Dissociation of the store-operated calcium current/CRAC and the Mg-nucleotide-regulated metal ion current MagNuM

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 muM GTP-gamma-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.