Lack of effect of Z-butylidenephthalide on presynaptic N-type Ca²⁺ channels in isolated Guinea-pig ileum

Z-Butylidenephthalide (Bdph) was reported to more potently inhibit electrically induced twitch responses than acetylcholine-induced tonic contraction in isolated Guinea-pig ileum (GPI). The aim of the present study was to investigate the inhibitory effects of Z-Bdph on Ca²⁺ and K⁺ channels on GPI. In Locke-Ringer's solution, both responses were isometrically recorded on a polygraph. Incubation of ω-conotoxin MVIIC, but not Z-Bdph, in the electrically stimulated GPI prior to adding ω-conotoxin GVIA, an irreversible blocker of N-type voltage-dependent Ca²⁺ channels (VDCCs), protected the binding sites and resulted in the twitch responses reversible by washing, suggesting that Z-Bdph did not bind to the N-type VDCCs. Interestingly, we found Z-Bdph concentration dependently delayed the onsets of K⁺-induced twitch responses, suggesting that Z-Bdph may be a blocker of K⁺ channels to interfere extracellular K⁺ across through the pre-junctional membrane of nerve ending in K⁺-free medium. Z-Bdph similar to nifedipine non-competitively inhibited cumulative ACh-induced phasic contractions, suggesting that Z-Bdph may bind to L-type of inositol-1,4,5-trisphosphate (IP₃)-sensitive Ca²⁺ channels on the endoplasmic reticulum (ER) membrane. In the presence of verapamil, a L-type Ca²⁺ channel blocker or Z-Bdph, the twitch inhibitions by either were effectively reversed by exogenous Ca²⁺, suggesting that they may freely pass through pre-junctional N-type, but not L-type which was blocked at least a part by either, of VDDCs open when each electrical coaxial stimulation (ECS) into intracellular space of cholinergic nerve terminal and trigger release of transmitters. In conclusion, results confirm that Z-Bdph more potently inhibits ECS-induced twitch responses than ACh-induced PCs in GPI and suggest that this effect is not mediated by interaction with presynaptic N-type VDCCs.