The effects of inositol 1, 4, 5-trisphosphate (InsP
3) released from caged InsP
3 by flash photolysis on free intracellular Ca
2+ concentration, [Ca
2+]
i, and outward K
+ current were simultaneously examined in a single smooth muscle cell of guinea pig small intestine using a patch pipette solution containing Indo-1 (0.1 mM), caged InsP
3 (50 μM) and KCl (130 mM). At a holding potential of −50 mV, a depolarizing pulse to + 10 mV for 200 msec caused a transient Ca
2+ current and an increase in [Ca
2+]
i. The amplitude of the Ca
2+-transient was positively correlated with the peak Ca
2+ current and negatively correlated with resting [Ca
2+]
i. InsP
3 produced increases in [Ca
2+]
i and outward K
+ current in most of the cells at −50 and −30 mV. The outward K
+ current response reached a peak sooner and decayed more quickly than the Indo-1 signal. Both responses to InsP
3 were resistant to the removal of extracellular Ca
2+. The Ca
2+-transient and outward K
+ current responses to InsP
3 at −30 mV were larger than those at −50 mV. The InsP
3-induced Ca
2+-transient was increased by increasing resting [Ca
2+]
i at −30 mV but not at −50 mV. These results suggest that InsP
3-induced Ca
2+ release from stores is potentiated by slight increases in [Ca
2+]
i via membrane depolarization.
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