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Mechanism of inhibition of delayed rectifier K+ current by 4‐aminopyridine in rabbit coronary myocytes.
Author(s) -
Remillard C V,
Leblanc N
Publication year - 1996
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.1996.sp021223
Subject(s) - 4 aminopyridine , chemistry , repolarization , kinetics , biophysics , washout , time constant , electrophysiology , patch clamp , potassium channel blocker , steady state (chemistry) , myocyte , potassium channel , medicine , endocrinology , biochemistry , receptor , biology , physics , engineering , quantum mechanics , electrical engineering
1. The mechanisms involved in the 4‐aminopyridine (4‐AP)‐induced block of delayed rectifier K+ current (IK(V)) in vascular smooth muscle cells were studied in cells enzymatically isolated from the rabbit coronary artery. 2. 4‐AP inhibited slowly inactivating IK(V) in a dose‐dependent manner (concentration producing half‐maximal inhibition, K1/2, = 1.37 mM), and shifted the steady‐state activation and inactivation curves of IK(V) by +9 and +16 mV, respectively. 3. The time constant of activation was significantly increased by 4‐AP at +20 mV; deactivation kinetics were unaffected upon repolarization to ‐40 mV. The fast (tau f approximately 1 s) and slow (tau s approximately 5 s) time constants of inactivation (0 and +20 mV), and the recovery kinetics (tau r approximately 6 s) at ‐60 mV were not significantly affected by 0.5 mM 4‐AP. However, tau f disappeared in the presence of 2 mM 4‐AP while tau s remained unaffected. 4. Use‐dependent unblock of IK(V) was revealed at potentials > or = ‐10 mV from analyses of the voltage dependence of 4‐AP‐sensitive currents and the frequency‐dependent changes (‘reverse use dependence’) of IK(V) during the application of repetitive steps (‐60 to +20 mV for 250 ms at a rate of 0.25 Hz) in control conditions, in the presence of 0.5 mM 4‐AP, and after washout of the drug. These results suggested that 4‐AP preferentially binds to the channel in the closed state, and unbinding is promoted by transitions to the open state. 5. The channel was modelled as a simple three‐state mathematical loop model incorporating single closed, open and inactivated states. The block by 4‐AP was modelled as a state‐dependent interaction with 4‐AP primarily binding to the closed state. Computer simulations support the hypothesis that 4‐AP‐induced block of the delayed rectifier K+ (KV) channel in the closed state is relieved during membrane depolarization. 6. Closed state binding of 4‐AP to the KV channel depolarizes vascular smooth muscle cells by shifting the activation curve of these channels to more positive potentials.