Desensitization of the response to thyrotropin-releasing hormone in Xenopus oocytes is an amplified process that precedes calcium mobilization

Consecutive challenges with thyrotropin-releasing hormone (TRH) of oocytes expressing the TRH receptor (TRH-R) resulted in a pronounced desensitization, manifested as a decrease in chloride current amplitude and an increase in response latency. Exposure to low concentrations of TRH resulted in a marked decrease in the amplitude of the subsequent response to a higher concentration of the agonist, even though the second challenge was given before the onset of the response to the first challenge (within 3-15 s). Cellular calcium concentration ([Ca]_i) did not increase within this interval, suggesting that calcium was not involved in the desensitization process. The latency of the second response, however, was either unchanged or shortened, implying additive effects of processes initiated by the first challenge. A longer interval (30 s) between the two challenges brought about a more pronounced decrease in amplitude and a prolongation of response latency. The calcium mobilization initiated by a second challenge with a high concentration of the agonist exhibited a longer latency, a lower rate of [Ca]_i increase and a lower amplitude. Stimulation of coexpressed cholinergic-muscarinic ml receptors with a low concentration of acetylcholine resulted in a pronounced desensitization of the TRH response (heterologous desensitization). Activation of protein kinase C by β-phorbol 12-myristate,13-acetate resulted in a dose-dependent inhibition of the response to TRH, suggesting that protein kinase C was involved in desensitization. Chelerythrine, a specific inhibitor of protein kinase C, abolished a large part of the desensitization. A mutant of the TRH-R that lacks protein kinase C concensus phosphorylation sites in the C-terminal region, exhibited desensitization. Hence, desensitization is not targeted at this part of the receptor molecule. Our results suggest that a very low receptor occupancy activates an amplification step that results in heterologous desensitization. This process is mediated, at least partly, by the activation of protein kinase C, acting on a target proximal to calcium mobilization.