Calcium release‐activated calcium current in rat mast cells.

Abstract

SUMMARY1. Whole-cell patch clamp recordings of membrane currents and fura-2 measurements of free intracellular calcium concentration ([Ca21]i) were used to study the biophysical properties of a calcium current activated by depletion of intracellular calcium stores in rat peritoneal mast cells.2. Calcium influx through an inward calcium release-activated calcium current (ICRAC) was induced by three independent mechanisms that result in store depletion: 6. The selectivity of 'CRAC for Ca2+ was assessed by using fura-2 as the dominant intracellular buffer (at a concentration of 2 mM) and relating the absolute changes in the calcium-sensitive fluorescence (390 nm excitation) with the calcium current integral. This relationship was almost identical to the one determined for Ca2+ influx through voltage-activated calcium currents in chromaffin cells, suggesting a similar selectivity. Replacing Na' and K+ by N-methyl-D-glucamine (with Ca2+ ions as exclusive charge carriers) reduced the amplitude of 'CRAC by only 9% further suggesting a high specificity for Ca2+ ions. M1S 1625M. HOTH AND -R. PENNER 7. The current amplitude was not greatly affected by variations of external Mg2" in the range of 0-12 mm. Even at 12 mm Mg2+ the current amplitude was reduced by only 23 %.8. 1CRAC was dose-dependently inhibited by Cd2+. The concentration-response relationship for Cd21 could be described by a Michaelis-Menten function with an apparent KD of 0 24 mm and a Hill coefficient of 1.9. All other tested divalent ions also dose-dependently and reversibly inhibited ICRAC The order of potency was determined by the relative blocking efficacy of 1 mm of the respective ions: Ba2+ % Sr2+ < Ni2 < Mn2+Co2-" Be2 < Cd2 < Zn2rThe trivalent ion La3+ was the most potent blocker of 1CRAC' 10. 1CRAc excluded monovalent ions in the presence of divalent ions. Complete removal of divalent ions typically resulted in a triphasic conductance change: an initial decrease in the calcium current, an abrupt increase in inward current with modest inward rectification due to passage of monovalent ions, and a subsequent decrease in total current with a linear current-voltage relationship. At the same time, these changes were accompanied by a shift in the reversal potential from > +50 to 0 mV.11. While all the features of 1CRAC are compatible with an ion channel mechanism, there was no significant increase in current noise associated with its activation. 12. Our results suggest that the calcium current activated by depletion of intracellular calcium stores is a highly selective pathway for calcium entry into mast cells and may constitute one of the mechanisms underlying the plateau phase of elevated cytosolic calcium concentration following receptor-mediated release of intracellular calcium.

Depletion of intracellular calcium stores activates an outward potassium current in mast and RBL‐1 cells that is correlated with CRAC channel activation

“…Previously we showed that ICRAC in mast cells was reduced by 9 + 2% (n = 4) when replacing external Na + and K + by Nmethyl-D-glucamine (membrane potential -40 mV [14]). A reduction of 10.9 + 6.1% (n = 6) was also seen at more negative potentials (-82 mV) indicating the voltage independence of this reduction.…”

“…However, if one compares the data published on permeation and selectivity for both channel types, it becomes clear that CRAC and trp are not identical. The Ca z+ selectivity over monovalent cations for CRAC channels in mast, RBL, and Jurkat T-cells was found to be 1000/1 or greater [14,15] whereas for trp it is about 40/1 [11]. Comparing the endogenous insect SOC channels with expressed trp channels in Sf9 cells, Vaca et al [29] showed that this SOC channel and the trp channel were clearly different regarding the permeation properties of Ca 2+, Ba 2+, Mg 2+, and Na +.…”

“…It has been shown that different maneuvers that deplete Ca 2+ stores lead to the activation of ICRAC in mast cells [13][14][15], RBL-2H3 cells [6,15,23,30] and RBL-1 cells [15]. As shown in Fig.…”

“…The observed following increase of Ca 2+ currents is probably mediated by a recently described positive feedback effect of [Ca2+]o on CRAC channels [3,32]. The slow inactivation of Na + currents [14,19] in the absence of extracellular divalent cations and the slow inactivation of Ba 2+ currents in the presence of 2 mM extracellular Mg 2+ [15] could both be explained by the reverse of the [Ca2+]o-dependent positive feedback mechanism.…”

“…(1) Outward Cs + currents through VOC channels have been measured in different cell types [8,10,12,18] whereas CRAC channels seemed not to carry any Cs + current. (2) Only the removal of external Ca 2+ was necessary to obtain huge monovalent currents through VOC channels [10,12] whereas all divalent cations (including Mg 2+) had to be removed to allow large monovalent inward currents through CRAC channels [14,19].…”