Inhibition by monensin of dog thyroid secretion in vitro.

Monensin, a carboxylic ionophore, raises the pH of prelysosomal and lysosomal compartments and inhibits lysosomal protein degradation. We tested this drug in dog thyroid slices to ascertain the role of lysosomal pH in thyroglobulin hydrolysis and hormone secretion. Monensin (10(-7)-10(-5) M) and another carboxylic ionophore, nigericin (10(-7)-10(-5) M), inhibited in a concentration-dependent manner TSH-stimulated secretion of T4, T3, and iodide. This inhibition was not toxic since: 1) 10(-5) M monensin did not affect TSH stimulation of protein iodination and cAMP accumulation; and 2) the inhibition was reversible. Secretion was blocked at a postphagocytotic, presumably lysosomal step because the time lag for the fall in secretion rate after 10(-5) M monensin addition was 19 min +/- (SD) 3 min (six experiments), i.e. the same as after lysosomotropic amine addition and significantly shorter than after addition of cytochalasin B (time lag, 43 min +/- 7 min, nine experiments), an inhibitor of phagocytosis. In addition, 10(-5) M monensin blocked the TSH-induced formation of apical pseudopods and colloid droplets and induced a swelling of the Golgi structures. In conclusion, monensin interfered with phagocytosis and with a postphagocytotic, presumably lysosomal, step in secretion by dog thyroid in vitro. Our data provide the first biochemical evidence, in the intact cell, that an acidic pH in the prelysosomal and/or lysosomal compartment is necessary for thyroid hormone secretion.