A Kinetic Model of Single and Clustered IP 3 Receptors in the Absence of Ca 2+ Feedback

Abstract

Ca 2+ liberation through inositol 1,4,5-trisphosphate receptor (IP 3R) channels generates complex patterns of spatiotemporal cellular Ca 2+ signals owing to the biphasic modulation of channel gating by Ca 2+ itself. These processes have been extensively studied in Xenopus oocytes, where imaging studies have revealed local Ca 2+ signals (“puffs”) arising from clusters of IP 3R, and patch-clamp studies on isolated oocyte nuclei have yielded extensive data on IP 3R gating kinetics. To bridge these two levels of experimental data, we developed an IP 3R model and applied stochastic simulation and transition matrix theory to predict the behavior of individual and clustered IP 3R channels. The channel model consists of four identical, independent subunits, each of which has an IP 3-binding site together with one activating and one inactivating Ca 2+-binding site. The channel opens when at least three subunits undergo a conformational change to an “active” state after binding IP 3 and Ca 2+. The model successfully reproduces patch-clamp data; including the dependence of open probability, mean open duration, and mean closed duration on [IP 3] and [Ca 2+]. Notably, the biexponential distribution of open-time duration and the dependence of mean open time on [Ca 2+] are explained by populations of openings involving either three or four active subunits. As a first step toward applying the single IP 3R model to describe cellular responses, we then simulated measurements of puff latency after step increases of [IP 3]. Assuming that stochastic opening of a single IP 3R at basal cytosolic [Ca 2+] and any given [IP 3] has a high probability of rapidly triggering neighboring channels by calcium-induced calcium release to evoke a puff, optimal correspondence with experimental data of puff latencies after photorelease of IP 3 was obtained when the cluster contained a total of 40–70 IP 3Rs.