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.

This content is only available as a PDF.
You do not currently have access to this content.