Effect of a second dose of thyrotropin on exocytosis and endocytosis in the rat thyroid gland.

The effect of TSH on exocytosis and endocytosis in thyroid follicle cells was studied with electron microscopic stereology. Groups of rats, pretreated with T4 for 2 d, were injected iv with TSH (500 mU) 20 min, 2, 4, 6, and 8 h before perfusion fixation. Parallel groups were given a second dose of TSH 2, 4, 6 and 8 h after the first one and the thyroids were fixed after 20 min. In controls, injected with saline, the membrane surface area (msa) of exocytotic vesicles was about the same as that of the apical plasma membrane. Pseudopods and colloid droplets were not present. TSH induced exocytosis. The msa of exocytotic vesicles was reduced by about 85% already after 20 min and was even more reduced after 2 h. At later times after injection of TSH the msa of exocytotic vesicles increased gradually to reach control levels after 8 h. TSH also induced endocytosis. After 20 min the increase in msa of endocytotic structures (pseudopods, colloid droplets and micropinocytotic vesicles) corresponded to the decrease in msa of exocytotic vesicles during the same time interval. The msa of endocytotic structures remained high at 2 h but decreased gradually at later time intervals and after 8 h no pseudopods and colloid droplets remained in the apical cell region. A second injection of TSH given 2 h after the first one when few exocytotic vesicles were present did not influence the msa of endocytotic structures. At later time intervals a second dose of TSH stimulated exocytosis as well as endocytosis.(ABSTRACT TRUNCATED AT 250 WORDS)