Block of heart calcium channels by amlodipine: influence of drug charge on blocking activity.

Changes in pH0 were used to to vary the ratio of neutral to ionized amlodipine (acid dissociation constant = 10(-8.6). The behavior of neutral and charged drug blockade of calcium channel current (ICa) was tested in the context of the modulated receptor hypothesis. ICa was recorded at room temperature from enzymatically isolated guinea pig ventricular cells using the whole-cell arrangement of the patch-clamp technique. When amlodipine was predominantly charged (pH0 = 7.4), trains of pulses that induced multiple channel openings enhanced block, but inhibition of ICa was also promoted by depolarizing changes in holding potential. Neutral amlodipine (pH0 = 10.0), blocked ICa at depolarized membrane potentials without channel openings. This form of the drug resembled other previously described neutral dihydropyridine (DHP) blockers in its voltage dependence. Recovery from block by ionized drug molecules was very slow and incomplete, whereas block by neutral molecules was always reversible at hyperpolarized membrane potentials. We conclude that amlodipine, like other DHP calcium channel blockers, preferentially blocks calcium channels in depolarized cells. At pH0 7.4 amlodipine molecules gain access to the DHP receptor more readily when channels open, but channel openings are not required for this interaction. Recovery from block by ionized drug is almost irreversible, suggesting that channel openings are needed for this process or that the ionized drug stabilizes the calcium channel in a nonconducting state.