Fluoroaluminates stimulate histamine secretion in the digitonin-permeabilized rat mast cells

Heterotrimeric G-protein Gq/11 localized on pancreatic zymogen granules is involved in calcium-regulated amylase secretion

The heterotrimeric G-protein Gq/11 was identified on pancreatic acinar zymogen granules and its function in calcium-regulated exocytosis was examined. Western blotting showed alphaq/11, but not alphas or alphao, to be localized to the zymogen granule membrane along with G-protein beta-subunit; all three alpha subunits were present in a plasma membrane fraction and the alphaq/11 signal was 30-fold more enriched in the plasma membrane as compared with granule membrane. Neither CCK receptors nor alpha subunits of the sodium pump, both plasma membrane markers were present on granule membranes. Immunohistochemistry of pancreatic lobules showed that alphaq/11 localized to the zymogen granule-rich apical region of acinar cells together with a much stronger signal at the basolateral plasma membrane. When the substance-P-related peptide GPAnt-2a, an antagonist of Gq/11, was introduced into streptolysin-O permeabilized acini to bypass the plasma membrane, the amylase release induced by 10 microM free calcium was potentiated in a concentration-dependent manner. By contrast, another substance-P-related peptide, GPAnt-1, an antagonist of Go and Gi, showed no effect on calcium-induced amylase release from permeabilized acini. GPAnt-2a peptide also exerted an inhibitory effect on the total GTPase activity of the purified zymogen granules and a larger inhibitory effect on the GTPase activity of the Gq/11 protein immunopurified from zymogen granules. GPAnt-1, however, did not inhibit GTPase activity of either zymogen granules or immunopurified Gq/11. These results suggest that GPAnt-2a peptide augmented calcium-induced amylase release from permeabilized acini by inhibiting GTPase activity of the Gq/11 protein on zymogen granules. We conclude that Gq/11 protein on zymogen granules plays a tonic inhibitory role in calcium-regulated amylase secretion from pancreatic acini.

Kinetic characterization of guanine-nucleotide-induced exocytosis from permeabilized rat mast cells

We have measured the time course of secretion of hexosaminidase from rat mast cells permeabilized (in simple buffered NaCl solutions) in response to guanine nucleotides [GTP or guanosine 5'-[gamma-thio]triphosphate (GTP[S])] and Ca2+. In these experiments, ATP was excluded from the system (and the cells were pretreated with metabolic inhibitors). For cells permeabilized in the absence of Mg2+ but in the presence of Ca2+, secretion commences promptly in response to addition of GTP; when Mg2+ (2 mM) is provided, secretion commences after an extended delay, much higher concentrations of GTP are required, and the final extent of secretion is decreased. Ongoing secretion due to GTP and Ca2+ is abruptly terminated by addition of Mg2+ to cells initially stimulated in its absence. In contrast, although Mg2+ has no effect on the sensitivity to the non-hydrolysable analogue GTP[S], its absence does nevertheless cause delays in the onset of secretion triggered by the addition of GTP[S] to cells initially permeabilized in the presence of Ca2+ (micromolar range, again in the absence of ATP). However, exocytosis from cells triggered with Ca2+ after permeabilization in the presence of high concentrations of GTP[S] is instantaneous. The delays due to triggering by GTP[S] have GTP[S]-concentration-dependent and -independent components. The guanine-nucleotide-concentration-dependent component is expressed as an extended duration of delay as the concentration of GTP[S] is decreased, and may reflect the binding of GTP[S] to GE. The concentration-independent component is manifested as a limiting delay which cannot be further diminished by increasing the guanine nucleotide concentration. The duration of the limiting delay is sensitive to the identity of the stimulating nucleotide (GTP < GTP[S] < p[NH]ppG) and may reflect the time taken for an activating conformational change to occur after binding. Since both components of the delays are abolished by the presence of Mg2+, both the binding of guanine nucleotide and the activation of GE appear to be Mg(2+)-dependent. We therefore conclude that nucleotide binding, activation and the GTPase activity of GE are strongly dependent on Mg2+, in common with the same three processes in Gs and Gi.

Late events in regulated exocytosis

To understand the intracellular mechanisms that control exocytosis it is necessary to have access to the cell interior. This is achieved by plasma membrane permeabilisation or by application of patch-pipettes. These conditions permit control over the cytosol composition and also allow leakage of soluble factors that may have roles in the exocytotic mechanism. Different permeabilisation methods allow different extents of leakage and therefore provide complementary data. The exocytotic machinery itself remains intact and can be activated by providing Ca2+ and/or a guanine nucleotide. In some cells there is evidence for the participation of two guanine nucleotide-binding proteins (GP and GE), as well as a Ca(2+)-binding protein. In others Ca2+ is the only requirement. In a number of cell types, ATP is not required for the late steps in the secretory pathway.

Fluoride is a contractile agent of guinea pig airway smooth muscle

1. Guinea pig parenchymal lung strips and tracheal smooth muscle contract potently after NaF-addition. Maximal contractions of lung strips and tracheal rings induced by NaF were 208 +/- 17% (n = 6) and 151 +/- 8% (n = 4) of the maximal histamine response respectively. 2. The -log EC50-value for NaF on lung strips and tracheal rings was 2.38 +/- 0.01 (n = 6) and 2.28 +/- 0.01 (n = 4) respectively. 3. Contractions induced by NaF were augmented after Al3+ pretreatment, suggesting the involvement of a G-protein. NaF responses were not affected by blockade of H1-, muscarinic-, leukotriene C4- or leukotriene D4-receptors, indicating that mast cell degranulation or nerve activation is most probably not implicated. 4. Contractions after NaF-addition were relatively insensitive to removal of extracellular calcium and were reversed via cAMP- and cGMP-mediated pathways. 5. Relaxation studies with (-)isoprenaline and 8-bromo-cGMP on lung strips, precontracted to similar levels with either a H1-agonist, KCl or NaF, showed that the level of relaxation depends on the contractile agent that is used. 6. After precontraction with KCl (-)isoprenaline relaxes lung strips only to 58 +/- 9% (n = 5) of the initial contraction, whereas lung strips precontracted with NaF or a H1-agonist relax 114 +/- 8% (n = 4) and 120 +/- 7% (n = 5) respectively with (-)isoprenaline. 7. Similar results were obtained with relaxation induced with 8-bromo-cGMP. 8. These findings suggest that NaF-induced contractions are elicited via a mechanism, that is probably similar to that of the H1-receptor. The involvement of a G-protein in the observed NaF-responses is therefore likely.