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M Nenquin

Publications and source records attributed to M Nenquin.

33 records · Page 2Linked to original sources
PubMed1987

Stimulation of insulin release by benzoic acid derivatives related to the non-sulphonylurea moiety of glibenclamide: structural requirements and cellular mechanisms.

A decrease in membrane permeability to K+ is the first critical event occurring in pancreatic B-cells upon stimulation by hypoglycemic sulphonylureas. Compound HB 699 (4-[2-(5-chloro-2-methoxybenzamido)ethyl]benzoic acid), the non-sulphonylurea moiety of glibenclamide stimulates B-cells by the same mechanisms as glibenclamide itself. Selected derivatives of HB 699 were used to test, with isolated mouse islets, whether this property is due to the benzoic acid end of the molecule (not present in glibenclamide) or to another active site (also present in glimenclamide). Of the two halves of HB 699, p-ethylbenzoic acid, but not 5-Cl-2-methoxybenzamide, was weakly effective. Replacement of the carboxyl group of HB 699 by various non-acidic groups decreased but did not abolish the ionic and secretory effects on B-cells. Modifications of the other end of the molecule altered the efficacy in both directions. Removal of the substituents on the benzamide ring decreased the efficacy, whereas replacement of the 5-Cl-2-methoxybenzyl group by a 1,1-diphenylethyl group or a 9-fluorenylmethyl group led to substantially more active compounds. Their cellular mode of action was however not modified. It is concluded that compound HB 699 contains two active sites, both of which can trigger insulin release by decreasing K+ permeability of the B-cell membrane. K channels appear to possess, not a sulphonylurea receptor, but a target site for various chemical groups. The chemical environment of the latter may also determine their efficacy by modulating their access to the channel.

Animals↗
PubMed1987

Distinct mechanisms for two amplification systems of insulin release.

The mechanisms whereby activation of the cyclic AMP-dependent protein kinase A or the Ca2+-phospholipid-dependent protein kinase C amplifies insulin release were studied with mouse islets. Forskolin and the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) were used to stimulate adenylate cyclase and protein kinase C respectively. The sulphonylurea tolbutamide was used to initiate insulin release in the presence of 3 mM-glucose. Tolbutamide alone inhibited 86Rb+ efflux, depolarized beta-cell membrane, triggered electrical activity, accelerated 45Ca2+ influx and efflux and stimulated insulin release. Forskolin alone only slightly inhibited 86Rb+ efflux, but markedly increased the effects of tolbutamide on electrical activity, 45Ca2+ influx and efflux, and insulin release. In the absence of Ca2+, only the inhibition of 86Rb+ efflux persisted. TPA (100 nM) alone slightly accelerated 45Ca2+ efflux and insulin release without affecting 45Ca2+ influx or beta-cell membrane potential. It increased the effects of tolbutamide on 45Ca2+ efflux and insulin release without changing 86Rb+ efflux, 45Ca2+ influx or electrical activity. Omission of extracellular Ca2+ suppressed all effects due to the combination of TPA and tolbutamide, but not those of TPA alone. Though ineffective alone, 10 nM-TPA amplified the releasing action of tolbutamide without affecting its ionic and electrical effects. In conclusion, the two amplification systems of insulin release involve at least partially distinct mechanisms. The cyclic AMP but not the protein kinase C system initiating signal (Ca2+ influx) triggered by the primary secretagogue.

Animals↗
PubMed1987

The ionic, electrical, and secretory effects of protein kinase C activation in mouse pancreatic B-cells: studies with a phorbol ester.

The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) was used to study the effects of protein kinase C activation on stimulus-secretion coupling in mouse pancreatic B-cells. At a nonstimulatory concentration of glucose (3 mM), 100 nM TPA, but not 10 nM TPA, slightly and slowly increased insulin release and 45Ca2+ efflux and decreased 86Rb+ efflux, but did not affect the membrane potential of B-cells. At a threshold concentration of glucose (7 mM), 100 nM TPA markedly increased insulin release without triggering electrical activity in B-cells. At a stimulatory concentration of glucose (10 mM), TPA caused a dose-dependent irreversible increase in insulin release, 45Ca2+ efflux, and 86Rb+ efflux and slightly augmented islet cAMP levels. Omission of extracellular Ca2+ abolished the effects of 10 nM TPA and partially inhibited those of 100 nM TPA on insulin release and 45Ca2+ efflux. In contrast, their effect on 86Rb+ efflux was paradoxically augmented. Glucose-induced electrical activity in B-cells was only marginally affected by TPA; the duration of the slow waves with spikes was not modified, but a small shortening of the polarized intervals raised their frequency and slightly increased the overall activity. This increase was significant only with 10 nM TPA, whereas only 100 nM TPA brought about a minute increase in 45Ca2+ influx. These results thus show that TPA induces insulin release or potentiates glucose-induced insulin release without mimicking or amplifying the initial ionic and electrical signals triggered by glucose. They suggest that protein kinase C activation affects stimulus-secretion coupling by modulating intracellular and/or nonelectrogenic membrane events.

Animals↗
PubMed1986

Magnesium uptake by pancreatic islet cells is modulated by stimulators and inhibitors of the B-cell function.

28Mg2+ uptake by rat islets was measured during incubation with various stimulators or inhibitors of insulin release. D-Glucose induced a dose-dependent increase in 28Mg2+ uptake after 10 min or 120 min. The threshold concentration was around 6 mM and the maximum effect was observed with 15-20 mM glucose. After 120 min 28Mg2+ uptake was also stimulated by the metabolized sugars mannose, N-acetylglucosamine or glyceraldehyde, was unaffected by the non-metabolized or poorly metabolized L-glucose, galactose, 3-O-methylglucose, 2-deoxyglucose, fructose or mannoheptulose and was inhibited by glucosamine. The effect of glucose was markedly impaired by mannoheptulose, glucosamine, aminooxyacetate and NH4Cl, but was only partially decreased by D600 or diazoxide, which were ineffective in a glucose-free medium. Tolbutamide or KCl slightly increased 28Mg2+ uptake. Alanine, leucine alone or with glutamine, and ketoisocaproate also stimulated 28Mg2+ uptake, whereas arginine and lysine decreased it. These changes in 28Mg2+ uptake, brought about by various modifiers of the B-cell function, are thus similar but not identical to the changes in Ca2+ uptake, and are not the consequence of insulin release. The stimulatory effect of glucose requires glucose metabolism by islet cells, but is only partially due to depolarization of the B-cell membrane.

Amino Acids↗
PubMed1985

Effects of a calcium channel agonist on the electrical, ionic and secretory events in mouse pancreatic B-cells.

The changes in pancreatic B-cell function produced by a Ca channel agonist, the dihydropyridine derivative CGP 28392, have been studied with mouse islets. CGP 28392 (5 microM) modified the electrical activity induced in B-cells by 10 mM glucose: the duration and the amplitude of the slow waves of membrane potential increased, but the overall spike activity decreased. Simultaneously, CGP 28392 markedly increased insulin release and 45Ca2+ efflux, and slightly accelerated 86Rb+ efflux from islet cells. These latter effects were abolished by omission of extracellular Ca2+. Qualitatively similar changes were observed at 15 mM glucose, whereas CGP 28392 was ineffective at 3 mM glucose. These results strongly suggest that an influx of Ca2+ contributes to the slow waves of membrane potential triggered by glucose, and underline the importance of this influx of Ca2+ for the control of insulin release by the sugar.

Action Potentials↗
PubMed1985

Nutrient insulin secretagogues decrease 45Ca2+ efflux from islet cells by a mechanism other than the inhibition of the Na+-Ca2+ countertransport.

The mechanism whereby nutrient insulin secretagogues decrease 45Ca2+ efflux from islet cells is controversial. It was studied with mouse islets perifused with Ca2+-free solutions. In the presence of Na+, glucose and ketoisocaproate inhibited 45Ca2+ efflux by about 50%. Substitution of choline+ salts for Na+ salts decreased the efflux rate by 45%, but did not prevent glucose from decreasing it further. Ketoisocaproate also inhibited 45Ca2+ efflux, but less markedly than in an Na+ medium. Omission of Na+ decreased the efflux rate even when it was already lowered by glucose or ketoisocaproate. It is thus clear that nutrient insulin secretagogues decrease 45Ca2+ efflux from islet cells by a mechanism other than the inhibition of the Na+-Ca2+ countertransport, possibly by increasing sequestration of the ion in cellular organelles.

Animals↗
PubMed1985

Sparteine increases insulin release by decreasing the K+ permeability of the B-cell membrane.

The effects of sparteine on the pancreatic B-cell function have been studied with mouse islets. In the presence of a non-stimulatory concentration of glucose (3 mM), sparteine (0.2-1 mM) decreased the rate of 86Rb+ efflux from islet cells, depolarized the B-cell membrane, induced a glucose-like electrical activity and stimulated insulin release. This increase in release was observed over a large range of glucose concentrations (3-20 mM), and was most marked in the presence of 10 mM glucose. At this concentration of glucose, the effect of sparteine was already detected with 0.02 mM and was maximal with 0.5 mM. Higher concentrations of sparteine only had a transient effect on insulin release. In the presence of 10 mM glucose, 0.2 mM sparteine decreased 86Rb+ efflux and increased 45Ca2+ efflux from islet cells. The effect on 86Rb+ efflux was only transient in the presence of extracellular calcium, whereas the effect on 45Ca2+ efflux required the presence of extracellular calcium. The electrical activity induced by glucose in B-cells was augmented by sparteine which, at a concentration of 0.5 mM, produced a persistent depolarization with continuous spike activity. The potentiation of insulin release by sparteine was not reversible, but was inhibited by adrenaline and completely blocked by omission of extracellular calcium. Sparteine reversed the increase in 86Rb+ efflux and the decrease in insulin release caused by diazoxide. These results show that sparteine increases insulin release by reducing the K+-permeability of the B-cell membrane.

Animals↗
PubMed1985

The effects of cesium chloride on insulin release, ionic fluxes and membrane potential in pancreatic B-cells.

Cs+ decreases K+ permeability in nerve and muscle cells. Its effects on the pancreatic B-cell function were studied with mouse islets. In the presence of 3 mM glucose, Cs+ substitution for K+ steadily inhibited 86Rb+ efflux and hyperpolarized the B-cell membrane. Addition of Cs+ to a K+-medium also inhibited 86Rb+ efflux, but depolarized the B-cell membrane. None of these changes altered insulin release. Substitution of Cs+ for K+ in a medium containing 10 mM glucose caused a Ca2+-dependent stimulation of insulin release and 45Ca2+ efflux, produced an initial fall and a secondary rise in 86Rb+ efflux and augmented the electrical activity in B-cells. Reintroduction of K+ to the medium was followed by a marked and transient inhibition of insulin release, that was blocked by ouabain and accompanied by an inhibition of 45Ca2+ and 86Rb+ efflux and by a hyperpolarization of the B-cell membrane. Addition of Cs+ to a K+ medium containing 10 mM glucose stimulated insulin release, 45Ca2+ efflux and 86Rb+ efflux. It also increased the electrical activity in B-cells. In the absence of Ca2+, however, Cs+ addition decreased the rate of 86Rb+ efflux. The effects of Cs+ on the B-cell function may be explained by its ability to decrease K+ permeability of the plasma membrane, by its inability to activate the sodium pump, and by a third unidentified effect likely brought about by the accumulation of intracellular Cs+.

Animals↗
PubMed1985

Mechanism of the stimulation of insulin release in vitro by HB 699, a benzoic acid derivative similar to the non-sulphonylurea moiety of glibenclamide.

HB 699 is a benzoic acid derivative similar to the non-sulphonylurea moiety of glibenclamide. The mechanisms whereby it affects B-cell function have been studied in vitro with mouse islets. In the presence of 3 mmol/l glucose, HB 699 decreased 86Rb+ efflux and accelerated 45Ca2+ efflux from islet cells, depolarized the B-cell membrane and induced an electrical activity similar to that triggered by stimulatory concentrations of glucose, and increased insulin release. The changes in 45Ca2+ efflux and insulin release, but not the inhibition of 86Rb+ efflux, were abolished in the absence of Ca2+. In the presence of 10 mmol/l glucose, HB 699 increased 86Rb+ and 45Ca2+ efflux from the islets, caused a persistent depolarization of the B-cell membrane with continuous electrical activity and markedly potentiated insulin release. All these changes were suppressed by omission of extracellular Ca2+. In the presence of 15 mmol/l glucose, diazoxide increased 86Rb+ efflux, hyperpolarized the B-cell membrane, suppressed electrical activity and inhibited insulin release. HB 699 reversed these effects of diazoxide. It is suggested that HB 699 decreases K+ permeability of the B-cell membrane, thereby causing a depolarization which leads to activation of voltage-dependent Ca channels and Ca2+ influx, and eventually increases insulin release. A sulphonylurea group is thus not a prerequisite to trigger the sequence of events that is also thought to underlie the releasing effects of tolbutamide and glibenclamide.

Animals↗
PubMed1984

Distinct effects of acetylcholine and glucose on 45calcium and 86rubidium efflux from mouse pancreatic islets.

The similarities between the effects of acetylcholine and glucose on phospholipid metabolism in pancreatic islet cells prompted the comparison of their effects on ionic fluxes. Acetylcholine (1 microM) consistently increased 45Ca2+ efflux from mouse islets, whereas glucose increased it in the presence, but decreased it in the absence of extracellular Ca2+. Acetylcholine consistently accelerated 86Rb+ efflux, and this effect was augmented by Ca2+ omission. On the other hand, glucose markedly inhibited 86Rb+ efflux, except when its concentration was raised from 10 to 15 mM in the presence of Ca2+. Unlike their effects on phospholipid metabolism, the ionic effects of the two insulin-secretagogues are thus very different.

Acetylcholine↗
PubMed1984

Forskolin suppresses the slow cyclic variations of glucose-induced electrical activity in pancreatic B cells.

The membrane potential of mouse pancreatic B cells was recorded with microelectrodes. In certain cells, both the slow waves of depolarization and the intervals of repolarization triggered by glucose (10 or 15 mM) displayed regular oscillations in their duration, though the concentration of the sugar remained constant. When forskolin (0.2 microM), an activator of adenylate cyclase, was added to the medium, the electrical activity rapidly became very regular, with slow waves and intervals of constant duration. This effect was unrelated to the overall increase in activity also brought about by forskolin. The oscillations resumed in 75% of the cells after withdrawal of the drug. Under similar conditions, forskolin rapidly and reversibly raised the cAMP concentration in the islets. The data suggest that cAMP is an important modulator of the electrical activity triggered by glucose in insulin-secreting cells.

Animals↗
PubMed1983

Glucose modulates Mg2+ fluxes in pancreatic islet cells.

Magnesium, the most abundant intracellular divalent cation, is an essential cofactor for many enzyme systems, but it remains unknown as to whether variations in the cytoplasmic concentration of ionized Mg2+ directly control cellular processes. Experiments with adrenal medullary cells made 'leaky' by exposure to high electric fields provided evidence that Mg2+ could influence hormone release not only by competing with Ca2+ for entry into the cell, but also at intracellular sites controlling exocytosis. A similar conclusion was reached for insulin release in a study using isolated rat islets also subjected to high voltage discharges. There is no experimental evidence, however, that physiological stimuli influence Mg2+ movements in intact secretory cells. We report here that 28Mg2+ fluxes in pancreatic islet cells are markedly modified by glucose, the physiological stimulus of insulin release, but not by its non-insulinotropic analogue, 3-O-methylglucose.

3-O-Methylglucose↗