Figure 4.

4-CEP releases Ca²⁺ from ER in beta cells. [Ca²⁺]_i was measured in the presence of 1.28 mM Ca_o²⁺. (A) 4-CEP (500 μM) induced a gradual increase in [Ca²⁺]_i. Carbachol (CCh) added in the continued presence of 4-CEP released further Ca²⁺. Similar results were obtained in all of 16 experiments. (B) Intracellular Ca²⁺ pools were depleted by treating the cells with 250 nM thapsigargin for 35 min. Under these conditions, neither 4-CEP nor carbachol increased [Ca²⁺]_i, but depolarization with KCl did. The trace is representative of three experiments. (C) Caffeine (5 mM) was present in the perifusion medium. 4-CEP (500 μM), added in the presence of caffeine, induced Ca²⁺ spikes in three of five cells. In the absence of caffeine, 4-CEP did not induce a Ca²⁺ spike in any of the 16 cells examined (c.f. A). (D) When added in the presence of forskolin, 4-CEP induced Ca²⁺ spikes in 7 of 13 cells. 4-CEP alone did not induce a Ca²⁺ spike in any of the 16 cells tested.