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M Nenquin

Publications and source records attributed to M Nenquin.

At least 19 recordsLinked to original sources
PubMed1991

Vanadate stimulation of insulin release in normal mouse islets.

The effects of vanadate (Na3VO4) on pancreatic B-cell function were studied in normal mouse islets. Vanadate did not affect basal insulin release but potentiated the effect of 7-30 mM glucose at concentrations of 0.1-1 mM. This effect was progressive and slowly reversible. It was abolished by omission of extracellular Ca2+ but unaffected by blockers of adrenergic or muscarinic receptors. Comparison of the changes in membrane potential, 86Rb efflux and 45Ca efflux that vanadate and ouabain produced in B-cells made it possible to exclude the hypothesis that vanadate increases insulin release by blocking the sodium pump. Vanadate was also without effect on cAMP levels. On the other hand, it markedly changed the characteristics of the Ca(2+)-dependent electrical activity and of the oscillations of cytoplasmic Ca2+ recorded in B-cells stimulated by 15 mM glucose. In the steady state, Ca2+ influx was increased by vanadate, and this resulted in a rise in cytoplasmic Ca2+. The exact mechanisms underlying these changes could not be established but a blockade of K channels was excluded. In the presence of LiCl, vanadate markedly increased inositol phosphate levels in islet cells. This effect was attenuated but not suppressed by omission of Ca2+. A small increase in inositol bisphosphate was still produced by vanadate in the absence of LiCl. These results suggest that vanadate both stimulates phosphoinositide breakdown and inhibits inositol phosphate degradation. In conclusion, vanadate does not induce insulin release, but markedly potentiates the stimulation by glucose. This property is not due to an inhibition of the sodium pump or to a rise in cAMP concentration. It results from a complex interplay between changes in B-cell membrane potential, phosphoinositide metabolism and Ca2+ handling.

Animals↗
PubMed1990

Mechanisms of the stimulation of insulin release by arginine-vasopressin in normal mouse islets.

The mechanisms by which arginine-vasopressin (AVP) affects pancreatic B-cell function were studied in normal mouse islets. AVP produced a dose-dependent (0.1-1000 nM; EC50 approximately 1-2 nM) amplification of glucose-induced insulin release. This amplification was of slow onset and reversibility. AVP was ineffective when the concentration of glucose was less than 7 mM, but was still very effective in 30 mM glucose. The increase in insulin release produced by AVP was accompanied by small accelerations of 86Rb and 45Ca efflux from islet cells. Omission of extracellular Ca2+ accentuated the effect of AVP on 86Rb efflux, attenuated that on 45Ca efflux, and abolished that on release. Under no condition did AVP inhibit 86Rb efflux. AVP did not significantly affect cAMP levels, but increased inositol phosphate levels in islet cells, even in the absence of extracellular Ca2+. AVP did not affect the membrane potential in unstimulated B-cells and augmented glucose-induced electrical activity only slightly. This was not due to a direct action on ATP-sensitive K+ channels as revealed by patch-clamp recordings (whole cell and outside-out patches). In conclusion, AVP is not an initiator of insulin release, but it potently amplifies glucose-induced insulin release in normal mouse B-cells. This effect involves a stimulation of phosphoinositide metabolism, and presumably an activation of protein kinase C, rather than a change in cAMP levels or a direct control of the membrane potential.

Acetylcholine↗
PubMed1990

Galanin and epinephrine act on distinct receptors to inhibit insulin release by the same mechanisms including an increase in K+ permeability of the B-cell membrane.

The mechanisms by which galanin and epinephrine affect pancreatic B-cell function were studied in normal mouse islets. In the presence of 15 mM glucose and 2.5 mM Ca2+, galanin (50 nM) and epinephrine (100 nM) hyperpolarized the B-cell membrane and suppressed electrical activity only transiently. These changes were accompanied by a decrease in 86Rb+ efflux from islet cells and nearly complete inhibition of insulin release. Both agents also decreased 86Rb+ efflux in the absence of Ca2+. Low concentrations (10-15 microM) of diazoxide, an activator of ATP-sensitive K+ channels, mimicked some effects of galanin and epinephrine. However, insulin release was more markedly inhibited by galanin or epinephrine than by diazoxide when electrical activity was similarly decreased, and diazoxide had no effect on 86Rb+ efflux in the absence of Ca2+. When the permeability to K+ was increased by 100 microM diazoxide and the hyperpolarization reversed by high extracellular K+, galanin and epinephrine still inhibited insulin release, but did not affect the membrane potential or 86Rb+ efflux. Galanin and epinephrine decreased glucose utilization and oxidation in islet cells by about 10%, whereas diazoxide had no effect. Blockade of alpha 2-adrenoceptors by yohimbine suppressed the effects of epinephrine, but not those of galanin. It is concluded that activation of galanin and alpha2-adrenergic receptors inhibits insulin release by the same mechanisms. These may involve an increase in K+ permeability of the B-cell membrane by opening ATP-sensitive K+ channels and an additional effect independent of the membrane potential.

Animals↗
PubMed1989

Comparison of the inhibition of insulin release by activation of adenosine and alpha 2-adrenergic receptors in rat beta-cells.

Rat islets were used to compare the mechanisms whereby adenosine and adrenaline inhibit insulin release. Adenosine (1 microM-2.5 mM) and its analogue N6(-)-phenylisopropyladenosine (L-PIA) (1 nM-10 microM) caused a concentration-dependent but incomplete (45-60%) inhibition of glucose-stimulated release. L-PIA was more potent than D-PIA [the N6(+) analogue], but much less than adrenaline, which caused nearly complete inhibition (85% at 0.1 microM). 8-Phenyltheophylline prevented the inhibitory effect of L-PIA and 50 microM-adenosine, but not that of 500 microM-adenosine or of adrenaline. In contrast, yohimbine selectively prevented the inhibition by adrenaline. Adenosine and L-PIA thus appear to exert their effects by activating membrane A1 receptors, whereas adrenaline acts on alpha 2-adrenergic receptors. Adenosine, L-PIA and adrenaline slightly inhibited 45Ca2+ efflux, 86Rb+ efflux and 45Ca2+ influx in glucose-stimulated islets. The inhibition of insulin release by adenosine or L-PIA was totally prevented by dibutyryl cyclic AMP, but was only attenuated when adenylate cyclase was activated by forskolin or when protein kinase C was stimulated by a phorbol ester. Adrenaline, on the other hand, inhibited release under these conditions. It is concluded that inhibition of adenylate cyclase, rather than direct changes in membrane K+ and Ca2+ permeabilities, underlies the inhibition of insulin release induced by activation of A1-receptors. The more complete inhibition mediated by alpha 2-adrenergic receptors appears to result from a second mechanism not triggered by adenosine.

Adenosine↗
PubMed1988

The muscarinic receptor subtype in mouse pancreatic B-cells.

Isolated mouse islets were used to identify the muscarinic receptor subtype present in pancreatic B-cells. We thus compared the inhibitory potencies of atropine (non-specific), of pirenzepine (specific for M1 receptors) and of compound AF-DX 116 (specific for cardiac M2 receptors) on acetylcholine-induced insulin release, 86Rb+ efflux and 45Ca2+ efflux. The three antagonists inhibited all effects of acetylcholine, but EC50 values were markedly different: atropine = 1.5-5 nM, pirenzepine = 0.6-1.7 microM and AF-DX 116 = 1.7-11 microM. The results did not suggest that the various effects of ACh could result from the activation of different subtypes of receptors. It is concluded that muscarinic receptors of pancreatic B-cells belong to an M2 subtype distinct from the cardiac M2 receptors.

Acetylcholine↗
PubMed1988

Effects of acute sodium omission on insulin release, ionic flux and membrane potential in mouse pancreatic B-cells.

The effects of acute omission of extracellular Na+ on pancreatic B-cell function were studied in mouse islets, using choline and lithium salts as impermeant and permeant substitutes, respectively. In the absence of glucose, choline substitution for Na+ hyperpolarized the B-cell membrane, inhibited 86Rb+ and 45Ca2+ efflux, but did not affect insulin release. In contrast, Li+ substitution for Na+ depolarized the B-cell membrane and caused a Ca2+-independent, transient acceleration of 45Ca2+ efflux and insulin release. Na+ replacement by choline in the presence of 10 mM glucose and 2.5 mM Ca2+ again rapidly hyperpolarized the B-cell membrane. This hyperpolarization was then followed by a phase of depolarization with continuous spike activity, before long slow waves of the membrane potential resumed. Under these conditions, 86Rb+ efflux first decreased before accelerating, concomitantly with marked and parallel increases in 45Ca2+ efflux and insulin release. In the absence of Ca2+, 45Ca2+ and 86Rb+ efflux were inhibited and insulin release was unaffected by choline substitution for Na+. Na+ replacement by Li+ in the presence of 10 mM glucose rapidly depolarized the B-cell membrane, caused an intense continuous spike activity, and accelerated 45Ca2+ efflux, 86Rb+ efflux and insulin release. In the absence of extracellular Ca2+, Li+ still caused a rapid but transient increase in 45Ca2+ and 86Rb+ efflux and in insulin release. Although not indispensable for insulin release, Na+ plays an important regulatory role in stimulus-secretion coupling by modulating, among others, membrane potential and ionic fluxes in B-cells.

Animals↗
PubMed1988

Muscarinic control of pancreatic B cell function involves sodium-dependent depolarization and calcium influx.

Mouse pancreatic islets were used to investigate the mechanisms and functional significance of the B cell membrane depolarization by acetylcholine (ACh). At low glucose (3mM), ACh (20 microM) increased 22Na+ influx, and slightly depolarized the B cell membrane but did not induce electrical activity or stimulate 45Ca2+ influx. ACh also accelerated 86Rb+ and 45Ca2+ efflux and barely affected basal insulin release. At a stimulatory concentration of glucose (10 mM), ACh stimulated 22Na+ influx, depolarized the B cell membrane, increased glucose-induced electrical activity, and stimulated 45Ca2+ influx. ACh also accelerated 86Rb+ and 45Ca2+ efflux and strongly potentiated insulin release. Omission of extracellular Ca2+ did not impair ACh stimulation of 22Na+ influx or 86Rb+ efflux, slightly modified the acceleration of 45Ca2+ efflux, and almost completely suppressed the increase in insulin release. Na+ omission (with N-methyl-D-glucamine as substitute) prevented the B cell membrane depolarization and the stimulation of 45Ca2+ influx, largely inhibited the acceleration of 86Rb+ efflux and insulin release, and suppressed the late phase of 45Ca2+ efflux otherwise produced by ACh. On the other hand, ACh stimulation of 3H efflux from islets prelabeled with myo-[2-3H]inositol was not affected by Na+ omission. All effects of ACh were blocked by atropine and unaffected by nicotinic antagonists. It is concluded that activation of muscarinic receptors depolarized the B cell membrane by increasing its permeability to Na+. When the membrane is already depolarized by glucose, this further depolarization augments Ca2+ influx and, hence, potentiates insulin release.

Acetylcholine↗
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↗