Contractile effects and intracellular Ca2+signalling induced by emodin in circular smooth muscle cells of rat colon

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 Ca2+([Ca2+]i) signalling was studied in smoothmuscle cells using Ca2+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 [Ca2+]i followed by a sustained elevation in [Ca2+]i.The emodin -induced increase in [Ca2+]i was unaffected bynifedipine,a voltage-gated Ca2+-channel antagonist,and thesustained phase of the rising of [Ca2+]i was attenuated byextracellular Ca2+removal with EGTA solution.Inhibiting Ca2+release from ryanodine-sensitive intracellular stores byryanodine reduced the peak increase in [Ca2+]i.Usingheparin,an antagonist of IP3R,almost abolished the peakincrease in [Ca2+]i.CONCLUSION: Emodin has a direct excitatory effect on circular smooth muscle cells in rat colon mediated via Ca2+/ CaM dependent pathways. Furthermore, emodin-induced peak [Ca2+]i increase may be attributable to the Ca2+release from IP3sensitive stores, which further promote Ca2+release from ryanodine-sensitive stores through CICR mechanism. Additionally, Ca2+influx from extracellular medium contributes to the sustained increase in [Ca2+]i.