Insulin release and cyclic AMP accumulation in response to glucose in pancreatic islets of fed and starved rats

TFEB and TFE3 control glucose homeostasis by regulating insulin gene expression

To fulfill their function, pancreatic beta cells require precise nutrient-sensing mechanisms that control insulin production. Transcription factor EB (TFEB) and its homolog TFE3 have emerged as crucial regulators of the adaptive response of cell metabolism to environmental cues. Here, we show that TFEB and TFE3 regulate beta-cell function and insulin gene expression in response to variations in nutrient availability. We found that nutrient deprivation in beta cells promoted TFEB/TFE3 activation, which resulted in suppression of insulin gene expression. TFEB overexpression was sufficient to inhibit insulin transcription, whereas beta cells depleted of both TFEB and TFE3 failed to suppress insulin gene expression in response to amino acid deprivation. Interestingly, ChIP-seq analysis showed binding of TFEB to super-enhancer regions that regulate insulin transcription. Conditional, beta-cell-specific, Tfeb-overexpressing, and Tfeb/Tfe3 double-KO mice showed severe alteration of insulin transcription, secretion, and glucose tolerance, indicating that TFEB and TFE3 are important physiological mediators of pancreatic function. Our findings reveal a nutrient-controlled transcriptional mechanism that regulates insulin production, thus playing a key role in glucose homeostasis at both cellular and organismal levels.

What Is the Metabolic Amplification of Insulin Secretion and Is It (Still) Relevant?

The pancreatic beta-cell transduces the availability of nutrients into the secretion of insulin. While this process is extensively modified by hormones and neurotransmitters, it is the availability of nutrients, above all glucose, which sets the process of insulin synthesis and secretion in motion. The central role of the mitochondria in this process was identified decades ago, but how changes in mitochondrial activity are coupled to the exocytosis of insulin granules is still incompletely understood. The identification of ATP-sensitive K⁺-channels provided the link between the level of adenine nucleotides and the electrical activity of the beta cell, but the depolarization-induced Ca²⁺-influx into the beta cells, although necessary for stimulated secretion, is not sufficient to generate the secretion pattern as produced by glucose and other nutrient secretagogues. The metabolic amplification of insulin secretion is thus the sequence of events that enables the secretory response to a nutrient secretagogue to exceed the secretory response to a purely depolarizing stimulus and is thus of prime importance. Since the cataplerotic export of mitochondrial metabolites is involved in this signaling, an orienting overview on the topic of nutrient secretagogues beyond glucose is included. Their judicious use may help to define better the nature of the signals and their mechanism of action.

β-Cell Control of Insulin Production During Starvation-Refeeding in Male Rats

Mammalian metabolism has evolved to adapt to changes in nutrient status. Insulin, the key anabolic hormone, facilitates intracellular storage of nutrient fuels and plays a pivotal role in the transition away from catabolism upon refeeding. Although circulating insulin relative to nutrient levels has been well characterized during fasting and refeeding, how pancreatic β-cell biology caters to acute changes in insulin demand has not been sufficiently addressed. Here, we examined the dynamics of (pro)insulin production and associated changes in β-cell ultrastructure during refeeding after a 72-hour fast in male rats. We found that fasted β-cells had marked degranulation, which inversely coordinated with the upregulation of autophagolysomal and lysosomal organelles. There was also expanded Golgi that correlated with enhanced (pro)insulin biosynthetic capacity but, conversely, blunted in vivo insulin secretion. Within 4 to 6 hours of refeeding, proinsulin biosynthesis, cellular ultrastructure, in vivo insulin secretion, and glucose tolerance normalized to levels near those of fed control animals, indicating a rapid replenishment of normal insulin secretory capacity. Thus, during a prolonged fast, the β-cell protects against hypoglycemia by markedly reducing insulin secretory capacity in vivo but is simultaneously poised to efficiently increase (pro)insulin production upon refeeding to effectively return normal insulin secretory capacity within hours.

cAMP signalling in insulin and glucagon secretion

The "second messenger" archetype cAMP is one of the most important cellular signalling molecules with central functions including the regulation of insulin and glucagon secretion from the pancreatic β- and α-cells, respectively. cAMP is generally considered as an amplifier of insulin secretion triggered by Ca²⁺ elevation in the β-cells. Both messengers are also positive modulators of glucagon release from α-cells, but in this case cAMP may be the important regulator and Ca²⁺ have a more permissive role. The actions of cAMP are mediated by protein kinase A (PKA) and the guanine nucleotide exchange factor Epac. The present review focuses on how cAMP is regulated by nutrients, hormones and neural factors in β- and α-cells via adenylyl cyclase-catalysed generation and phosphodiesterase-mediated degradation. We will also discuss how PKA and Epac affect ion fluxes and the secretory machinery to transduce the stimulatory effects on insulin and glucagon secretion. Finally, we will briefly describe disturbances of the cAMP system associated with diabetes and how cAMP signalling can be targeted to normalize hypo- and hypersecretion of insulin and glucagon, respectively, in diabetic patients.

Control of Insulin Secretion by Production of Reactive Oxygen Species: Study Performed in Pancreatic Islets from Fed and 48-Hour Fasted Wistar Rats

Mitochondria and NADPH oxidase are important sources of reactive oxygen species in particular the superoxide radical (ROS) in pancreatic islets. These molecules derived from molecular oxygen are involved in pancreatic β-cells signaling and control of insulin secretion. We examined the involvement of ROS produced through NADPH oxidase in the leucine- and/or glucose-induced insulin secretion by pancreatic islets from fed or 48-hour fasted rats. Glucose-stimulated insulin secretion (GSIS) in isolated islets was evaluated at low (2.8 mM) or high (16.7 mM) glucose concentrations in the presence or absence of leucine (20 mM) and/or NADPH oxidase inhibitors (VAS2870–20 μM or diphenylene iodonium—DPI—5 μM). ROS production was determined in islets treated with dihydroethidium (DHE) or MitoSOX Red reagent for 20 min and dispersed for fluorescence measurement by flow cytometry. NADPH content variation was examined in INS-1E cells (an insulin secreting cell line) after incubation in the presence of glucose (2.8 or 16.7 mM) and leucine (20 mM). At 2.8 mM glucose, VAS2870 and DPI reduced net ROS production (by 30%) and increased GSIS (by 70%) in a negative correlation manner (r = -0.93). At 16.7 mM glucose or 20 mM leucine, both NADPH oxidase inhibitors did not alter insulin secretion neither net ROS production. Pentose phosphate pathway inhibition by treatment with DHEA (75 μM) at low glucose led to an increase in net ROS production in pancreatic islets from fed rats (by 40%) and induced a marked increase (by 144%) in islets from 48-hour fasted rats. The NADPH/NADP⁺ ratio was increased when INS-1E cells were exposed to high glucose (by 4.3-fold) or leucine (by 3-fold). In conclusion, increased ROS production through NADPH oxidase prevents the occurrence of hypoglycemia in fasting conditions, however, in the presence of high glucose or high leucine levels, the increased production of NADPH and the consequent enhancement of the activity of the antioxidant defenses mitigate the excess of ROS production and allow the secretory process of insulin to take place.

Rp-cAMPS Prodrugs Reveal the cAMP Dependence of First-Phase Glucose-Stimulated Insulin Secretion

cAMP-elevating agents such as the incretin hormone glucagon-like peptide-1 potentiate glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells. However, a debate has existed since the 1970s concerning whether or not cAMP signaling is essential for glucose alone to stimulate insulin secretion. Here, we report that the first-phase kinetic component of GSIS is cAMP-dependent, as revealed through the use of a novel highly membrane permeable para-acetoxybenzyl (pAB) ester prodrug that is a bioactivatable derivative of the cAMP antagonist adenosine-3',5'-cyclic monophosphorothioate, Rp-isomer (Rp-cAMPS). In dynamic perifusion assays of human or rat islets, a step-wise increase of glucose concentration leads to biphasic insulin secretion, and under these conditions, 8-bromoadenosine-3',5'-cyclic monophosphorothioate, Rp-isomer, 4-acetoxybenzyl ester (Rp-8-Br-cAMPS-pAB) inhibits first-phase GSIS by up to 80%. Surprisingly, second-phase GSIS is inhibited to a much smaller extent (≤20%). Using luciferase, fluorescence resonance energy transfer, and bioluminescence resonance energy transfer assays performed in living cells, we validate that Rp-8-Br-cAMPS-pAB does in fact block cAMP-dependent protein kinase activation. Novel effects of Rp-8-Br-cAMPS-pAB to block the activation of cAMP-regulated guanine nucleotide exchange factors (Epac1, Epac2) are also validated using genetically encoded Epac biosensors, and are independently confirmed in an in vitro Rap1 activation assay using Rp-cAMPS and Rp-8-Br-cAMPS. Thus, in addition to revealing the cAMP dependence of first-phase GSIS from human and rat islets, these findings establish a pAB-based chemistry for the synthesis of highly membrane permeable prodrug derivatives of Rp-cAMPS that act with micromolar or even nanomolar potency to inhibit cAMP signaling in living cells.

New insights concerning the molecular basis for defective glucoregulation in soluble adenylyl cyclase knockout mice

Recently published findings indicate that a knockout (KO) of soluble adenylyl cyclase (sAC, also known as AC-10) gene expression in mice leads to defective glucoregulation that is characterized by reduced pancreatic insulin secretion and reduced intraperitoneal glucose tolerance. Summarized here are current concepts regarding the molecular basis for this phenotype, with special emphasis on the potential role of sAC as a determinant of glucose-stimulated insulin secretion. Highlighted is new evidence that in pancreatic beta cells, oxidative glucose metabolism stimulates mitochondrial CO₂production that in turn generates bicarbonate ion (HCO(3)(-)). Since HCO(3)(-) binds to and directly stimulates the activity of sAC, we propose that glucose-stimulated cAMP production in beta cells is mediated not simply by transmembrane adenylyl cyclases (TMACs), but also by sAC. Based on evidence that sAC is expressed in mitochondria, there exists the possibility that beta-cell glucose metabolism is linked to mitochondrial cAMP production with consequent facilitation of oxidative phosphorylation. Since sAC is also expressed in the cytoplasm, sAC catalyzed cAMP production may activate cAMP sensors such as PKA and Epac2 to control ion channel function, intracellular Ca²⁺ handling, and Ca²⁺-dependent exocytosis. Thus, we propose that the existence of sAC in beta cells provides a new and unexpected explanation for previously reported actions of glucose metabolism to stimulate cAMP production. It seems possible that alterations of sAC activity might be of importance when evaluating new strategies for the treatment of type 2 diabetes (T2DM), or when evaluating why glucose metabolism fails to stimulate insulin secretion in patients diagnosed with T2DM. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Roles of Ca2+ ions in the control of ChREBP nuclear translocation

Carbohydrate-responsive element binding protein (ChREBP (MLXIPL)) is emerging as an important mediator of glucotoxity both in the liver and in the pancreatic β-cells. Although the regulation of its nuclear translocation and transcriptional activation by glucose has been the subject of intensive research, it is still not fully understood. We have recently uncovered a novel mechanism in the excitable pancreatic β-cell where ChREBP interacts with sorcin, a penta-EF-hand Ca(2)(+)-binding protein, and is sequestered in the cytosol at low glucose concentrations. Upon stimulation with glucose and activation of Ca(2)(+) influx, or application of ATP as an intracellular Ca(2)(+)-mobilising agent, ChREBP rapidly translocates to the nucleus. In sorcin-silenced cells, ChREBP is constitutively present in the nucleus, and both glucose and Ca(2)(+) are ineffective in stimulating further ChREBP nuclear shuttling. Whether an active Ca(2)(+)-sorcin element of ChREBP activation also exists in non-excitable cells is discussed.

Glucose-induced nuclear shuttling of ChREBP is mediated by sorcin and Ca(2+) ions in pancreatic β-cells

Carbohydrate-responsive element-binding protein (ChREBP) is a regulator of pancreatic β-cell gene expression and an important mediator of glucotoxicity. Glucose increases the activity and nuclear localization of ChREBP by still ill-defined mechanisms. Here we reveal, using both MIN6 and primary mouse β-cells, a unique mechanism behind ChREBP nuclear translocation. At low glucose concentrations, ChREBP interacts with sorcin, a penta EF hand Ca(2+) binding protein, and is sequestered in the cytosol. Sorcin overexpression inhibits ChREBP nuclear accumulation at high glucose and reduced the activity of L-type pyruvate kinase (L-PK) and TxNIP promoters, two well-characterized ChREBP target genes. Sorcin inactivation by RNA interference increases ChREBP nuclear localization and in vivo binding to the L-PK promoter at low glucose concentrations. Ca(2+) influx was essential for this process since Ca(2+) chelation with EGTA, or pharmacological inhibition with diazoxide and nifedipine, blocked the effects of glucose. Conversely, mobilization of intracellular Ca(2+) with ATP caused the nuclear accumulation of ChREBP. Finally, sorcin silencing inhibited ATP-induced increases in intracellular Ca(2+) and glucose-stimulated insulin secretion. We therefore conclude that sorcin retains ChREBP in the cytosol at low glucose concentrations and may act as a Ca(2+) sensor for glucose-induced nuclear translocation and the activation of ChREBP-dependent genes.

Rats treated with oleoyl-oestrone maintain glucidic homeostasis: comparisons with a pair-fed model

To determine whether or not the weight (and fat) loss induced by oleoyl-oestrone treatment results only as a consequence of decreased food intake, we compared treated animals with a pair-fed model. To this end, Wistar female rats received daily oral gavages of 10 mumol/kg per d oleoyl-oestrone in sunflower oil, or vehicle alone for 10 or 20 d. A second group of rats received the gavage of sunflower oil and the same amount of food ingested as the oleoyl-oestrone-treated animals (pair-fed group). Rats treated with oleoyl-oestrone maintained glucidic metabolism homeostasis despite a marked decrease in adipose tissue weight (P<0.001). Pair-fed rats exhibited a different pattern, comparable to short-term starvation, with greatly decreased glycogen stores (P<0.0001). The most significant effects were detected in the 10 d period groups. Oleoyl-oestrone affected the activity of the ponderostat system not only by decreasing appetite but also by modifying energy partition: treated animals maintained their glucose and energy homeostasis despite decreased food intake and the massive depletion of lipid stores.

Role of oscillations in membrane potential, cytoplasmic Ca2+, and metabolism for plasma insulin oscillations

A model for the relationship between ionic and metabolic oscillations and plasma insulin oscillations is presented. It is argued that the pancreatic beta-cell in vivo displays two intrinsic frequencies that are important for the regulation of plasma insulin oscillations. The rapid oscillatory activity (2--7 oscillations [osc] per minute), which is evident in both ionic and metabolic events, causes the required elevation in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) for the exocytosis of insulin granules. This activity is important for regulation of the amplitude of plasma insulin oscillations. The frequency of the rapid oscillatory ionic activities is regulated by glucose and allows the beta-cell to respond in an analogous way, with gradual changes in [Ca(2+)](i) and insulin release in response to the alterations in glucose concentration. The slower oscillatory activity (0.2--0.4 osc/min), which is evident in the metabolism of the beta-cell, has a frequency corresponding to the frequency observed in plasma insulin oscillations. The frequency is not affected by changes in the glucose concentration. This activity is suggested to generate energy in a pulsatile fashion, which sets the frequency of the plasma insulin oscillations. It is proposed that the slow oscillations in [Ca(2+)](i) observed in vitro are a manifestation of the metabolic oscillations and do not represent an in vivo phenomenon.

Impairment of the priming effect of glucose on insulin secretion from isolated islets of aging rats

The time-dependent potentiation (TDP) of insulin release or priming effect exerted by glucose was evaluated in the islets of Langerhans of mature and old rats. Islets isolated from 12- and 26-month-old male Sprague-Dawley rats and incubated for two consecutive 60-min periods in the presence of various stimulating agents were unable to enhance their insulin responsiveness significantly during the second incubation period and showed other abnormalities in their sensitivity to secretagogues compared with islets from 3-month-old animals. The priming action of glucose plus arginine or isobutylmethylxanthine (IBMX) was not observed in islets from 12-month-old rats, but surprisingly, islets from senescent rats showed a restoration of the beta-cell memory in the presence of IBMX. Interestingly, the islets isolated from 2-month-old animals previously exposed to an intravenous glucose load in vivo released approximately twice as much insulin as the islets taken from fed rats not subjected to the load. This potentiation exerted by the intravenous glucose administration was reduced but not abolished in the islets of glucose-intolerant, 12-month-old rats. In conclusion, the glucose TDP of insulin secretion is impaired in islets of mature and old rats, confirming an early loss of sensitivity of beta-cells to secretagogues during aging.

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.

Glucose stimulates insulin release without altering cyclic AMP production or inositolphospholipid turnover in freshly obtained human insulinoma cells

Glucose, forskolin, IBMX and carbachol all stimulated insulin release from freshly obtained human insulinoma cells. In these same cells, cellular cyclic AMP levels were raised by forskolin and IBMX but not by glucose and carbachol. On the other hand, of all the insulin secretagogues examined, only carbachol stimulated the formation of 3H-inositol trisphosphate in these cells. Thus, in these insulinoma cells, glucose apparently induces insulin secretion without altering cyclic AMP production or inositolphospholipid turnover.

Effects of prolonged maternal fast on the pancreas of 18-21-day-old foetal rats. Ultrastructural and morphometric study of the beta cells

After maternal fasting for 72 h the pancreatic beta cells of 18-day-old foetal rats show a conspicuous enrichment in secretory material, with an increase of pancreatic insulin concentration and a marked development of the rough endoplasmic reticulum and the Golgi apparatus. The morphometric analysis shows that the intracytoplasmic migration of the secretory granules is inhibited, principally inside the cell web. Consequently the number of secretory granules fused with plasma membrane decreases and this is associated with a decreased foetal plasma insulin. The difference in the ultrastructural aspect of the beta cells of foetuses from fasting mothers and of foetuses from fed mothers is less conspicuous at 19 days of gestation and progressively disappears at 20 and 21 days. The modifications in ultrastructural aspect and in functional state are discussed.

Effects of fasting on beta-cell function, body fat, islet volume, and total pancreatic insulin content

The effect of 24 to 48 hours of fasting on beta-cell function, islet area, islet number, and total pancreatic insulin content was examined in mice. Estimation of total body fat mass was based on the known relationship between weight of the gonadal fat pad and percentage of total body fat. The body fat decreased by 31% after 24 hours and 68% after 48 hours (P = 0.003) of fasting. Glucose-stimulated insulin release from the isolated, perfused mouse pancreas decreased by 50% and 76% after 24 and 48 hours, respectively, without affecting the dynamic pattern of insulin secretion. The fall in secreted amount of insulin was closely related to the decrease in the body fat (P = 0.002). The islet area was determined by planimetric quantitation of the perfusion-stained pancreatic islets and was related to the total pancreatic insulin content. Neither islet area nor islet number changed significantly during the fasting periods, whereas a higher pancreatic insulin content was found in mice fasted for 24 hours. A close connection existed between total pancreatic insulin content and islet area (P = 0.003). Our data suggest that the decrement in body fat induced by fasting is synchronized with a progressive loss of the capability of the pancreatic beta cells to release insulin.

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.

Evidence for priming and inhibitory effects of glucose on insulin secretion from isolated islets of Langerhans

Biphasic insulin secretion from perifused rat islets of Langerhans was affected in three ways by the islet glucose environment prior to stimulation. (i) The secretory response to glucose was diminished if the basal concentration of glucose in the medium was reduced from 5.5 to 2.7 mmol/l for 2 h prior to stimulation. First phase secretion was affected more than the second. (ii) Secretion was potentiated if islets had been previously exposed to a stimulatory concentration of glucose of 22.2 mmol/l. Again first phase secretion was particularly affected and there was a positive correlation between the magnitude of the secretory response and the duration of the initial stimulus. (iii) In contrast, both phases of secretion were proportionately reduced if islets had been previously exposed to stimulatory concentrations of glucose of 8.3 mmol/l.

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.

The metabolism of cyclic AMP and glucose in isolated islets from Acomys cahirinus

Glucose-induced cyclic (3H) AMP accumulation, insulin secretory responses and the metabolism of glucose were studied in pancreatic islets from Acomys cahirinus. 27.7 mmol/l of glucose stimulated neither islet cyclic (3H) AMP accumulation nor insulin release during the first 5 min of incubation. Stimulation by glucose of cyclic (3H) AMP was observed after 15 min of incubation and insulin release was markedly stimulated between 15 and 30 min. The utilization of glucose, measured as the production of (3H)2O from (5--3H) glucose was stimulated by glucose after 10 min and proceeded at an apparently linear rate during a 20 min incubation period. In incubations of 5 min, glibenclamide, glucagon or chloromercuribenzene-p-sulphonic acid failed to stimulate islet cyclic (3H) AMP accumulation. 3-isobutyl-l-methylxanthine in a concentration of 1.0 mmol/l was the only agent tested that elevated rapidly (1 min) islet cyclic (3H) AMP. None of the agents tested elicited an insulin secretory response in 5 min incubations. It is concluded that 1) no gross defect is apparent in the utilization of glucose by Acomys islets, 2) the secretory derangement of the Acomys is associated with a delayed cyclic AMP response to glucose, 3) however a decreased level of cyclic AMP cannot be the sole explanation for the delayed insulin secretion in the Acomys.

Medical complications of obesity

Obesity leads to several complications that affect many body systems. This paper focuses mainly on the cardiovascular complications, which include coronary heart disease, cerebrovascular disease and stroke, and congestive heart failure; the last may be secondary not only to advanced coronary atherosclerosis, but also to other pathogenetic factors. The increased frequency of coronary heart disease in the obese is largely attributable to the commonly associated hypertension, diabetes mellitus and lipoprotein abnormalities, rather than the adiposity. The lipoprotein disorders that have a role in atherogenesis are decreased plasma concentrations of high-density lipoproteins and elevated plasma concentrations of low-density lipoproteins. Abnormalities in cholesterol metabolism are responsible for the increased frequency of cholelithiasis in obese persons. The factors that mediate the development of cardiovascular and gallbladder complications are correctable by an appropriate program of meal planning and physical activity.

Opposite effects of starvation on oxidation of [14C]adenosine and adenosine-induced insulin release by isolated mouse pancreatic islets

To test further the hypothesis that ribonucleosides stimulate insulin secretion and biosynthesis by producing metabolic signals, the effects of starvation on adenosine-stimulated insulin production and the oxidation of adenosine by isolated mouse pancreatic islets were examined. No direct correlation was found between the metabolic flux and insulin secretion, since the starvation-induced impairment of the adenosine-stimulated insulin secretion was accompanied by an increased rate of adenosine oxidation. Adenosine-stimulated insulin biosynthesis was, however, unaffected by starvation.

Cyclic AMP and insulin release

The role of cyclic adenosine-3',5'-monophosphate (cAMP) for insulin secretion has been investigated. In isolated islets of Langerhans from the rat, glucose increases cAMP concomitant with insulin secretion. Stimulation of these two parameters is likewise reversible in parallel. The minimal and maximal concentrations of glucose eliciting cAMP and insulin responses are similar. Isomers and epimers of glucose influence insulin and cAMP in a parallel fashion as do sulfonylurea compounds (tolbutamide and glibenclamide). On the contrary, the time-dependent potentiation of glucose-induced insulin secretion is not accompanied by gross changes in cAMP. Reciprocally, in the absence of glucose islet cAMP can be markedly elevated by other agents (methyl xanthines, cholera toxin) without major insulin responses. The results indicate that metabolism of cAMP in the beta-cell is intimately linked to the glucose (and sulfonylurea) action on insulin secretion, although other factors influenced by the hexose are also necessary for the release process. The finding that the cAMP response is impaired in fasting, during the neonatal period and in diabetes mellitus (in the Chinese hamster) suggests an important role for the nucleotide in physiological and pathophysiological states characterized by decreased insulin release.

Decreased cyclic AMP and insulin responses to glucose in pancreatic islets of diabetic Chinese hamsters

The dose as well as the time kinetics of insulin and adenosine-3',5'-monophosphate (cyclic AMP) responses to glucose were compared in pancreatic islets isolated from normal and diabetic Chinese hamsters. The insulin content in diabetic islets was about one-half that in normal islets. Insulin release in diabetic islets incubated for 10 min with glucose 60-1000 mg/100 ml was from one-third to one-half that in normal islets. Glucose 1000 mg/100 ml stimulated three-fold increases in insulin release without increasing the accumulation of [3H] cyclic AMP in either normal or diabetic islets prelabelled with [3H] adenine. However, in the presence of 1.0 mM of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX), glucose 150 mg/100 ml elicited significant increases of insulin release (+ 134%) and [3H] cyclic AMP accumulation in islets (+ 44%) and incubation medium (+ 48%) of islets of normal but not diabetic hamsters. Also, in perifusion experiments with 0.1 mM IBMX, glucose 500 mg/100 ml produced threefold greater increases in insulin release and two-fold greater increases in efflux of cyclic AMP in normal than diabetic islets. By contrast with the lesser effects of glucose in diabetic islets, 1.0 mM IBMX increased islet and medium cyclic AMP, as well as insulin release, similarly in normal and diabetic islets. It is suggested that the impairment of glucose-induced insulin release in islets of the diabetic Chinese hamster may be due to a defective interaction of glucose with the adenylate cyclase-cyclic AMP system in the pancreatic B cell.