Gj. Stephens et al., ON THE MECHANISM OF 4-AMINOPYRIDINE ACTION ON THE CLONED MOUSE-BRAIN POTASSIUM CHANNEL MKV1.1, Journal of physiology, 477(2), 1994, pp. 187-196
1. This study used the whole-cell patch clamp technique to investigate
the mechanism of action of the K+ channel blocker 4-aminopyridine (4-
AP) on the cloned K+ channel mouse Kv1.1 (mKv1.1) expressed in Chinese
hamster ovary cells. 2. Cells transfected with mKv1.1 expressed a non
-inactivating, delayed rectifier-type K+ current. 4-AP induced a dose-
, voltage- and use-dependent block of mKv1.1. 3. 4-AP blockade of mKv1
.1 was similar whether 4-AP was administered extracellularly (IC50 = 1
47 mu M) or intracellularly (IC50 = 117 mu M). 4. Inclusion of the fir
st twenty amino acids of the N-terminus sequence of the Shaker B K+ ch
annel ('inactivation peptide') in the patch electrode transformed mKv1
.1 into a rapidly inactivating current. The time constant of decay for
the modified current was dependent on the concentration of inactivati
on peptide, and under these conditions extracellular 4-AP had a reduce
d potency (IC50 values of 471 and 537 mu M for 0.5 and 2 mg ml(-1) ina
ctivation peptide, respectively) 5. A permanently charged analogue of
4-AP, 4-aminopyridine methiodide (4-APMI), was found to block mKv1.1 w
hen applied inside the cell, but was without effect when administered
externally. 6. Decreasing the intracellular pH (pH(i)) to 6.4 caused a
n increase in 4-AP potency (IC50 = 76 mu M), whereas at pH(i) 9.0, the
4-AP potency fell (IC50 = 295 mu M). Conversely, increasing extracell
ular pH (pH(o)) to 9.0 caused an increase in 4-AP potency (IC50 = 93 m
u M), whereas at pH(o) 6.4, 4-AP potency decreased (IC50 = 398 mu M).
7. Taken together, these findings support the hypotheses that the unch
arged form of 4-AP crosses the membrane, and that it is predominantly
the cationic form which acts on mKv1.1 channels intracellularly, possi
bly at or near to the binding site for the inactivation peptide.