Dissociation of the store‐operated calcium current ICRAC and the Mg‐nucleotide‐regulated metal ion current MagNuM

Abstract

Rat basophilic leukaemia cells (RBL-2H3-M1) were used to study the characteristics of the store-operated Ca²⁺ release-activated Ca²⁺ current (I_CRAC) and the magnesium-nucleotide-regulated metal cation current (MagNuM) (which is conducted by the LTRPC7 channel). Pipette solutions containing 10 mm BAPTA and no added ATP induced both currents in the same cell, but the time to half-maximal activation for MagNuM was about two to three times slower than that of I_CRAC. Differential suppression of I_CRAC was achieved by buffering free [Ca²⁺]_i to 90 nm and selective inhibition of MagNuM was accomplished by intracellular solutions containing 6 mm Mg.ATP, 1.2 mm free [Mg²⁺]_i or 100 μm GTP-γ-S, allowing investigations on these currents in relative isolation. Removal of extracellular Ca²⁺ and Mg²⁺ caused both currents to be carried significantly by monovalent ions. In the absence or presence of free [Mg²⁺]_i, I_CRAC carried by monovalent ions inactivated more rapidly and more completely than MagNuM carried by monovalent ions. Since several studies have used divalent-free solutions on either side of the membrane to study selectivity and single-channel behaviour of I_CRAC, these experimental conditions would have favoured the contribution of MagNuM to monovalent conductance and call for caution in interpreting results where both I_CRAC and MagNuM are activated.