Figure 2

cAMP augments InsP ₃ -induced Ca²⁺ release from β-escin permeabilized cells, as shown by intraluminal Ca²⁺ measurements with Mag-Fura-2. (A) cAMP does not induce Ca²⁺ release from the ER (n = 4). (B) Application of 5 μM InsP₃ induces Ca²⁺ release from the ER and is augmented by 100 μM cAMP. (C, D) Ca²⁺ release induced by 5 μM InsP₃ is neither enhanced by application of fresh InsP₃ solution (C) nor by mock stimulation with Rp-cAMPS (D). (E) Quantification of the cAMP-dependent augmentation of Ca²⁺ release induced by 5 μM InsP₃ from experiments as shown in B. 0% Ca²⁺ release is the intraluminal Mag-fura-2 ratio at time zero of the recording; 100% Ca²⁺ release is the fluorescence ratio after complete loss of intraluminal Ca²⁺ following ionomycin application. A sigmoidal dose-response curve fitted to mean values (R² = 0.4) of the InsP₃(+cAMP)-induced Ca²⁺ release gives an EC_{50, cAMP} of 2.6 μM. (F) Dose-response relationship for InsP₃-induced Ca²⁺ release in the presence (triangles) and absence (squares) of 100 μM cAMP. The leftward shift of the dose-response relationship indicates sensitization of InsP₃-induced Ca²⁺ release for InsP₃ by cAMP. (E, F) The number of measurements for every data point is given in brackets. Means ± S.D.