Kinetic Control of Multiple Forms of Ca2+ Spikes by Inositol Trisphosphate in Pancreatic Acinar Cells

Abstract

The mechanisms of agonist-induced Ca2+ spikes have been investigated using a caged inositol 1,4,5-trisphosphate (IP3) and a low-affinity Ca2+ indicator, BTC, in pancreatic acinar cells. Rapid photolysis of caged IP3 was able to reproduce acetylcholine (ACh)-induced three forms of Ca2+ spikes: local Ca2+ spikes and submicromolar (<1 μM) and micromolar (1–15 μM) global Ca2+ spikes (Ca2+ waves). These observations indicate that subcellular gradients of IP3 sensitivity underlie all forms of ACh-induced Ca2+ spikes, and that the amplitude and extent of Ca2+ spikes are determined by the concentration of IP3. IP3-induced local Ca2+ spikes exhibited similar time courses to those generated by ACh, supporting a role for Ca2+-induced Ca2+ release in local Ca2+ spikes. In contrast, IP3- induced global Ca2+ spikes were consistently faster than those evoked with ACh at all concentrations of IP3 and ACh, suggesting that production of IP3 via phospholipase C was slow and limited the spread of the Ca2+ spikes. Indeed, gradual photolysis of caged IP3 reproduced ACh-induced slow Ca2+ spikes. Thus, local and global Ca2+ spikes involve distinct mechanisms, and the kinetics of global Ca2+ spikes depends on that of IP3 production particularly in those cells such as acinar cells where heterogeneity in IP3 sensitivity plays critical role.

Nuclear and cytosolic calcium are regulated independently

“…Because the microinjection volume was Ͻ10% of the total cell volume, the maximum intracellular concentration of NPEInsP 3 was 100 M. Maximal uncaging occurred with flashes Ͼ300 msec in duration, so the concentration of free InsP 3 after an initial flash was estimated by (100 M) ϫ (flash duration)͞ (300 msec), up to a maximum of 100 M. The amount of InsP 3 liberated by subsequent flashes was determined by the same formula, but reducing the maximum releasable InsP 3 by the amount that already had been released. These estimates are similar to those described by others using similar techniques (39).…”

Ca2+ signalling and membrane current activated by cADPr in starfish oocytes

“…The subcellular pattern of the [Ca 2+ ] i rise following the activation of InsP 3 Rs and cADPr/RyRs has been investigated using high-resolution imaging techniques and depends on both the cell type and the intracellular receptor stimulated. In pancreatic acinar cells, for instance, photolysis of high doses of InsP 3 elicits a Ca 2+ "spot", which is initiated in the secretory pole (the trigger zone), and then spreads throughout the cell as a global Ca 2+ wave [22]. On the other hand, the injection of cADPr in T-lymphocytes initially induces a cADPr/RyRs-dependent rapid rise in [Ca 2+ ] i throughout the cell, followed by Ca 2+ influx that elicits a slow Ca 2+ wave starting from sub-membrane "hot spots".…”

Calcium wave pacemakers in eggs

“…In several somatic cells, the location of the Ca 2+ wave pacemakers corresponds to the area of the cell that is most sensitive to Ins(1,4,5)P3 (Ito et al, 1999;Thomas et al, 1999;Petersen et al, 1999), and Ins(1,4,5)P3, which diffuses rapidly in the cytoplasm, is thought to act as a global messenger (Albritton et al, 1992;Kasai and Petersen, 1994).…”