Calcium is transported into the lumen of pig thyroid follicles by fluid phase basolateral to apical transcytosis

• Published in: Journal of cellular physiology 1997-04, Vol.171 (1), p.43-51
• Main Authors: Fonlupt, Pierre, Audebet, Christine, Gire, Veronique, Bernier-Valentin, Francoise, Rousset, Bernard
• Format: Article
• Language: English
• Subjects: Animals; Autoradiography; Calcium - metabolism; Cell Polarity; Cells, Cultured; Ion Transport; Isoquinolines; Microscopy, Phase-Contrast; Swine; Thyroid Gland - cytology; Thyroid Gland - metabolism; Thyroid Gland - ultrastructure
• ISSN: 0021-9541; 1097-4652
• DOI: 10.1002/(SICI)1097-4652(199704)171:1<43::AID-JCP6>3.0.CO;2-H

The lumen of thyroid follicles contains a high concentration of thyroglobulin, the thyroid prohormone and a high concentration of calcium (Ca2+). As thyroglobulin binds Ca2+, intraluminal Ca2+ is expected to be in free and protein‐bound forms. In the present work, we have investigated the mechanism(s) by which Ca2+ could enter the lumen of thyroid follicles. 45Ca2+ 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. 45Ca2+ 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‐1 = 1.71 ± 0.28 hr‐1 and k‐2 = 0.20 ± 0.05 hr‐1 (n = 10). The slow uptake process accounted for 20–50% of the total RTF‐associated Ca2+ after 24 hr. 45Ca2+ 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‐1 value of 1.64 ± 0.15 hr‐1 (n = 10) which was not statistically different from the k‐1 obtained for 45Ca2+ uptake by RTF. We then compared the kinetics of 45Ca2+ uptake by RTF with the kinetics of transport of fluid phase markers: [14C]‐sucrose and Lucifer Yellow from the medium to the lumen of RTF. [14C]‐sucrose and Lucifer Yellow uptakes by RTF appeared as slow processes compatible with the entry in a single compartment with k‐1 values of 0.32 ± 0.06 hr‐1 (n = 3) and 0.23 ± 0.015 hr‐1 (n = 3), respectively. These values were not significantly different from the k‐2 value obtained for 45Ca2+ uptake by RTF. These data suggest that thyroid follicles would possess two independent Ca2+ compartments: cells and lumen, and that the entry of Ca2+ into the lumen of follicles probably could take place by fluid phase basolateral to apical transcytosis. J. Cell. Physiol. 171:43–51, 1997. © 1997 Wiley‐Liss, Inc.