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. Ca-45(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. Ca-45(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(-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. Ca-45(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(-1) value of 1.64 +/- 0.15 hr(-1) (n = 10) which was not statistically different from the k(-1) obtained for Ca-45(2+) uptake by RTF. We then compared the kinetics of Ca-45(2+) uptake by RTF with the kinetics of transport of fluid phase markers: [C-14]-sucrose and Lucifer Yellow from the medium to the lumen of RTF. [C-14]-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 Ca-45(2+) 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 rake place by fluid phase basolateral to apical transcytosis. (C) 1997 Wiley-Liss, Inc.