Waiting time distributions for clusters of receptors

Inositol 1,4,5-trisphosphate ( IP 3 R ) receptor channels control release of Ca 2 + from the endoplasmic reticulum into the cytosol of a cell. The binding of both 1,4,5-trisphosphate ( IP 3 ) and activating Ca 2 + is required for the channel to open. At high Ca 2 + concentrations, IP 3 R s are inhibited. IP 3 R s are composed of four identical subunits and form in clusters. Many models have been proposed to describe how the binding of IP 3 and Ca 2 + to subunits results in the opening and closing of IP 3 R s. Here we compare the opening and closing probability distributions for clusters of IP 3 R s, resulting from three different models. The distributions are calculated both analytically, using a method we have developed, and with simulations. We found significant differences in the behavior of the three models as the Ca 2 + and IP 3 concentrations are varied.