Effects of extracellular calcium on fASIC1 currents
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Fish ASIC1 (fASIC1) cloned from Opsanus tau, unlike the rat ASICs, requires Ca2+, in the extracellular preconditioning solution (pH 7.4) to be activated. Here we show that fASIC1 is interacting with Ca2+ in the same way as mammalian ASICs: extracellular Ca2+, is increasing the proportion of channels available for activation by stabilizing the closed state of the channel; in the activation process Ca2+ is released; H+ compete for the binding site of Ca2+ making the gating mechanism both Ca2+ and H+ dependent; H+ stabilizes the desensitized state; Ca2+ blocks the fASIC1 channel; and the affinity of the block is also modulated by H+. The "Ca2+ activation requirement" of fASICI reflects its greater affinity for steady-state desensitization by H+ compared to mammalian ASIC1.