Extracellular Ca2+ sensitivity of mGluR1alpha associated with persistent glutamate response in transfected CHO cells

We previously reported that the metabotropic glutamate receptor 1alpha (mGluR1alpha) can be activated not only by glutamate but also by extracellular Ca2+ (Ca2+o), and that Ser 166 in the extracellular domain determines the sensitivity to Ca2+o. In the present study, we investigated by intracellular Ca2+ (Ca2+i) imaging, the effect of Ca2+o on the glutamate responses of Chinese Hamster Ovary (CHO) cells stably expressing mGluR1alpha wild-type (CHO-wt). As a negative control, we carried out similar experiments using CHO cells expressing Ser166Asp mutant of mGluR1alpha (CHO-S166D) or the substance P receptor (CHO-SPR), which were not activated by Ca2+o application. We observed a remarkable prolongation of the duration of the glutamate response in CHO-wt cells in a Ca2+o concentration dependent manner. In CHO-S166D cells and CHO-SPR cells, only a small sustained component of the glutamate response was observed in the presence of Ca2+o. These sustained components were blocked by SKF-96365, a blocker of receptor-operated Ca2+-influx. Thus, it was concluded that the Ca2+o-sensing function of mGluR1alpha-wt induced the persistent opening of the receptor-operated Ca2+-permeable channels, probably by persistent activation of the receptor by glutamate. We additionally observed that the dose-response relationship of CHO-S166D and CHO-SPR shifted significantly by changing Ca2+o concentration, i.e. Ca2+o was required to maintain the normal ligand responses of these receptors.