Inhibition of cyclic 3',5' monophosphate synthesis in rat testis by L-triiodothyronine.

Cytosolic adenylate cyclase activity from rat seminiferous tubules is inhibited by L-triiodothyronine (L-T3). In a typical dose-response curve, using Mn-ATP as substrate, no effect is observed at 10(-10) M L-T3; about 15 to 25% inhibition is found in the range between 10(-9) and 10(-6) M L-T3 and finally a sharp enzyme inhibition is evident at increasing hormone concentrations from 10(-6) to 10(-4) M. Incubation of decapsulated testes with L-T3 leads to a decrease of intracellular cyclic AMP levels. Dose-response relationships for such effect are similar to those found for adenylate cyclase activity. In this case a clear response is observed at 10(-8) M L-T3.