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 ooc ytes expressing the TRH receptor (TRH-R) resulted in a pronounced dese nsitization, manifested as a decrease in chloride current amplitude an d an increase in response latency. Exposure to low concentrations of T RH resulted in a marked decrease in the amplitude of the subsequent re sponse to a higher concentration of the agonist, even though the secon d challenge was given before the onset of the response to the first ch allenge (within 3 - 15 s). Cellular calcium concentration ([Ca](i)) di d not increase within this interval, suggesting that calcium was not i nvolved in the desensitization process. The latency of the second resp onse, however, was either unchanged or shortened, implying additive ef fects of processes initiated by the first challenge. A longer interval (30 s) between the two challenges brought about a more pronounced dec rease in amplitude and a prolongation of response latency. The calcium mobilization initiated by a second challenge with a high concentratio n of the agonist exhibited a longer latency a lower rate of [Ca](i) in crease and a lower amplitude. Stimulation of coexpressed cholinergic-m uscarinic mi receptors with a low concentration of acetylcholine resul ted in a pronounced desensitization of the TRH response (heterologous desensitization). Activation of protein kinase C by beta-phorbol 12-my ristate, 13-acetate resulted in a dose-dependent inhibition of the res ponse to TRH, suggesting that protein kinase C was involved in 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, desensitiz ation is not targeted at this part of the receptor molecule. Our resul ts suggest that a very low receptor occupancy activates an amplificati on step that results in heterologous desensitization. This process is mediated, at least partly, by the activation of protein kinase C, acti ng on a target proximal to calcium mobilization.