Studies on the Stromal, Luminal and Cellular Compartments of the Thyroid

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Abstract
The volumes of the stromal (interstitial), luminal and follicular cell compartments of rat and guinea pig thyroid glands in various experimental conditions have been measured by radioactive indicators and the results have been compared with volumes determined by histological measurements. The volume of distribution of C14-inulin was found to be the same as that of the stroma as determined by a histological procedure. Thus, inulin is a measure of stromal volume in the thyroid gland. Neither TSH (thyroid stimulating hormone), chronic PTU (propylthiouracil) treatment, nor hypophysectomy altered the inulin space in the thyroid of rats or guinea pigs. In rats the volume of distribution of S35O4= corresponded with the combined stromal and luminal volumes as determined by inulin and histological measurements, respectively. In guinea pigs the S35O4= space was less than the sum of the inulin space and the histologically determined luminal volume. The difference is explained by the large amount of negatively charged colloid protein in the lumen of the guinea pig as compared to the rat thyroid. The colloid protein anion sets up a Donnan distribution between lumen and stroma and thereby allows unequal distribution of the diffusible anions SO4= and Cl- so that their concentration in the stroma is greater than in the lumen. TSH and PTU, which decrease the colloid protein content of the guinea pig thyroid lumen, caused an increase in SO4= and Cl- spaces as a resultof the reduced Donnan distribution ratio. In this situation the S35O4 space approached the combined inulin and histologically determined luminal volumes. The S35O4= uptake curve of the guinea pig thyroid contains 2 components, a fast and a slow one. The fast component corresponds to penetration into the stroma and its zero-time volume is equal to the inulin space. The slower component corresponds to penetration into the lumen. The volume of distribution of Cl36- was found to be larger than that of S35O4= in both species of animals. The excess Cl36- is located in follicular cells at a concentration that corresponds with that predicted from the assumption that this ion is distributed across cells passively, i.e., in equilibrium with the transmembrane potential. The uptake curve of Cl36- by the guinea pig thyroid contains 3 components. The first component has a short half-life and represents penetration into stroma and cells, whereas the last 2, with longer half-lives, represent penetration into the lumen. TSH increased the amount and the rate of Cl- penetration into the lumen as a consequence of reducing the colloid protein content, as described above. Implications of these results for the calculation of electrolyte concentration in the various thyroid compartments are discussed.