The calcium-independent transient outward potassium current in isolated ferret right ventricular myocytes. II. Closed state reverse use-dependent block by 4-aminopyridine.

Abstract

Block of the calcium-independent transient outward K + current, Ito, by 4-aminopyridine (4-AP) was studied in ferret right ventricular myocytes using the whole cell patch clamp technique. 4-AP reduces/to through a closed state blocking mechanism displaying "reverse use-dependent" behavior that was inferred from: (a) development of tonic block at hyperpolarized potentials; (b) inhibition of development of tonic block at depolarized potentials; (c) appearance of "crossover phenomena" in which the peak current is delayed in the presence of 4-AP at depolarized potentials; (d) relief of block at depolarized potentials which is concentration dependent and parallels steady-state inactivation for low 4-AP concentrations (V1/~ ~ -10 mV in 0.1 mM 4-AP) and steady-state activation at higher concentrations (VI/2 --+7 mV in 1 mM 4-AP, +15 mV in 10 mM 4-AP); and (e) reassociation of 4-AP at hyperpolarized potentials. No evidence for interaction of 4-AP with either the open or inactivated state of the/to channel was obtained from measurements of kinetics of recovery and deactivation in the presence of 0.5-1.0 mM 4-AP. At hyperpolarized potentials (-30 to -90 mV) 10 mM 4-AP associates slowly (time constants ranging from ~ 800 to 1,300 ms) with the closed states of the channel (apparent Kd ~ 0.2 mM). From -90 to -20 mV the affinity of the /to channel for 4-AP appears to be voltage insensitive; however, at depolarized potentials (+20 to +100 mV) 4-AP dissociates with time constants ranging from ~ 350 to 150 ms. Consequently, the properties of 4-AP binding to the/to channel undergo a transition in the range of potentials over which channel activation and inactivation occurs (-30 to +20 mV). We propose a closed state model of/to channel gating and 4-AP binding kinetics, in which 4-AP binds to three closed states. In this model 4-AP has a progressively lower affinity as the channel approaches the open state, but has no intrinsic voltage dependence of binding.Address reprint requests to Dr.

Block by 4‐aminopyridine of a Kv1.2 delayed rectifier K+ current expressed in Xenopus oocytes.

“…It is now accepted that 4-AP acts on the cytoplasmic side of K+ channels (Hermann & Gorman, 1981;Choquet & Korn, 1992;, but the actual mechanism of block has not been fully characterized. (Campbell, Rasmusson & Strauss, 1993) and adult rat (Castle & Slawsky, 1993) (Thompson, 1982;Kehl, 1990) in which increasing the frequency, magnitude or duration of depolarizations decreased the degree of block by 4-AP. In contrast, the opening of It.…”

Inhibition of Neuronal Degenerin/Epithelial Na+ Channels by the Multiple Sclerosis Drug 4-Aminopyridine

“…Perhaps not a coincidence, such a description also defines the conductive pore of the channel as it runs through the plasma membrane. Kv channels also have a single binding site for 4-AP (4,6,48,49). This binding site is within the pore, enabling 4-AP to be a pore blocker.…”

Effects of diabetic cardiomyopathy on regional electrophysiologic characteristics of rat ventricle

“…The blocking effect of 4-AP on I to shows reverse use-dependence at fast stimulation rates [20]. To avoid the reverse use-dependence, we applied test pulses at a frequency of 0.05 Hz.…”