Plasma membrane Ca²⁺ release-activated Ca²⁺ channels with a high selectivity for Ca²⁺ identified by patch-clamp recording in rat liver cells

Repetitive waves of increased cytoplasmic Ca²⁺ concentration play a central role in the process by which hormones regulate liver function. Maintenance of these Ca²⁺ waves requires Ca²⁺ inflow through store-operated Ca²⁺ channels. The properties and mechanism(s) of activation of these channels are not well understood. Store-operated Ca²⁺ channels (SOCs) in the H4-IIE rat liver cell line were studied by whole-cell patch clamping. Depletion of Ca²⁺ in intracellular stores by intracellular perfusion with either inositol 1, 4, 5-trisphosphate (InsP₃) or thapsigargin in the presence of 10 mmol/L ethylene glycol-bis(β-aminoethyl ether)-N, N-tetraacetic acid (EGTA), or with 10 mmol/L EGTA alone, activated an inward current that reversed at a membrane potential above +40 mV. In physiologic extracellular medium, this inward current was carried exclusively by Ca²⁺ and was blocked by a variety of di- and trivalent cations. In the absence of extracellular Ca²⁺ and Mg²⁺, the inward current was carried by monovalent cations. This current was 10 to 30 times larger than that observed in the presence of extracellular Ca²⁺, and permitted the detection of single-channel events that corresponded to a single-channel conductance of about 40 pS. Both the Ca²⁺ and Na⁺ inward currents were blocked by the calmodulin antagonist, N-(6-amino hexyl)-5-chloro-1-naphthalenesulphonamide (W7), but not by calmidazolium or calmodulin-dependent protein kinase II fragment 290-309. It is concluded that liver cells possess plasma membrane Ca²⁺ channels that have a high selectivity for Ca²⁺, are activated by a decrease in the concentration of Ca²⁺ in intracellular stores through a mechanism that is unlikely to involve calmodulin, and are involved in re-filling intracellular Ca²⁺ stores during Ca²⁺ signaling.