The role of Ca2+ feedback in shaping InsP(3)-evoked Ca2+ signals in mouse pancreatic acinar cells

1. Cytosolic Ca2+ has been proposed to act as both a positive and a negativ e feedback signal on the inositol trisphosphate (InsP(3)) receptor. However , it is unclear how this might affect the Ca2+ response in vivo. 2. Mouse pancreatic acinar cells were whole-cell patch clamped to record th e Ca2+-dependent chloride (Cl-(Ca)) current spikes and imaged to record the cytosolic Ca2+ spikes elicited by the injection of Ins(2,4,5)P-3. Increasi ng concentrations of Ca2+ buffer (up to 200 mu M EGTA or BAPTA) were associ ated with the appearance of steps in the current activation phase and a pre valence of smaller-amplitude Cl-(Ca) spikes. Imaging experiments showed tha t with increased buffer the secretory pole cytosolic Ca2+ signal became fra gmented and spatially discrete Ca2+ release events were observed. 3. At higher buffer concentrations (200-500 mu M), increasing concentration s of EGTA increased spike frequency and reduced spike amplitude. In contras t, BAPTA decreased spike frequency and maintained large spike amplitudes. 4. We conclude that, during InsP(3)-evoked spiking, long-range Ca2+ feedbac k (similar to 2-4 mu m) shapes the rising phase of the Ca2+ signal by actin g to co-ordinate discrete Ca2+ release events and short-range (similar to 4 0 nm) Ca2+ feedback acts to inhibit further Ca2+ release.