Previous studies have established that antigen-stimulated hydrolysis of inositol phospholipids through the action of phospholipase C initiates a cascade of signals which ultimately lead to secretion in RBL-2H3 cells. Hydrolysis leads not only to the generation of Ca2+-mobilizing inositol polyphosphates, but also to the generation of diacgltylycerol which causes the activation of protein kinase C. This in turn not only causes the phosphorylation of critical contractile proteins and induces an increase of cytosolic pH which facilitates secretion, but it also modulates the phosphoinositide-response probably at the level of the receptor/G protein complex. The phosphoinositide response and as a consequence secretion can also be enhanced through activation of stimulatory adenosine receptors an enhancement mimicked by cholera toxin - possibly through modulation of phospholipase C. This enhancement does not appear to be mediated by cyclic nucleotides or Ca2+.
