Characterization of nifedipine block of the human heart delayed rectifier, hKv1.5

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 Kd values of 18.6 +/- 2.7 and 6.3 +/- 0.5 microM 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 (tau2) for nifedipine block of hKv1.5 were concentration and voltage dependent. At +40 mV, tau2 was 16.7 +/- 0.8 (10 microM), and 4.8 +/- 0.6 msec (50 microM), (n = 4-8). Using a first order kinetic analysis, apparent binding constants were 5.64 x 10(6) M(-1) s(-1) (k(+1), on-rate) and 37.5 s(-1) (k(-1), off-rate), with a Kd of 6.65 microM, 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 approximately = 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.