Inhibition of cardiac L-type calcium channels by quaternary amlodipine: implications for pharmacokinetics and access to dihydropyridine binding site.
We have used whole cell patch clamp procedures to investigate the inhibition of L-type calcium channel currents in guinea pig ventricular cells by the permanently charged dihydropyridine (DHP)compound UK-118,434-05 (quaternary amlodipine, QA). The location of the charge group of this drug molecule is approximately three times closer to the active DHP moiety than is the case for SDZ-207-180, the only other previously-investigated quaternary DHP molecule. Like SDZ-207-180, QA inhibits channel activity only by external application, consistent with an externally, but not internally, accessible binding site, and once blocked, channels do not recover availability by membrane hyperpolarization independent of extracellular pH. However inhibition by QA occurs at roughly 20 x lower potency than comparable inhibition by SDZ-207-180. Low affinity binding to the DHP binding site was confirmed directly with radioligand binding. The permanently charged amlodipine derivative inhibited radioligand DHP binding in partially purified rabbit skeletal muscle transverse tubule membranes with a pseudo-Hill slope close to unity and an IC50 value of 4.2 +/- 0.6 microM. These results indicate that the characteristically slow pharmacokinetics of tertiary amlodipine are due to the unusually stable inhibition of L-channels caused by the ionized fraction of drug molecules. Furthermore, because the distance between the ionized head group and the DHP moiety is so short, the low affinity binding and channel inhibition by QA suggests that the DHP binding site is not on the extracellular domain of the L-channel alpha 1 subunit, but instead must reside within the bilayer or channel pore at a location closer to the extracellular rather than the intracellular face of the membrane.
Kwan
Y W
YW
Department of Physiology, University of Rochester School of Medicine and Dentistry, NY 14642-8642, USA.
Bangalore
R
R
Lakitsh
M
M
Glossmann
H
H
Kass
R S
RS
D017311:Amlodipine / Q000494:pharmacology* / Q000627:therapeutic use; D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000378:metabolism / Q000502:physiology*; D002478:Cells, Cultured; D004095:Dihydropyridines / Q000378:metabolism*; D006168:Guinea Pigs; D006352:Heart Ventricles; D006801:Humans; D017275:Isradipine / Q000378:metabolism; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D018482:Muscle, Skeletal / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism*; D011817:Rabbits
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D004095:Dihydropyridines; D017311:Amlodipine; D017275:Isradipine