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+ chann els. The properties and mechanism(s) of activation of these channels are no t well understood. Store-operated Ca2+ channels (SOCs) in the H4-IIE rat li ver cell line were studied by whole-cell patch clamping. Depletion of Ca2in intracellular stores by intracellular perfusion with either inositol 1,4 ,5-trisphosphate (InsP(3)) or thapsigargin in the presence of 10 mmol/L eth ylene glycol-bis(beta -aminoethyl ether)-N,N-tetraacetic acid (EGTA), or wi th 10 mmol/L EGTA alone, activated an inward current that reversed at a mem brane potential above +40 mV. In physiologic extracellular medium, this inw ard 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 t o 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 c urrents were blocked by the calmodulin antagonist, N-(6-amino hexyl)-5-chlo ro-1-naphthalenesulphonamide (W7), but not by calmidazolium or calmodulin-d ependent protein kinase II fragment 290-309. It is concluded that liver cel ls possess plasma membrane Ca2+ channels that have a high selectivity for C a2+, are activated by a decrease in the concentration of Ca2+ In intracellu lar stores through a mechanism that is unlikely to involve calmodulin, and are involved in re-tilling intracellular Ca2+ stores during Ca2+ signaling.