Dexamethasone treatment in vivo counteracts the functional pancreatic islet alterations caused by malnourishment in rats

Geriatric nutritional risk index is correlated with islet function but not insulin resistance in elderly patients with type 2 diabetes: A retrospective study

The geriatric nutritional risk index (GNRI) is a simple nutritional assessment tool that can predict poor prognosis in elderly subjects. The aim of this study was to evaluate the association between GNRI and both islet function and insulin sensitivity in patients with type 2 diabetes mellitus. This research carries significant implications for the integrated treatment and nutritional management of this patient population. A total of 173 patients with type 2 diabetes mellitus, aged 60 years or older, who were hospitalized in the Endocrinology Department at Hebei General Hospital from February 2018 to June 2021, were selected as the research subjects. These subjects were divided into 4 groups according to the quartile of their GNRI values: T1 (GNRI < 99.4, n = 43), T2 (99.4 ≤ GNRI < 103, n = 43), T3 (103 ≤ GNRI < 106.3, n = 43), and T4 (GNRI ≥ 106.3, n = 44). Glucose, insulin, and C-peptide concentrations were tested at 0, 30, 60, 120, and 180 minutes during a 75 g oral glucose tolerance test. The homeostasis model assessment for insulin resistance and the homeostasis model assessment for β cell function index were calculated. As the GNRI value increased, the levels of total protein, albumin, hemoglobin, alanine transaminase, aspartate aminotransferase, and 25-hydroxyvitamin D increased significantly. The area under the curve for blood glucose decreased significantly across the 4 groups, while the AUCs for insulin and C-peptide showed an overall increasing trend. β Cell function index increased significantly with the increase of GNRI; meanwhile, both the early-phase insulin secretion index and the late-phase insulin secretion index increased significantly. Although there was an increasing trend, homeostasis model assessment for insulin resistance did not change significantly among the 4 groups. This study indicates that elderly type 2 diabetes patients with higher nutritional risk have worse islet function, while insulin sensitivity is not associated with nutritional risk.

Protein malnutrition in BALB/C mice: A model mimicking clinical scenario of marasmic-kwashiorkor malnutrition

Protein malnutrition continues to be a major global issue. A stable animal model to address protein malnutrition and its effect on various disease conditions is necessary. In the present study, we have formulated and standardized a low protein diet (LPD) to develop a protein malnutrition model using Balb/C mice. Healthy male Balb/C mice were weaned and exposed to LPD combinations while another group exposed to normal diet (18% protein). Animal survival, change in body weight, body mass index (BMI), biochemical parameters, antioxidant status, and liver histopathology were used to confirm the development of malnourished mice model (marasmic-kwashiorkor). Mice receiving 10% protein diet showed moderate weight gain, higher BMI, and no mortality compared to the 6% protein group. The former group showed remarkable differences in BMI, biochemical and antioxidant parameters. Further, histopathological changes against the normal group at weeks 20 and 30 confirmed the development of protein malnutrition in mice on 10% protein diet. The study confirms the development of a stable, economical, reproducible, and clinically relevant protein malnutrition model using the formulated 10% protein diet. Further, the model can be used for short and long-term studies to investigate the pathophysiology of malnutrition in any disease/condition.

Pep19 Has a Positive Effect on Insulin Sensitivity and Ameliorates Both Hepatic and Adipose Tissue Phenotype of Diet-Induced Obese Mice

Peptide DIIADDEPLT (Pep19) has been previously suggested to improve metabolic parameters, without adverse central nervous system effects, in a murine model of diet-induced obesity. Here, we aimed to further evaluate whether Pep19 oral administration has anti-obesogenic effects, in a well-established high-fat diet-induced obesity model. Male Swiss mice, fed either a standard diet (SD) or high-fat diet (HFD), were orally administrated for 30 consecutive days, once a day, with saline vehicle or Pep19 (1 mg/kg). Next, several metabolic, morphological, and behavioral parameters were evaluated. Oral administration of Pep19 attenuated HFD body-weight gain, reduced in approximately 40% the absolute mass of the endocrine pancreas, and improved the relationship between circulating insulin and peripheral insulin sensitivity. Pep19 treatment of HFD-fed mice attenuated liver inflammation, hepatic fat distribution and accumulation, and lowered plasma alanine aminotransferase activity. The inguinal fat depot from the SD group treated with Pep19 showed multilocular brown-fat-like cells and increased mRNA expression of uncoupling protein 1 (UCP1), suggesting browning on inguinal white adipose cells. Morphological analysis of brown adipose tissue (BAT) from HFD mice showed the presence of larger white-like unilocular cells, compared to BAT from SD, Pep19-treated SD or HFD mice. Pep19 treatment produced no alterations in mice behavior. Oral administration of Pep19 ameliorates some metabolic traits altered by diet-induced obesity in a Swiss mice model.

Maternal vitamin D administration attenuates metabolic disturbances induced by prenatal exposure to dexamethasone in a sex-dependent manner

PURPOSE

The overexposure to synthetic glucocorticoids (GC) during pregnancy can predispose to metabolic diseases during adulthood. Vitamin D is not only crucial for fetal development, but also exerts direct effects on the GC sensitivity and down-regulates GC receptors. Given the vitamin D effects on glucocorticoid-related parameters, we aimed to investigate a possible protective role of maternal vitamin D administration on the glucose homeostasis of rats exposed to dexamethasone in utero.

METHODS

Pregnant rats received dexamethasone (0.1 mg/kg, Dex) daily between the 14th and 19th days of pregnancy. A subgroup of dexamethasone-treated dams received oral administration of vitamin D (500UI, DexVD) during the whole gestation. The corresponding control groups of dams were included (CTL and VD groups, respectively). Male and female offspring were evaluated at 3, 6 and 12 months of age.

RESULTS

Prenatal exposure to dexamethasone caused metabolic disruption in an age and sex-dependent manner being the older male offspring more susceptible to insulin resistance, fatty liver and beta-cell mass expansion than females. Furthermore, we demonstrated that prenatal GC led to glucose intolerance in male and female offspring in an age-dependent manner. Maternal vitamin D administration did not influence glucose intolerance but attenuated the insulin resistance, liver lipid accumulation and prevented the beta-cell mass expansion caused by prenatal dexamethasone in the male offspring.

CONCLUSION

Maternal vitamin D administration mitigates metabolic disturbances that occur later in life in male rats exposed to GC in utero. Moreover, our data suggest vitamin D as an important nutritional supplement for pregnant overexposed to GC during gestation.

Impact of combined long-term fructose and prednisolone intake on glucose and lipid homeostasis in rats: benefits of intake interruption or fish oil administration

We investigated whether combined long-term fructose and prednisolone intake would be more detrimental to the glucose homeostasis than if ingested separately. We also evaluated whether fish oil administration or interruption of treatments has any positive impact. For this, male adult Wistar rats ingested fructose (20%) (F) or prednisolone (12.5 µg/mL) (P) or both (FP) through drinking water for 12 weeks. A separate group of fructose and prednisolone-treated rats received fish oil treatment (1 g/kg) in the last 6 weeks. In another group, the treatment with fructose and prednisolone was interrupted after 12 weeks, and the animals were followed for more 12 weeks. Control groups ran in parallel (C). The F group had higher plasma TG (+42%) and visceral adiposity (+63%), whereas the P group had lower insulin sensitivity (-33%) and higher insulinemia (+200%). Only the the FP group developed these alterations combined with higher circulating uric acid (+126%), hepatic triacylglycerol content (+16.2-fold), lipid peroxidation (+173%) and lower catalase activity (-32%) that were associated with lower protein kinase B content and AMP-activated protein kinase (AMPK) phosphorylation in the liver, lower AMPK phosphorylation in the adipose tissue and higher beta-cell mass. Fish oil ingestion attenuated the elevation in circulating triacylglycerol and uric acid values, while the interruption of sugar and glucocorticoid intake reverted almost all modified parameters. In conclusion, long-term intake of fructose and prednisolone by male rats are more detrimental to glucose and lipid homeostasis than if ingested separately and the benefits of treatment interruption are broader than fish oil treatment.

The Atypical Cannabinoid Abn-CBD Reduces Inflammation and Protects Liver, Pancreas, and Adipose Tissue in a Mouse Model of Prediabetes and Non-alcoholic Fatty Liver Disease

Background and Aims: The synthetic atypical cannabinoid Abn-CBD, a cannabidiol (CBD) derivative, has been recently shown to modulate the immune system in different organs, but its impact in obesity-related meta-inflammation remains unstudied. We investigated the effects of Abn-CBD on metabolic and inflammatory parameters utilizing a diet-induced obese (DIO) mouse model of prediabetes and non-alcoholic fatty liver disease (NAFLD). Materials and Methods: Ten-week-old C57Bl/6J mice were fed a high-fat diet for 15 weeks, following a 2-week treatment of daily intraperitoneal injections with Abn-CBD or vehicle. At week 15 mice were obese, prediabetic and developed NAFLD. Body weight and glucose homeostasis were monitored. Mice were euthanized and blood, liver, adipose tissue and pancreas were collected and processed for metabolic and inflammatory analysis. Results: Body weight and triglycerides profiles in blood and liver were comparable between vehicle- and Abn-CBD-treated DIO mice. However, treatment with Abn-CBD reduced hyperinsulinemia and markers of systemic low-grade inflammation in plasma and fat, also promoting white adipose tissue browning. Pancreatic islets from Abn-CBD-treated mice showed lower apoptosis, inflammation and oxidative stress than vehicle-treated DIO mice, and beta cell proliferation was induced. Furthermore, Abn-CBD lowered hepatic fibrosis, inflammation and macrophage infiltration in the liver when compared to vehicle-treated DIO mice. Importantly, the balance between hepatocyte proliferation and apoptosis was improved in Abn-CBD-treated compared to vehicle-treated DIO mice. Conclusions: These results suggest that Abn-CBD exerts beneficial immunomodulatory actions in the liver, pancreas and adipose tissue of DIO prediabetic mice with NAFLD, thus protecting tissues. Therefore, Abn-CBD and related compounds could represent novel pharmacological strategies for managing obesity-related metabolic disorders.

Glucose Homeostasis Is Not Affected in a Murine Model of Parkinson's Disease Induced by 6-OHDA

There is a mutual relationship between metabolic and neurodegenerative diseases. However, the causal relationship in this crosstalk is unclear and whether Parkinson’s disease (PD) causes a posterior impact on metabolism remains unknown. Considering that, this study aimed to evaluate the appearance of possible changes in metabolic homeostasis due to 6-hydroxydopamine (6-OHDA) administration, a neurotoxin that damage dopaminergic neurons leading to motor impairments that resemble the ones observed in PD. For this, male Wistar rats received bilateral 6-OHDA administration in the dorsolateral striatum, and the motor and metabolic outcomes were assessed at 7, 21, or 35 days post-surgical procedure. Dexamethasone, a diabetogenic glucocorticoid (GC), was intraperitoneally administered in the last 6 days to challenge the metabolism and reveal possible metabolic vulnerabilities caused by 6-OHDA. Controls received only vehicles. The 6-OHDA-treated rats displayed a significant decrease in locomotor activity, exploratory behavior, and motor coordination 7 and 35 days after neurotoxin administration. These motor impairments paralleled with no significant alteration in body mass, food intake, glucose tolerance, insulin sensitivity, and biochemical parameters (plasma insulin, triacylglycerol, and total cholesterol levels) until the end of the experimental protocol on days 35–38 post-6-OHDA administration. Moreover, hepatic glycogen and fat content, as well as the endocrine pancreas mass, were not altered in rats treated with 6-OHDA at the day of euthanasia (38th day after neurotoxin administration). None of the diabetogenic effects caused by dexamethasone were exacerbated in rats previously treated with 6-OHDA. Thus, we conclude that bilateral 6-OHDA administration in the striatum causes motor deficits in rats with no impact on glucose and lipid homeostasis and does not exacerbate the adverse effects caused by excess GC. These observations indicate that neurodegeneration of dopaminergic circuits in the 6-OHDA rats does not affect the metabolic outcomes.

Prevention of Elevation in Plasma Triacylglycerol with High-Dose Bezafibrate Treatment Abolishes Insulin Resistance and Attenuates Glucose Intolerance Induced by Short-Term Treatment with Dexamethasone in Rats

OBJECTIVE

Fibrates are used as lipid-lowering drugs and are well tolerated as cotreatments when glucose metabolism disturbances are also present. Synthetic glucocorticoids (GCs) are diabetogenic drugs that cause dyslipidemia, dysglycemia, glucose intolerance, and insulin resistance when in excess. Thus, we aimed to describe the potential of bezafibrate in preventing or attenuating the adverse effects of GCs on glucose and lipid homeostasis.

METHODS

Male Wistar rats were treated with high-dose bezafibrate (300 mg/kg, body mass (b.m.)) daily for 28 consecutive days. In the last five days, the rats were also treated with dexamethasone (1 mg/kg, b.m.).

RESULTS

Dexamethasone treatment reduced the body mass gain and food intake, and bezafibrate treatment exerted no impact on these parameters. GC treatment caused an augmentation in fasting and fed glycemia, plasma triacylglycerol and nonesterified fatty acids, and insulinemia, and bezafibrate treatment completely prevented the elevation in plasma triacylglycerol and attenuated all other parameters. Insulin resistance and glucose intolerance induced by GC treatment were abolished and attenuated, respectively, by bezafibrate treatment.

CONCLUSION

High-dose bezafibrate treatment prevents the increase in plasma triacylglycerol and the development of insulin resistance and attenuates glucose intolerance in rats caused by GC treatment, indicating the involvement of dyslipidemia in the GC-induced insulin resistance.

Glucose homeostasis in two degrees of sepsis lethality induced by caecum ligation and puncture in mice

Sepsis is associated with high mortality. Both critically ill humans and animal models of sepsis exhibit changes in their glucose homeostasis, that is, hypoglycaemia, with the progression of infection. However, the relationship between basal glycaemia, glucose tolerance and insulin sensitivity is not well understood. Thus, we aimed to evaluate this glucose homeostasis triad at the late stage of sepsis (24 h after surgery) in male Swiss mice subjected to lethal and sublethal sepsis by the caecal ligation and puncture (CLP) model. The percentage of survival 24 h after CLP procedure in the Lethal and Sublethal groups was around 66% and 100% respectively. Both Lethal and Sublethal groups became hypoglycaemic in fasting and fed states 24 h after surgery. The pronounced fed hypoglycaemia in the Lethal group was not due to worsening anorexic behaviour or hepatic inability to deliver glucose in relation to the Sublethal group. Reduction in insulin sensitivity in CLP mice occurred in a lethality-dependent manner and was not associated with glucose intolerance. Analysis of oral and intraperitoneal glucose tolerance tests, as well as the gastrointestinal motility data, indicated that CLP mice had reduced intestinal glucose absorption. Altogether, we suggest cessation of appetite and intestinal glucose malabsorption are key contributors to the hypoglycaemic state observed during experimental severe sepsis.

Impact of Fish Oil Supplementation and Interruption of Fructose Ingestion on Glucose and Lipid Homeostasis of Rats Drinking Different Concentrations of Fructose

Background. Continuous fructose consumption may cause elevation of circulating triacylglycerol. However, how much of this alteration is reverted after the removal of fructose intake is not known. We explored this question and compared the efficacy of this approach with fish oil supplementation. Methods. Male Wistar rats were divided into the following groups: control (C), fructose (F) (water intake with 10% or 30% fructose for 9 weeks), fish oil (FO), and fructose/fish oil (FFO). Fish oil was supplemented only for the last 33 days of fructose ingestion. Half of the F group remained for additional 8 weeks without fructose ingestion (FR). Results. Fructose ingestion reduced food intake to compensate for the increased energy obtained through water ingestion, independent of fructose concentration. Fish oil supplementation exerted no impact on these parameters, but the removal of fructose from water recovered both ingestion behaviors. Plasma triacylglycerol augmented significantly during the second and third weeks (both fructose groups). Fish oil supplementation did not attenuate the elevation in triacylglycerol caused by fructose intake, but the interruption of sugar consumption normalized this parameter. Conclusion. Elevation in triacylglyceridemia may be recovered by removing fructose from diet, suggesting that it is never too late to repair improper dietary habits.

Glucose homeostasis in rats treated with 4-vinylcyclohexene diepoxide is not worsened by dexamethasone treatment

4-vinilcyclohexene diepoxide (4-VCD) causes premature ovarian failure and may result in estrogen deficiency, characterizing the transition to estropause in rodents (equivalent to menopause in women). Estropause/menopause is associated with metabolic derangements such as glucose intolerance and insulin resistance. Glucocorticoids (GCs) are known to exert diabetogenic effects. Thus, we aimed to investigate whether rats with premature ovarian failure are more prone to the diabetogenic effects of GC. For this, immature female rats received daily injections of 4-VCD [160mg/kg body weight (b.w.), intraperitoneally (i.p.)] for 15 consecutive days, whereas control rats received vehicle. After 168days of the completion of 4-VCD administration, rats were divided into 4 groups: CTL-received daily injections of saline (1mL/kg, b.w., i.p.) for 5days; DEX-received daily injections of dexamethasone (1mg/kg, b.w., i.p.) for 5days; VCD-treated as CTL group; VCD+DEX-treated as DEX group. Experiments and euthanasia occurred one day after the last dexamethasone injection. 4-VCD-treated rats exhibited ovary hypotrophy and reduced number of preantral follicles (p<0.05). Premature ovarian failure had no impact on the body weight gain or food intake, but both were reduced by the effects of dexamethasone. The increase in blood glucose, plasma insulin and triacylglycerol levels as well as the reduction in insulin sensitivity caused by dexamethasone treatment was not exacerbated in the VCD+DEX group of rats. Premature ovarian failure did change neither the hepatic content of glycogen and triacylglycerol nor the glycerol release from perigonadal adipose tissue. Glucose intolerance was observed in the VCD group after an ipGTT (p<0.05), but not after an oral glucose challenge. Glucose intolerance and compensatory pancreatic β-cell mass caused by GC were not modified by ovarian failure in the VCD+DEX group. We conclude that reduced ovarian function has no major implications on the diabetogenic effects promoted by GC treatment, indicating that other factors related to aging may make rats more vulnerable to GC side effects on glucose metabolism.

Age- and gender-related changes in glucose homeostasis in glucocorticoid-treated rats

The disruption to glucose homeostasis upon glucocorticoid (GC) treatment in adult male rats has not been fully characterized in older rats or in females. Thus, we evaluated the age- and gender-related changes in glucose homeostasis in GC-treated rats. We injected male and female rats at 3 months and 12 months of age with either dexamethasone (1.0 mg/kg body mass, intraperitoneally) or saline, daily for 5 days. All of the GC-treated rats had decreased body mass and food intake, and adrenal hypotrophy. Increased glycemia was observed in all of the GC-treated groups and only the 3-month-old female rats were not glucose intolerant. Dexamethasone treatment resulted in hyperinsulinemia and hypertriacylglyceridemia in all of the GC-treated rats. The glucose-stimulated insulin secretion (GSIS) was higher in all of the dexamethasone-treated animals, but it was less pronounced in the older animals. The β-cell mass was increased in the younger male rats treated with dexamethasone. We conclude that dexamethasone treatment induces glucose intolerance in both the 3- and 12-month-old male rats as well as hyperinsulinemia and augmented GSIS. Three-month-old female rats are protected from glucose intolerance caused by GC, whereas 12-month-old female rats developed the same complications that were present in 3- and 12-month-old male rats.

Long-term dexamethasone treatment alters the histomorphology of acinar cells in rat parotid and submandibular glands

Glucocorticoids (GCs) induce insulin resistance (IR), a condition known to alter oral homeostasis. This study investigated the effects of long-term dexamethasone administration on morphofunctional aspects of salivary glands. Male Wistar rats received daily injections of dexamethasone [0.1 mg/kg body weight (b.w.), intraperitoneally] for 10 days (DEX), whereas control rats received saline. Subsequently, glycaemia, insulinaemia, insulin secretion and salivary flow were analysed. The parotid and submandibular glands were collected for histomorphometric evaluation and Western blot experiments. The DEX rats were found to be normoglycaemic, hyperinsulinaemic, insulin resistant and glucose intolerant (P < 0.05). DEX rat islets secreted more insulin in response to glucose (P < 0.05). DEX rats had significant reductions in the masses of the parotid (29%) and submandibular (16%) glands (P < 0.05) that was associated with reduced salivary flux rate. The hypotrophy in both glands observed in the DEX group was associated with marked reduction in the volume of the acinar cells in these glands of 50% and 26% respectively (P < 0.05). The total number of acinar cells was increased in the submandibular glands of the DEX rats (P < 0.05) but not in the parotid glands. The levels of proteins related to insulin and survival signalling in both glands did not differ between the groups. In conclusion, the long-term administration of dexamethasone caused IR, which was associated with significant reductions in both mass and flux rate of the salivary glands. The parotid and submandibular glands exhibited reduced acinar cell volume; however, the submandibular glands displayed acinar hyperplasia, indicating a gland-specific response to GCs. Our data emphasize that GC-based therapies and insulin-resistant states have a negative impact on salivary gland homeostasis.

Glucose intolerance induced by glucocorticoid excess is further impaired by co-administration with β-hydroxy-β-methylbutyrate in rats

Glucocorticoid (GC) excess alters glucose homeostasis and promotes modifications in murinometric and anthropometric parameters in rodents and humans, respectively. β-hydroxy-β-methylbutyrate (HMB), a leucine metabolite, has been proposed as a nutritional strategy for preventing muscle wasting, but few data regarding its effects on glucose homeostasis are available. Here, we analyzed whether the effects of GC excess on glucose homeostasis may be attenuated or exacerbated by the concomitant ingestion of HMB. Adult Wistar rats (90-days-old) were assigned to four groups: (1) vehicle treated (Ctl), (2) dexamethasone (DEX) treated (Dex), (3) HMB treated (Hmb), and (4) DEX plus HMB treated (DexHmb). Dex groups received DEX (1 mg·kg body weight (BW)−1, intraperitoneal) for 5 consecutive days. HMB groups ingested HMB (320 mg·kg BW−1, oral gavage) for the same 5 days. HMB ingestion did not attenuate the effects of DEX on food intake and body weight loss, changes in masses of several organs, insulin resistance, and glucose intolerance (p > 0.05). In fact, in DexHmb rats, there was increased fasting glycemia and exacerbated glucose intolerance with the main effect attributed to DEX treatment (p < 0.05). HMB exerted no attenuating effect on plasma triacylglycerol levels from DexHmb rats, but it seems to attenuate the lipolysis induced by β-adrenergic stimulation (20 μmol·L−1isoproterenol) in fragments of retroperitoneal adipose tissue from DexHmb rats. Therefore, HMB does not attenuate the diabetogenic characteristics of GC excess. In fact, the data suggest that HMB may exacerbate GC-induced glucose intolerance.

Glucose homoeostasis in rats exposed to acute intermittent hypoxia

AIM

Chronic exposure to intermittent hypoxia commonly induces the activation of sympathetic tonus and the disruption of glucose homoeostasis. However, the effects of exposure to acute intermittent hypoxia (AIH) on glucose homoeostasis are not yet fully elucidated. Herein, we evaluated parameters related to glucose metabolism in rats exposed to AIH.

METHODS

Male adult rats were submitted to 10 episodes of hypoxia (6% O2 , for 45 s) interspersed with 5-min intervals of normoxia (21%), while the control (CTL) group was kept in normoxia.

RESULTS

Acute intermittent hypoxia rats presented higher fasting glycaemia, normal insulinaemia, increased lactataemia and similar serum lipid levels, compared to controls (n = 10, P < 0.05). Additionally, AIH rats exhibited increased glucose tolerance (GT) (n = 10, P < 0.05) and augmented insulin sensitivity (IS) (n = 10, P < 0.05). The p-Akt/Akt protein ratio was increased in the muscle, but not in the liver and adipose tissue of AIH rats (n = 6, P < 0.05). The elevated glycaemia in AIH rats was associated with a reduction in the hepatic glycogen content (n = 10, P < 0.05). Moreover, the AIH-induced increase in blood glucose concentration, as well as reduced hepatic glycogen content, was prevented by prior systemic administration of the β-adrenergic antagonist (P < 0.05). The effects of AIH on glycaemia and Akt phosphorylation were transient and not observed after 60 min.

CONCLUSIONS

We suggest that AIH induces an increase in blood glucose concentration as a result of hepatic glycogenolysis recruitment through sympathetic activation. The augmentation of GT and IS might be attributed, at least in part, to increased β-adrenergic sympathetic stimulation and Akt protein activation in skeletal muscles, leading to a higher glucose availability and utilization.

Leucine supplementation augments insulin secretion in pancreatic islets of malnourished mice

OBJECTIVES

We investigated the influence of leucine supplementation on insulin secretion and on some proteins related to insulin secretion in malnourished mice.

METHODS

Swiss mice (aged 21 days) received isocaloric normo-17% (NP) or 6% low-protein (LP) diet for 120 days. Half of the NP and LP mice received 1.5% leucine in the drinking water during the last 30 days (NPL and LPL, respectively).

RESULTS

The LP mice were hypoinsulinemic compared with the NP group, whereas LPL mice exhibited increased insulinemia in the fed state versus LP mice. The LP mouse islets were less responsive to 22.2 mM glucose, 100 microM carbachol (Cch), and 10 mM leucine than the NP group. However, LPL islets were more responsive to all these conditions compared with the LP group. The muscarinic type 3 receptor, (M3R) Cabeta2, and PKC-alpha protein contents were reduced in LP compared with NP islets but significantly higher in LPL than LP islets. The p-AKT/AKT ratio was higher in LPL compared with LP islets.

CONCLUSIONS

Leucine supplementation increases insulin secretion in response to glucose and leucine and to agents that potentiate secretion, such as Cch, in malnourished mice. The enhanced levels of M3R, Cabeta2, and PKC-alpha proteins, as well as of the p-AKT/AKT ratio, may play a role in this process.

Reduced insulin secretion in protein malnourished mice is associated with multiple changes in the beta-cell stimulus-secretion coupling

The mechanism by which protein malnutrition impairs glucose-stimulated insulin secretion in the pancreatic beta-cell is not completely known but may be related to alterations in the signaling events involved in insulin release. Here, we aimed to study the stimulus-secretion coupling of beta-cells from mice fed with low-protein (LP) diet or normal-protein (NP) diet for 8 wk after weaning. Patch-clamp measurements in isolated cells showed that beta-cells from LP mice had a resting membrane potential that was more hyperpolarized than controls. Additionally, depolarization and generation of action potentials in response to stimulatory glucose concentrations were also impaired in beta-cells of LP mice. All these alterations in the LP group were most likely attributed to higher ATP-dependent K(+) (K(ATP)) channel activity in resting conditions and lower efficiency of glucose to induce the closure of these channels. Moreover, a Western blot analysis revealed higher protein levels of the sulphonylurea receptor of the K(ATP) channel in islets of LP mice. Because beta-cell Ca(2+) signals depend on electrical activity, intracellular Ca(2+) oscillations were measured by fluorescence microscopy in intact islets, indicating a lower response to glucose in the LP group. Finally, cell-to-cell synchrony of Ca(2+) signals was analyzed by confocal microscopy. Islets from LP mice exhibited a decreased level of coupling among beta-cells, which was probably due to the low expression levels of connexin 36. Therefore, low-protein diet leads to several alterations in the stimulus-secretion coupling of pancreatic beta-cells that might explain the diminished insulin secretion in response to glucose in this malnutrition state.

Augmentation of insulin secretion by leucine supplementation in malnourished rats: possible involvement of the phosphatidylinositol 3-phosphate kinase/mammalian target protein of rapamycin pathway

A regimen of low-protein diet induces a reduction of pancreatic islet function that is associated with development of metabolic disorders including diabetes and obesity afterward. In the present study, the influence of leucine supplementation on metabolic parameters, insulin secretion to glucose and to amino acids, as well as the levels of proteins that participate in the phosphatidylinositol 3-phosphate kinase (PI3K) pathway was investigated in malnourished rats. Four groups were fed with different diets for 12 weeks: a normal protein diet (17%) without (NP) or with leucine supplementation (NPL) or a low (6%)-protein diet without (LP) or with leucine supplementation (LPL). Leucine was given in the drinking water during the last 4 weeks. As indicated by the intraperitoneal glucose tolerance test, LPL rats exhibited increased glucose tolerance as compared with NPL group. Both NPL and LPL rats had higher circulating insulin levels than controls. The LPL rats also showed increased insulin secretion by pancreatic islets in response to glucose or arginine compared with those observed in islets from LP animals. Glucose oxidation was significantly reduced in NPL, LP, and LPL isolated islets as compared with NP; but no alteration was observed for leucine and glutamate oxidation among the 4 groups. Western blotting analysis demonstrated increased PI3K and mammalian target protein of rapamycin protein contents in LPL compared with LP islets. A significant increase in insulin-induced insulin receptor substrate 1-associated PI3K activation was also observed in LPL compared with LP islets. These findings indicate that leucine supplementation can augment islet function in malnourished rats and that activation of the PI3K/mammalian target protein of rapamycin pathway may play a role in this process.