ISSN:
1432-2013
Keywords:
Node of Ranvier
;
Voltage clamp
;
4-aminopyridine
;
K channel
;
Voltage-dependent block
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Summary 1. Voltage clamp experiments were done on single myelinated nerve fibres of the frog,Rana esculenta. 2. 53 μM 4-aminopyridine (4-AP) reducedI K to about one-fifth if tested with infrequent (1/min) and short (10 ms) depolarizing pulses; the onset time constant under these circumstances was ca. 160 s (14–15° C). After prolonged treatment the effect was virtually irreversible. 3. At equilibrium with 4-AP, increasing the frequency of short pulses removed part of the block, the block removal accelerating with inceasing pulse duration and frequency. 4. In 53 μM 4-AP unblocking of K channels during long (0.8 s) depolarizing pulses proceeded with a time constant, τ r , of ca. 0.2 s. Restoration of block at the resting potential proceeded with a much larger time constant, τ′ r , of ca. 1 min. 5. The stationary fraction,r ∞, of K channels conducting in 53 μM 4-AP was 0.66, 0.41, and 0.24 at V=120, 50, and 0 mV, respectively. 6. In a series of experiments with [4-AP] varying between 13.3 and 848 μM, τ r decreased from 0.25 to 0.10 s (V=130 mV, ca. 17°C) whiler ∞ followed the empirical relation 1/r ∞=1+c t +c v exp(−0.77 EF/RT) with E=V-70 mV.c t andc v are dimensionless quantities that increase with [4-AP] and reflect the voltage-independent and voltage-dependent component, respectively, of block. 7. Block of K channels and partial removal are also observed with inwardI K at raised [K+]0. Removal proceeds on depolarization even ifI K is additionally but temporarily suppressed by tetraethylammonium. Hence neither direction nor amplitude ofI K but only the pulse potential seems to determine the extent of block for a given [4-AP].
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00583659
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