Figure 2.

Effect of extracellular calcium, G protein, and calcium intracellular stores on the depolarization-evoked calcium signals in primary muscle cultures. (A) Relative fluorescence variation in an ROI after depolarization of a cell incubated in an external medium with no added calcium and 0.5 mM EGTA. Both fast and slow calcium signals are similar to control signals in normal calcium (see text for statistical analysis). (B) Time course of relative fluorescence after potassium depolarization in a cell previously incubated with 1 μg/ml pertussis toxin. Under this condition, an absence of a slow calcium transient was observed. Note that in most cases (e.g., Fig. 1 C, and Fig. 2 A) Ca²⁺ levels do not return completely to basal levels after the end of the slow transient peak. (C) Series of relative fluorescence analysis in independent cells after the addition of ryanodine (20 μM); after potassium depolarization in cells pretreated for 15–30 min with ryanodine (20 μM); or nifedipine (10 μM) plus ryanodine (20 μM) and after thapsigargin addition in ryanodine- plus nifedipine-treated cells.