AIM:To investigate whether emodin has any effects oncircular smooth muscle cells of rat colon and to examine themechanism underlying its effect.METHODS:Smooth muscle cells were isolated from thecircular muscle layer of Wistar rat colon and the cell lengthwas measured by computerized image micrometry.Intracellular Ca~(2+)([Ca~(2+)]i) signalling was studied in smoothmuscle cells using Ca~(2+)indicator Fluo-3 AM on a laser-scanning confocal microscope.RESULTS:Emodin dose-dependently induced smoothmuscle cells contraction.The contractile responses inducedby emodin were inhibited by preincubation of the cells withML-7 (an inhibitor of MLCK).Emodin caused a large,transientincrease in [Ca~(2+)]i followed by a sustained elevation in [Ca~(2+)]i.The emodin -induced increase in [Ca~(2+)]i was unaffected bynifedipine,a voltage-gated Ca~(2+)-channel antagonist,and thesustained phase of the rising of [Ca~(2+)]i was attenuated byextracellular Ca~(2+) removal with EGTA solution.Inhibiting Ca~(2+)release from ryanodine-sensitive intracellular stores byryanodine reduced the peak increase in [Ca~(2+)]i.Usingheparin,an antagonist of IP_3R,almost abolished the peakincrease in [Ca~(2+)]i.CONCLUSION: Emodin has a direct excitatory effect on circular smooth muscle cells in rat colon mediated via Ca~(2+)/ CaM dependent pathways. Furthermore, emodin-induced peak [Ca~(2+)]i increase may be attributable to the Ca~(2+)release from IP_3 sensitive stores, which further promote Ca~(2+) release from ryanodine-sensitive stores through CICR mechanism. Additionally, Ca~(2+ )influx from extracellular medium contributes to the sustained increase in [Ca~(2+)]i. AIM:To investigate whether emodin has any effects oncircular smooth muscle cells of rat colon and to examine theme chanism underlying its effect.METHODS:Smooth muscle cells were isolated from thecircular muscle layer of Wistar rat colon and the cell lengthwas measured by computerized image micrometry.Intracellular Ca~(2+)([Ca~(2+)]i) signalling was studied in smoothmuscle cells using Ca~(2+)indicator Fluo-3 AM on a laser-scanning confocal microscope.RESULTS:Emodin dose-dependently induced Smoothmuscle cells contraction.The contractile responses induced by emodin were inhibited by preincubation of the cells with ML-7 (an inhibitor of MLCK).Emodin caused a large, transient increase in [Ca 2+ ]i followed by a sustained elevation in [Ca ~(2+)]i.The emodin -induced increase in [Ca~(2+)]i was unaffected bynifedipine, a voltage-gated Ca~(2+)-channel antagonist, and the sustained period of the rising of [Ca ~(2+)]i was attenuated byextracellular Ca~(2+) removal with EGTA solution.Inhibiting Ca~(2+)relea Se from ryanodine-sensitive intracellular stores byryanodine reduced the peak increase in [Ca~(2+)]i.Usingheparin, an antagonist of IP_3R, almost abolished the peakincrease in [Ca~(2+)]i.CONCLUSION: Emodin has a The transient citatory effect on circular smooth muscle cells in rat colon mediated via Ca 2+ / CaM dependent pathways., emodin-induced peak [Ca 2+ ]i increase may be attributable to the Ca 2+ )release from IP_3 sensitive stores, which further promote Ca~(2+) release from ryanodine-sensitive stores through CICR mechanism.In addition, Ca~(2+)influx from extracellular medium contributes to the cumulative increase in [Ca~(2+) )]i.