| Description |
The water volumes and the electrolyte concentrations in the three compartments (interstitial, cellular, and luminal) of the thyroid gland were determined in rates, guinea pigs, and turtles in different functional states. The determinations were based on data obtained from (1) tracer distribution, (2) histometric measurements, (3) chemical analysis, and (4) specific liquid ion-exchange microelectrode studies. From the characteristic features of electrolyte distribution in different compartment of the thyroid gland, the nature of the transport system in the basal and apical membranes of the thyroid follicles is discussed. As in most other organs, the K+ concentration is high and the Na+ concentration is low in the cellular compartment of the thyroid gland. The luminal compartment of the thyroid gland is an "extra" extracellular space, and as such contains high Na+ and low K+ concentrations. However, the water and electrolyte concentration in the luminal and cellular fluids depend on the amount of colloid present in the follicular lumen which, in turn, is related to the functional state of the thyroid gland. The thyroid gland of adult rats contains a small amount of colloid in the follicular lumen; hence, the concentrations of Na+, K+, and C1¯ luminal fluid are the same as or very close to those in the interstitial fluid. Treatment with a single injection of TSH or with chronic administration of PTU increases intracellular H2O K+ and C1¯concentrations and decreases intracellular Na+ concentration. Hypophysectomy produces changes in the opposite direction. Compartmental volumes and electrolyte concentrations of the thyroid gland in young rats (under the age of three weeks) are like those of hypophysectomized rats. In guinea pig, the 3H2O-uptake curve of the thyroid gland could be resolved graphically into three components. The values of the zero-time intercept of the components were 0.165, 0.230 and 0.305, which are close to the fluid volumes of interstitial, cellular and luminal spaces, respectively, in the same animals as calculated from data obtained from total H2O content, 14C-inulin space, and histometric measurements of follicles of the thyroid gland. The Na+ concentration of thyroid luminal fluid from nephrectomized guinea pigs as calculated from the 22Na+-uptake data is lower than the interstitial sodium concentration. However, the 22Na+ concentration in thyroidal luminal water of intact guinea pigs is slightly higher than that in interstitial water determined by the measurement of radioactivity in both plasma and in luminal fluid obtained directly by the micropuncture technic. By use of specific liquid ion-exchanger microelectrodes, the K+ activity in the luminal fluid of turtle thyroid glands was found to be the same as the interstitial fluid, but the C1¯ activity in the luminal fluid is much lower than that in the interstitial fluid. The calculated C1¯ equilibrium potential between interstitial and luminal fluids is lower than the directly measure intraluminal potential in both turtle and guinea pig thyroid glands. This has lead to speculation that C1¯may be actively transported out of the luminal fluid in exchange for some anions in the epithelial cells of the thyroid gland. |
