Calcium is transported into the lumen of pig thyroid follicles by fluid phase basolateral to apical transcytosis.
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 = 1.71 +/- 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 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.
Fonlupt
P
P
Institut National de la Santé et de la Recherche Médicale, Unité 369, Faculté de Médecine Lyon RTH-Laënnec, France.
Audebet
C
C
Gire
V
V
Bernier-Valentin
F
F
Rousset
B
B
D000818:Animals; D001345:Autoradiography; D002118:Calcium / Q000378:metabolism*; D016764:Cell Polarity; D002478:Cells, Cultured; D017136:Ion Transport; D007546:Isoquinolines; D008858:Microscopy, Phase-Contrast; D013552:Swine; D013961:Thyroid Gland / Q000166:cytology / Q000378:metabolism* / Q000648:ultrastructure
D007546:Isoquinolines; C017475:lucifer yellow; D002118:Calcium