Mechanisms of desensitization and resensitization of G protein-coupled neurokinin1 and neurokinin2 receptors.

We compared the desensitization of neurokinin1 and neurokinin2 (NK1 and NK2) receptors expressed in Chinese hamster ovary cells to substance P and neurokinin A, respectively. Substance P and neurokinin A stimulated a rapid increase in intracellular Ca2+ concentration ([Ca2+]i) for both receptors, which was due to release of Ca2+ from intracellular stores. This was followed by a plateau in [Ca2+]i, which was due to influx of extracellular Ca2+, and was more sustained for the NK2 receptor. When Ca2+ was present in the extracellular solution, the Ca2+ response of the NK1 receptor, but not the NK2 receptor, rapidly desensitized and slowly resensitized to two exposures to agonist. In contrast, the [Ca2+]i response, measured in Ca2+-free solution, and inositol triphosphate generation desensitized and resensitized similarly for the NK1 and NK2 receptors. Thus, differences in desensitization between the NK1 receptor and the NK2 receptor may be related to differences in entry of extracellular Ca2+. We compared endocytosis of the NK1 and NK2 receptors to determine whether disparities could account for differences in desensitization. Fluorescent and radiolabeled substance P and neurokinin A were internalized similarly by cells expressing NK1 and NK2 receptors. Thus, disparities in internalization cannot account for differences in desensitization. We used inhibitors to examine the contribution of endocytosis, recycling, and phosphatases to desensitization and resensitization of the NK1 receptor. Desensitization did not require endocytosis. However, resensitization required endocytosis, recycling, and phosphatase activity. This suggests that the NK1 receptor desensitizes by phosphorylation and resensitizes by dephosphorylation in endosomes and recycling.