Effects of diltiazem on human nicotinic acetylcholine and GABA(A) receptors.

Effects of the L-type calcium channel antagonist diltiazem on recombinant human GABA(A) receptor (alpha1beta2gamma2s) or on muscle (alpha1beta1deltagamma and alpha1beta1delta(epsilon)) or neuronal (alpha7 and alpha4beta2) nicotinic acetylcholine receptors expressed in Xenopus oocytes were examined using two-electrode voltage-clamp. Diltiazem inhibited the function of both muscle and neuronal nicotinic receptors, but it had no effect on GABA(A) receptors. The extent of functional inhibition of nicotinic receptors depended on the receptor subtype, and the order of inhibition potency by diltiazem was alpha7>alpha4beta2 approximately alpha1beta1deltagamma approximately alpha1beta1delta(epsilon). Inhibition of alpha7 receptor function was non-competitive and voltage-independent, and it occurred at concentrations far lower than those needed to inhibit (never completely) binding of (125)I-alpha-bungarotoxin to heterologously expressed alpha7 receptors in mammalian cells. Pre-incubation in diltiazem before concomitant application with acetylcholine increased inhibition of function and slowed recovery from inhibition. Verapamil, a phenylalkylamine antagonist of L-type Ca(2+) channels also fully inhibited alpha7 receptor function and partially inhibited (125)I-alpha-bungarotoxin binding to alpha7 receptors, but was less potent than diltiazem. Effects on both alpha7 receptor function and (125)I-alpha-bungarotoxin binding by verapamil plus diltiazem suggest separate sites for verapamil and diltiazem on alpha7 receptors. These results provide further evidence that L-type Ca(2+) channel drugs inhibit ligand-gated cationic channels and suggest that caution should be applied when using these compounds to study systems in which L-type Ca(2+) channels and ligand-gated cationic channels co-exist.