Calcium is transported into the lumen of pig thyroid follicles by fluid phase basolateral to apical transcytosis
✍ Scribed by Pierre Fonlupt; Christine Audebet; Veronique Gire; Francoise Bernier-Valentin; Bernard Rousset
- Publisher
- John Wiley and Sons
- Year
- 1997
- Tongue
- English
- Weight
- 372 KB
- Volume
- 171
- Category
- Article
- ISSN
- 0021-9541
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✦ Synopsis
The lumen of thyroid follicles contains a high concentration of thyroglobulin, the thyroid prohormone and a high concentration of calcium (Ca 2/ ). As thyroglobulin binds Ca 2/ , intraluminal Ca 2/ is expected to be in free and protein-bound forms. In the present work, we have investigated the mechanism(s) by which Ca 2/ could enter the lumen of thyroid follicles. 45 Ca 2/ uptake studies were carried out on reconstituted pig thyroid follicles (RTF) and pig thyroid cell monolayers (TCM) in primary culture, representing experimental systems with two compartments (cells / lumina) and one compartment, respectively. 45 Ca 2/ accumulation in RTF was rapid during the first hour of incubation and then slowly increased. Analysis of the uptake data with a ''two compartments'' model gave two kinetic constant values: k 01 Å 1.71 { 0.28 hr 01 and k 02 Å 0.20 { 0.05 hr 01 (n Å 10). The slow uptake process accounted for 20-50% of the total RTF-associated Ca 2/ after 24 hr. 45 Ca 2/ uptake by TCM was rapid and reached a stable level within 1-2 hr. Experimental data fitted with a ''single compartment'' model and gave a k 01 value of 1.64 { 0.15 hr 01 (n Å 10) which was not statistically different from the k 01 obtained for 45 Ca 2/ uptake by RTF. We then compared the kinetics of 45 Ca 2/ uptake by RTF with the kinetics of transport of fluid phase markers: [ 14 C]-sucrose and Lucifer Yellow from the medium to the lumen of RTF. [ 14 C]-sucrose and Lucifer Yellow uptakes by RTF appeared as slow processes compatible with the entry in a single compartment with k 01 values of 0.32 { 0.06 hr 01 (n Å 3) and 0.23 { 0.015 hr 01 (n Å 3), respectively. These values were not significantly different from the k 02 value obtained for 45 Ca 2/ uptake by RTF. These data suggest that thyroid follicles would possess two independent Ca 2/ compartments: cells and lumen, and that the entry of Ca 2/ into the lumen of follicles probably could take place by fluid phase basolateral to apical transcytosis.