Allosteric regulation by cytoplasmic Ca2+ and IP3 of the gating of IP3 receptors in permeabilized guinea‐pig vascular smooth muscle cells

1 The potentiation by Ca2+ of inositol 1,4,5‐trisphosphate (IP3)‐induced Ca2+ release was studied by measuring luminal Ca2+ concentrations of the Ca2+ stores using a fluorescent Ca2+ indicator, furaptra, in permeabilized smooth muscle cells. 2 Ca2+ release at 10 μM IP3 was potentiated by an increase in the cytoplasmic Ca2+ concentration in the presence of 10 mM EGTA. This effect was not due to the pharmacological effect of EGTA, because changes in the EGTA concentration at a constant Ca2+ concentration had no effect on the Ca2+ release rate. 3 With an increase in the cytoplasmic Ca2+ concentration from 30 to 630 nM, the Ca2+ release rate at a saturating IP3 concentration increased 110‐fold and the EC50 for IP3 increased from 0.07 to 1.0 μM. It was also indicated that the relationship between Ca2+ concentration and Ca2+ release rate was shifted towards higher Ca2+ concentrations at higher IP3 concentrations. 4 These results suggest that IP3 and submicromolar concentrations of Ca2+ allosterically lower the affinity of the IP3 receptor for each other and are both required for IP3 receptor activation. These properties enable the IP3 receptors to detect simultaneous increases in IP3 and Ca2+ concentrations.