Interaction of cyclic AMP and alloxan on insulin secretion in isolated rat islets perifused in vitro.

The interaction of alloxan and cyclic AMP on glucose-induced insulin secretion was studied with the use of isolated rat islet perifusion. Simultaneous perifusion with cyclic AMP and alloxan did not protect the islets against the effect of alloxan. However, addition of dibutyryl cyclic AMP to the alloxan solution produced 40% protection of glucose-induced insulin secretion. Partial protection was obtained with either theophylline (41%) or caffeine alone (54%), and the addition of 1 mg/ml glucose to the theophylline or caffeine solution provided greater than 68% protection. The levels of islet tissue cyclic AMP were more than 2.1 times that of islets not protected against the alloxan effect, when partial or nearly complete reversal of the inhibitory action of alloxan on glucose-induced insulin secretion was effected by theophylline or caffeine. These results suggest that cyclic AMP affords partial protection against the effect of alloxan on glucose-induced insulin secretion.     

The role of cyclic AMP in the pathogenesis of glucose desensitization in rat pancreatic islets

Adenosine-3',5'-cyclic monophosphate (cyclic AMP) promotes exocytosis of insulin in pancreatic beta cells. This study was performed to investigate the role of cyclic AMP in the pathogenesis of glucose desensitization in rat pancreatic islets. In islets cultured with high glucose for 48 hours, 27 mmol/L glucose-induced insulin release was markedly impaired, while 3.3 mmol/L glucose-or arginine-induced insulin release was enhanced, indicating glucose desensitization. Islet cyclic AMP content was 190% enhanced in high glucose-culture islets for 48 hours. In islets cultured with dibutyryl-cyclic AMP (dbcAMP) or 3-isobutyl methy-xanthine (IBMX), islet insulin content or 27 mmol/L glucose-induced insulin release was deteriorated. In contrast, 3.3 mmol/L glucose- or arginine-induced insulin release was increased, which was similar to glucose-desensitized islets. Wash-out of dbc AMP for the last 24 hours of the 48-hour culture period restored impaired high glucose-induced insulin release in the same manner as wash-out of high glucose. Diazoxide, the KATP channel opener, also restored impaired high glucose-induced insulin release from dbcAMP-cultured islets. The data suggest that enhancement of cyclic AMP in high glucose-culture islets may be one of the pathogenesis of glucose desensitization.     

A 48-hour exposure of pancreatic islets to calpain inhibitors impairs mitochondrial fuel metabolism and the exocytosis of insulin

Genetic variation in the gene for a cytosolic cysteine protease, calpain-10, increases the susceptibility to type 2 diabetes apparently by altering levels of gene expression. In view of the importance of altered beta-cell function in the pathophysiology of type 2 diabetes, the present study was undertaken to define the effects on insulin secretion of exposing pancreatic islets to calpain inhibitors for 48 hours. Exposure of mouse islets to calpain inhibitors (ALLN, ALLM, E-64-d, MDL 18270, and PD147631) of different structure and mechanism of action for 48 hours reversibly suppresses glucose-induced insulin secretion by 40% to 80%. Exposure of islets to inhibitors of other proteases, ie, cathepsin B and proteasome, did not affect insulin secretion. The 48-hour incubation with calpain inhibitors also attenuates insulin secretory responses to the mitochondrial fuel alpha-ketoisocaproate (KIC). The same incubation also suppresses glucose metabolism and intracellular calcium ([Ca(2+)](i)) responses to glucose or KIC in islets. In summary, long-term inhibition of islet calpain activity attenuates insulin secretion possibly by limiting the rate of glucose metabolism. A reduction of calpain activity in islet could contribute to the development of beta-cell failure in type 2 diabetes thereby providing a link between genetic susceptibility to diabetes and the pathophysiologic manifestations of the disease.     

Effect of low concentrations of glucagon on insulin release and cyclic AMP content in isolated rat islets

Studies on hormone action in isolated islets have generally been carried out using concentrations far above physiologic levels. This study investigates whether glucagon at concentrations close to the physiologic level is insulinotropic in isolated islets and how this relates to islet cyclic AMP levels. Collagenase isolated rat islets were tested directly after isolation or after a 24-hour tissue culture. Insulin release and islet cyclic AMP content were determined during a 30-minute incubation by radioimmunoassay. After maintenance in tissue culture glucose-induced (16.7 mmol/L) insulin release was clearly enhanced by glucagon concentrations between 2 and 1,000 ng/mL in a dose-related manner. Islet cyclic AMP was increased only by glucagon 1 mumol/L (3.8 micrograms/mL). When phosphodiesterases were inhibited (0.1 mmol/L 3-isobutyl-1-methylxanthine) insulin release was stimulated by 1 ng/mL and cyclic AMP by 100 ng/mL. By contrast, in freshly isolated islets, glucagon concentrations in the range of 1 to 100 micrograms/mL were needed to augment glucose-induced insulin release. These findings demonstrate that the hormone sensitivity of collagenase isolated islets is markedly improved by short-term maintenance in tissue culture. The threshold level for a detectable effect on islet cyclic AMP is considerably higher than for glucose-induced insulin release. Since in hepatocytes two signal transduction systems for glucagon have recently been demonstrated, the results could mean that at low concentrations glucagon effects may not be mediated via cyclic AMP.     

Prolactin regulates adenylyl cyclase and insulin secretion in rat pancreatic islets

A role for prolactin (PRL) in the regulation of adenylyl cyclase (AC), cyclic AMP (cAMP) formation and insulin secretion was studied in isolated rat pancreatic islets cultured for 4 days at 5.5 mM glucose in the absence (control) or presence of PRL (500 ng/ml). In PRL-treated islets, stimulation by glucose (8 mM), carbamylcholine chloride (CCh) and phorbol dibutyrate increased cAMP levels 40, 89, and 151%, respectively, above similarly stimulated control islets without PRL. Moreover, insulin secretion in PRL-treated islets was more than doubled in response to 8 mM glucose plus glucagon-like peptide 1 compared with control islets. PRL also increased protein kinase C (PKC) activity in cultured islets. When islets were cultured at an insulin secretion desensitizing concentration of glucose (11 mM) for 4 days, there was a decrease in forskolin-stimulated cAMP production. However, the presence of PRL with 11 mM glucose prevented the glucose-induced decrease in cAMP production. Insulin secretion in response to 17 mM glucose was also higher (P<0.02) in islets cultured with 11 mM glucose plus PRL compared with islets cultured with 11 mM glucose alone. Islet AC types -III, -V, and -VI mRNA levels increased relative to 18s rRNA following PRL treatment. In contrast, culture at 11 mM glucose decreased relative AC-III, -V and -VI mRNA levels by as much as 50%. Culture with PRL prevented the decrease in AC expression during islet culture with 11 mM glucose, and the mRNA levels remained similar to control islets cultured at 5.5 mM glucose. Thus, PRL not only increased islet AC expression and activity and insulin secretory responsiveness, but also protected islets from chronic glucose-induced inhibition of these beta-cell activation parameters.     

Cyclic AMP phosphodiesterases in human lymphocytes

The function of lymphocytes, like platelets, has been shown to be inhibited by agents which increase intracellular cyclic AMP. Two high-affinity cAMP phosphodiesterases (PDEs), the cyclic GMP-inhibited cAMP phosphodiesterase, PDE3, and the cAMP-specific phosphodiesterase PDE4, are known to regulate cAMP concentration in haemopoietic cells by degrading cAMP to AMP. We characterized the relative contribution of the two PDEs to total lymphocyte PDE activity. We then determined which of the different gene products, PDE3A, typical of myocardium and platelets, or PDE3B, typical of adipocytes, were present in lymphocytes. The PDE3-specific inhibitor, milrinone, and the PDE4 inhibitor, rolipram, suppressed hydrolysis by 70% and 30% respectively, which indicated that both PDE4 and PDE3 were present, and that PDE3 was predominant. RT-PCR yields the expected size fragment for the primer pair PDE3B and not for PDE3A. The DNA sequence obtained had > 95% identity with PDE3B. PDE3B appears to be the major cAMP PDE in lymphocytes. In contrast to human platelets, human lymphocytes appear to contain the PDE3B subtype. Since PDE3B in adipocytes is subject to hormonal regulation, lymphocytes may be similarly modulated. Understanding the role of cAMP regulation and the involvement of cAMP in lymphocyte function may have important implications in drug development.     

Reduced expression of the liver/beta-cell glucose transporter isoform in glucose-insensitive pancreatic beta cells of diabetic rats.

Rats injected with a single dose of streptozocin at 2 days of age develop non-insulin-dependent diabetes 6 weeks later. The pancreatic beta islet cells of these diabetic rats display a loss of glucose-induced insulin secretion while maintaining sensitivity to other secretagogues such as arginine. We analyzed the level of expression of the liver/beta-cell glucose transporter isoform in diabetic islets by immunofluorescence staining of pancreas sections and by Western blotting of islet lysates. Islets from diabetic animals have a reduced expression of this beta-cell-specific glucose transporter isoform and the extent of reduction is correlated with the severity of hyperglycemia. In contrast, expression of this transporter isoform in liver is minimally modified by the diabetes. Thus a decreased expression of the liver/beta-cell glucose transporter isoform in beta cells is associated with the impaired glucose sensing characteristic of diabetic islets; our data suggest that this glucose transporter may be part of the beta-cell glucose sensor.     

Not all insulin secretagogues sensitize pancreatic islets to recombinant human interleukin 1 beta.

The purpose of this study was to test the influence of different insulin secretagogues on interleukin 1 beta mediated injury to isolated rat pancreatic islets. Islets were exposed to interleukin 1 beta for 6 days. During exposure, beta-cells were stimulated with glucose (11 mmol/l vs 3.3 mmol/l) or with non-nutrients as tolbutamide (250 mumols/l), iso-butyl 1-methyl-xanthine (50 mumols/l), or glucagon (10 mg/l). At 3.3 mmol/l of glucose, 60,000 U/l of interleukin 1 beta caused an inhibition of medium insulin accumulation to 62 +/- 5% of control from 48 h to 6 days of exposure, whereas islet DNA content was unaffected. At 11 mmol/l of glucose, interleukin 1 beta dose-dependently decreased medium insulin accumulation (e.g. 60,000 U/l of interleukin 1 beta, 12 +/- 3% of control) and islet content of DNA (60,000 U/l of interleukin 1 beta, 60 +/- 8% of control). During beta-cell stimulation with tolbutamide, interleukin 1 beta caused inhibition of insulin accumulation to 36 +/- 9% of control. In contrast, on islets stimulated with iso-butyl 1-methyl-xanthine or glucagon, the effects of interleukin 1 beta were equivalent to those on non-stimulated islets. These differences were paralleled by differences in the interleukin 1 beta effect on islet morphology. In conclusion, high beta-cell activity (as measured by islet insulin release) may increase islet susceptibility to interleukin 1 beta, however, depending upon the intracellular pathway through which insulin secretion is activated.     

Interleukin-1β effects on cyclic GMP and cyclic AMP in cultured rat islets of Langerhans — arginine — dependence and relationship to insulin secretion

Summary When islets were cultured with interleukin-1 (1 or 100 pmol/l) for 12 h in arginine-containing medium, cyclic GMP levels were increased 1.6- and 4.5-fold respectively. The arginine analogue, N--nitro-l-arginine methyl ester, which blocks nitric oxide formation and partially reverses inhibition of insulin secretion by 100 pmol/l interleukin-1, largely, but not completely, blocked generation of cyclic GMP. Treatment of islets with 100 pmol/l interleukin-1 for 12 h significantly decreased islet cyclic AMP generation in the absence of isobutylmethylxanthine (from 13.1±0.7 to 9.3±0.8 fmol/g islet protein), this fall was arginine-dependent and may have resulted from an effect on a cyclic AMP phosphodiesterase, since it was masked if isobutylmethylxanthine was present. Isobutylmethylxanthine (0.4 mmol/l) reduced the inhibitory potency of interleukin-1 in 15 h slightly but significantly from 80.5 to 59.0%. The morpholinosydnonimine SIN-1, which is a nitric oxide donor, inhibited insulin secretion, raised islet cyclic GMP and lowered cyclic AMP; its effects were similar to those of interleukin-1. However, 6-anilinoquinoline-5,8-quinone, [LY83583 (1–10 mol/l)], inhibited insulin secretion, and significantly decreased cyclic GMP while 8-bromocyclic GMP stimulated insulin secretion. Both low- and high-dose interleukin-1 treatment give a large arginine-dependent and a small, yet significant, arginine-independent increase in cyclic GMP. The inhibitory effect of SIN-1 or interleukin-1 on insulin secretion seems to depend to a small extent on decreased islet cyclic AMP, though sustained increases in nitric oxide or depleted islet GTP may directly affect the secretory process.     

Evidence for differential effects of noradrenaline and somatostatin on intracellular messenger systems in rat islets of Langerhans

The mechanisms involved in inhibition of insulin secretion by somatostatin and noradrenaline were compared in order to establish whether the receptors for these agents are coupled to similar effector systems in the pancreatic B cell. Both agents significantly reduced forskolin-induced adenylate cyclase activity in islet homogenates, although noradrenaline was more effective than somatostatin. The capacity of noradrenaline to inhibit insulin secretion was largely unaffected by agents that increase intracellular cyclic AMP, whereas the effect of somatostatin as an inhibitor was markedly reduced under these conditions. Both noradrenaline and somatostatin inhibited the stimulation of insulin secretion induced by K+ depolarization, but different mechanism were involved. Somatostatin significantly inhibited K(+)-stimulated 45Ca2+ efflux and influx in islets, while noradrenaline exerted only a minor influence on these processes. The data indicate that noradrenaline controls insulin secretion by a mechanism which operates beyond the level of intracellular messenger generation. In contrast, somatostatin exerts at least part of its inhibitory effect on insulin secretion by directly controlling islet cell Ca2+ influx in a manner which may be regulated by cyclic AMP.     

Long term inhibitory effects of pancreastatin and diazepam binding inhibitor on pancreatic beta-cell deoxyribonucleic acid replication, polyamine content, and insulin secretion

The two peptides pancreastatin and diazepam binding inhibitor (DBI) were recently demonstrated in pancreatic islets and were shown to inhibit insulin secretion in short term experiments. In the present study we investigated long term effects of pancreastatin and DBI on the DNA synthesis, polyamine content, and insulin secretion of pancreatic beta-cells in tissue culture. For this purpose fetal rat pancreatic islets enriched in beta-cells were isolated and cultured for 3 days at different concentrations of rat pancreastatin and porcine DBI. It was found that pancreastatin dose-dependently decreased beta-cell DNA synthesis, reaching maximal inhibition at 100 nM. In parallel with this, pancreastatin also decreased insulin secretion and the islet contents of insulin and the polyamines spermidine and spermine. These effects were abolished by a high glucose concentration or addition of GH. Also, DBI evoked a dose-dependent inhibition of beta-cell DNA synthesis but affected neither the islet contents of insulin or polyamines nor insulin secretion. Like pancreastatin, DBI was ineffective in preventing the increased beta-cell DNA synthesis, insulin content, or secretion in response to high glucose or GH. It is concluded that pancreastatin and DBI inhibit beta-cell DNA synthesis and function in vitro. In the case of pancreastatin these inhibitory effects may be mediated by a decrease in islet polyamine content. It is suggested that pancreastatin and DBI may influence beta-cell replication and function in vivo in an autocrine or paracrine fashion.     

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

Summary 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-1-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.     

Attenuation of insulin secretion by insulin-like growth factor 1 is mediated through activation of phosphodiesterase 3B

Both insulin and insulin-like growth factor 1 (IGF-1) are known to reduce glucose-dependent insulin secretion from the β cells of pancreatic islets. In this paper we show that the mechanism by which IGF-1 mediates this effect is in large part through activation of a specific cyclic nucleotide phosphodiesterase, phosphodiesterase 3B (PDE3B). More specifically, in both isolated pancreatic islets and insulin-secreting HIT-T15 cells, IGF-1 inhibits insulin secretion that has been increased by glucose and glucagonlike peptide 1 (GLP-1). Moreover, IGF-1 decreases cAMP levels in parallel to the reduction of insulin secretion. Insulin secretion stimulated by cAMP analogs that activate protein kinase A and also are substrates for PDE3B is also inhibited by IGF-1. However, IGF-1 does not inhibit insulin secretion stimulated by nonhydrolyzable cAMP analogs. In addition, selective inhibitors of PDE3B completely block the ability of IGF-1 to inhibit insulin secretion. Finally, PDE3B activity measured in vitro after immunoprecipitation from cells treated with IGF-1 is higher than the activity from control cells. Taken together with the fact that pancreatic β cells express little or no insulin receptor but large amounts of IGF-1 receptor, these data strongly suggest a new regulatory feedback loop model for the control of insulin secretion. In this model, increased insulin secretion in vivo will stimulate IGF-1 synthesis by the liver, and the secreted IGF-1 in turn feedback inhibits insulin secretion from the β cells through an IGF-1 receptor-mediated pathway. This pathway is likely to be particularly important when levels of both glucose and secretagogues such as GLP-1 are elevated.     

Effects of biotin on glucotoxicity or lipotoxicity in rat pancreatic islets

Biotin (vitamin H) plays an important role as a cofactor in glucose or lipid metabolism. We showed that biotin potentiated glucose-induced insulin release in isolated rat islets, while biotin alone did not affect insulin release. Coculture with biotin in islets for 48 hours significantly enhanced glucose-induced insulin release or islet insulin content. Similarly, preproinsulin or pancreatic/duodenal homeobox-1 (PDX-1) mRNA was also enhanced in islets cultured with biotin for 48 hours. Furthermore, we measured effects of biotin on beta-cell function under glucotoxic or lipotoxic states. In islets cultured with high glucose or palmitate for 48 hours, glucose-induced insulin release or islet insulin content deteriorated. Coculture with biotin significantly restored glucose-induced insulin release or islet insulin content together with the restoration of preproinsulin or PDX-1 mRNA. We conclude that biotin exerts its beneficial effects on beta-cell dysfunction induced by glucose or free fatty acids probably through the enhancement of insulin biosynthesis.     

Monomethylated-adenines potentiate glucose-induced insulin production and secretion via inhibition of phosphodiesterase activity in rat pancreatic islets

Monomethyladenines have effects on DNA repair, G-protein-coupled receptor antagonism and autophagy. In islet ß-cells, 3-methyladenine (3-MA) has been implicated in DNA-repair and autophagy, but its mechanism of action is unclear. Here, the effect of monomethylated adenines was examined in rat islets. 3-MA, N6-methyladenine (N6-MA) and 9-methyladenine (9-MA), but not 1- or 7-monomethylated adenines, specifically potentiated glucose-induced insulin secretion (3-4 fold; p ≤ 0.05) and proinsulin biosynthesis (～2-fold; p ≤ 0.05). Using 3-MA as a ‘model’ monomethyladenine, it was found that 3-MA augmented [cAMP]_i accumulation (2-3 fold; p ≤ 0.05) in islets within 5 minutes. The 3-, N6- and 9-MA also enhanced glucose-induced phosphorylation of the cAMP/protein kinase-A (PKA) substrate cAMP-response element binding protein (CREB). Treatment of islets with pertussis or cholera toxin indicated 3-MA mediated elevation of [cAMP]_i was not mediated via G-protein-coupled receptors. Also, 3-MA did not compete with 9-cyclopentyladenine (9-CPA) for adenylate cyclase inhibition, but did for the pan-inhibitor of phosphodiesterase (PDE), 3-isobutyl-1-methylxanthine (IBMX). Competitive inhibition experiments with PDE-isoform specific inhibitors suggested 3-MA to have a preference for PDE4 in islet ß-cells, but this was likely reflective of PDE4 being the most abundant PDE isoform in ß-cells. In vitro enzyme assays indicated that 3-, N6- and 9-MA were capable of inhibiting most PDE isoforms found in ß-cells. Thus, in addition to known inhibition of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3′K)/m Target of Rapamycin (mTOR) signaling, 3-MA also acts as a pan-phosphodiesterase inhibitor in pancreatic ß-cells to elevate [cAMP]_i and then potentiate glucose-induced insulin secretion and production in parallel.     

Nicotinamide partially reverses the interleukin-1 beta inhibition of glucose-induced insulin release in pancreatic islets.

Interleukin-1 beta (IL-1 beta) is known to inhibit glucose-induced insulin release by pancreatic islets. We studied the effect of nicotinamide, an inhibitor of poly[adenosine diphosphate (ADP)-ribose] synthetase and a free-radical scavenger, on this IL-1 beta-induced inhibition using rat pancreatic islets. In static experiments, groups of five islets were incubated for 24 hours in culture medium CMRL-1066, with or without 50 U/mL IL-1 beta, in the presence or absence of nicotinamide (dose range, 0 to 50 mmol/L), and then exposed for 1 hour to either 1.4 or 19.4 mmol/L glucose, 10 mmol/L arginine, or 10 mumols/L glyburide. Basal insulin secretion was 183 +/- 32 pg/islet/h (mean +/- SE, n = 7) and 176 +/- 39 (n = 7) in control islets and in islets exposed to 50 U/mL IL-1 beta, respectively. Glucose-stimulated insulin secretion was significantly reduced (185 +/- 41) in IL-1 beta-exposed islets in comparison to control islets (2,037 +/- 363). In parallel, arginine-stimulated insulin release was inhibited by IL-1 beta exposure (166 +/- 31 pg/islet/h, mean +/- SE, n = 3) in comparison to control islets (1,679 +/- 307). In contrast, IL-1 beta exposure did not significantly reduce glyburide-induced insulin secretion (1,516 +/- 231 and 1,236 +/- 214 in control and IL-1 beta-exposed islets, respectively; mean +/- SE, n = 3). When islets were simultaneously exposed to IL-1 beta and increasing concentrations of nicotinamide, a dose-dependent recovery of glucose-induced insulin secretion was observed, with the maximum effect at 25 mmol/L nicotinamide (1,007 +/- 123, P less than .001).(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo administration of interleukin-1 inhibits glucose-stimulated insulin release

Recombinant interleukin-1 beta (IL-1 beta) was administered intraperitoneally for 3 days to normal C57BL/6ByJ (B6) mice. The islets from IL-1-treated and control animals were isolated and glucose-stimulated insulin secretion studied in the perifusion system. The total islet insulin content and the ultrastructure of the islets isolated from the animals treated with IL-1 did not differ from those seen in control animals. However, glucose-stimulated insulin release was significantly impaired after 3 days of in vivo administration of IL-1, either 3 micrograms/animal/day or 0.3 micrograms/animal/day. The administration of IL-1 inhibited an acute phase of glucose-induced insulin release, whereas neither basal insulin secretion nor insulin release from 10-30 min of perifusion with glucose was impaired. There was an only partial (27%) and non-significant restoration of the insulin secretory response to glucose stimulation 4 days after discontinuation of IL-1 treatment. We conclude that IL-1 administered in vivo is capable of adversely affecting pancreatic islet response to glucose stimulation. After 3 days of administration, these changes are confined to the process of insulin release, with the islet cell morphology and total insulin content being unaffected.     

Pathophysiology of insulin secretion

Defects in pancreatic islet beta-cell function play a major role in the development of diabetes mellitus. Type 1 diabetes is caused by a more or less rapid destruction of pancreatic beta cells, and the autoimmune process begins years before the beta-cell destruction becomes complete, thereby providing a window of opportunity for intervention. During the preclinical period and early after diagnosis, much of the insulin deficiency may be the result of functional inhibition of insulin secretion that may be at least partially and transiently reversible. Type 2 diabetes is characterized by a progressive loss of beta-cell function throughout the course of the disease. The pattern of loss is an initial (probably of genetic origin) defect in acute or first-phase insulin secretion, followed by a decreasing maximal capacity of insulin secretion. Last, a defective steady-state and basal insulin secretion develops, leading to almost complete beta-cell failure requiring insulin treatment. Because of the reciprocal relation between insulin secretion and insulin sensitivity, valid representation of beta-cell function requires interpretation of insulin responses in the context of the prevailing degree of insulin sensitivity. This appropriate approach highlights defects in insulin secretion at the various stages of the natural history of type 2 diabetes and already present in individuals at risk to develop the disease. To date none of the available therapies can stop the progressive beta-cell defect and the progression of the metabolic disorder. The better understanding of the pathophysiology of the disease should lead to the development of new strategies to preserve beta-cell function in both type 1 and type 2 diabetes mellitus.