Inhibition of cyclic 3′5′ monophosphate synthesis in rat testis by L-triiodothyronine

Summary

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.