Mode switching is the major mechanism of ligand regulation of InsP3 receptor calcium release channels.
The inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) 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(3)R 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(3)R. Long channel activity durations of the Sf9-InsP(3)R have now enabled identification of a novel aspect of InsP(3)R gating: modal gating. Using a novel algorithm to analyze channel modal gating kinetics, InsP(3)R 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(3)R channel activity, with implications for the kinetics of Ca(2+) release events in cells.
Ionescu
Lucian
L
Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA.
White
Carl
C
Cheung
King-Ho
KH
Shuai
Jianwei
J
Parker
Ian
I
Pearson
John E
JE
Foskett
J Kevin
JK
Mak
Don-On Daniel
DO
D000465:Algorithms; D000818:Animals; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D003627:Data Interpretation, Statistical; D004594:Electrophysiology; D066298:In Vitro Techniques; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000187:drug effects / Q000378:metabolism*; D015640:Ion Channel Gating / Q000502:physiology; D007700:Kinetics; D008024:Ligands; D018408:Patch-Clamp Techniques; D018411:Spodoptera
D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D008024:Ligands