Figure 3.

The SP-induced increase in [Ca²⁺]_i in the absence of extracellular Ca²⁺. SP (1 nM) was added to cells in Ca²⁺-free HBS (containing 1 mM EGTA) buffer as indicated and the calcium release from its intracellular store was measured. Extracellular Ca²⁺ level was restored to 2 mM at 180 sec and the calcium influx was measured as the change in [Ca²⁺]_i. Addition of Ca²⁺ (2 mM) to the Ca²⁺-free HBS buffer in the absence of SP causes only a slight increase in basal level of [Ca²⁺]_i. The traces are from representative experiments repeated at least three times.