. TSH is thought to increase the iodide concentration in both cellular and luminal fluids by enhancing active I^- transport across the basal cell membrane and increasing the passive leak of I^- across the apical cell membrane.