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Single voltage‐activated Na+ and K+ channels in the somata of rat motoneurones.
Author(s) -
Safronov B V,
Vogel W
Publication year - 1995
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1995.sp020863
Subject(s) - membrane potential , conductance , chemistry , biophysics , electrophysiology , patch clamp , steady state (chemistry) , neuroscience , physics , biochemistry , biology , condensed matter physics
1. Voltage‐activated Na+ and K+ channels were investigated in the soma membrane of motoneurones using the patch‐clamp technique applied to thin slices of neonatal rat spinal cord. 2. One type of TTX‐sensitive Na+ channel, with a conductance of 14.0 pS, was found to underlie the macroscopic Na+ conductance in the somata of motoneurones. These channels activated within a potential range between ‐60 and ‐20 mV with a potential of half‐maximal activation (E50) of ‐38.9 mV and steepness factor (k) of 6.1 mV. 3. Kinetics of Na+ channel inactivation could be fitted with a single exponential function at all potentials investigated. The curve of the steady‐state inactivation had the following parameters: a half‐maximal potential (Eh,50) of ‐81.6 mV and k of ‐10.2 mV. 4. Kinetics of recovery of Na+ channels from inactivation at a potential of ‐80 mV were double exponential with fast and slow components of 16.2 (76%) and 153.7 ms (24%), respectively. It is suggested that the recovery of Na+ channels from inactivation plays a major role in defining the limiting firing frequency of action potentials in motoneurones. 5. Whole‐cell K+ currents consisted of transient (A)‐ and delayed‐rectifier (DR)‐components. The A‐component activated between ‐60 and +20 mV with an E50 of ‐33.3 mV and k of 15.7 mV. The curve of steady‐state inactivation was best fitted with an Eh,50 of ‐82.5 mV and k of ‐10.2 mV. The DR‐component of K+ current activated smoothly at more positive potentials. E50 and k for DR‐currents were +1.4 and 16.9 mV, respectively. 6. The most frequent single K+ channel found in the somata of motoneurones was the fast inactivating A‐channel with a conductance of 19.2 pS in external Ringer solution. In symmetrical high‐K+ solutions the conductance was 50.9 and 39.6 pS for inward and outward currents, respectively. The channel activation took place between ‐60 and +20 mV. The curve of steady‐state inactivation of single A‐channels had an Eh,50 of ‐87.1 mV and k of ‐12.8 mV. In high‐Ko+ solution A‐channels demonstrated a rapid deactivation at potentials between ‐110 and ‐60 mV. The time constant of the channel deactivation depended on the membrane potential and changed from 1.5 ms at ‐110 mV to 6.3 ms at ‐60 mV. 7. Delayed‐rectifier K+ channels were found in the soma membrane at a moderate density.(ABSTRACT TRUNCATED AT 250 WORDS)