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In secretory epithelial cells, complex patterns of Ca2+ signals regulate physiological processes. How these patterns are generated is still not fully understood. In particular, the basis of global Ca2+ waves is not clear. We have studied regional differences in InsP3-evoked Ca2+ release in single mouse pancreatic acinar cells, using high-speed (approximately 90 frames s-1), high-sensitivity Ca2+ imaging combined with rapid (10 ms) spot photolysis (2 micrometer diameter) of caged InsP3. Within a single region we measured Ca2+ response latency and rate of rise to construct an InsP3 dose-response relationship. Spot InsP3 liberation in the secretory pole region consistently elicited a dose-dependent, rapid release of Ca2+. Spot InsP3 liberation in the basal pole region of approximately 50% of cells elicited a similar dose-response relationship but with a lower apparent InsP3 affinity than in the secretory pole. In the other cells, basal pole InsP3 liberation did not elicit active Ca2+ release, even at the highest stimulus intensities we employed, although these same cells did respond when the stimulus spot was moved to different regions. We conclude that in the basal pole active sites of rapid Ca2+ release have a lower functional affinity for InsP3 than those in the secretory pole and are spread out in discrete sites across the basal pole. These properties explain the propagation of Ca2+ waves across the basal pole that are only observed at higher stimulus levels.

InsP(3)-evoked elementary Ca(2+) release events have been postulated to play a role in providing the building blocks of larger Ca(2+) signals. In pancreatic acinar cells, low concentrations of acetylcholine or the injection of low concentrations of InsP(3) elicit a train of spatially localized Ca(2+) spikes. In this study we have quantified these responses and compared the Ca(2+) signals to the elementary events shown in Xenopus oocytes. The results demonstrate, at the same concentrations of InsP(3), Ca(2+) signals consisting of one population of small transient Ca(2+) release events and a second distinct population of larger Ca(2+) spikes. The signal mass amplitudes of both types of events are within the range of amplitudes for the elementary events in Xenopus oocytes. However, the bimodal Ca(2+) distribution of Ca(2+) responses we observe is not consistent with the continuum of event sizes seen in Xenopus. We conclude that the two types of InsP(3)-dependent events in acinar cells are both elementary Ca(2+) signals, which are independent of one another. Our data indicate a complexity to the organization of the Ca(2+) release apparatus in acinar cells, which might result from the presence of multiple InsP(3) receptor isoforms, and is likely to be important in the physiology of these cells.

1. Cytosolic Ca2+ has been proposed to act as both a positive and a negative feedback signal on the inositol trisphosphate (InsP3) 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 the Ca2+-dependent chloride (Cl(Ca)) current spikes and imaged to record the cytosolic Ca2+ spikes elicited by the injection of Ins(2,4,5)P3. Increasing concentrations of Ca2+ buffer (up to 200 microM EGTA or BAPTA) were associated with the appearance of steps in the current activation phase and a prevalence of smaller-amplitude Cl(Ca) spikes. Imaging experiments showed that with increased buffer the secretory pole cytosolic Ca2+ signal became fragmented and spatially discrete Ca2+ release events were observed. 3. At higher buffer concentrations (200-500 microM), increasing concentrations of EGTA increased spike frequency and reduced spike amplitude. In contrast, BAPTA decreased spike frequency and maintained large spike amplitudes. 4. We conclude that, during InsP3-evoked spiking, long-range Ca2+ feedback ( approximately 2-4 microm) shapes the rising phase of the Ca2+ signal by acting to co-ordinate discrete Ca2+ release events and short-range ( approximately 40 nm) Ca2+ feedback acts to inhibit further Ca2+ release.