TY - JOUR T1 - Characterization of Nifedipine Block of the Human Heart Delayed Rectifier, hKv1.5 JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther SP - 1247 LP - 1256 VL - 281 IS - 3 AU - Xue Zhang AU - James W. Anderson AU - David Fedida Y1 - 1997/06/01 UR - http://jpet.aspetjournals.org/content/281/3/1247.abstract N2 - Nifedipine antagonizes L-type Ca++ channels found throughout the cardiovascular system, but also blocks Kv channels, which are members of the same supergene family. We have examined nifedipine actions on the human heart K+ channel (hKv1.5) expressed in human embryonic kidney cells. Peak and steady-state currents on depolarization were reduced by nifedipine withK d values of 18.6 ± 2.7 and 6.3 ± 0.5 μM respectively at +40 mV, and with Hill coefficients of 0.75 ± 0.04 and 0.93 ± 0.03. Block increased rapidly between -10 mV and +10 mV, coincident with channel opening and suggested an open channel block mechanism, which was confirmed by tail current crossover on repolarization (unblock on channel closing). At more positive potentials than +20 mV, block was relieved. The time constants (τ2) for nifedipine block of hKv1.5 were concentration and voltage dependent. At +40 mV, τ2 was 16.7 ± 0.8 (10 μM), and 4.8 ± 0.6 msec (50 μM), (n = 4–8). Using a first order kinetic analysis, apparent binding constants were 5.64 × 106M− 1 s− 1(k+1, on-rate) and 37.5 s− 1(k− 1, off-rate), with aK d of 6.65 μM, close to that obtained from the dose-response curve. An increase in the off-rate (k− 1) could explain relief of block >+20 mV. The rank order of block under different patch configurations was whole-cell ≈ outside-out > inside-out ≫ cell-attached macropatches. Together, these suggested a binding site for nifedipine at the extracellular pore of hKv1.5 or at a hydrophobic channel domain within the lipid bilayer at a site that is more accessible from the extracellular side. The American Society for Pharmacology and Experimental Therapeutics ER -