Effect of cooling on intracellular transport and secretion of thyroglobulin

The effect of cooling to 20° C on the intracellular transport and secretion of thyroglobulin was studied by incubating open thyroid follicles isolated from porcine thyroid tissue. Follicles were labeled with 3H-leucine or 3H-galactose and the secretion of labeled thyroglobulin into the incubation medium was followed by chase incubations under various experimental conditions. The observations indicate that the transport of thyroglobulin is inhibited at three sites of the intracellular pathway by cooling to 20° C, i.e., between the RER cisternae and the Golgi cisternae, between the latter and the exocytic vesicles, and between these vesicles and the extracellular space (corresponding to the follicle lumen). The secretion of 3H-leucine-labeled thyroglobulin decreased linearly between 37° and 20° C; within this temperature range the activation energy for secretion, calculated from Arrhenius plots, was found to be 37 kcal/mol. Below 20° C the secretion was scarcely measurable. It is suggested that the three transport blocks at 20° C result mainly from inhibition of membrane fission and fusion due to phase transition in membrane lipids.