Regulation of type 1 inositol 1,4,5-trisphosphate-gated calcium channels by InsP(3) and calcium - Simulation of single channel kinetics based on ligand binding and electrophysiological analysis

Cytosolic calcium ac ts as bo th a coagonist and an inhibitor of the type 1 inositol 1,4,5-trisphosphate (InsP(3))-gated Ca channel, resulting in a be ll-shaped Ca dependence of channel activity (Bezprozvanny, I., J. Watras, a nd B.E. Ehrlich. 1991. Nature. 351:751-754; Finch, E.A., TJ. Turner, and S. M. Goldin. 1991. Science. 252: 443-446; Iino, M. 1990. J. Gen. Physiol. 95: 1103-1122). The ability of Ca to inhibit channel activity, however, varies dramatically depending on InsP3 concentration (Combettes, L., Z. Hannaert-M erah,J.F. Coquil, C. Rousseau, M. Claret, S. Swillens, and P. Champeil. 199 4. J. Biol. Chem. 269:17561-17571; Kaftan, E.J., B.E. Ehrlich, and J. Watra s. 1997. J. Gen. physiol. 110:529-538). In the present report, we have exte nded the characterization of the effect of cytosolic Ca on both InsP(3) bin ding and InsP(3)-gated channel kinetics, and incorporated these data into a mathematical model capable of simulating channel kinetics. We Found that c ytosolic Ca increased the K-d of InsP(3) binding similar to 3.5-fold, but d id not influence the maximal number of binding sites. The ability of Ca to decrease InsP(3) binding is consistent with the rightward shift in the bell -shaped Ca dependence of InsP(3)-gated Ca channel activity. High InsP(3) co ncentrations are able to overcome the Ca-dependent inhibition of channel ac tivity, apparently due to a low affinity InsP(3) binding site (Kaftan, E.J. , B.E. Ehrlich, and J. Watras. 1997. J. Gen. Physiol. 110:529-538). Constan ts from binding analyses and channel activity determinations were used to d evelop a mathematical model that fits the complex Ca-dependent regulation o f the type 1 InsP(3)-gated Ca channel. This model accurately simulated both steady state data (channel open probability and InsP(3) binding) and kinet ic data (channel activity and open time distributions), and yielded testabl e predictions with regard to the regulation of this intracellular Ca channe l. Information gained from these analyses, and our current molecular model of this Ca channel, will be important for understanding the basis and regul ation of intracellular Ca waves and oscillations in intact cells.