Extracellular acid increases the open probability of transduction channels in spider mechanoreceptors

Ion channels of the epithelial sodium channel, degenerin and acid-sensitive channel (ENaC/DEG/ASIC) family share a number of structural and functional homologies. Several members of this group have been linked to mechanoreception and nociception, but there is no direct evidence that these molecules cause the transduction of mechanical stimuli in any mechanoreceptor. The receptor channels of a spider mechanoreceptor, the VS-3 slit-sense organ of Cupiennius salei, show several similarities to ENaC/DEG/ASIC channels, including Na⁺ selectivity and amiloride blockade. We recorded the receptor current under voltage clamp in VS-3 neurons at different extracellular pH values. Acid pH partially blocked the delayed rectifier K⁺ current and increased the receptor current in these cells. Noise analysis of the receptor current showed that low pH increased the open probability of the receptor channels. Therefore, acid sensitivity is a further similarity between these mechanoreceptor channels and the ENaC/DEG/ASIC family.