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 ints-acellular Ca ²⁺ 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, I_min, I_max, and I _NS; 2) I_CRAC channels are highly Ca²⁺-selective (P_Ca/Cs > 1000) aad I_CRAC single channel conductance is too small for single channel analysis; 3) I_min channels in HEK293 cells display functional properties identical to I_min 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) I_min channels in HEK293 cells are activated by InsP ₃ and inhibited by phosphatidylinositol 4,5-bisphosphate, but store-independent; 5) when compared with I_min, I_max channels have higher conductance for divalent (17 pS) and monovalent (33 pS) cations, but less selective for divalent cations (P_Ba/K = 4), 6) I_max 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 iniracellular Ca²⁺ stores. Our findings provide novel information about endogenous Ca²⁺ channels supporting receptor-operated and store-operated Ca²⁺ influx pathways in HEK293 cells.