Cytosolic inositol 1,4,5-trisphosphate dynamics during intracellular calcium oscillations in living cells

We developed genetically encoded fluorescent inositol 1,4,5-trisphosphate (IP(3)) sensors that do not severely interfere with intracellular Ca(2+) dynamics and used them to monitor the spatiotemporal dynamics of both cytosolic IP(3) and Ca(2+) in single HeLa cells after stimulation of exogenously expressed metabotropic glutamate receptor 5a or endogenous histamine receptors. IP(3) started to increase at a relatively constant rate before the pacemaker Ca(2+) rise, and the subsequent abrupt Ca(2+) rise was not accompanied by any acceleration in the rate of increase in IP(3). Cytosolic [IP(3)] did not return to its basal level during the intervals between Ca(2+) spikes, and IP(3) gradually accumulated in the cytosol with a little or no fluctuations during cytosolic Ca(2+) oscillations. These results indicate that the Ca(2+)-induced regenerative IP(3) production is not a driving force of the upstroke of Ca(2+) spikes and that the apparent IP(3) sensitivity for Ca(2+) spike generation progressively decreases during Ca(2+) oscillations.