BIS(BENZYLISOQUINOLINE) ANALOGS OF TETRANDRINE BLOCK L-TYPE CALCIUM CHANNELS - EVIDENCE FOR INTERACTION AT THE DILTIAZEM-BINDING SITE

Bis(benzylisoquinoline) alkaloids block Cal+ uptake through the L-type Ca2+ channel and modulate binding of ligands to four distinct sites (dihydropyridine, benzothiazepine, aralkylamine, and (diphenylbutyl)piperidine) in the Ca2+ entry blocker receptor complex of the channel. These alkaloids are structural analogs of tetrandrine, which has previously been demonstrated to block the L-type Ca2+ channel through interaction at the benzothiazepine (diltiazem) site (King et al., 1988). Different alkaloid conformational classes display either alpha-beta, beta-alpha, alpha-alpha, or beta-beta stereochemistry at the two chiral isoquinoline carbons. Compounds from all four classes were tested for their ability to interact with Ca2+ entry blocker ligands. All analogs completely inhibit diltiazem binding, but many only partially inhibit D-600 and fluspirilene binding. For dihydropyridine binding, the compounds show either stimulation or inhibition or exhibit no effect. This profile is quite different from the interaction displayed by diltiazem or tetrandrine. Scatchard analyses show effects predominantly on K(d) for diltiazem, D-600, and PN200-110 binding. Representative conformers do not effect diltiazem dissociation rates but alter dissociation kinetics of ligands which bind to the other three sites. A correlation of the ability of these compounds to inhibit Ca2+ uptake through the L-type Ca2+ channel in GH3 cells exists only with their inhibition of diltiazem binding but not with inhibition of binding of ligands representing other classes of Ca2+ entry blockers. These data, taken together, indicate that a variety of bis(benzylisoquinoline) congeners act to block the L-type Ca2+ channel by binding to the benzothiazepine site on the channel. The novel patterns exhibited by these agents in affecting allosteric coupling between the benzothiazepine site and the other receptors of the Ca2+ entry blocker receptor complex define a new class of Ca2+ entry blocker with binding properties distinct from diltiazem.