Effect of graded doses of thyrotropin on exocytosis and early phase of endocytosis in the rat thyroid.

Our previous studies on the follicle cell have shown that exocytosis and endocytosis induced by a large dose of TSH (500 mU) are functionally related elements of a redistribution of membrane in the apical cell region. To explore the significance of exocytosis at submaximal levels of stimulation, T4-treated rats were injected iv with 5 or 50 mU TSH 5, 10, or 20 min before perfusion fixation of the thyroid. Additional groups were given 20 or 100 mU TSH or saline 20 min before fixation. Electron microscopic stereology showed that TSH induced exocytosis and endocytosis and a redistribution of membrane in the apical part of the follicle cell. Exocytosis preceded endocytosis. The redistribution of membrane was quantitatively different but qualitatively similar to that previously observed after a large dose of TSH. The rate of exocytosis was linearly correlated with the logarithm of the TSH dose. The size, but not the number (counted in the light microscope), of pseudopods was related to the TSH dose; at 20 min, the membrane surface area of a pseudopod induced by 100 mU was about twice that of a pseudopod induced by 5 mU. The total membrane surface area of endocytotic structures (pseudopods, colloid droplets, and micropinocytotic vesicles) was related to the surface area of the membrane added to the apical plasma membrane by exocytosis. The findings indicate that exocytosis is part of the normal response of the follicle cell to stimulation and represents the first step in the redistribution of membranes in the apical part of the follicle cell induced by TSH. At all levels of stimulation, the membrane of the exocytotis vesicles, rather than the apical plasma membrane, is the membrane reserve used to make endocytotic structures. (Endocrinology 108: 399, 1981)