Direct Observation of Calcium-Independent Intercellular ATP Signaling in Astrocytes

Adenosine triphosphate (ATP) is assumed to be involved in the regulation of many extracellular signaling systems including calcium wave propagation. So far all supportive evidence is indirect, such as monitoring changes in intracellular calcium on application of extracellular ATP or off-site measurement of ATP from superfusates. Furthermore, the causal relationships among the various signaling agents are still unclear. A novel chemiluminescence dynamic imaging method was developed to monitor ATP release from living biological cells. The assay has linear response over 3 orders of magnitude for fixed concentrations of enzyme and cofactors, with a correlation coefficient of 0.999. The detectability of ATP is down to 10^-8 M at millisecond exposure times with an intensified charge-coupled device camera. The direct imaging of ATP waves in astrocyte cultures was performed together with Fluo-3-Ca imaging at millisecond temporal resolution and micrometer-scale spatial resolution. We discovered that extracellular ATP mediates intercellular calcium wave propagation, but surprisingly, release and propagation of ATP are not calcium dependent. Therefore, ATP rather than Ca or IP₃ is the primary intercellular signaling messenger.