Parallel reduction of pancreas insulin content and insulin secretion in 48-h tolbutamide-infused normoglycemic rats

Nitrite increases glucose-stimulated insulin secretion and islet insulin content in obese type 2 diabetic male rats

PURPOSE

Reduced bioavailability of nitric oxide (NO) is associated with pathogenesis of type 2 diabetes. Nitrite can act as a substrate for generation of systemic NO. The aim of this study was to examine the effects of nitrite administration on glucose-stimulated insulin secretion (GSIS) and islet insulin content in obese type 2 diabetic rats.

METHODS

Male rats were divided into 4 groups: Control, control + nitrite, diabetes, and diabetes + nitrite. Sodium nitrite (50 mg/L in drinking water) was administered for 8 weeks. Diabetes was induced using high-fat diet and low-dose of streptozotocine. Serum levels of fasting glucose, insulin, and lipid profile were measured and the insulin resistance/sensitivity indices were calculated every 2 weeks. Glycated hemoglobin (HbA_1C) was measured every month. At the end of the study, tissue levels of glucose transporter 4 (GLUT4) protein and serum interleukin-1 beta (IL-1β) were measured as well as glucose and insulin tolerance test were done. GSIS from isolated pancreatic islets and islet insulin content were also determined.

RESULTS

Nitrite administration significantly increased insulin secretion in both control and diabetic rats in presence of 16.7 mM glucose. Nitrite also significantly increased islet insulin content by 27% and 39% in both control and diabetic rats, respectively. Nitrite decreased elevated serum IL-1β in diabetic rats (4.0 ± 0.2 vs. 2.9 ± 0.2 pg/mL, P = 0.001). In diabetic rats, nitrite also significantly increased tissue levels of GLUT4 by 22% and 26% in soleus muscle and epididymal adipose tissue, respectively. In addition, nitrite significantly improved glucose and insulin tolerance, insulin sensitivity, lipid profile, and decreased fasting glucose and insulin, but had no effect on HbA_1C.

CONCLUSIONS

Long-term nitrite administration increased both insulin secretion and insulin content in obese type 2 diabetic rats. In addition, nitrite therapy had favorable effects on glucose tolerance, insulin resistance, inflammation, and dyslipidemia in type 2 diabetic rats.

Transgenic expression of the human growth hormone minigene promotes pancreatic β-cell proliferation

Transgenic mouse models are designed to study the role of specific proteins. To increase transgene expression the human growth hormone (hGH) minigene, including introns, has been included in many transgenic constructs. Until recently, it was thought that the hGH gene was not spliced, transcribed, and translated to produce functional hGH protein. We generated a transgenic mouse with the transcription factor Forkhead box M1 (FoxM1) followed by the hGH minigene, under control of the mouse insulin promoter (MIP) to target expression specifically in the pancreatic β-cell. Expression of FoxM1 in isolated pancreatic islets in vitro stimulates β-cell proliferation. We aimed to investigate the effect of FoxM1 on β-cell mass in a mouse model for diabetes mellitus. However, we found inadvertent coexpression of hGH protein from a spliced, bicistronic mRNA. MIP-FoxM1-hGH mice had lower blood glucose and higher pancreatic insulin content, due to increased β-cell proliferation. hGH signals through the murine prolactin receptor, and expression of its downstream targets tryptophan hydroxylase-1 (Tph1), tryptophan hydroxylase-2 (Tph2), and cytokine-inducible SH2 containing protein (Cish) was increased. Conversely, transcriptional targets of FoxM1 were not upregulated. Our data suggest that the phenotype of MIP-FoxM1-hGH mice is due primarily to hGH activity and that the FoxM1 protein remains largely inactive. Over the past decades, multiple transgenic mouse strains were generated that make use of the hGH minigene to increase transgene expression. Our work suggests that each will need to be carefully screened for inadvertent hGH production and critically evaluated for the use of proper controls.

Prenatal tolbutamide treatment alters plasma glucose and insulin concentrations and negatively affects the postnatal performance of chickens

To examine the relationship of insulin and glucose, broiler embryos were subjected to acute or prolonged hypoglycemia during the late embryonic phase by, respectively, injecting once (at embryonic day [ED] 16 or 17) or on 3 consecutive days (ED 16, 17, and 18) with tolbutamide (80 μg/g embryo weight), a substance that stimulates insulin secretion from the pancreas. After 1 tolbutamide injection, a prolonged (32 h) decrease of plasma glucose and a profound acute increase in plasma insulin were observed. The 3 consecutive tolbutamide injections induced hypoglycemia for 4 days (from ED 16 to ED 19). The postnatal performance after 3 consecutive tolbutamide injections in broiler embryos was also investigated. Body weight was lower in tolbutamide-treated chickens from hatch to 42 d compared with sham (P = 0.001) and control (P < 0.001) chickens. Feed intake was lower in the tolbutamide group from hatch to 42 d as compared with sham (P = 0.007) and control (P = 0.017) animals. In addition, at 42 d, plasma glucose concentrations, after an insulin injection challenge (50 μg/kg body weight), were higher in tolbutamide-treated chickens compared with the sham and the control group as were their basal glucose levels (P value of group effect <0.001). In conclusion, tolbutamide treatment during the late embryonic development in broilers resulted in prolonged hypoglycemia in this period and negatively influenced the posthatch performance.

Overexpression of glutathione peroxidase 4 prevents β-cell dysfunction induced by prolonged elevation of lipids in vivo

We have shown that oxidative stress is a mechanism of free fatty acid (FFA)-induced β-cell dysfunction. Unsaturated fatty acids in membranes, including plasma and mitochondrial membranes, are substrates for lipid peroxidation, and lipid peroxidation products are known to cause impaired insulin secretion. Therefore, we hypothesized that mice overexpressing glutathione peroxidase-4 (GPx4), an enzyme that specifically reduces lipid peroxides, are protected from fat-induced β-cell dysfunction. GPx4-overexpressing mice and their wild-type littermate controls were infused intravenously with saline or oleate for 48 h, after which reactive oxygen species (ROS) were imaged, using dihydrodichlorofluorescein diacetate in isolated islets, and β-cell function was assessed ex vivo in isolated islets and in vivo during hyperglycemic clamps. Forty-eight-hour FFA elevation in wild-type mice increased ROS and the lipid peroxidation product malondialdehyde and impaired β-cell function ex vivo in isolated islets and in vivo, as assessed by decreased disposition index. Also, islets of wild-type mice exposed to oleate for 48 h had increased ROS and lipid peroxides and decreased β-cell function. In contrast, GPx4-overexpressing mice showed no FFA-induced increase in ROS and lipid peroxidation and were protected from the FFA-induced impairment of β-cell function assessed in vitro, ex vivo and in vivo. These results implicate lipid peroxidation in FFA-induced β-cell dysfunction.

Antidiabetic effects of SGLT2-selective inhibitor ipragliflozin in streptozotocin-nicotinamide-induced mildly diabetic mice

Sodium-glucose cotransporter (SGLT) 2 plays an important role in renal glucose reabsorption, and inhibition of renal SGLT2 activity represents an innovative strategy for the treatment of hyperglycemia in diabetic patients. The present study investigated the antidiabetic effects of ipragliflozin, a SGLT2-selective inhibitor, in streptozotocin-nicotinamide-induced mildly diabetic mice, which exhibited a mild decline in glucose tolerance associated with the loss of early-phase insulin secretion. Oral administration of ipragliflozin increased urinary glucose excretion in a dose-dependent manner, an effect which was significant at doses of 0.3 mg/kg or higher and lasted over 12 h. In addition, ipragliflozin dose-dependently improved hyperglycemia and glucose intolerance with concomitant decreases in plasma insulin levels without causing hypoglycemia. Once-daily dosing of ipragliflozin (0.1 - 3 mg/kg) for 4 weeks attenuated hyperglycemia, glucose intolerance, and impaired insulin secretion. These results suggest that the SGLT2-selective inhibitor ipragliflozin increases urinary glucose excretion by inhibiting renal glucose reabsorption, improves hyperglycemia in streptozotocin-nicotinamide-induced mildly diabetic mice, and may be useful for treating type 2 diabetes.

Initial combination therapy for type 2 diabetes mellitus: is it ready for prime time?

The increased prevalence of type 2 diabetes mellitus is primarily being driven by the increasing global rates of overweight/obesity. Given the magnitude of this epidemic, we can expect these metabolic abnormalities to play an increasing role in the development of cardiovascular disease. In a pathophysiologic sense, type 2 diabetes is a multiorgan, multifactorial condition, characterized by β-cell dysfunction, insulin resistance in peripheral tissues and the liver, defective incretin activity, and elevated levels of free fatty acids and proinflammatory mediators. Despite the considerable burden of disease associated with type 2 diabetes, most patients are not at, or are unable to achieve, recommended glycemic control guideline targets. In part, this is because of the relentlessly progressive nature of the disease, but it may also be attributable to the current diabetes treatment paradigm, which is characterized by ineffective lifestyle interventions, followed by monotherapy and frequent early treatment failure with prolonged periods of elevated glucose as a consequence of clinical inertia. Thus, it is most appropriate to rethink the current treatment paradigm for type 2 diabetes in the context of a more aggressive initial therapy; specifically with early initiation of combination therapy. Our current understanding of the complex pathophysiology of the disease and the progressive deterioration in glycemic control over time supports the philosophy of earlier intervention with a more comprehensive initial therapy. Thus, while control of hyperglycemia remains the paramount goal, focusing on the underlying pathophysiology of type 2 diabetes is increasingly becoming the therapeutic strategy, with the aim of potentially providing disease modification. Although this is a logical approach, it remains to be demonstrated that early combination therapy will result in disease modification in a clinical setting. Not surprisingly, the incretin-based therapies have gained a great deal of attention in the context of being a component of initial combination therapy, given their potential beneficial effects on β-cell function with lowered risk of weight gain and hypoglycemia.

The dopamine receptor D2 agonist bromocriptine inhibits glucose-stimulated insulin secretion by direct activation of the alpha2-adrenergic receptors in beta cells

Treatment with the dopamine receptor D2 (DRD2) agonist bromocriptine improves metabolic features in obese patients with type 2 diabetes by a still unknown mechanism. In the present study, we investigated the acute effect of bromocriptine and its underlying mechanism(s) on insulin secretion both in vivo and in vitro. For this purpose, C57Bl6/J mice were subjected to an intraperitoneal glucose tolerance test (ipGTT) and a hyperglycemic (HG) clamp 60min after a single injection of bromocriptine or placebo. The effects of bromocriptine on glucose-stimulated insulin secretion (GSIS), cell membrane potential and intracellular cAMP levels were also determined in INS-1E beta cells. We report here that bromocriptine increased glucose levels during ipGTT in vivo, an effect associated with a dose-dependent decrease in GSIS. During the HG clamp, bromocriptine reduced both first-phase and second-phase insulin response. This inhibitory effect was also observed in INS-1E beta cells, in which therapeutic concentrations of bromocriptine (0.5-50nM) decreased GSIS. Mechanistically, neither cellular energy state nor cell membrane depolarization was affected by bromocriptine whereas intracellular cAMP levels were significantly reduced, suggesting involvement of G-protein-coupled receptors. Surprisingly, the DRD2 antagonist domperidone did not counteract the effect of bromocriptine on GSIS, whereas yohimbine, an antagonist of the alpha2-adrenergic receptors, completely abolished bromocriptine-induced inhibition of GSIS. In conclusion, acute administration of bromocriptine inhibits GSIS by a DRD2-independent mechanism involving direct activation of the pancreatic alpha2-adrenergic receptors. We suggest that treatment with bromocriptine promotes beta cells rest, thereby preventing long-lasting hypersecretion of insulin and subsequent beta cell failure.

Parathyroid cell resistance to fibroblast growth factor 23 in secondary hyperparathyroidism of chronic kidney disease

Although fibroblast growth factor 23 (FGF23) acting through its receptor Klotho-FGFR1c decreases parathyroid hormone expression, this hormone is increased in chronic kidney disease despite an elevated serum FGF23. We measured possible factors that might contribute to the resistance of parathyroid glands to FGF23 in rats with the dietary adenine-induced model of chronic kidney disease. Quantitative immunohistochemical and reverse transcription-PCR analysis using laser capture microscopy showed that both Klotho and FGFR1 protein and mRNA levels were decreased in histological sections of the parathyroid glands. Recombinant FGF23 failed to decrease serum parathyroid hormone levels or activate the mitogen-activated protein kinase signaling pathway in the glands of rats with advanced experimental chronic kidney disease. In parathyroid gland organ culture, the addition of FGF23 decreased parathyroid hormone secretion and mRNA levels in control animals or rats with early but not advanced chronic kidney disease. Our results show that because of a downregulation of the Klotho-FGFR1c receptor complex, an increase of circulating FGF23 does not decrease parathyroid hormone levels in established chronic kidney disease. This in vivo resistance is sustained in parathyroid organ culture in vitro.

Deletion of the vitamin D receptor specifically in the parathyroid demonstrates a limited role for the receptor in parathyroid physiology

1,25(OH)2D3 decreases parathyroid hormone (PTH) gene transcription through the vitamin D receptor (VDR). Total body VDR−/− mice have high PTH levels, hypocalcemia, hypophosphatemia, and bone malformations. To investigate PTH regulation by the VDR specifically in the parathyroid, we generated parathyroid-specific VDR knockout mice ( PT-VDR−/−). In both strains, there was a decrease in parathyroid calcium receptor (CaR) levels. The number of proliferating parathyroid cells was increased in the VDR−/− mice but not in the PT-VDR−/− mice. Serum PTH levels were moderately but significantly increased in the PT-VDR−/− mice with normal serum calcium levels. The sensitivity of the parathyroid glands of the PT-VDR−/− mice to calcium was intact as measured by serum PTH levels after changes in serum calcium. This indicates that the reduced CaR in the PT-VDR−/− mice enables a physiologic response to serum calcium. Serum C-terminal collagen crosslinks, a marker of bone resorption, were increased in the PT-VDR−/− mice with no change in the bone formation marker, serum osteocalcin, consistent with a resorptive effect due to the increased serum PTH levels in the PT-VDR−/− mice. Therefore, deletion of the VDR specifically in the parathyroid decreases parathyroid CaR expression and only moderately increases basal PTH levels, suggesting that the VDR has a limited role in parathyroid physiology.

Dynamics of insulin sensitivity, -cell function, and -cell mass during the development of diabetes in fa/fa rats

Both male Zucker Fatty (mZF) and lower-fat-fed female Zucker diabetic fatty (LF-fZDF) rats are obese but remain normoglycemic. Male ZDF (mZDF) and high-fat-fed female ZDF rats (HF-fZDF) are also obese but develop diabetes between 7 and 10 wk of age. Although these models have been well studied, the mechanisms governing the adaptations to obesity in the normoglycemic animals, and the failure of adaptation in the animals that develop diabetes, remain unclear. Here we use quantitative morphometry and our recently developed coupled beta-cell mass (beta(m)), insulin, and glucose model to elucidate the dynamics of insulin sensitivity (S(I)), beta-cell secretory capacity (beta(sc)), and beta(m) in these four animal models. Both groups that remained normoglycemic with increasing obesity (mZF, LF-fZDF) exhibited increased beta(m) and constant beta(sc) in response to a falling S(I). In rats that developed hyperglycemia (mZDF, HF-fZDF), there was a greater reduction in S(I) and slower expansion of beta(m), with constant beta(sc). beta(sc) decreased after glucose levels rose above 20 mM. Taken together, these data suggest that excessive insulin resistance and insufficient beta(m) adaptation play a primary role in the pathogenesis of diabetes.

beta-Cell adaptation to insulin resistance. Increased pyruvate carboxylase and malate-pyruvate shuttle activity in islets of nondiabetic Zucker fatty rats

The beta-cell biochemical mechanisms that account for the compensatory hyperfunction with insulin resistance (so-called beta-cell adaptation) are unknown. We investigated glucose metabolism in isolated islets from 10-12-week-old Zucker fatty (ZF) and Zucker lean (ZL) rats (results expressed per mg/islet of protein). ZF rats were obese, hyperlipidemic, and normoglycemic. They had a 3.8-fold increased beta-cell mass along with 3-10-fold increases in insulin secretion to various stimuli during pancreas perfusion despite insulin content per milligram of beta-cells being only one-third that of ZL rats. Islet glucose metabolism (utilization and oxidation) was 1.5-2-fold increased in the ZF islets despite pyruvate dehydrogenase activity being 30% lowered compared with the ZL islets. The reason was increased flux through pyruvate carboxylase (PC) and the malate-pyruvate and citrate-pyruvate shuttles based on the following observations (% ZL islets): increased V(max) of PC (160%), malate dehydrogenase (170%), and malic enzyme (275%); elevated concentrations of oxaloacetate (150%), malate (250%), citrate (140%), and pyruvate (250%); and 2-fold increased release of malate from isolated mitochondria. Inhibition of PC by 5 mm phenylacetic acid markedly lowered glucose-induced insulin secretion in ZF and ZL islets. Thus, our results suggest that PC and the pyruvate shuttles are increased in ZF islets, and this accounts for glucose mitochondrial metabolism being increased when pyruvate dehydrogenase activity is reduced. As the anaplerosis pathways are implicated in glucose-induced insulin secretion and the synthesis of glucose-derived lipid and amino acids, our results highlight the potential importance of PC and the anaplerosis pathways in the enhanced insulin secretion and beta-cell growth that characterize beta-cell adaptation to insulin resistance.

Desensitization of insulin secretion

Desensitization of insulin secretion describes a reversible state of decreased secretory responsiveness of the pancreatic beta-cell, induced by a prolonged exposure to a multitude of stimuli. These include the main physiological stimulator, glucose, but also other nutrients like free fatty acids and practically all pharmacological stimulators acting by depolarization and Ca2+ influx into the beta-cell. Desensitization of insulin secretion appears to be an important step in the manifestation of type 2 diabetes and in the secondary failure of oral antidiabetic treatment. In this commentary, the basic concepts and the controversial issues in the field will be outlined. With regard to glucose-induced desensitization, two fundamentally opposing concepts have emerged. The first is that desensitization is the consequence of functional changes in the beta-cell that impair glucose-recognition. The second is that long-term increased secretory activity leads to a depletion of releasable insulin, often in spite of increased insulin synthesis. The latter concept is more appropriately termed beta-cell exhaustion. The same dichotomy applies to the desensitization evoked by pharmacological stimuli: again the relative contributions of a decreased insulin content versus alterations in signal transduction are in dispute. The action of tolbutamide on beta-cells may be an example of desensitization caused by a lack of releasable insulin since the signaling mechanisms are nearly unchanged, whereas the action of phentolamine, an imidazoline, induces a strong desensitization without reducing insulin content or secretory granules, apparently by abolishing Ca2+ influx. With pharmacological agents it seems that both, alterations in signal transduction and decreased availability of releasable insulin, can contribute to the desensitized state of the beta-cell, the relative contribution being variable depending upon the exact nature of the secretory stimulus.

Expression profiling of pancreatic beta cells: glucose regulation of secretory and metabolic pathway genes

Pancreatic beta cells respond to changes in blood glucose by secreting insulin and increasing insulin synthesis. To identify genes used in these responses, we have carried out expression profiling of beta cells exposed to high (25 mM) or low (5.5 mM) glucose by using oligonucleotide microarrays. Functional clustering of genes that averaged a 2.2-fold or greater change revealed large groups of secretory pathway components, enzymes of intermediary metabolism, cell-signaling components, and transcription factors. Many secretory pathway genes were up-regulated in high glucose, including seven members of the endoplasmic reticulum (ER) translocon. In agreement with array analysis, protein levels of translocon components were increased by high glucose. Most dramatically, the alpha subunit of the signal recognition particle receptor was increased over 20-fold. These data indicate that the translocon and ribosome docking are major regulatory targets of glucose in the beta cell. Analysis of genes encoding enzymes of intermediary metabolism indicated that low glucose brought about greater utilization of amino acids as an energy source. This conclusion was supported by observations of increased urea production under low-glucose conditions. The above results demonstrate genome-wide integration of beta-cell functions at the level of transcript abundance and validate the efficacy of expression profiling in identifying genes involved in the beta-cell glucose response.

Addition of low-dose rosiglitazone to sulphonylurea therapy improves glycaemic control in Type 2 diabetic patients

AIMS

This study was designed to test the efficacy and safety of low-dose rosiglitazone, a potent, insulin-sensitizing thiazolidinedione, in combination with sulphonylurea in Type 2 diabetic patients.

METHODS

For the intention-to-treat analysis, 574 patients (59% male, mean age 61 years) were available, randomized to receive 26 weeks of twice-daily placebo (n = 192), rosiglitazone 1 mg (n = 199) or rosiglitazone 2 mg (n = 183) in addition to existing sulphonylurea treatment with gliclazide (47.6% of patients), glibenclamide (41.8%) or glipizide (9.4%) (two patients were taking carbutamide or glimepiride). Change in haemoglobin A1c (HbA1c), fasting plasma glucose (FPG), fructosamine, insulin, C-peptide, albumin, and lipids were measured, and safety was evaluated.

RESULTS

Mean baseline HbA1c was 9.2% and FPG was 11.4 mmol/l. Rosiglitazone at doses of 1 and 2 mg b.d. plus sulphonylurea produced significant decreases, compared with sulphonylurea plus placebo, in HbA1c (-0.59% and -1.03%, respectively; both P<0.0001) and FPG (1.35 mmol/l and 2.44 mmol/l, respectively; both P<0.0001). Both HDL-cholesterol and LDL-cholesterol increased and potentially beneficial decreases in non-esterified fatty acids and gamma glutamyl transpeptidase levels were seen in both rosiglitazone groups. The overall incidence of adverse experiences was similar in all three treatment groups, with no significant cardiac events, hypoglycaemia or hepatotoxicity.

CONCLUSIONS

Overall, the combination of rosiglitazone and a sulphonylurea was safe, well tolerated and effective in patients with Type 2 diabetes.

Alterations of insulin secretion from mouse islets treated with sulphonylureas: perturbations of Ca2+ regulation prevail over changes in insulin content

1. To determine how pretreatment with sulphonylureas alters the beta cell function, mouse islets were cultured (18 - 20 h) without (controls) or with (test) 0.01 microM glibenclamide. Acute responses to glucose were then determined in the absence of glibenclamide. 2. Test islets were insensitive to drugs (sulphonylureas and diazoxide) acting on K+-ATP channels, and their [Ca2+]i was already elevated in the absence of stimulation. 3. Insulin secretion was increased in the absence of glucose, and mainly stimulated between 0 - 10 instead of 7 - 20 mM glucose in controls. The maximum response was halved, but this difference disappeared after correction for the 45% decrease in the islet insulin content. 4. The first phase of glucose-induced insulin secretion was abrogated because of a paradoxical decrease of the high basal [Ca2+]i in beta cells. The second phase was preserved but occurred with little rise of [Ca2+]i. These abnormalities did not result from alterations of glucose metabolism (NADPH fluorescence). 5. In islets cultured with 50 microM tolbutamide, glucose induced biphasic increases in [Ca2+]i and insulin secretion. The decrease in the secretory response was matched by the decrease in insulin content (45%) except at maximal glucose concentrations. Islets pretreated with tolbutamide, however, behaved like those cultured with glibenclamide if tolbutamide was also present during the acute functional tests. 6. In conclusion, treatment with a low glibenclamide concentration causes long-lasting blockade of K+-ATP channels and rise of [Ca2+]i in beta cells. Glucose-induced insulin secretion occurs at lower concentrations, is delayed and is largely mediated by a modulation of Ca2+ action on exocytosis. It is suggested that glucose regulation of insulin secretion mainly depends on a K+-ATP channel-independent pathway during in vivo sulphonylurea treatment.

Impaired phasic insulin and amylin secretion in diabetic rats

We have proposed that a hyperstimulated insulin secretion causing beta-cell degranulation is the basis for the impaired glucose-potentiated insulin secretion in type 2 diabetes ("overworked beta-cell"). To confirm this idea, we previously investigated tolbutamide-infused euglycemic rats. Two novel kinds of beta-cell dysfunction were observed: altered phasic glucose-potentiated insulin secretion with preferential sparing of the first phase and a raised secreted ratio of amylin to insulin. The current study tested these parameters in 90% (intact beta-cell insulin stores) and 95% (markedly lowered insulin stores) pancreatectomized (Px) diabetic rats. Rats underwent pancreas perfusion 5-6 wk postsurgery. Controls showed nonchanging insulin secretion during a 20-min perfusion of 16.7 mM glucose + 10 mM arginine. In contrast, both Px groups showed an altered phasic pattern, with the first phase being supernormal (for the beta-cell mass) but the second phase reduced in tandem with the insulin content. Amylin secretion from control and 90% Px rats paralleled the insulin output, so that the amylin-to-insulin ratio averaged 0. 12 +/- 0.03% in the controls and 0.16 +/- 0.01% in the 90% Px rats over the two secretory phases. In contrast, the amylin-to-insulin ratio in 95% Px rats equaled that of controls during the first phase (0.12 +/- 0.1%) but was twice normal during the second phase (0.32 +/- 0.4%). These results confirm the validity of the overworked beta-cell schema by showing identical beta-cell functional defects in Px rats and tolbutamide-infused normoglycemic rats.