SUBCELLULAR CA2-CELLS( SIGNALS UNDERLYING WAVES AND GRADED RESPONSES IN HELA)

Background: Many agonist-evoked intracellular Ca2+ signals have a comp lex spatio-temporal arrangement, and are observed as repetitive Ca2+ s pikes and Ca2+ waves. The key to revealing how these complex signals a re generated lies in understanding the functional structure of the int racellular Ca2+ pool, Previous imaging studies, using relatively large cells such as oocytes and myocytes, have identified subcellular eleme ntary Ca2+ signals, indicating that the intracellular Ca2+ pool releas es Ca2+ from functionally discrete sites. However, it is unclear wheth er the intracellular Ca2+ pool in smaller cells has a similar architec ture, and how such subcellular signals would contribute to global spik es and waves. Results: We detected subcellular Ca2+ signals during the response of single Fura2-loaded HeLa cells to histamine, The spatio-t emporal properties of some of these signals were similar to the elemen tary Ca2+ signals observed in other cells, Subcellular Ca2+ signals we re particularly obvious during the 'pacemaker' Ca2+ rise that preceded the regenerative Ca2+ wave. During this pacemaker, the Ca2+ signals w ere observed initially in the region from which the Ca2+ wave originat ed, but became more widespread and frequent until a Ca2+ wave was spaw ned, Similar localized signals were seen during the post-wave Ca2+ inc rease, and during the low-amplitude Ca2+ responses evoked by threshold histamine concentrations. Conclusions: The intracellular Ca2+ pool in HeLa cells is composed of many functionally discrete units. Upon stim ulation, these units produce localized Ca2+ signals. The sequential ac tivation and summation of these units results in Ca2+ wave propagation and, furthermore, the differential recruitment of these units may und erlie the graded amplitude of the intracellular Ca2+ signals.