title: Verapamil block of T-type calcium channels.
abstract: Verapamil is a prototypical phenylalkylamine (PAA), and it was the first calcium channel blocker to be used clinically. It tonically blocks L-type channels in the inner pore with micromolar affinity, and its affinity increases at depolarized membrane potentials. In T-type calcium channels, verapamil blocks with micromolar affinity and has modestly increased affinity at depolarized potentials. We found that a related PAA, 4-desmethoxyverapamil (D888), is comparable with verapamil both in affinity and in state-dependence. Permanently charged verapamil was more effective intracellularly than neutral verapamil. Charged PAAs were able to access their binding site from both inside and outside the cell. Furthermore, membrane-impermeant [2-(trimethylammonium)ethyl]methanethiosulfonate was able to access the inner pore from outside of the cell. We examined a homology model of the T-type calcium channel to look for possible routes of drug entry. Mutation of L1825W produced a channel that was blocked significantly more slowly by charged verapamil from the outside, with an increase in apparent affinity when the drug was applied from the inside. Data suggest that T-type channels have a back pathway through which charged drugs can access the inner pore of the channel without passing through the plasma membrane.
authors.affiliation: Department of Medicine, University of Chicago, Chicago, IL 60637, USA.
authors.forename: Pamela
authors.initials: P
authors.lastname: Bergson
authors.forename: Gregory
authors.initials: G
authors.lastname: Lipkind
authors.forename: Steven P
authors.initials: SP
authors.lastname: Lee
authors.forename: Mark-Eugene
authors.initials: ME
authors.lastname: Duban
authors.forename: Dorothy A
authors.initials: DA
authors.lastname: Hanck
mesh_terms: D002121:Calcium Channel Blockers / Q000494:pharmacology*; D020747:Calcium Channels, T-Type / Q000737:chemistry / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D005110:Extracellular Space / Q000187:drug effects; D057809:HEK293 Cells; D006801:Humans; D008564:Membrane Potentials / Q000187:drug effects; D016297:Mutagenesis, Site-Directed; D017434:Protein Structure, Tertiary; D014700:Verapamil / Q000031:analogs & derivatives / Q000494:pharmacology*
chemical_list: D002121:Calcium Channel Blockers; D020747:Calcium Channels, T-Type; C043184:4-desmethoxyverapamil; D014700:Verapamil