Subcellular Ca2+ signals underlying waves and graded responses in HeLa cells

BACKGROUND Many agonist-evoked intracellular Ca2+ signals have a complex spatio-temporal arrangement, and are observed as repetitive Ca2+ spikes and Ca2+ waves. The key to revealing how these complex signals are generated lies in understanding the functional structure of the intracellular Ca2+ pool. Previous imaging studies, using relatively large cells such as oocytes and myocytes, have identified subcellular elementary Ca2+ signals, indicating that the intracellular Ca2+ pool releases Ca2+ from functionally discrete sites. However, it is unclear whether the intracellular Ca2+ pool in smaller cells has a similar architecture, and how such subcellular signals would contribute to global spikes and waves. RESULTS We detected subcellular Ca2+ signals during the response of single Fura2-loaded HeLa cells to histamine. The spatio-temporal properties of some of these signals were similar to the elementary Ca2+ signals observed in other cells. Subcellular Ca2+ signals were particularly obvious during the 'pacemaker' Ca2+ rise that preceded the regenerative Ca2+ wave. During this pacemaker, the Ca2+ signals were observed initially in the region from which the Ca2+ wave originated, but became more widespread and frequent until a Ca2+ wave was spawned. Similar localized signals were seen during the post-wave Ca2+ increase, 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 stimulation, these units produce localized Ca2+ signals. The sequential activation and summation of these units results in Ca2+ wave propagation and, furthermore, the differential recruitment of these units may underlie the graded amplitude of the intracellular Ca2+ signals.