The "calcium antagonist" TMB-8 [3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester] is a potent, non-competitive, functional antagonist at diverse nicotinic acetylcholine receptor subtypes.

[3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester] (TMB-8) has seen wide use as an "intracellular Ca2+ antagonist." However, this study shows that TMB-8 acts as a noncompetitive, functional antagonist at diverse nicotinic acetylcholine receptor (nAChR) subtypes with potencies that exceed those for other reported effects of TMB-8, including inhibition of intracellular Ca2+ mobilization. TMB-8 is a potent inhibitor (IC50 approximately 400 nM) of agonist-stimulated ion flux mediated by functional human muscle nAChR or ganglionic alpha 3 beta 4-nAChR subtypes expressed by TE671/RD or SH-SY5Y cells. TMB-8 is also a potent inhibitor (IC50 approximately 500 nM) of a functional, central nervous system nAChR subtype that mediates nicotinic agonist-stimulated [3H]dopamine release from rat brain synaptosomes. TMB-8 is much less potent (IC50 approximately 30-200 microM) as an inhibitor of high-affinity 3H-labeled acetylcholine or 125I-labeled alpha-bungarotoxin binding to human muscle nAChR, ganglionic alpha 3 beta 4-nAChR, or ganglionic alpha 7-nAChR subtypes. Moreover, functional inhibition by TMB-8 of muscle-type nAChR is due to a reduction in agonist efficacy, but not potency, and is proportionately stronger with increasing agonist concentration, thereby suggesting that TMB-8 acts as a noncompetitive inhibitor. Similar effects are observed for local anesthetics such as tetracaine and procaine (functional IC50 values of approximately 5 and approximately 50 microM, respectively), although TMB-8 is the most potent of these agents. Studies with TMB-8 or BAPTA [1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid] analogues indicate that the amino group of TMB-8 is essential and that Ca2+ chelation is not required for inhibition of nAChR function.(ABSTRACT TRUNCATED AT 250 WORDS)