Fig. 4 . Thapsigargin inhibits [Ca²⁺]_i oscillations at non-SERCA-specific doses. (A) Application of 5 µM thapsigargin to cells generating oscillations after progesterone stimulation had no effect in most instances (). In just 2.5% of these cells there was a failure to clear Ca²⁺ from the cytoplasm between cycles, such that oscillation amplitude was reduced (). Upon washout of thapsigargin the amplitude of oscillations recovered. (B) Two single cell records from cells stimulated with 3 µM progesterone followed by application of 10 µM thapsigargin. In these two cells oscillations arrested at peak. (C) Records from two cells generating progesterone-induced oscillations which responded to 10 µM thapsigargin as in B. Subsequent application of 10 µM A23187 caused a further significant increase in fluorescence, showing that the `stable' elevation of [Ca²⁺]_i was not an artefact due to saturation of the fluorescent probe. (D) Records from three cells in an experiment in which cells were exposed to a series of increasing doses of thapsigargin. In these cells 10 µM thapsigargin failed to inhibit [Ca²⁺]_i oscillations but raising the concentration to 30 µM caused [Ca²⁺]_i to stabilise at an elevated level.