Plasma membrane Ca 2+ release–Activated Ca 2+ channels with a high selectivity for Ca 2+ identified by patch‐clamp recording in rat liver cells

Repetitive waves of increased cytoplasmic Ca 2+ concentration play a central role in the process by which hormones regulate liver function. Maintenance of these Ca 2+ waves requires Ca 2+ inflow through store‐operated Ca 2+ channels. The properties and mechanism(s) of activation of these channels are not well understood. Store‐operated Ca 2+ channels (SOCs) in the H4‐IIE rat liver cell line were studied by whole‐cell patch clamping. Depletion of Ca 2+ in intracellular stores by intracellular perfusion with either inositol 1,4,5‐trisphosphate (InsP 3 ) 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 Ca 2+ and was blocked by a variety of di‐ and trivalent cations. In the absence of extracellular Ca 2+ and Mg 2+ , the inward current was carried by monovalent cations. This current was 10 to 30 times larger than that observed in the presence of extracellular Ca 2+ , and permitted the detection of single‐channel events that corresponded to a single‐channel conductance of about 40 pS. Both the Ca 2+ 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 Ca 2+ channels that have a high selectivity for Ca 2+ , are activated by a decrease in the concentration of Ca 2+ in intracellular stores through a mechanism that is unlikely to involve calmodulin, and are involved in re‐filling intracellular Ca 2+ stores during Ca 2+ signaling.