Fig. 8. Inhibition of Ca²⁺ wave initiation following prolonged agonist stimulation of HeLa cells. Typical responses of single HeLa cells to 20 µM InsP₃BM either before (A) or after (B) a 4-hour incubation with 1 mM histamine. (A) An example in which the pacemaking Ca²⁺ puff activity was able to drive the cell towards the threshold for triggering a regenerative global Ca²⁺ wave. (B) The failure of the Ca²⁺ puffs to trigger a regenerative response in a cell pre-incubated with 1 mM histamine. The cell shown in (B) was chosen because it displayed three active pacemaking Ca²⁺ puff sites and was one of the most active cells following the histamine preincubation. However, despite the considerable activity of the pacemaking Ca²⁺ puff sites, they caused only modest changes in the global Ca²⁺ concentration. The cells shown in this figure were from the same passage and are typical of the responses from three independent experiments. Coloured circles on the inset cell images depict the positions of the pacemaking Ca²⁺ puffs.