Functional properties of endogenous receptor- and store-operated calcium influx channels in HEK293 cells

Activation of phospholipase C (PLC)-mediated signaling pathways in non-excitable cells causes the release of calcium (Ca2+) from inositol 1,4,5-trisphosphate (InsP3)-sensitive intracellular Ca2+ stores and activation of Ca2+ influx via plasma membrane Ca2+ channels. The properties and molecular identity of plasma membrane Ca2+ influx channels in non-excitable cells is a focus of intense investigation. In the previous studies we used patch clamp electrophysiology to describe the properties of Ca2+ influx channels in human carcinoma A431 cell lines. Now we extend our studies to human embryonic kidney HEK293 cells. By using a combination of Ca2+ imaging and whole cell and single channel patch clamp recordings we discovered that: 1) HEK293 cells contain four types of plasma membrane Ca2+ influx channels: I(CRAC), Imin, Imax, and I(NS); 2) I(CRAC) channels are highly Ca2+-selective (P(Ca/Cs)>1000) and I(CRAC) single channel conductance is too small for single channel analysis; 3) Imin channels in HEK293 cells display functional properties identical to Imin channels in A431 cells, with single channel conductance of 1.2 pS for divalent cations, 10 pS for monovalent cations, and divalent cation selectivity P(Ba/K)=20; 4) Imin channels in HEK293 cells are activated by InsP3 and inhibited by phosphatidylinositol 4,5-bisphosphate, but store-independent; 5) when compared with Imin, Imax channels have higher conductance for divalent (17 pS) and monovalent (33 pS) cations, but less selective for divalent cations (P(Ba/K)=4), 6) Imax channels in HEK293 cells can be activated by InsP3 or by Ca2+ store depletion; 7) I(NS) channels are non-selective (P(Ba/K)=0.4) and display a single channel conductance of 5 pS; and 8) I(NS) channels are not gated by InsP3 but activated by depletion of intracellular Ca2+ stores. Our findings provide novel information about endogenous Ca2+ channels supporting receptor-operated and store-operated Ca2+ influx pathways in HEK293 cells.