Block of potassium channels of the nodal membrane by 4-aminopyridine and its partial removal on depolarization.

1. Voltage clamp experiments were done on single myelinated nerve fibres of the frog, Rana esculenta. 2. 53 muM 4-aminopyridine (4-AP) reduced IK 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 degrees 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 increasing pulse duration and frequency. 4. In 53 muM 4-AP unblocking of K channels during long (0.8 s) depolarizing pulses proceeded with a time constant, taur, of ca. 0.2 s. Restoration of block at the resting potential proceeded with a much larger time constant, tau'r, of ca. 1 min. 5. The stationary fraction, rinfinity, of K channels conducting in 53 muM 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 muM, taur decreased from 0.25 to 0.10 s (V = 130 mV, ca. 17 degrees C) while rinfinity followed the empirical relation 1/rinfinity = 1 + ct + cv exp(-0.77 EF/RT) with E = V - 70 mV. ct and cv 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 inward IK at raised [K+]O. Removal proceeds on depolarization even if IK is additionally but temporarily suppressed by tetraethylammonium. Hence neither direction nor amplitude of IK but only the pulse potential seems to determine the extent of block for a given [4-AP].