Inositol 1,4,5-trisphosphate (InsP(3)) and calcium interact to increase the dynamic range of InsP(3) receptor-dependent calcium signaling

The inositol 1,4,5-trisphosphate (InsP(3))-gated Ca channel in cerebellum is tightly regulated by Ca (Bezprorvanny, 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 mu M InsP(3), was described by a bell-shaped curve (Bezprozvanny, I., J. Watras, and B.E. Ehrlich. 1991. Nature (Lend.). 351:751-754). We report here that, when we used lower InsP(3) concentrations, the peak of the Ca-dependence curve shifted to lower Ca concentrations. Unexpectedly, when we used high InsP, concentrations, channel activity persisted at Ca concentrations as high as 30 mu M. To explore this unexpected response of the channel, we measured InsP, binding over a broad range of InsP, concentrations. We found the well-characterized high affinity InsP, binding sites (with K-d <1 and 50 nM) (Maeda, N., M. Niinobe, and K. Mikoshiba. 1990. EMBO (Eur. Mol. Biol. Organ.) J.9:61-61; 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 (Lend.). 339:468-470) and a low affinity InsP(3) binding site (K-d = 10 mu M). Using these InsP, binding sites, we developed a new model that accounts for the shift in the Ca-dependence curve at low InsP, levels and the maintained channel activity at high Ca and InsP, levels. The observed Ca dependence of the InsP,-gated Ca channel allows the cell to abbreviate the rise of intracellular Ca in the presence of low levels of InsP,, but also provides a means of maintaining high intracellular Ca during periods of prolonged stimulation.