Extracellular calcium-sensing-receptor (CaR)-mediated opening of an outward K+ channel in murine MC3T3-E1 osteoblastic cells: Evidence for expressionof a functional CaR

The existence in osteoblasts of the G-protein-coupled extracellular calcium (Ca-o(2+))-sensing receptor (CaR) that was originally cloned from parathyr oid and kidney remains controversial. In our recent studies, we utilized mu ltiple detection methods to demonstrate the expression of CaR transcripts a nd protein in several osteoblastic cell lines, including murine MC3T3-E1 ce lls. Although we and others have shown that high Ca-o(2+) and other polycat ionic CaR agonists modulate the function of MC3T3-E1 cells, none of these a ctions has been unequivocally shown to be mediated by the CaR, Previous inv estigations using neurons and lens epithelial cells have shown that activat ion of the CaR stimulates Ca2+- activated K+ channels. Because osteoblastic cells express a similar type of channel, we have examined the effects of s pecific "calcimimetic" CaR activators on the activity of a Ca2+-activated K + channel in MC3T3-E1 cells as a way of showing that the CaR is not only ex pressed in those cells but is functionally active. Patch-clamp analysis in the cell-attached mode showed that raising Ca-o(2+) from 0.75 to 2.75 mmol/ L elicited about a fourfold increase in the open state probability (P-o) of an outward K+ channel with a conductance of similar to 92 pS. The selectiv e calcimimetic CaR activator, NPS R-467 (0.5 mu mol/L), evoked a similar ac tivation of the channel, while its less active stereoisomer, NPSS-467 (0.5 mu mol/L), did not, Thus, the CaR is not only expressed in MC3T3-E1 cells, but is also functionally coupled to the activity of a Ca2+-activated K+ cha nnel. This receptor, therefore, could transduce local or systemic changes i n Ca-o(2+) into changes in the activity of this ion channel and related phy siological processes in these and perhaps other osteoblastic cells. (Bone 2 7:21-27; 2000) (C) 2000 by Elsevier Science Inc. All rights reserved.