Effect of interferon and double-stranded RNA on B-cell function in mouse islets of Langerhans.

The direct effects of alpha- and beta-interferons on isolated mouse pancreatic islets were investigated in vitro and found to be similar. After 7 h incubation with interferon concentrations above 350 units/ml, glucose-stimulated (pro)insulin biosynthesis was significantly inhibited, with only a slight inhibition of total protein biosynthesis. Inhibition could be abolished in the additional presence of an anti-interferon antibody. Interferon did not affect insulin release, total insulin content, or glucose oxidation of the islets. The stimulation of (pro)insulin biosynthesis by adenosine, D-glyceraldehyde, mannose, N-acetylglucosamine and leucine was also inhibited by interferon, with no effect on insulin release. At concentrations of dsRNA (double-stranded RNA) said to induce interferon (1-100 micrograms/ml), glucose-stimulated (pro)insulin biosynthesis was inhibited without significantly affecting insulin release. The dsRNA may itself inhibit stimulated (pro)insulin biosynthesis or may function indirectly by the induction of interferon.     

Interrelationship of islet metabolism, adenosine triphosphate content and insulin release

The oxidation of some exogenous substrates and their effects on ATP content and insulin release in mouse pancreatic islets were measured. The ATP concentration of islets incubated without exogenous substrate shows a gradual decrease, which can be prevented by glucose or mannose (20mm) or leucine (2.5mm); d-glyceraldehyde (5mm) is as effective as glucose (5mm); fructose or N-acetylglucosamine (20mm), pyruvate (10mm) and dl-3-hydroxybutyrate (2mm) are less effective; galactose (20mm), acetate (10mm), octanoate (2mm) and succinate (10mm) have no ATP-maintaining ability. Islets oxidize glucose, mannose, glyceraldehyde, leucine and, less readily, N-acetylglucosamine and glucosamine; galactose, however, is poorly metabolized. Mannoheptulose inhibits the oxidation of glucose but not of glyceraldehyde. Insulin release, measured over a 2h incubation, is stimulated by glucose, mannose, leucine, glyceraldehyde or glucosamine but not by fructose or N-acetylglucosamine. The latter, however, potentiates the effects of glucose or glyceraldehyde (5mm) or leucine (2.5mm) on release; the potentiating effects are inhibited by mannoheptulose, which also blocks glucose-, but not glyceraldehyde- or leucine-stimulated release. In the presence of glucose (20mm), metabolic inhibitors depress insulin release and islet ATP content in parallel. However, rates of insulin release and ATP content measured after incubation with various combinations of exogenous substrates do not appear to be correlated. Sulphonylureas stimulate insulin release but decrease islet ATP concentrations. These results provide further evidence of a close association between the metabolic activity of exogenous substrates and their ability to initiate insulin release. Glucoreceptor models are formulated in the light of these observations and discussed.     

The effect of N-acylglucosamines on the biosynthesis and secretion of insulin in the rat.

The effects of N-acylglucosamines on insulin release have been studied. N-Acylglucosamines stimulated insulin release from rat islets in vitro only if a sub-stimulatory concentration of glucose was also present, and this secretory response was abolished by mannoheptulose. In perifused islets the rapidity of the secreotry response to N-acetyl-D-glucosamine was similar to that observed with D-glucose. Increasing acyl-chain length from N-acetyl- to N-hexanoyl-D-glucosamine impaired the secretory response; however, N-dichloroacetyl-D-glucosamine was a more potent stimulator of release than was N-acetyl-D-glucosamine. Polymers of N-acetyl-D-glucosamine containing two to six monomers linked alpha1-4 did not stimulate insulin release; glucosamine linked to dextran via a propionyl or hexanoyl spacer group was also without insulin-releasing ability. N-Acylglucosamines were also effective in eliciting insulin release in vivo when injected into conscious rats. At the dose used (86 mumol), N-acetylgucosamine elicited a rapid rise in plasma-insulin concentration; N-butyrylglucosamine was less effective, and there was little or no response to N-hexanoylglucosamine. The response to N-dichloroacetyl-glucosamine was greater than that to N-acetylglucosamine; an increase in plasma insulin concentration could be elicited by N-dichloroacetylglucosamine at a dose (17 mumol) at which neither glucose nor N-acetylglucosamine was effective. The secretory response to acetylglucosamine is not mediated by conversion into glucose. Rates of (pro)-insulin biosynthesis by rat islets have been measured (Pro)-insulin biosynthesis was stimulated by glucose, and this response was abolished by mannoheptulose. N-Acetylglucosamine also stimulated (pro)-insulin biosynthesis; this effect of N-acetylglucosamine did not require the presence of glucose, and was not abolished by mannoheptulose. It is concluded that there are differences in signal reception and/or transduction for the processes of insulin biosynthesis and release.     

Short- and long-term effects of dimethyl sulfoxide on mouse pancreatic islet B-cell function in vitro

Dimethylsulfoxide has been used as a cryoprotectant for the endocrine pancreas. To explore possible harmful effects of Me₂SO influenced neither glucose-stimulated insulin release, nor islet glucose oxidation. On the other hand, 1 M but not 0.25 M Me₂SO decreased glucose-stimulated (pro)insulin and total protein biosynthesis in acute experiments. In islet culture experiments with Me₂SO-supplemented culture media there was no obvious effects on glucose-stimulated (pro)insulin biosynthesis. It is suggested that Me₂SO in the range 0.01–0.5 M does not affect islet B-cell function in vitro.     

Studies of insulin release and rat pancreatic islet metabolism with diastereomers of D-glucose

Studies of insulin release with diastereomers and other analogues of D-glucose demonstrated that only sugars which undergo oxidation to CO₂ stimulate insulin release by the pancreatic islet. None of the non-metabolizable diastereomers of glucose stimulated insulin release in the presence of a substimulatory concentration of glucose for fuel. Although 5.5 mM glucose formed 77% as much CO₂ as 16.7 mM mannose and twice that of 16.7 mM fructose, 5.5 mM glucose did not stimulate insulin release whereas 16.7 mM mannose and fructose did stimulate insulin release. These results indicate that the important stimulus for glucose-induced insulin release involves metabolism of glucose, but that the stimulus does not involve solely a fuel function of glucose.     

Role of nitric oxide synthase isoforms in glucose-stimulated insulin release

The role of islet constitutive nitric oxide synthase (cNOS) in insulin-releasing mechanisms is controversial. By measuring enzyme activities and protein expression of NOS isoforms [i.e., cNOS and inducible NOS (iNOS)] in islets of Langerhans cells in relation to insulin secretion, we show that glucose dose-dependently stimulates islet activities of both cNOS and iNOS, that cNOS-derived nitric oxide (NO) strongly inhibits glucose-stimulated insulin release, and that short-term hyperglycemia in mice induces islet iNOS activity. Moreover, addition of NO gas or an NO donor inhibited glucose-stimulated insulin release, and different NOS inhibitors effected a potentiation. These effects were evident also in K+-depolarized islets in the presence of the ATP-sensitive K+ channel opener diazoxide. Furthermore, our results emphasize the necessity of measuring islet NOS activity when using NOS inhibitors, because certain concentrations of certain NOS inhibitors might unexpectedly stimulate islet NO production. This is shown by the observation that 0.5 mmol/l of the NOS inhibitor N(G)-monomethyl-L-arginine (L-NMMA) stimulated cNOS activity in parallel with an inhibition of the first phase of glucose-stimulated insulin release in perifused rats islets, whereas 5.0 mmol/l of L-NMMA markedly suppressed cNOS activity concomitant with a great potentiation of the insulin secretory response. The data strongly suggest, but do not definitely prove, that glucose indeed has the ability to stimulate both cNOS and iNOS in the islets and that NO might serve as a negative feedback inhibitor of glucose-stimulated insulin release. The results also suggest that hyperglycemia-evoked islet NOS activity might be one of multiple factors involved in the impairment of glucose-stimulated insulin release in type II diabetes mellitus.     

Regulation of RNA metabolism in relation to insulin production and oxidative metabolism in mouse pancreatic islets in vitro

This study was undertaken to investigate the long-term effects of different substrates, in particular glucose, on the regulation of islet RNA metabolism and the relationship of this regulation to the metabolism and insulin production of the islet B-cell. For this purpose collagenase-isolated mouse islets were used either in the fresh state or after culture for 2 or 5 days in RPMI 1640 plus 10% calf serum supplemented with various test compounds. Islets cultured with 16.7 mM glucose contained more RNA than those cultured with 3.3 mM glucose. Culture of islets in glucose at low concentrations inhibited glucose-stimulated RNA synthesis and this inhibitory effect was reversed by prolonged exposure to high glucose concentrations. Culture with 10 mM leucine and 3.3 mM glucose or with 10 mM 2-ketoisocaproate and 3.3 mM glucose increased the total RNA content of islets as compared to that of islets cultured with 3.3 mM glucose alone. Islets cultured with 5 mM theophylline maintained a high RNA content in the presence of 3.3 mM glucose. Theophylline also increased the islet RNA content when added together with 16.7 mM glucose, as compared to 16.7 mM glucose alone. Theophylline probably exerted this effect by decreasing the rate of RNA degradation. Changes in islet RNA metabolism showed a close correlation to changes in islet total protein biosynthesis, whereas islet (pro)insulin biosynthesis and insulin release exhibited different glucose-dependency patterns. The response of islet oxygen uptake to glucose was similar to that of islet RNA and protein biosynthesis. It is concluded that the RNA content of the pancreatic islets is controlled at the levels of both synthesis and degradation. Glucose stimulates the RNA synthesis and inhibits its degradation. Moreover, the results suggest that regulation of RNA synthesis may be mediated through islet metabolic fluxes and the cAMP system.     

Protection of the pancreatic β-cell glucoreceptor mechanism for insulin secretion during culture in chemically defined medium

High concentrations of glucose have a protective effect on the glucoreceptor mechanism for insulin secretion during culture of pancreatic islets in chemically defined media. To study at what level glucose exerts this effect, insulin secretion from beta-cell-rich mouse pancreatic islets was measured before and after culture for 1 week in the presence of different substances. Before culture, glucose and inosine were potent stimulators, mannose and fructose were less potent and xylitol had no effect on secretion. Culture in 3mm-glucose resulted in a 10-fold decrease in the insulin response to glucose stimulation. A less marked decrease was noted after culture in 20mm- or 30mm-glucose. Inosine-stimulated secretion was much decreased after culture in high concentrations of glucose, whereas the responses to mannose or fructose were unchanged. After culture in 30mm-mannose, glucose-stimulated secretion was similar to that observed after culture in high concentrations of glucose, whereas the response to mannose had much decreased. There were no secretory responses to glucose or fructose after culture in 30mm-fructose, or to glucose or xylitol after culture in 30mm-xylitol. Culture in 10mm-inosine did not preserve any significant response to glucose or inosine. The insulin contents of islets and culture media were higher after culture in high concentrations of glucose, mannose or inosine than after culture in fructose, xylitol or low concentrations of glucose. It is suggested that glucose, and to some extent mannose, preserves the glucoreceptor mechanism for insulin secretion by influencing an early stage in glucose metabolism, presumably glucokinase activity.     

Pentitols and insulin release by isolated rat islets of Langerhans

1. Insulin secretion was studied in isolated islets of Langerhans obtained by collagenase digestion of rat pancreas. In addition to responding to glucose and mannose as do whole pancreas and pancreas slices in vitro, isolated rat islets also secrete insulin in response to xylitol, ribitol and ribose, but not to sorbitol, mannitol, arabitol, xylose or arabinose. 2. Xylitol and ribitol readily reduce NAD(+) when added to a preparation of ultrasonically treated islets. 3. Adrenaline (1mum) inhibits the effects of glucose and xylitol on insulin release. Mannoheptulose and 2-deoxy-glucose, however, inhibit the response to glucose but not that to xylitol. 4. The intracellular concentration of glucose 6-phosphate is increased when islets are incubated with glucose but not with xylitol, suggesting that xylitol does not promote insulin release by conversion into glucose 6-phosphate. 5. Theophylline (5mm) potentiates the effect of 20mm-glucose on insulin release from isolated rat islets of Langerhans, but has no effect on xylitol-mediated release. These results indicate that xylitol does not stimulate insulin release by alterations in the intracellular concentrations of cyclic AMP. 6. A possible role for the metabolism of hexoses via the pentose phosphate pathway in the stimulation of insulin release is discussed.     

Effects of cyclic AMP on DNA replication and protein biosynthesis in fetal rat islets of Langerhans maintained in tissue culture

The regulatory role of cyclic AMP (cAMP) in the growth and insulin production of the islet organ in vitro has been investigated. The effects of dibutyryl cyclic AMP (dbcAMP), theophylline , and 3-isobutyl-1-methylxanthine (IBMX) on DNA replication and on the biosynthesis of RNA and insulin in fetal rat islets of Langerhans maintained in tissue culture have been studied. Raising the glucose concentration from 2.7 mM to 16.7 mM caused a two-fold increase in DNA replication. Both dbcAMP and theophylline markedly inhibited the DNA replication at all glucose Concentrations studied. Low concentrations of IBMX stimulated DNA synthesis. However, at higher concentrations of this drug, known to considerably increase the islet cAMP levels , a marked inhibition of islet DNA replication was observed. Both (pro)insulin and total protein biosynthesis were stimulated by glucose, whereas dbcAMP stimulated only the (pro)insulin biosynthesis. Since glucose is known to raise islet intracellular levels of cAMP, which is known to be an inhibitor of cellular proliferation, the observed glucose stimulation of both islet-cell DNA replication and insulin production appeared conflicting. It is suggested that this dual effect of glucose may depend on a stimulation of proliferation in a limited pool of islet cells which may not exhibit an increase in cAMP.     

Glucose metabolism in mouse pancreatic islets

1. Rates of glucose oxidation, lactate output and the intracellular concentration of glucose 6-phosphate were measured in mouse pancreatic islets incubated in vitro. 2. Glucose oxidation rate, measured as the formation of (14)CO(2) from [U-(14)C]glucose, was markedly dependent on extracellular glucose concentration. It was especially sensitive to glucose concentrations between 1 and 2mg/ml. Glucose oxidation was inhibited by mannoheptulose and glucosamine but not by phlorrhizin, 2-deoxyglucose or N-acetylglucosamine. Glucose oxidation was slightly stimulated by tolbutamide but was not significantly affected by adrenaline, diazoxide or absence of Ca(2+) (all of which may inhibit glucose-stimulated insulin release), by arginine or glucagon (which may stimulate insulin release) or by cycloheximide (which may inhibit insulin synthesis). 3. Rates of lactate formation were dependent on the extracellular glucose concentration and were decreased by glucosamine though not by mannoheptulose; tolbutamide increased the rate of lactate output. 4. Islet glucose 6-phosphate concentration was also markedly dependent on extracellular glucose concentration and was diminished by mannoheptulose or glucosamine; tolbutamide and glucagon were without significant effect. Mannose increased islet fructose 6-phosphate concentration but had little effect on islet glucose 6-phosphate concentration. Fructose increased islet glucose 6-phosphate concentration but to a much smaller extent than did glucose. 5. [1-(14)C]Mannose and [U-(14)C]fructose were also oxidized by islets but less rapidly than glucose. Conversion of [1-(14)C]mannose into [1-(14)C]glucose 6-phosphate or [1-(14)C]glucose could not be detected. It is concluded that metabolism of mannose is associated with poor equilibration between fructose 6-phosphate and glucose 6-phosphate. 6. These results are consistent with the idea that glucose utilization in mouse islets may be limited by the rate of glucose phosphorylation, that mannoheptulose and glucosamine may inhibit glucose phosphorylation and that effects of glucose on insulin release may be mediated through metabolism of the sugar.     

Biosynthesis of insulin secretory granule membrane proteins. Control by glucose.

The biosynthesis of a component SGM 110, specifically localized to the membrane of insulin secretory granules, was studied in rat insulinoma cells and in normal islets of Langerhans. Cells or islets were labelled with [35S]methionine or [3H]mannose and SGM 110 was immunoprecipitated by using a monoclonal antibody. Pulse-chase experiments demonstrated that the nascent polypeptide was cotranslationally glycosylated to form a 97,000 Da peptide which in turn was processed to the mature 110,000 Da form. A 50,000 Da form detected by immunoblotting with the same antibody was not conspicuously labelled even after a 20 h chase incubation, suggesting that it represented late processing of SGM 110 in lysosomes. With insulinoma cells, an increase in medium glucose concentration from 3 mM to 20 mM was without effect on the secretion of insulin or on the biosynthesis of (pro)insulin or SGM 110. In normal islets, however, 20 mM-glucose produced a 17-fold increase in (pro)insulin biosynthesis and a 13-fold increase in SGM 110 biosynthesis, compared with only a 2-fold increase in total protein synthesis, as judged by incorporation of [35S]methionine during a 1 h incubation. The effect of glucose on both (pro)insulin and SGM 110 biosynthesis was blocked by the addition of mannoheptulose, but not by the removal of extracellular calcium, both of which conditions inhibit insulin secretion. In contrast tolbutamide, an agent which stimulates insulin secretion, did not enhance the biosynthesis of (pro)insulin or SGM 110. It is concluded that at least one protein component of the insulin secretory granule membrane is synthesized co-ordinately with proinsulin and is subject to similar regulatory mechanisms. Factors which acutely control insulin secretion may also control granule biogenesis, although the two processes are not coupled in an obligatory fashion.     

Inhibition of glucose-stimulated insulin release by pseudo-alpha-DL-glucose as a glucokinase inhibitor

Pseudo-alpha- and pseudo-beta-DL-glucose, the isomers of 5-hydroxymethyl-1,2,3,4-cyclohexanetetrol with alpha-gluco and beta-gluco configurations, were used as synthetic analogs of glucose anomers to study the mechanism of glucose-stimulated insulin release by pancreatic islets. Neither isomer was phosphorylated by liver glucokinase nor stimulated insulin release from islets. Incubation of islets with pseudo-alpha-DL-glucose resulted in a considerable accumulation of the glucose analog, probably the D form, in islets. The alpha-isomer, but not the beta-isomer, inhibited both glucose-stimulated insulin release (44% inhibition at 20 mM) and islet glucokinase activity (36% inhibition at 20 mM) in a concentration-dependent manner and to a comparable degree. These results strongly suggest that the inhibition of glucose-stimulated insulin release by pseudo-alpha-DL-glucose is due to the inhibition of islet glucokinase by the glucose analog, providing additional evidence for the essential role of islet glucokinase in glucose-stimulated insulin release.     

Insulin regulation of glucose metabolism in HT29 colonic adenocarcinoma cells: activation of glycolysis without augmentation of glucose transport

The effects of insulin on glucose transport and metabolism were examined in cultured HT29 human colonic adenocarcinoma cells. The presence of glucose transporters was verified by D-glucose displaceable [3H]cytochalasin B binding. The Kd and Bmax values from cytochalasin B binding studies were 190 +/- 30 nM and 8.4 +/- 1.4 pmol/mg protein, respectively. Glucose transport determined with 3-O-methylglucose showed saturable kinetics with a Km of 5.8 +/- 0.4 mM and a Vmax of 0.047 +/- 0.003 mumol/mg protein per min at 25 degrees C. Moreover, in HT29 cells, two classes of insulin binding sites were detected in radioligand binding experiments. Although insulin failed to stimulate glucose transport, it was found to activate glycolysis in HT29 cells. Glucose consumption increased from 0.33 +/- 0.03 mumol/mg protein per h to 0.49 +/- 0.05 mumol/mg protein per h and lactate production was augmented from 0.67 +/- 0.04 mumol/mg protein per h to 0.87 +/- 0.06 mumol/mg protein per h in response to 10(-7) to 10(-5) M insulin. Insulin also enhanced mannose metabolism. Apart from these two hexoses, HT29 cells exhibited a surprisingly narrow substrate specificity. With the possible exception of glyceraldehyde, little lactate was produced from alternative substrates, including adenosine, inosine, ribose, deoxyribose, dihydroxyacetone, galactose and fructose either with or without insulin. Despite its limited utilization by the glycolytic pathway, adenosine was readily salvaged for de novo synthesis of adenine nucleotides. These findings suggest that insulin directly influences substrate utilization through the glycolytic pathway in HT29 cells without activating the glucose transport pathway.     

The pentose cycle and insulin release in mouse pancreatic islets

1. Rates of insulin release, glucose utilization (measured as [(3)H]water formation from [5-(3)H]glucose) and glucose oxidation (measured as (14)CO(2) formation from [1-(14)C]- or [6-(14)C]-glucose) were determined in mouse pancreatic islets incubated in vitro, and were used to estimate the rate of oxidation of glucose by the pentose cycle pathway under various conditions. Rates of oxidation of [U-(14)C]ribose and [U-(14)C]xylitol were also measured. 2. Insulin secretion was stimulated fivefold when the medium glucose concentration was raised from 3.3 to 16.7mm in the absence of caffeine; in the presence of caffeine (5mm) a similar increase in glucose concentration evoked a much larger (30-fold) increase in insulin release. Glucose utilization was also increased severalfold as the intracellular glucose concentration was raised over this range, particularly between 5 and 11mm, but the rate of oxidation of glucose via the pentose cycle was not increased. 3. Glucosamine (20mm) inhibited glucose-stimulated insulin release and glucose utilization but not glucose metabolism via the pentose cycle. No evidence was obtained for any selective effect on the metabolism of glucose via the pentose cycle of tolbutamide, glibenclamide, dibutyryl 3':5'-cyclic AMP, glucagon, caffeine, theophylline, ouabain, adrenaline, colchicine, mannoheptulose or iodoacetamide. Phenazine methosulphate (5mum) increased pentose-cycle flux but inhibited glucose-stimulated insulin release. 4. No formation of (14)CO(2) from [U-(14)C]ribose could be detected: [U-(14)C]xylitol gave rise to small amounts of (14)CO(2). Ribose and xylitol had no effect on the rate of oxidation of glucose; ribitol and xylitol had no effect on the rate of glucose utilization. Ribose, ribitol and xylitol did not stimulate insulin release under conditions in which glucose produced a large stimulation. 5. It is concluded that in normal mouse islets glucose metabolism via the pentose cycle does not play a primary role in insulin-secretory responses.     

The stimulus-secretion coupling of glucose-induced insulin release. Effect of exogenous pyruvate on islet function.

1. In isolated pancreatic islets, pyruvate causes a shift to the left of the sigmoidal curve relating the rate of insulin release to the ambient glucose concentration. The magnitude of this effect is related to the concentration of pyruvate (5--90 mM) and, at a 30 mM concentration, is equivalent to that evoked by 2 mM-glucose. Pyruvate also enhances insulin release in the presence of fructose, leucine and 4-methyl-2-oxopentanoate. 2. In the presence of glucose 8 mM), the secretory response to pyruvate is an immediate process, displaying a biphasic pattern. 3. The insulinotropic action of pyruvate coincides with an inhibition of 45Ca efflux and a stimulation of 45Ca net uptake. The relationship between 45Ca uptake and insulin release displays its usual pattern in the presence of pyruvate. 4. Exogenous pyruvate rapidly accumulates in the islets in amounts close to those derived from the metabolism of glucose. The oxidation of [2-14C]pyruvate represents 64% of the rate of [1-14C]pyruvate decarboxylation and, at a 30 mM concentration, is comparable with that of 8 mM-[U-14C]glucose. 5. When corrected for the conversion of pyruvate into lactate, the oxidation of 30 mM-pyruvate corresponds to a net generation of about 314 pmol of reducing equivalents/120 min per islet. 6. Pyruvate does not affect the rate of glycolysis, but inhibits the oxidation of glucose. Glucose does not affect pyruvate oxidation. 7. Pyruvate (30 mM) does not affect the concentration of ATP, ADP and AMP in the islet cells. 8. Pyruvate (30 mM) increases the concentration of reduced nicotinamide nucleotides in the presence but not in the absence of glucose. A close correlation is seen between the concentration of reduced nicotinamide nucleotides and the net uptake of 45Ca. Menadione inhibits the effect of pyruvate on insulin release, without altering its rate of oxidation. 9. Pyruvate, like glucose, modestly stimulates lipogenesis. 10. Pyruvate, in contrast with glucose, markedly inhibits the oxidation of endogenous nutrients. The latter effect accounts for the apparent discrepancy between the rate of pyruvate oxidation and the magnitude of its insulinotropic action. 11. Dichloroacetate fails to affect glucose oxidation and glucose-stimulated insulin release. 12. It is concluded that the effect of pyruvate to stimulate insulin release depends on its ability to increase the concentration of reduced nicotinamide nucleotides in the islet cells.     

Insulin release: Reconciliation of the receptor and metabolic hypotheses

Summary Nutrients which stimulate insulin secretion are currently thought to initiate the series of cellular events eventually leading to insulin release either by interacting with a stereospecific receptor system (the regulatory site hypothesis) or by acting as a fuel (the substrate site hypothesis) in the pancreaticB-cell. The latter hypothesis is supported by a number of observations indicating that the capacity of nutrients to stimulate insulin release is indeed highly dependent on their capacity to increase catabolic fluxes in isolated pancreatic islets. However, these observations do not rule out the existence of nutrient receptors in islet cells. For instance, a nonmetabolized analog of L-leucine stimulates insulin release by causing allosteric activation of glutamate dehydrogenase, which should be considered, therefore, as a receptor for certain amino acids. Likewise, the increase in glycolytic flux, which is associated with the process of glucose-stimulated insulin release, is attributable not solely to a mass action phenomenon but also to the activation of phosphofructokinase by fructose 2.6-bisphosphate. The biosynthesis of this activator may involve a glucose receptor system. The fact that certain nutrient secretagogues (e.g D-glucose and L-leucine) act in the B-cell both as substrates and enzyme activators permits reconciliation of the substrate site and regulatory site hypotheses for insulin release.     

Effect of glucose on protein phosphorylation in rat pancreatic islets

Isolated rat pancreatic islets, incubated in the presence of extracellular ³²Pi to steady state ³²P incorporation into cellular phosphopeptides, were exposed to glucose for 10 min. Glucose (16.7 mM) significantly stimulated the phosphorylation of six phosphoproteins with molecular weights of 15,000, 35,000, 49,000, 64,000, 93,000 and 138,000. Mannoheptulose (16.7 mM) markedly inhibited glucose-stimulated phosphorylation of these six phosphoproteins. This protein phosphorylation might be important in mediating glucose-stimulated insulin release.