Interaction between phosphoinositide turnover system and cyclic AMP pathway for the secretion of pancreastatin and somatostatin from QGP-1N cells

Stimulation of cyclic AMP accumulation and phosphoinositide hydrolysis by M3 muscarinic receptors in the rat peripheral lung

The effects of oxotremorine-M (oxo-M), a muscarinic agonist, on cyclic AMP (cAMP) accumulation in slices of the rat peripheral lung were investigated. Oxo-M stimulated cAMP accumulation in a concentration-dependent manner with an EC50 value of 4.2 microM and a maximal effect of 2.4 +/- 0.39-fold over basal. In the presence of forskolin (25 microM), the maximal effect of oxo-M was increased to 14.1 +/- 4.0-fold over basal. Forskolin alone caused a 5.9 +/- 2.2-fold increase in cAMP relative to basal; therefore, the combination of both drugs was more than additive. The effects of oxo-M on cAMP accumulation were unaffected by tetrodotoxin, indicating that the action of oxo-M was not mediated by neuronal release of neurotransmitters. Oxo-M had a small inhibitory effect on cAMP in a homogenate preparation, indicating that the stimulatory response to oxo-M in slices of the lung is not due to direct stimulation of adenylyl cyclase. Characterization of the oxo-M potentiation of forskolin-stimulated cAMP accumulation using different muscarinic antagonists yielded calculated pKB values that agreed with binding affinities for the M3 subtype. Oxo-M elicited phosphoinositide hydrolysis in the lung, and the nature of the antagonism of this response was also consistent with that expected for an M3-mediated response. cAMP accumulation in the presence of oxo-M (100 microM), forskolin (12 microM), or both drugs combined was inhibited by indomethacin (1 microM). These results demonstrate that the M3 receptor stimulates cAMP accumulation and phosphoinositide hydrolysis in the rat peripheral lung, and the mechanism for cAMP stimulation may involve arachidonic acid metabolites.

Intracellular Ca2+ signals in human-derived pancreatic somatostatin-secreting cells (QGP-1N)

Single-cell microfluorimetry techniques have been used to examine the effects of acetylcholine (0.1-100 microM) on the intracellular free calcium ion concentration ([Ca2+]i) in a human-derived pancreatic somatostatin-secreting cell line, QGP-1N. When applied to the bath solution, acetylcholine was found to evoke a marked and rapid increase in [Ca2+]i at all concentrations tested. These responses were either sustained, or associated with the generation of complex patterns of [Ca2+]i transients. Overall, the pattern of response was concentration related. In general, 0.1-10 microM acetylcholine initiated a series of repetitive oscillations in cytoplasmic Ca2+, whilst at higher concentrations the responses consisted of a rapid rise in [Ca2+]i followed by a smaller more sustained increase. Without external Ca2+, 100 microM acetylcholine caused only a transient rise in [Ca2+]i, whereas lower concentrations of the agonist were able to initiate, but not maintain, [Ca2+]i oscillations. Acetylcholine-evoked Ca2+ signals were abolished by atropine (1-10 microM), verapamil (100 microM) and caffeine (20 mM). Nifedipine failed to have any significant effect upon agonist-evoked increases in [Ca2+]i, whilst 50 mM KCl, used to depolarise the cell membrane, only elicited a transient increase in [Ca2+]i. Ryanodine (50-500 nM) and caffeine (1-20 mM) did not increase basal Ca2+ levels, but the Ca(2+)-ATPase inhibitors 2,5-di(tert-butyl)-hydroquinone (TBQ) and thapsigargin both elevated [Ca2+]i levels. These data demonstrate for the first time cytosolic Ca2+ signals in single isolated somatostatin-secreting cells of the pancreas. We have demonstrated that acetylcholine will evoke both Ca2+ influx and Ca2+ mobilisation, and we have partially addressed the subcellular mechanism responsible for these events.

Production and secretion of chromogranin A and pancreastatin by the human pancreatic carcinoma cell line QGP-1N on stimulation with carbachol

Chromogranin A (CGA) is thought to be a precursor of pancreastatin (PST). Carbachol (Cch) stimulated the secretion of CGA and PST from QGP-1N cells derived from a human pancreatic islet cell tumor. Atropine inhibited the secretion of both. Sodium fluoride, phorbol ester, and calcium ionophore also stimulated the secretion of both. Cch (10(-5) M) stimulated inositol 1,4,5-trisphosphate production in QGP-1N cells. Stimulation with Cch increased the total amount of PST in the cells and the medium 1.7-fold and decreased the amount of CGA in the cells and medium. QGP-1N cells were labelled with [35S]methionine, and then CGA and PST in the cells and medium were immunoprecipitated with specific antisera, and separated by electrophoresis in polyacrylamide gel. Stimulation with Cch resulted in an increase in the intensity of PST-immunoreactive bands and a decrease in those of CGA-immunoreactive bands. Cch did not increase the cellular level of CGA messenger RNA. These results suggested that (1) the secretion of CGA and PST from QGP-1N cells is regulated mainly through muscarinic receptors coupled with activation of polyphosphoinositide breakdown by a G protein, with intracellular calcium ion and protein kinase C playing a role in the stimulus-secretion coupling and that (2) Cch may induce the secretion of PST and CGA and processing from CGA to PST.