Proton-gated sodium current in parasympathetic ganglion cells of frog heart.
1. The proton-gated current was investigated in whole-cell configuration of the neurons isolated from bullfrog heart parasympathetic ganglia with the use of the "concentration-clamp" technique, which combines intracellular perfusion and extremely rapid exchange of external solution within 1-2 ms, under a single-electrode voltage-clamp condition. 2. In all isolated neurons, a "step" decrease in extracellular pH (pHo) induced a transient inward current that was followed by a complete inactivation within 1 s. 3. The proton-gated current increased in a sigmoidal fashion as pHo decreased. In the external solution containing 2 mM Ca2+, the threshold of current activation was at pHo 7.4, and the maximum response appeared at pHo 6.5-5.5. The dissociation constant (Kd) and Hill coefficient were 7.1 and 3.2, respectively. 4. The proton-gated current was reduced by decreasing the extracellular Na+ concentration. In the absence of K+, the current produced by reduced extracellular pH reversed at the Na+ equilibrium potential (ENa), indicating that the current was carried by Na+. 5. Kinetics of both the activation and inactivation phases of proton-induced current were single exponential. The time constants of activation (tau a) and inactivation (tau i) had no potential dependence but decreased slightly by decreasing pHo. 6. In the inactivation curve of the proton-induced current obtained by decreasing pHo from various conditioning pHos to 6.5, the half-maximum inactivation occurred at pHo 7.75. 7. The proton-gated current was suppressed as the extracellular Ca2+ concentration [( Ca2+]o) increased from 0.1 to 10 mM, and the half inhibition occurred at greater than 10 mM [Ca2+]o.(ABSTRACT TRUNCATED AT 250 WORDS)
Kim
D K
DK
Department of Neurophysiology, Tohoku University School of Medicine, Sendai, Japan.
Tateishi
N
N
Akaike
N
N
D000818:Animals; D005726:Ganglia, Parasympathetic / Q000187:drug effects / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008564:Membrane Potentials; D011892:Rana catesbeiana / Q000502:physiology*; D012964:Sodium / Q000502:physiology*; D015222:Sodium Channels / Q000187:drug effects / Q000502:physiology*
D015222:Sodium Channels; D012964:Sodium