Early effects of thyroid stimulating hormone (TSH) on exocytosis and endocytosis in the thyroid

The early effect (3 min) of thyroid stimulating hormone (TSH) on exocytosis-endocytosis in the rat thyroid follicle cell was investigated by electron microscopic morphometry. All rats used were pre-treated with thyroxine for 2 days prior to the experiments. In control rats, the membrane surface area of exocytotic vesicles, the only type of vesicle present in the apical part of the follicle cell, approximately equaled that of the apical plasma membrane. Intravenous administration of TSH caused a significant decrease in the membrane surface area of exocytotic vesicles by about 20% and a corresponding, significant increase in the membrane surface area of the apical plasma membrane by about 20%. Pseudopods were present in 4 of 6 TSH-injected rats examined, but the membrane surface area of these structures was only 15% of the increase in the surface area of the apical plasma membrane. The total membrane surface area analyzed was not influenced by injection of TSH. We conclude that the initial effect of TSH on the thyroid follicle cell is a re-distribution of membrane from exocytotic vesicles to the apical plasma membrane. This conclusion is in harmony with our previous observations which indicate that the (membrane-requiring) endocytotic response after TSH is regulated by the amount of membrane material added to the apical plasma membrane by exocytosis.