Vol. 281, Issue 3, 1247-1256, 1997

Characterization of Nifedipine Block of the Human Heart Delayed Rectifier, hKv1.5¹

Xue Zhang, James W. Anderson and David Fedida

Department of Physiology, Botterell Hall, Queen's University, Kingston, Ontario, Canada

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 with K_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 (₂) for nifedipine block of hKv1.5 were concentration and voltage dependent. At +40 mV, ₂ 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 × 10⁶ M¹ s¹ (k₊₁, on-rate) and 37.5 s¹ (k₁, off-rate), with a K_d of 6.65 µM, close to that obtained from the dose-response curve. An increase in the off-rate (k₁) 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.