The stimulus-secretion coupling of glucose-induced insulin release. XX. fasting: a model for altered glucose recognition by the B-cell

Alterations in pancreatic β cell function and Trypanosoma cruzi infection: evidence from human and animal studies

The parasite Trypanosoma cruzi causes a persistent infection, Chagas disease, affecting millions of persons in endemic areas of Latin America. As a result of immigration, this disease has now been diagnosed in non-endemic areas worldwide. Although, the heart and gastrointestinal tract are the most studied, the insulin-secreting β cell of the endocrine pancreas is also a target of infection. In this review, we summarize available clinical and laboratory evidence to determine whether T. cruzi-infection-mediated changes of β cell function is likely to contribute to the development of hyperglycemia and diabetes. Our literature survey indicates that T. cruzi infection of humans and of experimental animals relates to altered secretory behavior of β cells. The mechanistic basis of these observations appears to be a change in stimulus-secretion pathway function rather than the loss of insulin-producing β cells. Whether this attenuated insulin release ultimately contributes to the pathogenesis of diabetes in human Chagas disease, however, remains to be determined. Since the etiologies of diabetes are multifactorial including genetic and lifestyle factors, the use of cell- and animal-based investigations, allowing direct manipulation of these factors, are important tools in testing if reduced insulin secretion has a causal influence on diabetes in the setting of Chagas disease. Long-term clinical investigations will be required to investigate this link in humans.

Mechanisms of β-cell functional adaptation to changes in workload

Insulin secretion must be tightly coupled to nutritional state to maintain blood glucose homeostasis. To this end, pancreatic β-cells sense and respond to changes in metabolic conditions, thereby anticipating insulin demands for a given physiological context. This is achieved in part through adjustments of nutrient metabolism, which is controlled at several levels including allosteric regulation, post-translational modifications, and altered expression of metabolic enzymes. In this review, we discuss mechanisms of β-cell metabolic and functional adaptation in the context of two physiological states that alter glucose-stimulated insulin secretion: fasting and insulin resistance. We review current knowledge of metabolic changes that occur in the β-cell during adaptation and specifically discuss transcriptional mechanisms that underlie β-cell adaptation. A more comprehensive understanding of how β-cells adapt to changes in nutrient state could identify mechanisms to be co-opted for therapeutically modulating insulin secretion in metabolic disease.

Plasma membrane Ca(2+)-ATPase overexpression reduces Ca(2+) oscillations and increases insulin release induced by glucose in insulin-secreting BRIN-BD11 cells

In the mouse beta-cell, glucose generates large amplitude oscillations of the cytosolic-free Ca(2+) concentration ([Ca(2+)](i)) that are synchronous to insulin release oscillations. To examine the role played by [ Ca(2+)](i) oscillations in the process of insulin release, we examined the effect of plasma membrane Ca(2+)-ATPase (PMCA) overexpression on glucose-induced Ca(2+) oscillations and insulin release in BRIN-BD11 cells. BRIN-BD11 cells were stably transfected with PMCA2wb. Overexpression could be assessed at the mRNA and protein level, with appropriate targeting to the plasma membrane assessed by immunofluorescence and the increase in PMCA activity. In response to K(+), overexpressing cells showed a markedly reduced rise in [Ca(2+)](i). In response to glucose, control cells showed large amplitude [Ca(2+)](i) oscillations, whereas overexpressing cells showed markedly reduced increases in [Ca(2+)](i) without such large oscillations. Suppression of [Ca(2+)](i) oscillations was accompanied by an increase in glucose metabolism and insulin release that remained oscillatory despite having a lower periodicity. Hence, [Ca(2+)] (i) oscillations appear unnecessary for glucose-induced insulin release and may even be less favorable than a stable increase in [ Ca(2+)](i) for optimal hormone secretion. [Ca(2+)](i) oscillations do not directly drive insulin release oscillations but may nevertheless intervene in the fine regulation of such oscillations.

Chronic blockade of NO synthase paradoxically increases islet NO production and modulates islet hormone release

Islet production of nitric oxide (NO) and CO in relation to islet hormone secretion was investigated in mice given the NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) in their drinking water. In these mice, the total islet NO production was paradoxically increased, reflecting induction of inducible NOS (iNOS) in background of reduced activity and immunoreactivity of constitutive NOS (cNOS). Unexpectedly, normal mice fasted for 24 h also displayed iNOS activity, which was further increased in L-NAME-drinking mice. Glucose-stimulated insulin secretion in vitro and in vivo was increased in fasted but unaffected in fed mice after L-NAME drinking. Glucagon secretion was increased in vitro. Control islets incubated with different NOS inhibitors at 20 mM glucose displayed increased insulin release and decreased cNOS activity. These NOS inhibitors potentiated glucose-stimulated insulin release also from islets of L-NAME-drinking mice. In contrast, glucagon release was suppressed. In islets from L-NAME-drinking mice, cyclic nucleotides were upregulated, and forskolin-stimulated hormone release, CO production, and heme oxygenase (HO)-2 expression increased. In conclusion, chronic NOS blockade evoked iNOS-derived NO production in pancreatic islets and elicited compensatory mechanisms against the inhibitory action of NO on glucose-stimulated insulin release by inducing upregulation of the islet cAMP and HO-CO systems.

Environmental modulation of the inhibitory action of D-mannoheptulose upon D-glucose metabolism in isolated rat pancreatic islets

In pancreatic islets prepared from fed rats and incubated at a low concentration (1.7 mM) of D-glucose, D-mannoheptulose (10.0 mM) virtually fails to affect the metabolism of the hexose. Likewise, in islets from starved rats, the relative extent of the inhibitory action of D-mannoheptulose upon D-glucose metabolism is much more marked at high (16.7 mM) than low (1.7 mM) hexose concentration. Nevertheless, despite decreasing the metabolism of D-glucose, starvation augments the sensitivity to D-mannoheptulose in the islets incubated at a low concentration of the hexose, D-galactose, but not D-fructose, also augments the inhibitory action of D-mannoheptulose upon D-glucose metabolism in islets prepared from fed rats and exposed to the low concentration of D-glucose. A comparable situation prevails in islets exposed to 2-ketoisocaproate. Forskolin, however, which decreases D-glucose catabolism in the islets from fed rats exposed to 1.7 mM D-glucose, fails to affect significantly the inhibitory action of D-mannoheptulose on D-glucose metabolism. It is proposed that hexoses transported by the same carrier as D-glucose and non-glucidic nutrient secretagogues somehow increase D-mannoheptulose uptake by the islet cells. The latter two conditions may be operative in islets exposed to a high concentration of D-glucose, this accounting for the exquisite sensitivity to D-mannoheptulose of glucose-stimulated islets.

Endoplasmic reticulum Ca2+ is important for the proteolytic processing and intracellular transport of proinsulin in the pancreatic beta-cell

The role of intracellular Ca2+ in the proteolytic processing and intracellular transport of secretory granule proproteins was investigated by pulse-chase radiolabelling of isolated rat islets of Langerhans. The conversion of proinsulin was inhibited by depletion of medium Ca2+ with EGTA and by blocking the transport of Ca2+ into cells with the Ca2+-channel antagonists verapamil, nifedipine and NiCl2. Proinsulin conversion was also reduced by the endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin, indicating that the process requires transport of Ca2+ into the endoplasmic reticulum. This was supported by the finding that proinsulin processing was inhibited when Ca2+ was depleted before or during pulse-labelling, but not after transport of the protein to post-endoplasmic-reticulum compartments. Similarly, the inhibition of proinsulin processing was reversed by re-introduction of medium Ca2+ around the time of radiolabelling, but not after 15 min of chase incubation. Ca2+ depletion also decreased proteolytic maturation of the prohormone convertases PC1, PC2 and carboxypeptidase H. Secretion experiments suggested that the rate and extent of proinsulin transport into secretory granules were inhibited marginally by Ca2+ depletion, whereas those of the convertases were markedly impeded. Inhibition of proinsulin conversion by Ca2+ depletion was thus not simply related to the Ca2+-dependencies of mature PC1 and PC2, but also to a requirement for endoplasmic reticulum Ca2+ in proteolytic maturation of the convertases and in their transfer to secretory granules. The results also suggest that the Ca2+ required for prohormone processing in the granules enters the secretory pathway via the endoplasmic reticulum.

Essentiality of circulating fatty acids for glucose-stimulated insulin secretion in the fasted rat

We asked whether the well known starvation-induced impairment of glucose-stimulated insulin secretion (GSIS) seen in isolated rat pancreas preparations also applies in vivo. Accordingly, fed and 18-24-h-fasted rats were subjected to an intravenous glucose challenge followed by a hyperglycemic clamp protocol, during which the plasma-insulin concentration was measured. Surprisingly, the acute (5 min) insulin response was equally robust in the two groups. However, after infusion of the antilipolytic agent, nicotinic acid, to ensure low levels of plasma FFA before the glucose load, GSIS was essentially ablated in fasted rats, but unaffected in fed animals. Maintenance of a high plasma FFA concentration by coadministration of Intralipid plus heparin to nicotinic acid-treated rats (fed or fasted), or further elevation of the endogenous FFA level in nonnicotinic acid-treated fasted animals by infusion of etomoxir (to block hepatic fatty acid oxidation), resulted in supranormal GSIS. The in vivo findings were reproduced in studies with the perfused pancreas from fed and fasted rats in which GSIS was examined in the absence and presence of palmitate. The results establish that in the rat, the high circulating concentration of FFA that accompanies food deprivation is a sine qua non for efficient GSIS when a fast is terminated. They also serve to underscore the powerful interaction between glucose and fatty acids in normal beta cell function and raise the possibility that imbalances between the two fuels in vivo could have pathological consequences.

Modulation of insulin secretion by feeding behavior and physical activity: possible beneficial effects on obese and aged rats

The high occurrence of diabetes in aged subjects is well known. In fact, the aging process is accompanied by obesity and presenting increased insulin release and lower peripheral responsiveness to this hormone. A recent study has shown that the changes in glucose utilization and insulin secretion during aging are abolished when obesity is somehow avoided. This fact justifies the effort to define new strategies to avoid the development of obesity during aging. In this review, beneficial effects of balanced diets, high frequency food intake and moderate exercise training on insulin secretion and its effect in rats are presented.

Mechanism of impaired glucose-potentiated insulin secretion in diabetic 90% pancreatectomy rats. Study using glucagonlike peptide-1 (7-37)

Chronic hyperglycemia causes a near-total disappearance of glucose-induced insulin secretion. To determine if glucose potentiation of nonglucose secretagogues is impaired, insulin responses to 10(-9) M glucagonlike peptide-1 (GLP-1) (7-37) were measured at 2.8, 8.3, and 16.7 mM glucose with the in vitro perfused pancreas in rats 4-6 wk after 90% pancreatectomy (Px) and sham-operated controls. In the controls, insulin output to GLP-1 was > 100-fold greater at 16.7 mM glucose versus 2.8 mM glucose. In contrast, the increase was less than threefold in Px, reaching an insulin response at 16.7 mM glucose that was 10 +/- 2% of the controls, well below the predicted 35-40% fractional beta-cell mass in these rats. Px and control rats then underwent a 40-h fast followed by pancreas perfusion using a protocol of 20 min at 16.7 mM glucose followed by 15 min at 16.7 mM glucose/10(-9) M GLP-1. In control rats, fasting suppressed insulin release to high glucose (by 90%) and to GLP-1 (by 60%) without changing the pancreatic insulin content. In contrast, in Px the insulin response to GLP-1 tripled in association with a threefold increase of the insulin content, both now being twice normal when stratified for the fractional beta-cell mass. The mechanism of the increased pancreas insulin content was investigated by assessing islet glucose metabolism and proinsulin biosynthesis. In controls with fasting, both fell 30-50%. In Px, the degree of suppression with fasting was similar, but the attained levels both exceeded those of the controls because of higher baseline (nonfasted) values. In summary, chronic hyperglycemia is associated with a fasting-induced paradoxical increase in glucose-potentiated insulin secretion. In Px rats, the mechanism is an increase in the beta-cell insulin stores, which suggests a causative role for a lowered beta-cell insulin content in the impaired glucose-potentiation of insulin secretion.

The stimulation of insulin secretion by D-glyceraldehyde correlates with its rate of oxidation in islet cells

D-Glyceraldehyde's capacity to mimic the effect of D-glucose on insulin secretion has not yet been sufficiently substantiated. It has been recently proposed, however, that they might act through different mechanisms in insulin-secreting tumoral cells. Therefore, we have performed a dose-related study of both the secretory and metabolic effects of D-glyceraldehyde on islets, which have been compared with those produced by D-glucose. D-Glyceraldehyde's capacity to stimulate secretion was paralleled in a dose-dependent manner by its rate of oxidation to 14CO2. Partial inhibition of D-glyceraldehyde oxidation by beta-iodoacetamide resulted in a proportional decrease in the secretory response. L-Glyceraldehyde, which was apparently very poorly oxidized by islets, did not stimulate secretion. The ratio of the maximum insulin responses D-glyceraldehyde and D-glucose (57%) correlated with the ratio of their respective maximum rates of oxidation (68%). At sub-maximal concentrations there was a potentiation of the secretagogue effects of the hexose by the triose, which was not apparent at a maximum effective dose of glucose. It is concluded that D-glyceraldehyde mimics the secretory effect of glucose because, similarly to the hexose, it is metabolized through islet aerobic glycolysis. The lower potency of D-glyceraldehyde as an insulin secretagogue than D-glucose is determined by the lower capacity of islets to oxidize the triose compared with the hexose. D-Glyceraldehyde, unlike D-glucose, is metabolized in islets to D-lactate. Alternative routes for the metabolism of D-glyceraldehyde might explain some of the secretagogue differences between the triose and the hexose.

SAM prevents impairment of glucose-stimulated insulin secretion caused by hexose deprivation or starvation

Succinic acid monomethyl ester (SAM) was recently proposed as an insulinotropic tool in non-insulin-dependent diabetes mellitus. Three models were now used to investigate whether SAM protects the B-cell against the impairment of glucose-stimulated insulin release caused by either glucose deprivation or starvation. In the first model, preincubation of the islets for 180 min at low glucose concentration in the presence of SAM prevented the decrease in the secretory response to D-glucose otherwise observed during a subsequent incubation. In the second model, an impaired secretory response to D-glucose was observed after 3-day culture at low (2.8 or 5.6 mM) as distinct from high (11.1 mM) hexose concentration and the presence of SAM in the culture medium again protected against this anomaly. In the third model, the infusion of SAM for 3 days to starved rats restored the secretory potential of isolated islets to a level comparable to that otherwise found in fed rats. Thus, during glucose deprivation or starvation, SAM is indeed able to maintain B-cell responsiveness to D-glucose.

Blood-brain glucose transfer in the mouse

The intracarotid injection method has been utilized to examine blood-brain barrier (BBB) glucose transport in normal mice, and after a 2-day fast. In anesthetized mice, cerebral blood flow (CBF) rates were reduced from 0.86 ml.min-1 x gm-1 in control to 0.80 ml.min-1 x gm-1 in fasted animals (p > 0.05). Brain Uptake Indices were significantly (p < 0.05) higher in fasted (plasma glucose = 4.7 mM) than control (plasma glucose = 6.5 mM) mice, while plasma glucose was significantly lower. The maximal velocity (Vmax) for glucose transport was 1562 +/- 303 nmoles.min-1 x g-1, and the half-saturation constant (Km =) 6.67 +/- 1.46 mM in normally fed mice. In fasted mice the Vmax was 2053 +/- 393 nmoles.min-1 x g-1 (p > 0.05), and the half-saturation constant (Km =) 7.40 +/- 1.60 mM (not significant, P > 0.05). A rabbit polyclonal antiserum to a synthetic peptide encoding the 13 C-terminal amino acids of the human erythrocyte glucose transporter (GLUT-1) immunocytochemically confirmed that the mouse brain capillary endothelial glucose transporter is a GLUT-1 transporter, and immunoreactivity was similar in brain endothelia from fed and fasted animals. In conclusion, after a 2-day fast in the mouse, we saw significant reductions in forebrain weight (7%), and plasma glucose levels (27%). Increased brain glucose extraction (25%, p < 0.05), and a 22% increase in the unsaturated permeability-surface area product (p < 0.05) was also observed.

Long-term regulation of pancreatic B-cell responsiveness to D-glucose by food availability, feeding schedule, and diet composition

The immediate metabolic, cationic, and secretory response of the insulin-producing B-cell to D-glucose is regulated, in a delayed or long-term manner, by nutritional factors such as food availability, feeding schedule, or diet composition. The B-cell keeps the memory of these nutritional manipulations so that the corresponding changes in its responsiveness to D-glucose can be documented in vitro in isolated pancreatic islets. The results of experiments conducted in starved rats, in animals exposed to an altered feeding schedule, and in rats given free access to a high-carbohydrate, high-protein, or high-lipid, as distinct from balanced, diet all suggest that a sufficient prandial hyperglycemia is essential for maintenance of an optimal metabolic and secretory behavior of the islet B-cell in response to a rise in D-glucose concentration.

Starvation is associated with changes in the elemental composition of the pancreatic beta-cell

By using the proton microprobe technique we have investigated the elemental composition of both pancreatic beta-cells and exocrine pancreas from fed and 24 h or 48 h starved obese hyperglycemic mice. Among the 15 elements measured in the beta-cells both Ca and Fe increased while Mg and S decreased significantly after 24 h of starvation, the effects being more pronounced after 48 h. When animals were starved for 48 h there was a decrease in the contents of Cl, Rb and Cu, whereas that of Al and Mn increased with 152 and 55%, respectively. There was an initial decrease in Na after 24 h of starvation, which was followed by an increase after 48 h. This is in contrast to Cd, which first increased and then decreased to a value lower than that obtained in the fed animal. The content of K showed a small decrease and that of Pb showed an increase only in the 24 h starved group. In the beta-cells the contents of Zn and P did not change subsequent to starvation. In the exocrine pancreas Na, Cl and P decreased after 24 h of starvation and except for Na, the decrease was maintained when the starvation period was increased to 48 h. After 24 h there was a significant, though transient, increase in K, Mg and Rb. With regard to the contents of Zn, Cu and S there was a progressive decrease as the starvation continued. In contrast to the endocrine pancreas the content of Al in the exocrine pancreas did not change after 48 h of starvation. There was no change in islet insulin content subsequent to starvation. The extent to which the observed changes in beta-cell elemental composition is involved in the impaired insulin release associated with starvation, merits further investigations.

Insulin secretion in the isolated islets of single-, regular-fasted and fed rats

To investigate the alterations in insulin secretion induced by habituation to single daily meal, adult rats were trained on a regular-fasting scheme (2-hr feeding/22-hr fasting) for 4 weeks. Insulin secretion induced by nutrient secretagogues (D-glucose and L-leucine) and the rates of 45Ca2+ outflow and 14C-glucose oxidation were studied in isolated islets obtained from these animals and in fed and 22-hr single-fasted rats. As expected, in the 22-hr fasted group, insulin secretion was drastically decreased although not abolished while 45Ca2+ outflow and 14C-glucose oxidation rates were only partially reduced. However, the regular-fasted rats did not secrete insulin in response to the nutrient secretagogues; calcium entry was not detected although a partial reduction in 14C-glucose oxidation rate was observed. These results suggest that regular fasting induces alterations in pancreatic B-cell glycolytic pathways leading to impairment of calcium efflux and insulin secretion. Such impairment is more pronounced than that induced by a single 22-hr fast.

Fasting-induced dissociation of cationic and secretory events in pancreatic islets

In pancreatic islets removed from 48 h-fasted rats, as distinct from fed animals, the release of insulin evoked by D-glucose is more severely impaired than that evoked by 2-ketoisocaproate. This decreased secretory response to D-glucose contrasts with an unimpaired cationic response to the sugar in terms of the glucose-induced decrease in both 86Rb and 45Ca outflow from pre-labelled islets. Likewise, fasting only causes a modest decrease of the secondary rise in 45Ca outflow evoked by D-glucose in islets perifused at normal Ca2+ concentration. The latter decrease appears more marked, however, if the cationic response to glucose is expressed relative to that evoked by 2-ketoisocaproate in islets removed from rats in the same nutritional state. It is concluded that, in the process of nutrient-stimulated insulin release, neither the decrease in K+ conductance (inhibition of 86Rb outflow) nor the sequestration of Ca2+ by intracellular organelles and/or direct inhibition of Ca2+ outward transport (decrease in 45Ca outflow) represent the sole determinant(s) of the subsequent gating of Ca2+ channels (secondary rise in 45Ca efflux).

Dissociation between pancreatic islet blood flow and insulin release in the rat

The blood flow to the pancreatic islets was estimated with the aid of microspheres in fed or starved (72 h) rats. The total pancreatic blood flow (PBF) in fed animals was 0.55 +/- 0.04 ml X min-1 X g pancreas and in the starved animals 0.30 +/- 0.04 ml X min-1 X g pancreas (P less than 0.001), and the corresponding islet blood flow (IBF) 82.0 +/- 12.4 and 50.5 +/- 9.7 microliter min-1 X g pancreas respectively (P greater than 0.05). Intraperitoneal injection of 2 ml of a 30% glucose solution caused a marked increase in IBF in both fed (P less than 0.05) and starved (P less than 0.01) animals to approximately the same level. The circulating insulin concentration remained unaffected by glucose in the starved rats but increased (P less than 0.001) in the fed rats, indicating that insulin release does not necessarily rise in parallel with an elevated IBF. Intraperitoneal injection of 2 ml of a 30% solution of mannoheptulose, an inhibitor of islet glucose metabolism, decreased the serum insulin concentrations although the serum glucose concentrations rose significantly in both fed (P less than 0.001) and starved (P less than 0.001) animals. This treatment, however, caused both IBF and PBF to increase significantly in both groups. The data support the view that islet blood flow is not necessarily related to the metabolic status of the islet cells or to the insulin release.

The effect of different kinds of refeeding on islet glucose phosphorylating activities

The aim of the present work has been to study the regulating effect of different kinds of diet on the activities of enzymes that phosphorylates glucose into glucose 6 phosphate in the islets of Langerhans. The metabolism of glucose in the B cell is controlled by two different enzymes, hexokinase and glucokinase, whose activities are lowered during fasting; this coincides with lowered levels of blood glucose and blood insulin and with a blocking of the insulin-secretory response toward glucose. Refeeding with a high-carbohydrate diet restores glucokinase activity in islet extracts, blood insulin, and blood glucose. By contrast, refeeding with a low-carbohydrate diet restores hexokinase activity in islet extracts, restores poorly blood insulin, and is unable to unblock the insulin secretory response toward glucose. These results support the important role that glucokinase plays in the regulation of glycolytic flux and on insulin secretion in the B cell. Hexokinase could play a role in the regulation of the glycolytic flux when the B cell responds to other secretagogues.

Starvation-induced changes of palmitate metabolism and insulin secretion in isolated rat islets stimulated by glucose

The influence of 48 h starvation on glucose-induced changes of palmitate metabolism and insulin release in isolated rat islets was investigated. (1) Islet insulin response to 20 mM-glucose was abolished after 48 h starvation, and it was restored by 0.25 mM-2-bromostearate, an inhibitor of fatty acid oxidation. (2) The increase in glucose concentration from 3 to 20 mM was accompanied by a 50% decrease in the oxidation rate of 0.5 mM-[U-14C]palmitate in control (fed) islets, and a concomitant increase (100%) in its incorporation into triacylglycerol and phospholipid fractions. (3) Starvation induced a higher basal (3 mM-glucose) rate of palmitate oxidation, which was resistant to inhibition by 20 mM-glucose. The latter also failed to increase palmitate incorporation into islet triacylglycerols and phospholipids. (4) 2-Bromostearate (0.25 mM) strongly inhibited the high oxidation rate of palmitate in islets of starved rats, and allowed a normal stimulation of its incorporation rate into islet lipids by 20mM-glucose. (5) The results suggest that starvation restricts islet esterification of fatty acids by inducing a higher rate of their oxidative degradation that is insensitive to regulation by glucose.

Fasting-induced impairment of glucose-1,6-bisphosphate synthesis in pancreatic islets

In pancreatic islets removed from rats fasted for 48 hours, the insulin secretory response to glucose is decreased. Although the activity of phosphoglucomutase is unaffected by fasting, the decrease in glucose-stimulated insulin release coincides with a suppression of the glucose-induced increment in both glucose-1,6-P2 content and lactate or pyruvate output. These findings are compatible with a regulatory role of glucose-1,6-P2 in the control of glycolysis in pancreatic islets.

Effects of glucose, leucine and adenosine on insulin release, 45Ca2+ net uptake, NADH/NAD ratios and oxygen consumption of islets isolated from fed and starved mice

In order to elucidate further the effects of starvation on islet metabolism and insulin release, pancreatic islets of mice were isolated and incubated in the presence of various nutrient secretagogues. Starvation for 60 h completely blocked the insulin release in response to either 16.7 mM glucose or 10 mM leucine. The further addition of 20 mM adenosine partly restored the insulin response. Glucose, adenosine, glucose + adenosine, glucose + leucine or leucine + adenosine all increased the NADH/NAD ratios over basal values in islets from both fed and starved mice. No effects of starvation were observed on islet NADH/NAD ratios in any of the above media, but when islets of starved animals were incubated in the absence of any metabolic substrates the NADH/NAD ratios were decreased. In the absence of exogenous substrates the respiratory rate was also lower in islets from starved animals. Respiratory stimulation evoked by either 16.7 mM glucose or 10 mM leucine + 10 mM glutamine was lower after starvation, whereas glucose + adenosine, glucose + leucine and adenosine all induced normal respiratory responses. No differences between the 45Ca2+ uptake of islets from either starved or fed mice were observed under any conditions. It is concluded that, in starvation, a dissociation between islet insulin release and metabolism (measured as NADH/NAD ratios, oxygen consumption and 45Ca2+ uptake) may exist in the presence of certain nutrient secretagogues.

The effect of glucose stimulation on 45calcium uptake of rat pancreatic islets and their total calcium content as measured by a fluorometric micro-method

Glucose-stimulated 45calcium uptake and total calcium content of rat pancreatic islets has been studied, using a new fluorometric micro-method to estimate total calcium. Extracellular calcium was separated from incubated tissue by a rapid micro-filtration procedure. Islets incubated up to 60 min with calcium chloride 2.5 mmol/l and glucose 2.5 mmol/l maintained the same calcium content (670 +/- 7.5 pmol/microgram DNA). When the glucose concentration was raised to 15 mmol/l no change in the total calcium content could be detected. On incubation with glucose 2.5 mmol/l in the absence of calcium, the calcium content decreased to 488 +/- 27 pmol/microgram DNA. On incubation with 45calcium chloride 2.5 mmol/l for 5 or 30 min at 2.5 mmol/l glucose, islets exchanged 21 +/- 2 and 28 +/- 1% of their total calcium content and, at 15 mmol/l glucose, 30 +/- 3 and 45 +/- 2%, respectively. Thus, islet calcium has a high turn-over rate. Glucose stimulation results in an increase of the calcium uptake without enhancing the total calcium content and hence must increase the calcium-exchangeable pool.

Calcium, zinc and other elements in islet and exocrine tissue of the rat pancreas as measured by histochemical methods and electron-probe micro-analysis, Effects of fasting and tolbutamide

Fasting for 24 or 72 h causes a strong decrease of pancreatic islet calcium content as detected by glyoxal-bis-(2-hydroxyanil), (GBHA). There is strong evidence that GBHA only detects ionized calcium and not total calcium (Wolters et al., 1979). Fasting does not influence the zinc content as detected by dithizone (DZN), and aldehyde-fuchsin (AF) staining intensity is only slightly decreased. After degranulation of islets by tolbutamide (which reduced the insulin content of the pancreas to 10% of the control value) the staining intensities of GBHA, DZN and AF were strongly depressed. Calcium (as well as other elements) were also measured by electron-probe micro-analysis (EPMA). It appeared that 24 or 72 h of fasting did neither affect the total content of Ca nor of Na, P, S, and K of the islets significantly. In exocrine tissue the Ca content increased gradually as a result of fasting. Thus, after 72 h of fasting the Ca content was 25% higher than in fed controls. On the other hand after 72 h of fasting the K content appeared to be decreased. EPMA revealed that after degranulation of islets the Ca content decreased markedly (35%). S appeared to be decreased by only 14%, whereas the content of the other elements was not changed. The results show that GBHA-detectable Ca is only a part of EPMA-detectable Ca. The GBHA-Ca "pool" which contains ionized Ca, is subjected to changes when the animals are fasted, the total Ca content as measured by EPMA does not change. Thus, at least two distinguishable pools of Ca exist within the islets (GBHA-detectable and not-GBHA-detectable). It is suggested that as a result of fasting Ca passes from one pool to another.

Regulation of cyclic AMP level and lactic acid production in Ehrlich ascites tumor cells

1. Quercetin (3.3',4',5,7-pentahydroxy flavone) at the concentration of 10(-4) M, as well as 2-10(-2) M theophylline and 1.5 - 10(-4) M prostaglandin E2 caused maximal rise of cyclic AMP in Ehrlich ascites tumor cells. 2. No additional increase of cyclic AMP level in these cells was found when both quercetin (10(-4) M) and theophylline (2-10(-2) M) were present in the incubation medium, while combination of quercetin (10(-4) M) and prostaglandin E2 (1.5 - 10(-4) M) has a synergistic effect on the level of cyclic AMP. 3. Degradation of cyclic AMP by homogenate of Ehrlich ascites tumor cells was inhibited by both quercetin and theophylline. 4. Quercetin, and to a smaller but significant extent theophylline, inhibited the lactic acid production in Ehrlich ascites tumor cells while prostaglandin E2 did not change the glycolytic rate in these cells. No synergistic inhibitory effect on lactic acid production was found when combinations of quercetin and prostaglandin E2, quercetin and theophylline or prostaglandin E2 and theophylline were tested. 5. Treatment of Ehrlich ascites tumor cells with dextran sulfate abolished the inhibitory effect of quercetin on lactic acid production, while the effect of the bioflavonoid on cyclic AMP levels was not altered.

The stimulus-secretion coupling of glucose-induced insulin release. XXII. Qualitative and quantitative aspects of glycolysis in isolated islets

When isolated islets of Langerhans are suddenly exposed to glucose, the entry of the hexose into islet cells first occurs at a high rate resulting in rapid equilibration of free glucose across the cell membrane; thereafter, the rate of net glucose uptake depends on its metabolism. More than 95% of the glucose taken up by the islets is converted to triosephosphate. The fractional contribution of the sorbitol and pentose-phosphate pathways to such a process does not exceed 10%. The output of lactate from the islets accounts for approximately half of the glycolytic flux. At increasing glucose concentrations up to 4.3 mM, the rate of glycolysis increases towards a first asymptotic value; at higher glucose levels (up to 27.8 mM), a sigmoidal pattern is seen tending towards a second saturation value. The total ATP content of the islets does not correlate with their insulin-secretory activity. It is suggested that, in the process of glucose-induced insulin release, glycolysis may regulate physiological processes possibly located in the micro-environment of the cell boundary.