Turnover of apical plasma membrane in thyroid follicle cells of normal and thyroxine-treated rats.

In thyroid follicle cells exocytotic vesicles transfer newly synthesized thyroglobulin to the follicle lumen and new membrane to the apical plasma membrane. In a previous study data obtained by quantitative electron microscopy were used to estimate the turnover of the pool of exocytotic vesicles in follicle cells of normal and thyroxine-treated (2 days) rats. In the present study, these kinetic data were combined with stereological measurements to calculate the amount of membrane added to the apical plasma membrane by exocytosis and, indirectly, to estimate the turnover of this membrane. In follicle cells of normal rats the area of the membrane added was about 240 micron2/h (180 micron2/h after correction for stereological overestimation) and in thyroxine-treated rats about 105 micron2/h (corrected 80 micron2/h). These areas corresponded to the addition of 1.2% and 0.7% respectively, of the apical plasma membrane per minute. In each cell the total volume of the exocytotic vesicles emptying their content into the follicle lumen in normal rats was 6.2 micron3/h (corrected 4.7 micron3/h) and in thyroxine-treated rats 3.0 micron3/h (corrected 2.3 micron3/h). Considerations based on the findings in the present study suggest that in normal rats macropinocytosis (formation of colloid droplets) is the most important endocytotic pathway for internalization of colloid, while micropinocytosis is the major pathway of membrane internalization. In thyroxine-treated rats macropinocytosis is inhibited. The low volume capacity of the micropinocytotic pathway probably explains the accumulation of colloid protein, mainly thyroglobulin, observed in such rats.