Inositol 1,4,5-Trisphosphate-Induced Ca2+-Transient and Outward K+ Current in Single Smooth Muscle Cells of Guinea Pig Small Intestine

The effects of inositol 1,4,5-trisphosphate (InsP3) released from caged InsP3 by flash photolysis on free intracellular Ca2+ concentration, [Ca2+]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 InsP3 (50 microM) and KCl (130 mM). At a holding potential of -50 mV, a depolarizing pulse to +10 mV for 200 msec caused a transient Ca2+ current and an increase in [Ca2+]i. The amplitude of the Ca(2+)-transient was positively correlated with the peak Ca2+ current and negatively correlated with resting [Ca2+]i. InsP3 produced increases in [Ca2+]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 InsP3 were resistant to the removal of extracellular Ca2+. The Ca(2+)-transient and outward K+ current responses to InsP3 at -30 mV were larger than those at -50 mV. The InsP3-induced Ca(2+)-transient was increased by increasing resting [Ca2+]i at -30 mV but not at -50 mV. These results suggest that InsP3-induced Ca2+ release from stores is potentiated by slight increases in [Ca2+]i via membrane depolarization.