cAMP potentiates InsP(3)-induced Ca(2+ )release from the endoplasmic reticulum in blowfly salivary glands

BACKGROUND: Serotonin induces fluid secretion from Calliphora salivary glands by the parallel activation of the InsP(3)/Ca(2+ )and cAMP signaling pathways. We investigated whether cAMP affects 5-HT-induced Ca(2+ )signaling and InsP(3)-induced Ca(2+ )release from the endoplasmic reticulum (ER). RESULTS: Increasing intracellular cAMP level by bath application of forskolin, IBMX or cAMP in the continuous presence of threshold 5-HT concentrations converted oscillatory [Ca(2+)](i )changes into a sustained increase. Intraluminal Ca(2+ )measurements in the ER of β-escin-permeabilized glands with mag-fura-2 revealed that cAMP augmented InsP(3)-induced Ca(2+ )release in a concentration-dependent manner. This indicated that cAMP sensitized the InsP(3 )receptor Ca(2+ )channel for InsP(3). By using cAMP analogs that activated either protein kinase A (PKA) or Epac and the application of PKA-inhibitors, we found that cAMP-induced augmentation of InsP(3)-induced Ca(2+ )release was mediated by PKA not by Epac. Recordings of the transepithelial potential of the glands suggested that cAMP sensitized the InsP(3)/Ca(2+ )signaling pathway for 5-HT, because IBMX potentiated Ca(2+)-dependent Cl(- )transport activated by a threshold 5-HT concentration. CONCLUSION: This report shows, for the first time for an insect system, that cAMP can potentiate InsP(3)-induced Ca(2+ )release from the ER in a PKA-dependent manner, and that this crosstalk between cAMP and InsP(3)/Ca(2+ )signaling pathways enhances transepithelial electrolyte transport.