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Physiologisches Institut, Justus-Liebig-Universität Giessen, Germany.
1. Voltage-dependent potassium channels were investigated in rat axonal membrane by means of the patch-clamp recording technique. Three different types of channels (F, I and S) have been characterized on the basis of their single-channel conductance, activation, deactivation and inactivation properties. 2. The fast (F) channels were activated smoothly at potentials (E) between -50 and 50 mV (E50 = 4.6 mV). They had a conductance of 55 pS for inward current and 30 pS for outward current in solutions containing 155 mM K+ (high K+) on both sides of the membrane at 21-23 degrees C. The F-channels demonstrated the fastest deactivation, within 1-2 ms, and inactivated in a few hundreds of milliseconds. The time constant of inactivation was 143 ms at E = +40 mV. 3. The intermediate (I) channels activated steeply between E = -70 and -50 mV (E50 = -64.2 mv) and had a single-channel conductance of 33 pS for inward and 18 ps for outward currents. The I-channels deactivated with intermediate kinetics with the time constants of 20.4 ms and 10.1 ms at E = -80 mV and E = -100 mV, respectively. Complete inactivation of the channels developed over tens of seconds. The time constant of inactivation was 7.4 s at E = +40 mV. 4. The slow (S) channels were active at potentials positive to -90 mV. Their conductance was 10 pS for inward currents. The time constant of activation of the S-channels was strongly potential dependent. At a holding potential of -100 mV the channels deactivated during a long time interval between 30 ms and 1 s, producing long-lasting tail currents. The mean time constant of deactivation for S-channels was 129 ms. 5. The conductances of F- and I-channels measured under normal physiological conditions (Ringer solution in bath) were 17 and 10 pS, respectively. 6. Tetraethylammonium (TEA), the classic blocker of potassium channels, suppressed F-, I- and S-channels. It gradually reduced the apparent amplitude of unitary currents in a dose-dependent manner with IC50 equal to 1.2 mM for F-channels, 0.6 mM for I-channels and 1.4 mM for S-channels. Dendrotoxin (DTX), a toxin from the green mamba snake, considerably inhibited the I tail currents at nanomolar concentrations (IC50 = 2.8 nM) while the amplitudes of single I-channel currents were not affected. 7. The K+ channels of F, I and S types form the basis of the potassium conductivity in mammalian peripheral myelinated axon.(ABSTRACT TRUNCATED AT 400 WORDS)
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