PROTON-INDUCED SODIUM CURRENT IN FROG ISOLATED DORSAL-ROOT GANGLION-CELLS

The proton-induced current was examined in isolated frog dorsal root ganglion (DRG) cells by the use of the 'concentration-clamp' technique, which allows intracellular perfusion and rapid change of external solution with various pH (pH(o)) within 2 ms under single-electrode voltage-clamp condition. Over one-half of the examined neurons showed no response for a 'step' reduction of pH(o) even in a Ca2+free external solution. In smaller neurons having a diameter < 20 μm, the persistent and reliable proton-induced responses were obtained, though the current amplitude and the activation and inactivation varied considerably for each cell. The decrease of external Na+ concentration ([Na+](o)) reduced the proton response. The proton response reversed the direction at the Na+ equilibrium potential (E(Na)). With decreasing pH(o) from 7.4, proton response increased in a sigmoidal fashion. The threshold was around pH 7.0 and the maximum response appeared at pH 5.2, whereas pK(a) and Hill coefficient were 6.0 and 1.97, respectively. The activation and inactivation phases of the proton-induced current behaved as a single exponential function. The time constants of activation (τ(a)) and inactivation (τ(i)) were not affected by changing either the holding membrane potential (V(H)) or the low external Ca2+ concentration ([Ca2+](o)) between 10-6 and 5 x 10-3 M. But the decrease of pH(o) up to 5.2 decreased both τ(a) and τ(i) in a saturable manner. In the inactivation curve of proton-induced current obtained by decreasing pH(o) from various conditioning pH(o) to 5.5, half inactivation occurred at pH(o) 7.45. The presence of external divalent cations suppressed the proton response in a dose-dependent manner. The half-inhibition dose (IC50) was 4.8 x 10-5 M for [Ca2+](o), 7 x 10-5 M for [Sr2+](o), 10-4 M for [Ba2+](o), and 7 x 10-3 M for [Mg2+](o). D-600, diltiazem, and Cd2+ dose dependently inhibited the proton response at high concentrations of 10-4-10-3 M. The results are consistent with the idea that proton-induced Na+ current flows through a proton-transformed Ca2+ channel in which dominancy seems to depend on the degree of acidification, as found in cultured chick sensory neurons. Unexpectedly, ω-conotoxin (10-7-10-5 M) dose dependently inhibited the proton response in frog DRG neurons without changing the activation and inactivation kinetics, whereas tetrodotoxin (10-6-10-5 M) suppressed the proton response with considerable acceleration of the inactivation phase. The mode of actions of these drugs on the proton response was discussed.