Mechanisms underlying spontaneous calcium spiking in aequorin-loaded ROS 17/2.8 cells

In earlier studies, McLeod and coworkers reported the detection of spontaneous calcium spiking in ROS 17/2.8 cells which they suggested was derived from individual cells progressing through mitosis or the cell cycle. They also indicated that the degree of spiking could be modulated by exposure of the cells to time-varying extremely low frequency electric fields. Given the implications of such observations for our understanding of the effects of electromagnetic fields on biological systems, it appeared important for mechanistic reasons to understand the basis of this spiking. In this study, we were able to confirm that spontaneous calcium spiking activity could be detected in ROS 17/2.8 cells and that this appeared to emanate from individual cells. We found this spiking to be completely dependent on extracellular calcium ions and to be independent of the inositol 1,4,5-trisphosphate-sensitive intracellular calcium store. This spiking is not reduced by treatments which slow down or block the passage of cells through the cell cycle. Further, we found that spiking was only detectable in the most highly aequorin-loaded subpopulation of cells whose growth rate is reduced and whose morphological appearance is abnormal. In conjunction with what is known about calcium spiking in other, nonexcitable mammalian cells in culture, the data presented strongly argue that the spontaneous calcium spiking observed in ROS 17/2.8 cells is unrelated to normal events of the cell cycle and most likely result from the damaging effects of excessive loading with aequorin.