Stereoselectivity of butylidenephthalide on non-adrenergic prejunctional voltage-dependent Ca²⁺ channels in prostatic portion of rat vas deferens

The naturally occurring and synthetic butylinenephthalide (Bdph) has two geometric isomers. Z- and E-Bdph were reported to have geometric stereoselectivity for voltage-dependent calcium channels (VDCCs) in Guinea-pig ileum. The aim of this study was to investigate whether the binding of Z- and E-Bdph on prejunctional VDCCs of rat vas deferens (RVD) is stereoselective. The twitch responses to electrical field stimulation (EFS, supramaximal voltage, 1 ms, 0.2 Hz) were recorded on a polygraph. Z- and E-Bdph concentration-dependently inhibited the twitch responses to EFS in full tissue, prostatic portion and epididymal portion of RVD. The pIC₅₀ value of Z-Bdph was greater than that of E-Bdph in the electrically stimulated prostatic portion of RVD, suggesting that the binding of Bdph on the non-adrenergic prejunctional VDCCs of cell membrane is stereoselective. In the prostatic portion, exogenous Ca²⁺ only partially reversed the twitch inhibition by Z-Bdph, but effectively reversed those by Ca²⁺ channel blockers, such as verapamil, diltiazem and aspaminol, suggesting that the action mechanisms may be different from those of Ca²⁺ channel blockers. K⁺ channel blockers, such as tetraethylammonium (TEA) and 4-aminopyridine (4-AP), may prolong duration of action potential to allow greater Ca²⁺ entry and induced more release of transmitters. Therefore both blockers via their prejunctional actions reversed the twitch inhibition induced by Z-Bdph in all preparations of RVD by a non-specific antagonism.