Proton‐induced transformation of calcium channel in chick dorsal root ganglion cells.

Abstract

1. In dissociated and cultured 2-5-day-old chick dorsal root ganglion cells, a large transient inward current could be activated in response to a ''step'' increase in [H+]0. 2. Using the single-electrode patch clamp technique in its whole-cell configuration, the proton-induced current was graded with [H+]0 and relaxed in 1-2 s. 3. The pH dependence of the current was sigmoid with activation occurring at around pH 7.0 (at [Ca2+]0 = 1 mM) and a maximum at pH 6.0-5.5. 4. Small increases of [H+]0, which by themselves failed to activate a significant amount of current, inactivated the proton-induced current. The half-maximum of inactivation occurred at pH 7.11 at [Ca2+]0 = 5 mM, but this changed to pH 7.32 at [Ca2+]0 = 1 mM. 5. the proton-induced inward current reversed direction at the Na+ equilibrium potential and was suppressed in the absence of [Na+]0. Measurement of the reversal potential at different [Na+]0 and/or [Na+]i showed a linear relation with a slope of 58 mV/decade as predicted from the Nernst equation. Thus, proton-induced current was carried by Na+ and was abbreviated as INa(H). 6. The membrane conductance associated with INa(H) showed no voltage dependence, but did change in parallel with the activation of the current. The membrane conductance increased by a factor of 10-20-fold at the peak of the inward current. 7. INa(H) was blocked by organic and inorganic Ca2+ channel blockers (diltiazem, Cd2+ and Ni2+), but was unaffeced by high concentrations of tetrodotoxin (TTX) or steady-state increases of the [Ca2+]i to 10-4 M or the [H+]i to 10-6 M. 8 In outside-out membrane patches, the single channel associated with the proton-induced current opened in bursts, with long pauses. The mean open time during the bursts was 1.26 ms and the channel had a conductance of 20-25 pS at -80 mV (120 mM [Na2+]0, 20 mM [Na2+]i). 9. Measurement of the voltage-gated Ca2+ current using short (30-50 ms) depolarizing pulses to zero showed that the Ca2+ current (Ica) but not the fast Na+ current (INa) was completely suppressed during the time course of activation of INa(H). 10 INa(H) was completely blocked by high (35-40 mM) [Ca2+]0. 11. Simulatenous elevation of [H+]0 and [Ca2+]0 failed to activate INa(H) but enhanced the voltage-gated Ca2+ channel. 12. Our data show that the proton-induced current is carried by Na+ flowing through a transformed Ca2+ channel. 13. An Na+ current with similar amplitude and kinetics to that of INa(H) could be activated on rapid withdrawal of extracellular Ca2+. However, the zero-Ca2+-induced INa required a critical [H+]0 for its activation. 14. These findings are consistent with a scheme in which removal of Ca2+ or its displacement by protons from the Ca2+ channel binding sites transforms the Ca2+ channel from a gated to a non-gated state and alters its permeability. Thus the Ca2+ channel may exist in two forms, a voltage-gated Ca2+-permeable state and a non-gated (proton-gated) Na+-permeable state.