AIM:To study the origin of calcium necessary foragonist-induced contraction of the distal colon in rats.METHODS:The change in intracellular calcium concentration ([Ca2+]i)evoked by elevating external Ca2+was detected by fura 2/AM fluorescence.Contractile activity was measured with a force displacement transducer.Tension was continuously monitored and recorded using a Powerlab 4/25T data acquisition system with an ML110 bridge bioelectric physiographic amplifier.RESULTS:Store depletion induced Ca2+ influx had an effect on [Ca2+]i.In nominally Ca2+-free medium,the sarco-endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin (1 μmol/L) increased [Ca2+]i from 68 to 241 nmol/L,and to 458 (P＜0.01) and 1006 nmol/L (P＜0.01),respectively,when 1.5 mmol/L and 3.0 mmol/L extracellular Ca2+ was reintroduced.Furthermore,the change in [Ca2+]1.was observed with verapamil (5 μmol/L),La3+(1 mmol/L) or KCI (40 mmol/L) in the bathing solution.These channels were sensitive to La3+(P＜0.01),insensitive to verapamil,and voltage independent.In isolated distal colons we found that in normal Krebs solution,contraction induced by acetylcholine (ACh) was partially inhibited by verapamil,and the inhibitory rate was 41% (P＜0.05).On the other hand,in Ca2+-free Krebs solution,ACh induced transient contraction due to Ca2+ release from the inLracellular stores.The transient contraction lasted until the Ca2+ store was depleted.Restoration of extracellular Ca2+ in the presence of atropine produced contraction,mainly due to Ca2+ influx.Such contraction was not inhibited by verapamil,but was decreased by La3+ (50 μmol/L) from 0.96 to 0.72 g (P＜0.01).CONCLUSION:The predominant source of activator Ca2+ for the contractile response to agonist is extracellular Ca2+,and intracellular Caz+ has little role to play in mediating excitation-contraction coupling by agonists in rat distal colon smooth muscle in vitro.The influx of extracellular Ca2+ is mainly mediated through voltage-,receptor- and store-operated Ca2+ channels,which can be used as an alternative to develop new drugs targeted on the dysfunction of digestive tract motility.