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 inhi bition of L-type calcium channel currents in guinea pig ventricular ce lls by the permanently charged dihydropyridine (DHP)compound UK-118,43 4-05 (quaternary amlodipine, QA). The location of the charge group of this drug molecule is approximately three times closer to the active D HP moiety than is the case for SDZ-207-180, the only other previously- investigated quaternary DHP molecule. Like SDZ-207-180, QA inhibits ch annel activity only by external application, consistent with an extern ally, but not internally, accessible binding site, and once blocked, c hannels do not recover availability by membrane hyperpolarization inde pendent of extracellular pH. However inhibition by QA occurs at roughl y 20 x lower potency than comparable inhibition by SDZ-207-180. Low af finity binding to the DHP binding site was confirmed directly with rad ioligand binding, The permanently charged amlodipine derivative inhibi ted radioligand DHP binding in partially purified rabbit skeletal musc le transverse tubule membranes with a pseudo-Hill slope close to unity and an IC50 value of 4.2 +/- 0.6 mu M. These results indicate that th e characteristically slow pharmacokinetics of tertiary amlodipine are due to the unusually stable inhibition of L-channels caused by the ion ized fraction of drug molecules. Furthermore, because the distance bet ween the ionized head group and the DHP moiety is so short, the low af finity binding and channel inhibition by QA suggests that the DHP bind ing 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.