Calcium-dependent protein phosphorylation during secretion by exocytosis in the mast cell

Role of adenylate cyclase in immunologic release of mediators from rat mast cells: agonist and antagonist effects of purine- and ribose-modified adenosine analogs

The initial monophasic rise in cyclic AMP beginning 5-15 sec after bridging of rat mast cell IgE-Fc receptors precedes the secretion of granule constituents, thereby implying a causal relationship. Direct evidence for a relationship between IgE-dependent transmembrane activation of adenylate cyclase and granule secretion was provided by the capacity of purine-modified (R site active) and ribose-modified (P site active) adenosine analogs, respectively, to augment and suppress mediator release while simultaneously increasing and decreasing the activity of adenylate cyclase. R site stimulation alone does not cause granule secretion but augments the rate and magnitude of IgE-Fc receptor-induced secretion, reflecting the coupled relationship of such receptors. Inhibition of adenylate cyclase at the P site attenuates the rise in cellular cyclic AMP and suppresses IgE-dependent mediator release in a parallel and superimposable dose-response fashion. Further, the relationship between the attenuation in the rise in cyclic AMP and the diminution in immunologic mediator release is linear with the regression line passing through the origin, indicating a direct relationship between the IgE-dependent activation of adenylate cyclase and preformed mediator release. Although not the only events in coupled mast cell activation--secretion, there is a sequential relationship among perturbation of IgE-Fc receptors, transmembrane activation of adenylate cyclase, elevation of cytoplasmic levels of cyclic AMP, activation of cyclic AMP-dependent protein kinase, and secretion of mast cell granules.

Antiallergic drug cromolyn may inhibit histamine secretion by regulating phosphorylation of a mast cell protein

Cromolyn inhibited histamine release from mast cells that was induced by a classic secretagogue and correspondingly increased incorporation of radioactive phosphate into a 78,000-dalton protein. These effects on histamine secretion and on protein phosphorylation were rapid in onset and both showed tachyphylaxis. Cromolyn may therefore act by altering the phosphorylation of a protein involved in the regulation of secretion.

Glucoreceptor mechanisms and the control of insulin release and biosynthesis

The models proposed for the means whereby the B-cell recognises glucose and related compounds as signals for insulin release and biosynthesis are discussed. The observed correlations between rates of metabolism and insulin release and biosynthesis are consistent with the substrate-site hypothesis. For glucose itself, the enzymes catalysing the phosphorylation of the sugar provide an explanation for the major characteristics of the islet responses, but for N-acetylglucosamine evidence is presented that the sugar transport system fulfils this discriminatory role. Possible mechanisms whereby sugar metabolism may be linked to changes in Ca2+-handling are considered and evidence is given supporting a role for the cytosolic NADPH/NADP+ ratio and the islet content of phosphoenolpyruvate. The nature of the targets for cyclic AMP and Ca2+ is discussed and some properties of islet cAMP-dependent protein kinase are summarised. Evidence is presented for the presence of calmodulin in islets and the possible involvement of calmodulin in stimulus-secretion coupling. On the basis of these considerations a speculative hypothesis for the mechanisms involved in the B-cell responses to glucose is outlined.

A study of protein phosphorylation in shape change and Ca++-dependent serotonin release by blood platelets

Upon treatment with agents such as thrombin, collagen or concanavalin A, blood platelets change shape, secrete serotonin and phosphorylate two proteins having molecular weights of approximately 20,000 and 40,000. We have analyzed the relationship of this protein phosphorylation to shape change and release aided by the fact that while shape change occurs independently of extracellular calcium, release of serotonin displays a rather strict calcium requirement. Under limited calcium conditions, where virtually no serotonin release occurs, (Con A)-stimulated phosphorylation is uninhibited. Divalent cations (Mg++, Co++ and Zn++) also inhibit release but not phosphorylation. The microtubule effectors colchicine and D2O show concomitant effects on release and phosphorylation, indicating a microtubule involvement prior to phosphorylation. Papaverine inhibits release and phosphorylation while not strongly influencing shape change, suggesting that shape change does not require phosphorylation. We therefore conclude that phosphorylation of these proteins takes place after shape change but prior to release, and although it may be required for secretion to occur, the two processes are easily separated. Thus phosphorylation of these proteins is not likely to be an integral component of the release mechanism.