A, during exposure to Ca2+-free medium containing 0.5 mm EGTA, ICS depletion resulted from addition of 10 μm CPA to the bathing medium. Following emptying of ICS calcium content, calcium add-back (1 mm) elicited a rise in calcium. Such an increase decreased with nickel chloride (0.5 mm), resulting from inhibition of SOC activation (n = 5). Washout of nickel chloride-containing solution failed to restore the F340 nm/F380 nm ratio to its pre-inhibited level. B, summary of results from five different cells.