The proton-activated inward current of rat sensory neurons includes a calcium component.

Many neurons possess a proton-activated conductance, IH, which supports a large transient inward current at negative potentials and thereby depolarizes cells during rapid drops in external pH. The channels underlying this conductance are permeant to monovalent cations, with a clear preference for sodium. In earlier experiments, it appeared that divalent cations were impermeant: increasing concentrations of extracellular Ca2+ actually decreased the current amplitude. Using whole-cell patch clamp recording techniques, we find that the proton-activated channel is permeant to Ca2+ ions. In the absence of monovalent cations, a substantial current is supported by divalent cations. The previously reported block results from competition between divalents and monovalents. This finding suggests that IH may provide a pathway for Ca2+ entry during the acidification that accompanies normal synaptic transmission, excessive electrical activity, and tissue ischemia.