Dual role for guanine nucleotides in stimulus-secretion coupling

We have made mast cells and neutrophils permeable to gain access to the cytosol and thus to manipulate the composition of the cytosol. Secretion from both cell types can be triggered by elevation of cytosol Ca2+ to concentrations approaching 10-6 M; alternatively, secretion from mast cells, and of beta-glucuronidase (but not lysozyme) from neutrophils, can be triggered in the absence of Ca2+ by introducing stable analogs of GTP. We propose that GTP acts at two intracellular guanine nucleotide regulatory proteins (N proteins) in the stimulus-secretion sequence. By interaction with Np located on the inner face of the plasma membrane, it activates polyphosphoinositide phosphodiesterase to yield inositol phosphates and diacylglycerol. By interaction with Ne, situated distal to the site of action of Ca2+ and protein kinase C, it directly activates the exocytotic process without intervention of the products of polyphosphoinositide hydrolysis.