Agonists of the Ca2+-Sensing Receptor (CaR) Activate Nonselective Cation Channels in HEK293 Cells Stably Transfected with the Human CaR

Abstract

Calcium (Ca2+) ions serve multiple roles both intra- and extracellularly. We recently cloned a cell surface, Ca2+o-sensing receptor (CaR) that plays a central role in Ca2+ohomeostasis by enabling direct regulation by Ca2+oof parathyroid hormone (PTH) secretion and the function of other tissues involved in mineral ion homeostasis. In parathyroid cells, the CaR activates phospholipase C, thereby raising the levels of inositol trisphosphate (IP3) and releasing Ca2+from intracellular stores. High Ca2+oalso activates Ca2+influx into parathyroid cells through poorly defined mechanisms that may involve Ca2+-permeable, nonselective cation channels (NCC). We now show that human embryonic kidney (HEK293) cells also have NCC and, furthermore, that these channels are regulated by the CaR. We have utilized the cell-attached configuration of the patch clamp technique to characterize the properties of these channels as well as their regulation by various CaR agonists added to the external bath solution. The polycationic CaR agonist, neomycin (100 μM), as well as an elevated concentration of Ca2+o(3 mM), both of which activate the cloned CaR, significantly increased the probability of channel opening (Po) in HEK cells stably transfected with the CaR but not in nontransfected HEK cells which do not contain the receptor. Thus, the activation of the CaR enhances the activity of Ca2+-permeable NCC in these cells, which could contribute to the sustained increase in Ca2+iin parathyroid cells which is observed in response to elevated Ca2+o. The CaR may also regulate the membrane functions of other CaR-expressing cells (e.g., those in the brain), at least in part, by modulating similar channels.