Ca2+ selectivity and fatty acid specificity of the noncapacitative, arachidonate-regulated Ca2+ (ARC) channels

The arachidonate-regulated, Ca2+-selective ARC channels represent a novel receptor-activated pathway for the entry of Ca2+ in nonexcitable cells that is entirely separate from the widely studied store-operated, Ca2+ release-activated Ca2+ channels. Activation of ARC channels occurs specifically at the low agonist concentrations typically associated with oscillatory Ca2+ signals and appears to provide the predominant mode of Ca2+ entry under these conditions (Mignen, O., Thompson, J. L., and Shuttleworth, T. J. (2001) J. Biol. Chem. 276, 35676-35683). In this study we demonstrate that ARC channels are present in a variety of different cell types including both cell lines and primary cells. Examination of their pharmacology revealed that currents through these channels are significantly inhibited by low concentrations (< 5 muM) of Gd3+, are unaffected by 100 pm 2-aminoethyoxydiphenyl borane, and are not activated by the diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (100 muM). Their selectivity for Ca2+ was assessed by determining the EC50 for external Ca2+ block of the monovalent currents observed in the absence of external divalent cations. The value obtained (150 nm) indicates that the Ca2+ selectivity of ARC channels is extremely high. Examination of the ability of various fatty acids, including arachidonic acid, to activate the ARC channels demonstrated that activation does not reflect any nonspecific membrane fluidity or detergent effects, shows a high degree of specificity for arachidonic acid over other fatty acids (especially monounsaturated and saturated fatty acids), and is independent of any arachidonic acid metabolite. Moreover, studies using the charged analogue arachidonyl coenzyme A demonstrate that activation of the ARC channels reflects an action of the fatty acid specifically at the internal face of the plasma membrane. Whether this involves a direct action of arachidonic acid on the channel protein itself or an action on some intermediary molecule is, at present, unclear.