Single-Channel Properties of Recombinant Acid-Sensitive Ion Channels Formed by the Subunits Asic2 and Asic3 from Dorsal Root Ganglion Neurons Expressed in Xenopus Oocytes

The acid-sensitive ion channels known as ASIC are gated by external protons. A set of these channels is expressed in dorsal root ganglion neurons where they may participate in the transduction of mechanical and nociceptive stimuli. Here, we have examined the single-channel properties of channels formed by the subunits ASIC2 and ASIC3 expressed in Xenopus oocytes using outside-out patches. The mean single-channel current-voltage relationship is linear with a slope conductance of 18 pS between −80 and −40 mV in 150 mM Na⁺ outside and 150 mM K⁺ inside the patch pipet. The selectivity for monovalent cations has the sequence Na⁺ > Li⁺ > K⁺. Divalent cations such as Ca²⁺ do not permeate, but instead block the channel when applied to the extracellular side. External protons increase the probability of channels being open to a maximum of 0.8 with an EC₅₀ of 16 ± 4 μM and a Hill coefficient of 2.7 ± 0.3, whereas the mean single-channel current amplitude is independent of external pH. Analysis of the kinetics of single channels indicates the presence of at least four modes of activity (Mod1 to Mod4) in addition to an inactivated state. Three of the modes exhibit distinct kinetics, and can be unambiguously identified on the basis of open probability (P_oMod1 = 0.5 ± 0.05; P_oMod2 > 0.9 ± 0.05; P_oMod3 < 0.1). Mode 4, which has a P_o in the range of 0.5–0.8, may constitute a distinct mode or alternatively, it represents transitions between the other three modes of activity. Increasing [H⁺]_o increases the frequency of entering the modes with high P_o (modes 1, 2, and 4) and the time the channel spends in the modes with high activity.