Two calcium-binding sites mediate the interconversion of liver inositol 1,4,5-trisphosphate receptors between three conformational states.

Cytosolic Ca2+ biphasically regulates Ins(1,4,5)P3-stimulated Ca2+ mobilization in liver [Marshall and Taylor (1993) J. Biol. Chem. 268, 13214-13220]. We have investigated the mechanisms underlying this biphasic control of Ca2+ mobilization in permeabilized hepatocytes by comparing the effects of Sr2+, Ba2+ and Ca2+ on the liver Ins(1,4,5)P3 receptor. Both Ca2+ and Sr2+ increased the binding of [3H]Ins(1,4,5)P3 to liver membranes by converting receptors from a low-affinity (KD approximately 35 nM) to a high-affinity (KD approximately 5 nM) state. Ba2+ (< or = 20 microM) did not affect [3H]Ins(1,4,5)P3 binding. At concentrations similar to those that caused an enhancement of [3H]Ins(1,4,5)P3 binding, Sr2+ (EC50 = 570 nM) and Ca2+ (EC50 = 200 nM) increased the sensitivity of the intracellular Ca2+ stores to Ins(1,4,5)P3. Further modest elevations in [Ca2+] (EC50 = 1.5 microM) inhibited Ins(1,4,5)P3-stimulated Ca2+ mobilization, whereas Sr2+ caused inhibition only when its concentration was very substantially increased (EC50 approximately 900 microM). Sr2+ is therefore only 3-fold less potent than Ca2+ in causing sensitization of Ins(1,4,5)P3-stimulated Ca2+ release, but 600-fold less potent in causing inhibition. Ba2+ neither sensitized ([Ba2+] < or = 20 microM) nor inhibited ([Ba2+] < or = 1 mM) Ins(1,4,5)P3-stimulated Ca2+ release, and did not inhibit either the sensitization of Ca2+ release evoked by Sr2+ or the inhibition of Ca2+ release evoked by Ca2+. Our results suggest that two distinct Ca(2+)-binding sites, which differ in their selectivities for bivalent cations, mediate the interconversion of Ins(1,4,5)P3 receptors between at least three different conformational states. These two Ca(2+)-binding sites, which may reside either on the Ins(1,4,5)P3 receptor itself or on distinct regulatory proteins, can be distinguished by their different selectivities for bivalent cations.