Mode switching is the major mechanism of ligand regulation of InsP 3 receptor calcium release channels

The inositol 1,4,5-trisphosphate (InsP 3) receptor (InsP 3R) plays a critical role in generation of complex Ca 2+ signals in many cell types. In patch clamp recordings of isolated nuclei from insect Sf9 cells, InsP 3R channels were consistently detected with regulation by cytoplasmic InsP 3 and free Ca 2+ concentrations ([Ca 2+] i) very similar to that observed for vertebrate InsP 3R. Long channel activity durations of the Sf9-InsP 3R have now enabled identification of a novel aspect of InsP 3R gating: modal gating. Using a novel algorithm to analyze channel modal gating kinetics, InsP 3R gating can be separated into three distinct modes: a low activity mode, a fast kinetic mode, and a burst mode with channel open probability (P o) within each mode of 0.007 ± 0.002, 0.24 ± 0.03, and 0.85 ± 0.02, respectively. Channels reside in each mode for long periods (tens of opening and closing events), and transitions between modes can be discerned with high resolution (within two channel opening and closing events). Remarkably, regulation of channel gating by [Ca 2+] i and [InsP 3] does not substantially alter channel P o within a mode. Instead, [Ca 2+] i and [InsP 3] affect overall channel P o primarily by changing the relative probability of the channel being in each mode, especially the high and low P o modes. This novel observation therefore reveals modal switching as the major mechanism of physiological regulation of InsP 3R channel activity, with implications for the kinetics of Ca 2+ release events in cells. © The Rockefeller University Press.