Sources of calcium in agonist-induced contraction of rat distal colon smooth muscle in vitro

Figure 1 Ca²⁺ influx through SOCCs in enzymatically dissociated smooth muscle cells of rats distal colon. After depletion of the SR by thapsigargin in the absence of extracellular Ca²⁺, subsequent rapid reintroduction of extracellular Ca²⁺ resulted in activation of CCE via SOCCs. CCE were insensitive to verapamil, but blocked by La³⁺. ^bP < 0.01 vs control.

Figure 2 CCE are not mediated via L-type Ca²⁺ channels. (A) Controls; (B) 40 mmol/L K⁺; (C) 60 mmol/L K⁺. Only 60 mmol/L K⁺ significantly decreased the Ca²⁺ influx mediated by CCE. ^aP < 0.05 vs control.

Figure 3 Role of Ca²⁺ in rat distal colon contraction. A: Representative results showing that removal of extracellular Ca²⁺ abolished contraction induced by high K⁺ and 5 &mgr;mol/L ACh. Summarized data showing that tension induced by high K⁺ and 5 &mgr;mol/L ACh before, during and after application of Ca²⁺-free Krebs solution. ^bP < 0.01, ^dP < 0.001 vs Control; B: Representative results showing that chelation of extracellular Ca²⁺ with 2.78 mmol/L EGTA abolished contraction induced by high K⁺ and 5 &mgr;mol/L ACh. Summarized data showing that tension induced by high K⁺ and 5 &mgr;mol/L ACh in the presence of EGTA ^bP < 0.01, ^fP < 0.001 vs-EG (before).

Figure 4 Effects of the VOCC blocker verapamil on distal colon contraction induced by high K⁺ and ACh. ^aP < 0.05, ^bP < 0.001 vs control.

Figure 5 Contribution of Ca²⁺ release from SR stores and Ca²⁺ influx via CCE in ACh-induced distal colon contraction. Aa: In the presence of atropine, restoration of extracellular Ca²⁺ induced contraction, most likely due to Ca²⁺ influx via CCE; Ab: VOCC blocker verapamil (5 &mgr;mol/L) negligibly affected CCE-mediated contraction in the presence of atropine; B: Summarized data showing that CCE-mediated contraction is about 0.46-fold greater than the contraction induced by SR Ca²⁺ release in ACh-induced rat distal colon contraction. ^bP < 0.001 vs Ca²⁺ release.

Figure 6 Inhibitory effects of La³⁺ on CCE-mediated distal colon contraction. ^bP < 0.01 vs control. Control: CCE-mediated contraction.