A bimodal pattern of InsP3-evoked elementary Ca2+ signals in pancreatic acinar

InsP(3)-evoked elementary Ca2+ release events have been postulated to play a role in providing the building blocks of larger Ca2+ signals. In pancreatic acinar cells, low concentrations of acetylcholine or the injection of low concentrations of InsP(3) elicit a train of spatially localized Ca2+ spikes. In this study we have quantified these responses and compared the Ca2+ signals to the elementary events shown in Xenopus oocytes. The results demonstrate, at the same concentrations of InsP(3), Ca2+ signals consisting of one population of small transient Ca2+ release events and a second distinct population of larger Ca2+ 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 Ca2+ distribution of Ca2+ 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 Ca2+ signals, which are independent of one another. Our data indicate a complexity to the organization of the Ca2+ 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.