Oxidation of glucose and fatty acids in normal and in A 2 -cell rich pancreatic islets isolated from guinea-pigs

Effect of phenylpyruvate on enzymes involved in fatty acid synthesis in rat brain

1. The effects of azaserine and nicotinamide, agents which inhibit and stimulate hepatic NAD synthesis respectively, on the content of NAD(+) and NADH in isolated rat islets of Langerhans incubated in vitro were studied. The effects of these compounds on the rates of insulin release, from isolated islets incubated in vitro, in response to various secretagogues were also measured. 2. Preincubation of islets in the presence of azaserine (0.3mm) caused a marked depletion of the normal islet-cell content of both NAD(+) and NADH and prevented the secretion of insulin in response to stimulatory concentrations of d-glucose, xylitol, d-xylulose, l-arginine hydrochloride and l-leucine. 3. Preincubation of islets in the presence of nicotinamide (2mm) increased the islet content of NAD(+) and enhanced the rate of release of insulin in response to d-glucose. Also when nicotinamide was present the inhibitory effect of azaserine on insulin release and the azaserine-induced depletion of the islet content of NAD(+) and NADH was prevented. 4. Preincubation with azaserine was without effect on the stimulation of insulin release caused by theophylline or dibutyryl cyclic AMP. 5. It is suggested that insulin release caused by sugars and amino acids is dependent on the maintenance of NAD concentrations, though this may not be the case for release due to theophylline and dibutyryl cyclic AMP.

Expression of novel organic cation/carnitine transporter (OCTN2) in the mouse pancreas

Among the organic cation transporters, OCTN2 is identified as the most important carnitine transporter owing to the ability to transport carnitine. Although the OCTN2 is previously found in various tissues, there have been no reports showing the OCTN2 in the pancreas. In this study, we examined the expression and localization of OCTN2 in the mouse pancreas by the aid of an in situ hybridization technique and immunohistochemistry with anti-OCTN2 antibody. As a result, the OCTN2 expression was found in the A-cells for the first time. OCTN2 was not expressed in B-cells, notwithstanding that the metabolism of long-chain fatty acids, which are transported into the mitochondria with the help of carnitine, was expected for fatty acid-stimulated insulin secretion. Thus, this study suggests the possibility of carnitine uptake in the pancreatic A-cells through OCTN2 and implies the presence of carnitine transporter(s) other than OCTN2 in the B-cell.

Glycerol-stimulated proinsulin biosynthesis in isolated pancreatic rat islets via adenoviral-induced expression of glycerol kinase is mediated via mitochondrial metabolism

In this study, we examined whether adenoviral-mediated glycerol kinase (AdV-CMV-GlyK) expression in isolated rat pancreatic islets could introduce glycerol-induced proinsulin biosynthesis. In AdV-CMV-GlyK-infected islets, specific glycerol-induced proinsulin biosynthesis translation and insulin secretion were observed in parallel from the same islets. The threshold concentration of glycerol required to stimulate proinsulin biosynthesis was lower (0.25-0.5 mmol/l) than that for insulin secretion (1.0-1.5 mmol/l), reminiscent of threshold differences for glucose-stimulated proinsulin biosynthesis versus insulin secretion. The dose-dependent glycerol-induced proinsulin biosynthesis correlated with the rate of glycerol oxidation in AdV-CMV-GlyK-infected islets, indicating that glycerol metabolism was required for the response. However, glycerol did not significantly increase lactate output from AdV-CMV-GlyK-infected islets, but the dihydroxyacetone phosphate (DHAP) to alpha-glycerophosphate (alpha-GP) ratio significantly increased in AdV-CMV-GlyK-infected islets incubated at 2 mmol/l glycerol compared with that at a basal level of 2.8 mmol/l glucose (P < or = 0.05). The DHAP:alpha-GP ratio was unaffected in AdV-CMV-GlyK-infected islets incubated at 2 mmol/l glycerol in the added presence of alpha-cyanohydroxycinnaminic acid (alpha-CHC), an inhibitor of the plasma membrane and mitochondrial lactate/pyruvate transporter. However, alpha-CHC inhibited glycerol-induced proinsulin biosynthesis and insulin secretion in AdV-CMV-GlyK-infected islets (>75%; P = 0.05), similarly to glucose-induced proinsulin biosynthesis and insulin secretion in AdV-CMV-GlyK-infected and control islets. These data indicated that in AdV-CMV-GlyK-infected islets, the importance of mitochondrial metabolism of glycerol was required to generate stimulus-response coupling signals to induce proinsulin biosynthesis and insulin secretion.

Differences in long-term effects of L-glutamine and D-glucose on insulin release from rat pancreatic islets

We have compared the effects of long-term exposure to L-glutamine or D-glucose on nutrient-induced insulin release from pancreatic islets of the rat. After 3 days of culture islets were finally tested in 1 h incubations for insulin responses to 16.7 mM of glucose, glutamine, leucine or a combination of leucine and glutamine. After culture at 11 mM glucose + 2 mM glutamine (A), glucose, leucine and glutamine stimulated release to a similar extent from islets. After culture at 1.7 mM glucose + 10 mM glutamine (B), only leucine stimulated insulin release. After culture at 11 mM glucose + 10 mM glutamine (C), both leucine and glutamine increased the insulin response. After culture at 1.7 mM glucose and 2 mM glutamine (D), only glutamine slightly stimulated release. After culture in high glutamine (B or C), a combination of leucine and glutamine significantly inhibited release as compared to leucine alone. A switch in culture media from B to A for 1 h prior to final incubations revived insulin release in response to glucose but not to glutamine. The reverse switch (A to B) abolished both subsequent glucose-and glutamine-induced insulin release. A switch from D to B revived an insulin response to leucine. Exposure of B-cells to 11 mM glucose during 30 min in another experimental system (perfused pancreas) induced a significant insulin response to subsequent stimulation with glutamine; this response was, however, only 17% of that to glucose per se observed in the same experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucagon physiology and pathophysiology in the light of new advances

Recent advances in the understanding of glucagon-insulin relationships at the level of the islets of Langerhans and of hepatic fuel metabolism are reviewed and their impact on our understanding of glucagon physiology and pathophysiology is considered. It now appears that alpha cells can respond directly to hyperglycaemia in the absence of insulin and beta cells, but that antecedent hyperglycaemia masks or attenuates this response. Insulin appears to exert ongoing release inhibition upon glucagon secretion, probably via the intra-islet microvascular system that connects beta cells to alpha cells. Diabetic hyperglucagonemia in insulin deficient states appears to be secondary to lack of the restraining influence of insulin. The alpha cell response to glucopenia, by contrast, may be in large part mediated by release of noradrenaline from nerve endings in contact with alpha cells. Glucagon's action on glucose and ketone production by hepatocytes is mediated by increase in cyclic-AMP-dependent protein kinase. The opposing action of insulin upon glucagon-mediated events probably occurs largely at this level. Consequently, when glucagon secretion or action is blocked, cyclic-AMP-dependent protein kinase activity is low even in the absence of insulin, explaining why marked glucose and ketone production is absent in bihormonal deficiency states.

Effects of ketone bodies on insulin release and islet-cell metabolism in the rat

Ketone bodies promote insulin secretion from isolated rat pancreatic islets in the presence of 5 mM-glucose, but are ineffective in its absence. At concentrations of 10 mM or less, the relative abilities of the ketone bodies to potentiate release are in the order D-3-hydroxybutyrate greater than DL-3-hydroxybutyrate greater than acetoacetate. The response curve relating insulin release to D-3-hydroxybutyrate concentration displays a threshold at 1 mM and a maximum at 10 mM. D-3-Hydroxybutyrate (5 mM, but not 10 mM) promotes insulin secretion in the presence of 5 mM concentrations of both L-arginine and DL-glyceraldehyde, but not with L-leucine, L-alanine, L-glutamate or 4-methyl-2-oxopentanoate. The oxidation rates of the exogenous ketone bodies do not correlate well with their capacities to promote insulin release. Moreover, the oxidation of 5 mM-D-3-hydroxybutyrate can be inhibited by 25% with methylmalonate (10 mM) without any diminution of release. The potentiation with D-3-hydroxybutyrate occurs without an observable increase in total islet cyclic AMP. However, a small net efflux matches the relative abilities of the ketone bodies to promote insulin release. With islets from 48 h-starved animals the insulin response is both diminished and less sensitive than in fed animals, since insulin secretion is not significantly raised until a threshold of 5 mM-D-3-hydroxybutyrate is reached. These results suggest that, in the rat at least, there should be a reappraisal of the physiological role of ketone bodies in the promotion of insulin release.

Respiration and insulin release in mouse pancreatic islets. Effects of L-leucine and 2-ketoisocaproate in combination with D-glucose and L-glutamine

In order to further evaluate the importance of B-cell metabolism for the stimulation of insulin release, respiration and insulin release were studied in mouse pancreatic islets. Leucine and 2-ketoisocaproate stimulated insulin release during an initial 1-h period, whereas there was no stimulation during two subsequent 1-h periods. This effect was in contrast to that of 16.7 mM glucose, which was a potent stimulator through all the 3 h. Furthermore, the presence of glucose (5.6 mM) or glutamine together with either leucine or 2-ketoisocaproate enhanced the insulin release and prolonged the stimulation. When the kinetics of islet respiration were studied both leucine and 2-ketoisocaproate exerted an initial stimulation on the O2 uptake which, however, was short-lived (less than 30 min). The presence of 5.6 mM glucose strongly delayed the respiratory retardation seen after the initial stimulation. Similarly, glutamine enhanced the leucine- and 2-ketoisocaproate-stimulated respiratory rates and prevented the respiratory retardation otherwise observed. Leucine (20 mM) and 2-ketoisocaproate (10 and 20 mM) stimulated the oxidation of glucose (5.6 mM). It is concluded that there is a strong correlation between respiratory stimulation and the enhancement of insulin release and that leucine and 2-ketoisocaproate depend on the presence of endogenous fuels for their ability to stimulate islet functions in vitro.

Alloxan reversibly impairs glucagon release and glucose oxidation by pancreatic A2-cells

Alloxan is known as a selective B-cell cytotoxic substance, and there is so far little evidence for a direct toxic effect on the other islet cell types. To elucidate further whether such effects occur, the actions of alloxan on glucagon release and glucose oxidation were studied in isolated normal or A(2)-cell-rich pancreatic islets of the guinea pig. The A(2)-cell-rich islets were obtained from animals injected with streptozotocin 1-2 weeks before islet isolation. After exposure to alloxan (2 or 5mm) in vitro for 30min at 4 degrees C, the islets were incubated in media containing either 1.7mm-glucose or 16.7mm-glucose plus 30m-i.u. of bovine insulin/ml. In both types of islet, alloxan abolished the ability of glucose and insulin both to decrease glucagon release and to increase the rate of glucose oxidation. A high concentration of glucose (28mm) during exposure to alloxan protected against these injurious effects. Tissue culture of alloxan-treated islets for 24h in 5.5mm-glucose restored neither the suppressive effect of glucose on glucagon release nor the inhibition of glucose oxidation of the A(2)-cells. However, culture for 1 week completely normalized both the glucagon-secretory response and glucose oxidation by both kinds of islets. It is therefore concluded that alloxan affects the secretory mechanism of not only the B-cell but also of the islet A(2)-cell, although this latter cell type is not primarily destroyed by the drug. The data furthermore support the concept of a relationship between glucose metabolism and the glucose-mediated glucagon release of the A(2)-cell.

Evidence for a limited role of NAD(P)H in the nutritional regulation of glucagon release: studies with menadione and NH4Cl

Menadione and NH4Cl were reported to lower the islet content of reduced pyridine nucleotides. They were used to investigate the possible significance of NAD(P)H in the regulation of glucagon release by glucose and arginine. Menadione (10-25 mumol/l) enhanced arginine-stimulated glucagon release at a low glucose concentration (3.3 mmol/l), but failed both to affect glucagon secretion in the sole presence of glucose (3.3 mmol/l) and to suppress the inhibitory action of glucose 11.1 mmol/l upon glucagon output. In contrast to menadione, NH4Cl inhibited arginine-stimulated glucagon release at the low glucose concentration. The inhibitory action of glucose in high concentration upon glucagon release was not suppressed by NH4Cl. These findings do not permit to extrapolate to the A2-cell the concept that reduced pyridine nucleotides represent a major coupling factor in the nutritional regulation of hormonal release.

Effect of oleic acid on arginine-induced glucagon secretion by the isolated perfused rat pancreas

The isolated perfused rat pancreas was used to investigate the effect of oleic acid on glucagon secretion in response to 10 mmol/l arginine. In the absence of oleic acid and at 2.5 mmol/l calcium, arginine induced a biphasic glucagon secretion. At lower extracellular calcium concentration (1.0 mmol/l), the second phase of glucagon release was reduced, the first phase being unchanged. In the presence of 1,500 mumol/l oleic acid, the glucagon response to arginine was also biphasic, but second phase release was markedly inhibited, the first phase glucagon release being unchanged. Such an effect was not obtained when oleic acid concentration in the medium was 750 mumol/l. These results demonstrate that high concentrations of oleic acid inhibit glucagon secretion in response to arginine from the isolated perfused rat pancreas and support the concept that circulating free fatty acid levels are involved in the control of glucagon secretion.

Interactions of alpha-ketoisocaproate, glucose and arginine in the secretion of glucagon and insulin from the perfused rat pancreas

The effects of alpha-ketoisocaproate (KIC, 10 mmol/l) on glucagon and insulin release were studied in the in vitro perfused rat pancreas. The experiments were performed at low glucose concentration (3.3 mmol/l) in the absence or presence of arginine (10 mmol/l). In all the experiments KIC induced a marked and not rapidly reversible inhibition of glucagon release. This inhibition was more pronounced in the absence (76 percent) than presence of arginine (61 percent). These inhibitory patterns closely duplicated those which were seen in parallel experiments which included a rise in the concentration of glucose (from 3.3 to 11.1 mmol/l). KIC was also a potent stimulator of insulin release. The results are compatible with the view that the intracellular metabolism of KIC and glucose plays an essential role in the regulation of glucagon release by exogenous substrates.

The role of calcium in glucagon release, interactions between glucose and calcium

The interrelationships between glucose and calcium in glucagon release were investigated using the dynamic system of the in vitro perfused rat pancreas. When calcium deprivation was induced in the presence of fixed concentrations of glucose prevailing throughout the experiments (3.3, 5.5, 8.3 and 16.6 mM), an enhancement of glucagon release invariably occurred, the shape and amplitude of such response differing in relation to the environmental glucose concentration. Such enhancement of glucagon release was readily reversible upon restoration of normal calcium levels. By contrast, during the period of calcium deprivation itself, glucagon release was little influenced by either raised (from 3.3 to 16.6 mM) or decreased (from 16.6 to 3.3 mM) glucose concentrations. These results clearly indicate that calcium plays, at least, a dual role - both inhibitory and permissive - in glucagon secretion, but the intimate mechanisms by which calcium exerts such a dual action are at present unknown.

Survival of isolated human islets of Langerhans maintained in tissue culture

Transplantation of human pancreatic islets to diabetic patients may require that donor islets be kept viable in vitro for extended time periods before transfer to the recipient. We have maintained isolated pancreatic islets obtained from the human cadaveric pancreas in tissue culture for 1-3 wk, after which we studied the structure and function of the islets. Electron micrographs of the cultured islets showed a satisfactory preservation of both beta-cells and alpha 2-cells. After culture for 1 wk, the islet oxygen uptake proceeded at a constant rate at a low glucose concentration (3.3 mM) and was significantly enhanced by raising the glucose concentration to 16.7 mM. Likewise, after culture for 1 wk, the islets responded with an increased insulin release when exposed to 16.7 mM glucose with or without added theophylline (10 mM). Islets cultured for 1-3 wk were able to incorporate [3H]leucine into proinsulin, as judged by gel filtration of acid-alcohol extracts. Glucagon release from the cultured islets was reduced significantly by 16.7 mM glucose alone, but stimulated by glucose (16.7 mM) plus theophylline (10 MM). It is concluded that viable pancreatic islets can be isolated from the pancreas of adult human donors and maintained in tissue culture for at least 1 wk without loss of the specific functions of the alpha 2- and beta-cells. It remains to be established whether such islets will survive and remain functionally competent after transplantation to human recipients.

The metabolism of lipids in mouse pancreatic islets. The oxidation of fatty acids and ketone bodies

The rate of oxidation of 14C-labelled fatty acids and of ketone bodies was measured in isolated pancreatic islets of obese-hyperglycaemic mice (ob/ob). The following main observations were made. 1. Octanoate, palmitate and oleate were all converted into CO2 by the pancreatic islets. Octanoate was oxidized with the highest rate followed by palmitate and oleate. 2. The rate of oxidation of 0.7 mM-palmitate was 3.1 pmol/h per mug drug weight. This was decreased by 50% in the presence of 16.7 mM-glucose. The rate of palmitate oxidation was also inhibited by 2-bromostearate. The palmitate oxidation showed a concentration-dependent increase, which was most marked between 0.25 and 1.0 mM. 3. Octanoate (5 mM) had no effect on the rate of oxidation of 3.3 mM- glucose. 4. Acetoacetate (5 mM) and D-3-hydroxybutyrate (5 mM) were oxidized at rates of 5.9 and 5.4 pmol/h per mug dry weight respectively. These rates were less than 10% of those found in kidney-cortex slices. The magnitude of the oxidation rates found for fatty acids and for ketone bodies suggest that these substrates represent important metabolic fuels for the pancreatic B-cells.

The heat production of pancreatic beta-cells stimulated by glucose

A recently developed batch microcalorimeter was used for studying heat production in beta-cell-rich pancreatic islets isolated from obese-hyperglycemic mice. In the absence of glucose, the rate of heat production was 50 nW/islet increasing to 90 nW/islet when the islets were exposed to 20 mM glucose. The data obtained are consistent with an increase in the caloric value of oxygen with glucose concentration, as might be expected when the beta-cells utilize proportionally more carbohydrate as a source of energy.

Characterization of the effects of arginine and glucose on glucagon and insulin release from the perfused rat pancreas

To characterize the mechanisms by which arginine and glucose affect pancreatic alpha and beta cell function, the effects of these agents over their full dose response, both alone and in various combinations, were studied using the perfused rat pancreas. Arginine (0-38 mM), in the absence of glucose, stimulated biphasic glucagon (IRG) secretion (Km approximately 3-4 mM) at concentrations less than 1 mM and caused nonphasic insulin (IRI) release (Km approximately 12-13 mM) but only at concentrations greater than 6 mM. Glucose (0-27.5 mM) alone stimulated biphasic IRI release (Km approximately 9-10 mM) at concentrations in excess of 5.5 mM and caused nonphasic inhibition of IRG secretion (Kt approximately 5-6 mM) at concentrations as low as 4.1 mM. These results demonstrate fundamental differences in pancreatic alpha and beta cell secretory patterns in response to glucose and arginine and suggest that glucagon secretion is more sensitive to the effect of both glucose and arginine. Various concentrations of arginine in the presence of 5.5 mM glucose stimulated biphasic IRG and IRI release: IRG responses were diminished and IRI responses were enhanced compared with those seen with arginine in the absence of glucose. Glucose (0-27.5 mM) in the presence of 3.2 or 19.2 mM arginine caused similar inhibition of IRG secretion (Km approximately 5-6 mM) and stimulation of IRI release (Km approximately 9-10 mM) as that seen with glucose alone, although greater IRG and IRI release occurred. This augmentation of IRI secretion was greater than that expected from mere additive effects of glucose and arginine. Classical Lineweaver-Burk analysis of these results indicates that glucose is a non-competitive inhibitor arginine-stimulated glucagon secretion and suggests that glucose and arginine affect pancreatic alpha and beta cell function via different mechanisms. In addition, comparison of simultaneous insulin and glucagon secretion patterns under various conditions suggests that endogenous insulin per se has little or no direct effect on IRG secretion and that endogenous glucagon does not appreciably affect pancreatic beta cell function.

Long-term effects of glucose on insulin release and glucose oxidation by mouse pancreatic islets maintained in tissue culture

Rates of glucose oxidation and insulin release in response to a wide range of glucose concentrations were studied in short-term experiments in isolated mouse pancreatic islets maintained in tissue culture for 6 days at either a physiological glucose concentration (6.7mm) or at a high glucose concentration (28mm). The curves relating glucose oxidation or insulin release to the extracellular glucose concentration obtained with islets cultured in 6.7mm-glucose displayed a sigmoid shape similar to that observed for freshly isolated non-cultured islets. By contrast islets that had been cultured in 28mm-glucose showed a linear relationship between the rate of glucose oxidation and the extracellular glucose concentration up to about 8mm-glucose. The maximal oxidative rate was twice that of the non-cultured islets and the glucose concentration associated with the half-maximal rate considerably decreased. In islets cultured at 28mm-glucose there was only a small increase in the insulin release in response to glucose, probably due to a depletion of stored insulin in those B cells that had been cultured in a high-glucose medium. It is concluded that exposure of B cells for 6 days to a glucose concentration comparable with that found in diabetic individuals causes adaptive metabolic alterations rather than degeneration of these cells.

Effects of alternations of plasma free fatty acid levels on pancreatic glucagon secretion in man

The present investigation was undertaken to ascertain whether alterations in plasma free fatty acids (FFA) affect pancreatic glucagon secretion in man since FFA have been reported to influence pancreatic alpha cell function in other species. Elevation of plasma FFA from a mean (+/-SE) basal level of 0.478+/-0.036 mM to 0.712+/-0.055 mM after heparin administration caused plasma glucagon levels to fall approximately 50%, from a basal value of 122+/-15 pg/ml to 59+/-14 pg/ml (P < 0.001). Lowering of plasma FFA from a basal level of 0.520+/-0.046 mM to 0.252+/-0.041 mM after nicotinic acid administration raised plasma glucagon from a basal level of 113+/-18 pg/ml to 168+/-12 pg/ml (P < 0.005). Infusion of glucose elevated plasma glucose levels to the same degree that heparin raised plasma FFA levels. This resulted in suppression of plasma glucagon despite the fact that plasma FFA levels also were suppressed. Glucagon responses to arginine were diminished after elevation of plasma FFA (P < 0.01) and during infusion of glucose (P < 0.01). Diminution of plasma FFA by nicotinic acid did not augment glucagon responses to arginine. These results thus demonstrate that rather small alterations in plasma FFA within the physiologic range have a significant effect on glucagon secretion in man. Although the effects of glucose appear to predominate over those of FFA, alterations in plasma FFA may nevertheless exert an important physiologic influence over human pancreatic alpha cell function, especially in the postabsorptive state.