Abstract
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.
Footnotes
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Send reprint requests to: Dr. David Fedida, Department of Physiology, Botterell Hall, Queen’s University, Kingston, Ontario, Canada, K7L 3N6.
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↵1 This work was supported by grants from the Medical Research Foundation of Canada, and the Heart and Stroke Foundation of Ontario, to D.F.
- Abbreviations:
- W/C
- whole cell recording
- C/A
- cell-attached recording
- O/O
- outside-out recording
- I/O
- inside-out recording
- Q
- gating charge
- HEK
- human embryonic kidney
- NMG
- N-methyl-d-glucamine
- Received November 21, 1996.
- Accepted February 10, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
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