The Store-operated Calcium Entry Pathways in Human Carcinoma A431 Cells : Functional Properties and Activation Mechanisms

Activation of phospholipase C (PLC)-mediated signaling pathways in nonexcitable cells causes the release of Ca²⁺ from intracellular Ca²⁺ stores and activation of Ca²⁺ influx across the plasma membrane. Two types of Ca²⁺ channels, highly Ca²⁺–selective I_CRAC and moderately Ca²⁺–selective I_SOC, support store-operated Ca²⁺ entry process. In previous patch-clamp experiments with a human carcinoma A431 cell line we described store-operated I_min/I_CRACL plasma membrane Ca²⁺ influx channels. In the present paper we use whole-cell and single-channel recordings to further characterize store-operated Ca²⁺ influx pathways in A431 cells. We discovered that (a) I_CRAC and I_SOC are present in A431 cells; (b) I_CRAC currents are highly selective for divalent cations and fully activate within 150 s after initiation of Ca²⁺ store depletion; (c) I_SOC currents are moderately selective for divalent cations (P_Ba/P_Cs = 14.5) and require at least 300 s for full activation; (d) I_CRAC and I_SOC currents are activated by PLC-coupled receptor agonists; (e) I_SOC currents are supported by I_min/I_CRACL channels that display 8.5–10 pS conductance for sodium; (f) I_CRAC single channel conductance for sodium is estimated at 0.9 pS by the noise analysis; (g) I_min/I_CRACL channels are activated in excised patches by an amino-terminal fragment of InsP₃R1 (InsP₃R1N); and (h) InsP₃ binding to InsP₃R1N is necessary for activation of I_min/I_CRACL channels. Our findings provide novel information about store-operated Ca²⁺ influx pathways in A431 cells.