Abstract
The effect of an indole-alkaloid mitragynine isolated from the Thai medicinal herb kratom ( Mitragyna speciosa) on neurogenic contraction of smooth muscle was studied in guinea-pig vas deferens. Mitragynine inhibited the contraction of the vas deferens produced by electrical transmural stimulation. On the other hand, mitragynine failed to affect the responses to norepinephrine and ATP. Mitragynine did not reduce KCl-induced contraction in the presence of tetrodotoxin, prazosin and α,β-methylene ATP. Mitragynine inhibited nicotine- or tyramine-induced contraction. By using the patch-clamp technique, mitragynine was found to block T- and L-type Ca 2+ channel currents in N1E-115 neuroblastoma cells. In the Ca 2+ measurement by a fluorescent dye method, mitragynine reduced KCl-induced Ca 2+ influx in neuroblastoma cells. The present results suggest that mitragynine inhibits the vas deferens contraction elicited by nerve stimulation, probably through its blockade of neuronal Ca 2+ channels.
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