Abstract

The benzothiazepine diltiazem blocks ionic current through L-type Ca²⁺ channels, as do the dihydropyridines (DHPs) and phenylalkylamines (PAs), but it has unique properties that distinguish it from these other drug classes. Wild-type L-type channels containing α_1CII subunits, wild-type P/Q-type channels containing α_1A subunits, and mutants of both channel types were transiently expressed in tsA-201 cells with β_1B and α₂δ subunits. Whole-cell, voltage-clamp recordings showed that diltiazem blocks L-type Ca²⁺ channels approximately 5-fold more potently than it does P/Q-type channels. Diltiazem blocked a mutant P/Q-type channel containing nine amino acid changes that made it highly sensitive to DHPs, with the same potency as L-type channels. Thus, amino acids specific to the L-type channel that confer DHP sensitivity in an α_1A background also increase sensitivity to diltiazem. Analysis of single amino acid mutations in domains IIIS6 and IVS6 of α_1CII subunits confirmed the role of these L-type-specific amino acid residues in diltiazem block, and also indicated that Y1152 of α_1CII, an amino acid critical to both DHP and PA block, does not play a role in diltiazem block. Furthermore, T1039 and Y1043 in domain IIIS5, which are both critical for DHP block, are not involved in block by diltiazem. Conversely, three amino acid residues (I1150, M1160, and I1460) contribute to diltiazem block but have not been shown to affect DHP or PA block. Thus, binding of diltiazem to L-type Ca²⁺channels requires residues that overlap those that are critical for DHP and PA block as well as residues unique to diltiazem.