EXTRACELLULAR AND INTRACELLULAR ACTION OF QUATERNARY DEVAPAMIL ON MUSCLE L-TYPE CA2-CHANNELS()

1 The quaternary derivative of the potent verapamiL-analogue, (-)-D888 , (qD888, 4-cyano-4-(3,4-dimethoxyphenyl)-N-[2-(3-methoxy phenyl)ethyl ]-N,N,5-trimethyl-1-hexanaminium) was synthesized as a novel membrane- impermeable probe to study the localization of phenylalkylamine (PAA) effector domains on L-type Ca2+ channels. Channel block by qD888 was i nvestigated in smooth muscle-like (A7r5) cells after extra- and intrac ellular application by use of the whole-cell configuration of the patc h clamp technique. 2 Extracellularly applied qD888 inhibited Sr2+ (I-S r) (IC50=90 mu M) and Na+ (IC50=27 mu m) inward currents through L-typ e Ca2+-channels mainly in a resting-state-dependent manner. Structural ly closely related quaternary PAAs (e.g. D890) were ineffective after extracellular application. 3 QD888 also blocked I-Sr from the cytoplas mic side, as did other quaternary PAAs (D890, D575). Intracellular blo ck was clearly dependent on channel opening, which resulted in pronoun ced use-dependence. 4 We conclude that qD888 blocks L-type Ca2+ channe ls not only from the intracellular side, via interaction with the clas sical PAA binding domain, but also from the extracellular channel surf ace. The properties of Ca2+ channel block together with previous bioch emical and structural data suggest that extracellular block may be med iated by a site that also confers tonic block by quaternary benzothiaz epines.