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

Kenzo Hirose, Shiro Kadowaki and Masamitsu Iino

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.

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