Inositol 1,4,5-Trisphosphate (InsP3) and Calcium Interact to Increase the Dynamic Range of InsP3 Receptor-dependent Calcium Signaling

The inositol 1,4,5-trisphosphate (InsP₃)-gated Ca channel in cerebellum is tightly regulated by Ca (Bezprozvanny, I., J. Watras, and B.E. Ehrlich. 1991. Nature (Lond.). 351:751–754; Finch, E.A., T.J. Turner, and S.M. Goldin. 1991. Science (Wash. DC). 252:443–446; Hannaert-Merah, Z., J.F. Coquil, L. Combettes, M. Claret, J.P. Mauger, and P. Champeil. 1994. J. Biol. Chem. 269:29642–29649; Iino, M. 1990. J. Gen. Physiol. 95:1103–1122; Marshall, I., and C. Taylor. 1994. Biochem. J. 301:591–598). In previous single channel studies, the Ca dependence of channel activity, monitored at 2 μM InsP₃, was described by a bell-shaped curve (Bezprozvanny, I., J. Watras, and B.E. Ehrlich. 1991. Nature (Lond.). 351:751–754). We report here that, when we used lower InsP₃ concentrations, the peak of the Ca-dependence curve shifted to lower Ca concentrations. Unexpectedly, when we used high InsP₃ concentrations, channel activity persisted at Ca concentrations as high as 30 μM. To explore this unexpected response of the channel, we measured InsP₃ binding over a broad range of InsP₃ concentrations. We found the well-characterized high affinity InsP₃ binding sites (with K _d < 1 and 50 nM) (Maeda, N., M. Niinobe, and K. Mikoshiba. 1990. EMBO (Eur. Mol. Biol. Organ.) J. 9:61–67; Mignery, G., T.C. Sudhof, K. Takei, and P. De Camilli. 1989. Nature (Lond.). 342:192–195; Ross, C.A., J. Meldolesi, T.A. Milner, T. Satoh, S. Supattapone, and S.H. Snyder. 1989. Nature (Lond.). 339:468–470) and a low affinity InsP₃ binding site (K _d = 10 μM). Using these InsP₃ binding sites, we developed a new model that accounts for the shift in the Ca-dependence curve at low InsP₃ levels and the maintained channel activity at high Ca and InsP₃ levels. The observed Ca dependence of the InsP₃-gated Ca channel allows the cell to abbreviate the rise of intracellular Ca in the presence of low levels of InsP₃, but also provides a means of maintaining high intracellular Ca during periods of prolonged stimulation.