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 pancreat ic acinar cells, low concentrations of acetylcholine or the injection of lo w concentrations of InsP(3) elicit a train of spatially localized Ca2+ spik es. In this study we have quantified these responses and compared the Ca2signals to the elementary events shown in Xenopus oocytes. The results demo nstrate, 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 w e observe is not consistent with the continuum of event sizes seen in Xenop us. We conclude that the two types of InsP(3)-dependent events in acinar ce lls are both elementary Ca2+ signals, which are independent of one another. Our data indicate a complexity to the organization of the Ca2+ release app aratus in acinar cells, which might result from the presence of multiple In sP(3) receptor isoforms, and is likely to be important in the physiology of these cells.