Low protein diet impairs glucose-induced insulin secretion from and 45Ca uptake by pancreatic rat islets

A low-protein maternal diet during gestation affects the expression of key pancreatic β-cell genes and the methylation status of the regulatory region of the MafA gene in the offspring of Wistar rats

Maternal nutrition during gestation has important effects on gene expression-mediated metabolic programming in offspring. To evaluate the effect of a protein-restricted maternal diet during gestation, pancreatic islets from male progeny of Wistar rats were studied at postnatal days (PND) 36 (juveniles) and 90 (young adults). The expression of key genes involved in β-cell function and the DNA methylation pattern of the regulatory regions of two such genes, Pdx1 (pancreatic and duodenal homeobox 1) and MafA (musculoaponeurotic fibrosarcoma oncogene family, protein A), were investigated. Gene expression analysis in the pancreatic islets of restricted offspring showed significant differences compared with the control group at PND 36 (P < 0.05). The insulin 1 and 2 (Ins1 and Ins2), Glut2 (glucose transporter 2), Pdx1, MafA, and Atf2 (activating transcription factor 2), genes were upregulated, while glucokinase (Gck) and NeuroD1 (neuronal differentiation 1) were downregulated. Additionally, we studied whether the gene expression differences in Pdx1 and MafA between control and restricted offspring were associated with differential DNA methylation status in their regulatory regions. A decrease in the DNA methylation levels was found in the 5' flanking region between nucleotides −8118 to −7750 of the MafA regulatory region in restricted offspring compared with control pancreatic islets. In conclusion, low protein availability during gestation causes the upregulation of MafA gene expression in pancreatic β-cells in the male juvenile offspring at least in part through DNA hypomethylation. This process may contribute to developmental dysregulation of β-cell function and influence the long-term health of the offspring.

Branched-chain amino acid supplementation restores reduced insulinotropic activity of a low-protein diet through the vagus nerve in rats

Background

Previously, we reported that a low-protein diet significantly reduced insulin secretion in response to feeding within 1 h in rats, suggesting that the insulinotropic effect of dietary protein plays an important role in maintaining normal insulin release. The current study aimed to elucidate whether deficiency of certain amino acids could diminish the insulinotropic activity and to investigate whether reduced insulin secretion in response to a low-protein diet is restored by supplementation with certain amino acids.

Methods

First, we fed male Wistar rats (5–6 rats per group) with diets deficient in every single amino acid or three branched-chain amino acids (BCAAs); within 1–2 h after the onset of feeding, we measured the plasma insulin levels by using an enzyme-linked immunosorbent assay (ELISA). As insulin secretion was reduced in BCAA-deficient groups, we fed low-protein diets supplemented with BCAAs to assess whether the reduced insulin secretion was restored. In addition, we treated the pancreatic beta cell line MIN6 with BCAAs to investigate the direct insulinotropic activity on beta cells. Lastly, we investigated the effect of the three BCAAs on sham-operated or vagotomized rats to assess involvement of the vagus nerve in restoration of the insulinotropic activity.

Results

Feeding a low-protein diet reduced essential amino acid concentrations in the plasma during an absorptive state, suggesting that reduced plasma amino acid levels can be an initial signal of protein deficiency. In normal rats, insulin secretion was reduced when leucine, valine, or three BCAAs were deficient. Insulin secretion was restored to normal levels by supplementation of the low-protein diet with three BCAAs, but not by supplementation with any single BCAA. In MIN6 cells, each BCAA alone stimulated insulin secretion but the three BCAAs did not show a synergistic stimulatory effect. The three BCAAs showed a synergistic stimulatory effect in sham-operated rats but failed to stimulate insulin secretion in vagotomized rats.

Conclusions

Leucine and valine play a role in maintaining normal insulin release by directly stimulating beta cells, and supplementation with the three BCAAs is sufficient to compensate for the reduced insulinotropic activity of the low-protein diet, through the vagus nerve.

Reduced glucose-induced insulin secretion in low-protein-fed rats is associated with altered pancreatic islets redox status

In the present study, we investigated the relationship between early life protein malnutrition-induced redox imbalance, and reduced glucose-stimulated insulin secretion. After weaning, male Wistar rats were submitted to a normal-protein-diet (17%-protein, NP) or to a low-protein-diet (6%-protein, LP) for 60 days. Pancreatic islets were isolated and hydrogen peroxide (H₂ O₂ ), oxidized (GSSG) and reduced (GSH) glutathione content, CuZn-superoxide dismutase (SOD1), glutathione peroxidase (GPx1) and catalase (CAT) gene expression, as well as enzymatic antioxidant activities were quantified. Islets that were pre-incubated with H₂ O₂ and/or N-acetylcysteine, were subsequently incubated with glucose for insulin secretion measurement. Protein malnutrition increased CAT mRNA content by 100%. LP group SOD1 and CAT activities were 50% increased and reduced, respectively. H₂ O₂ production was more than 50% increased whereas GSH/GSSG ratio was near 60% lower in LP group. Insulin secretion was, in most conditions, approximately 50% lower in LP rat islets. When islets were pre-incubated with H₂ O₂ (100 μM), and incubated with glucose (33 mM), LP rats showed significant decrease of insulin secretion. This effect was attenuated when LP islets were exposed to N-acetylcysteine.

Insulin injection restored increased insulin receptor substrate (IRS)-2 protein during short-term protein restriction but did not affect reduced insulin-like growth factor (IGF)-I mRNA or increased triglyceride accumulation in the liver of rats

Dietary protein restriction reduces insulin-like growth factor (IGF)-I synthesis and impairs growth. Moreover, insulin secretion is impaired and hepatic insulin signaling is activated presumably through upregulation of insulin receptor substrate (IRS)-2, which can stimulate lipogenesis thereby resulting in steatosis. In order to determine whether impaired insulin secretion is the primary cause of these changes, we injected insulin into protein-restricted rats and compensated for the reduction in insulin secretion for 1 and 7 d. Insulin infusion did not overcome the reduction in liver IGF-I mRNA nor the hepatic triglyceride accumulation. In contrast, it clearly suppressed the upregulation of hepatic IRS-2 on day 1, but not on day 7. Furthermore, insulin elimination increased IRS-2 in H4IIE-C3 cells. In summary, we found that reduced insulin secretion during protein restriction directly increased hepatic IRS-2 as a rapid response on day 1, while additional mechanisms contributed to the upregulation of IRS-2 on day 7.

Maternal moderate physical training during pregnancy attenuates the effects of a low-protein diet on the impaired secretion of insulin in rats: potential role for compensation of insulin resistance and preventing gestational diabetes mellitus

The effects of pregestational and gestational low-to-moderate physical training on insulin secretion in undernourished mothers were evaluated. Virgin female Wistar rats were divided into four groups as follows: control (C, n = 5); trained (T, n = 5); low-protein diet (LP, n = 5); trained with a low-protein diet (T + LP, n = 5). Trained rats ran on a treadmill over a period of 4 weeks before mate (5 days week⁻¹ and 60 min day⁻¹, at 65% of VO_2max). At pregnancy, the intensity and duration of the exercise were reduced. Low-protein groups were provided with an 8% casein diet, and controls were provided with a 17% casein diet. At third day after delivery, mothers and pups were killed and islets were isolated by collagenase digestion of pancreas and incubated for a further 1 h with medium containing 5.6 or 16.7 mM glucose. T mothers showed increased insulin secretion by isolated islets incubated with 16.7 mM glucose, whereas LP group showed reduced secretion of insulin by isolated islets when compared with both C and LP + T groups. Physical training before and during pregnancy attenuated the effects of a low-protein diet on the secretion of insulin, suggesting a potential role for compensation of insulin resistance and preventing gestational diabetes mellitus.

Developmental programming of aging of isolated pancreatic islet glucose-stimulated insulin secretion in female offspring of mothers fed low-protein diets in pregnancy and/or lactation

Diabetes predisposition is determined by pancreatic islet insulin secretion and insulin resistance. We studied female rat offspring exposed to low-protein maternal diet (50% control protein diet) in pregnancy and/or lactation at postnatal days 36, 110 and 450. Rats were fed either control 20% casein diet (C) or restricted diet (R – 10% casein) during pregnancy. After delivery, mothers received either C or R diet until weaning to provide four offspring groups: CC, RR, CR and RC (first letter denoting maternal pregnancy diet and the second lactation diet). Serum glucose, insulin and homeostatic model assessment (HOMA) were measured. Pancreatic islets were isolated and in vitro insulin secretion quantified in low glucose (5 mM) and high glucose (11 mM). Serum glucose, insulin and HOMA were similar in all groups at 36 and 110 postnatal days. HOMA was only higher in RR at 450 postnatal days. Only CC demonstrated differences in glucose sensitivity of β-cells to high and low doses at the three ages studied. At 36 days, RR, CR and RC and at 450 days RR and RC groups did not show glucose-stimulated insulin secretion differences between low and high glucose. Aging-associated glucose-stimulated insulin secretion loss was affected by maternal dietary history, indicating that developmental programming must be considered a major factor in aging-related development of predisposition to later-life dysfunctional insulin metabolism. Female offspring islets’ insulin secretion was higher than previously reported in males.

A low-protein diet during pregnancy alters glucose metabolism and insulin secretion

In pancreatic islets, glucose metabolism is a key process for insulin secretion, and pregnancy requires an increase in insulin secretion to compensate for the typical insulin resistance at the end of this period. Because a low-protein diet decreases insulin secretion, this type of diet could impair glucose homeostasis, leading to gestational diabetes. In pancreatic islets, we investigated GLUT2, glucokinase and hexokinase expression patterns as well as glucose uptake, utilization and oxidation rates. Adult control non-pregnant (CNP) and control pregnant (CP) rats were fed a normal protein diet (17%), whereas low-protein non-pregnant (LPNP) and low-protein pregnant (LPP) rats were fed a low-protein diet (6%) from days 1 to 15 of pregnancy. The insulin secretion in 2.8 mmol l(-1) of glucose was higher in islets from LPP rats than that in islets from CP, CNP and LPNP rats. Maximal insulin release was obtained at 8.3 and 16.7 mmol l(-1) of glucose in LPP and CP groups, respectively. The glucose dose-response curve from LPNP group was shifted to the right in relation to the CNP group. In the CP group, the concentration-response curve to glucose was shifted to the left compared with the CNP group. The LPP groups exhibited an "inverted U-shape" dose-response curve. The alterations in the GLUT2, glucokinase and hexokinase expression patterns neither impaired glucose metabolism nor correlated with glucose islet sensitivity, suggesting that β-cell sensitivity to glucose requires secondary events other than the observed metabolic/molecular events.

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.

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

The effects of dexamethasone (Dex) on the metabolic parameters, peripheral insulin, and glucose sensitivity in vivo as well as on islet function ex vivo of rats submitted to low-protein diet were analyzed. Dexamethasone (1.0 mg/kg body weight) was administered intraperitoneally daily to adult Wistar rats fed on a normal-protein diet or low-protein diet (LPD) for 5 days, whereas control rats fed on a normal-protein diet or low-protein diet (LP) received saline alone. At the end of the experimental period, LP rats showed a significant reduction in serum insulin, total serum protein, and serum albumin levels compared with rats fed on a normal-protein diet (P<.05). All these parameters tended to be normalized in LPD rats (P<.05); furthermore, these rats exhibited increased serum glucose and nonesterified fatty acid levels compared with LP rats (P<.05). Rats submitted to the low-protein diet demonstrated normal peripheral glucose sensitivity and improved peripheral insulin sensitivity, which was reversed by Dex treatment. A reduced area of islets from LP rats was partially recovered in LPD rats (P<.05). At 16.7 mmol/L glucose, insulin secretion from LPD islets was also partially recovered and was significantly higher than that from LP islets (P<.05). In conclusion, induction of insulin resistance by Dex treatment reverses most of the metabolic alterations in rats submitted to a low-protein diet. In addition, several islet functions were also improved by Dex, confirming the plasticity of pancreatic islets in adverse conditions.

Protein deficiency during pregnancy and lactation impairs glucose-induced insulin secretion but increases the sensitivity to insulin in weaned rats

We studied glucose homeostasis in rat pups from dams fed on a normal-protein (170 g/kg) (NP) diet or a diet containing 60 g protein/kg (LP) during fetal life and the suckling period. At birth, total serum protein, serum albumin and serum insulin levels were similar in both groups. However, body weight and serum glucose levels in LP rats were lower than those in NP rats. At the end of the suckling period (28 d of age), total serum protein, serum albumin and serum insulin were significantly lower and the liver glycogen and serum free fatty acid levels were significantly higher in LP rats compared with NP rats. Although the fasting serum glucose level was similar in both groups, the area under the blood glucose concentration curve after a glucose load was higher for NP rats (859 (sem 58) mmol/l per 120 min for NP rats v. 607 (sem 52) mmol/l per 120 min for LP rats; P < 0.005). The mean post-glucose increase in insulin was higher for NP rats (30 (sem 4.7) nmol/l per 120 min for NP rats v. 17 (sem 3.9) nmol/l per 120 min for LP rats; P < 0.05). The glucose disappearance rate for NP rats (0.7 (sem 0.1) %/min) was lower than that for LP rats (1.6 (sem 0.2)%/min; P < 0.001). Insulin secretion from isolated islets (1 h incubation) in response to 16.7mmol glucose/l was augmented 14-fold in NP rats but only 2.6-fold in LP rats compared with the respective basal secretion (2.8 mmol/l; P < 0.001). These results indicate that in vivo as well as in vitro insulin secretion in pups from dams maintained on a LP diet is reduced. This defect may be counteracted by an increase in the sensitivity of target tissues to insulin.

Treinamento físico durante a recuperação nutricional não afeta o metabolismo muscular da glicose de ratos

O presente estudo visou avaliar a ingestão alimentar, ganho de peso e metabolismo muscular da glicose em ratos submetidos ao treinamento aeróbio durante recuperação de desnutrição protéica. Para isso, 60 ratos da linhagem Wistar, machos, foram separados nos grupos normoprotéico (NP) e hipoprotéico (HP), de acordo com a dieta NP (17% de proteína) ou HP (6% de proteína), respectivamente, recebida do desmame (21 dias) aos 90 dias de idade. Todos os animais passaram então, a receber a dieta NP e foram submetidos (treinado TRE) ou não (sedentário - SED) ao treinamento físico, que consistiu de corrida em esteira rolante, 25m/min, 50 minutos ao dia, cinco dias na semana, durante 30 dias, compondo os grupos NP-SED, NP-TRE, HP/NP-SED e HP/NP-TRE. Foi avaliado o metabolismo da glicose em fatias de músculo sóleo incubado em presença de insulina (100miU/L) e glicose (5,5mM, contendo [C14] glicose e [H³] 2-deoxiglicose). A ingestão alimentar diária (g/100g de peso corporal) do grupo HP/NP-TRE (24,39 ± 4,07) foi maior do que o grupo HP/NP-SED (21,62 ± 4,69). O ganho de peso (g) foi semelhante nos grupos HP/NP-TRE (203,80 ± 34,03) e HP/NP-SED (214,43 ± 30,54). Não houve diferença entre estes dois grupos quanto aos parâmetros: captação de glicose, oxidação de glicose e síntese de glicogênio pelo músculo sóleo. Desse modo, pudemos concluir que o treinamento aeróbio não teve impacto sobre a recuperação nutricional, visto que não houve diferenças metabólicas ou somáticas entre animais recuperados em presença ou ausência do treinamento.

Low-protein diets reduce PKAalpha expression in islets from pregnant rats

We investigated the effect of protein restriction on insulin secretion and the expression of protein kinase (PK)Aalpha and PKCalpha in islets from control and pregnant rats. Adult control nonpregnant (CN) and control pregnant (CP) rats were fed a normal-protein diet (17%), whereas low-protein nonpregnant (LPN) and low-protein pregnant (LPP) rats were fed a low-protein diet (6%) for 15 d. In the presence of 2.8 and 8.3 mmol glucose/L, insulin secretion by islets of CP rats was higher than that by islets of CN rats. Compared with the CN groups, insulin secretion by islets of LPN rats was lower with 8.3 but not with 2.8 mmol glucose/L. The insulin secretion by islets of LPP rats was higher than by LPN rats at both glucose concentrations. IBMX (1 mmol/L), a phosphodiesterase inhibitor, increased insulin secretion by islets from pregnant rats, and this effect was greater in islets of CP rats than in LPP rats. Forskolin (0.01-100 micromol/L), a stimulator of adenylyl cyclase, increased insulin secretion only in islets of CN and CP rats, with a higher 50% effective concentration in islets of CP rats compared with CN rats. The insulin secretion induced by phorbol 12-myristate 13-acetate (a stimulator of PKC) was higher in islets of LPN and LPP rats than in the respective controls, especially at 8.3 mmol glucose/L. PKAalpha, but not PKCalpha, expression was lower in islets of rats fed low protein than in the controls, regardless of the physiological status of the rats. All endocrine cells of the islets, including beta-cells, expressed the PKAalpha isoform. The cytoplasmic distribution of this enzyme in beta-cells was not modified by pregnancy and/or protein restriction. In conclusion, our results indicate that the response of islets from rats fed low protein during pregnancy is similar to that of control rats, at least for physiologic glucose concentration. However, the decreased response to IBMX and forskolin indicates decreased production and/or sensitivity to cAMP; this was associated with a decrease in PKA expression, which may result in lower PKA activity.

Is gastric ulceration different in normal and malnourished rats?

Protein malnutrition can adversely affect all tissues. The aim of the present study was to test the hypothesis that protein deprivation influences gastric ulcer formation, as well as metabolism and organ growth, in rats. In the present study, there was a significant reduction in the body and organ weight of rats fed a low-protein diet (P<0.001). Malnourished rats were less susceptible to ulceration of the gastric mucosa in ethanol and indomethacin models of acute gastric ulcers when compared with rats fed a normoproteic diet (17 % protein). Mucus production and prostaglandin E2 formation increased in malnourished rats, possibly explaining the lower number of acute ulcers in these animals. Pylorus ligature altered gastric juice composition (increased pH and gastric volume, and decreased total acid concentration) in the animal group fed a low-protein diet compared with the group fed a diet containing 17 % protein (P<0.05). The gastric mucosa was more damaged in malnourished rats than in normal rats evaluated for 14 d after acetic acid injection (P<0.001). Malnourished rats exhibited resistance to acute gastric lesions, owing to an increase in prostaglandin GE2 release and mucus secretion, which protected their gastric mucosa. This phenomenon was not seen in subchronic gastric ulceration.

A low protein diet alters gene expression in rat pancreatic islets

Insulin secretion is regulated mainly by circulating nutrients, particularly glucose, and is also modulated by hormonal and neuronal inputs. Nutritional alterations during fetal and early postnatal periods, induced by either low protein or energy-restricted diets, produce beta-cell dysfunction. As a consequence, insulin secretion in response to different secretagogues is reduced, as is the number of beta-cells and the size and vascularization of islets. In this study, we used a cDNA macroarray technique and RT-PCR to assess the pattern of gene expression in pancreatic islets from rats fed isocaloric low (6 g/100 g, LP) and normal (17 g/100 g, NP) protein diets, after weaning. Thirty-two genes related to metabolism, neurotransmitter receptors, protein trafficking and targeting, intracellular kinase network members and hormones had altered expression (up- or down-regulated). RT-PCR confirmed the macroarray results for five selected genes, i.e., clusterin, secretogranin II precursor, eukaryotic translation initiation factor 2, phospholipase A(2) and glucose transporter. Thus, cDNA macroarray analysis revealed significant changes in the gene expression pattern in rats fed a low protein diet after weaning. The range of proteins affected indicated that numerous mechanisms are involved in the intracellular alterations in the endocrine pancreas, including impaired glucose-induced insulin secretion.

Impaired Pancreatic Endocrine and Exocrine Responses in Growth-Retarded Piglets

The alteration of pancreatic endocrine and exocrine secretory responses induced by secretagogues and neural input was investigated in post-weaning growth-retarded (GR) piglets. Blood and pancreatic juice were collected from these animals (6-8-weeks old). Plasma insulin and pancreatic digestive enzymes induced by nutrients, drugs and vagal stimulation were measured biochemically. The pancreas was inspected by immunohistochemical analysis. In GR piglets, the plasma glucose and insulin concentrations at the resting state were very low, and the secretory response was also markedly reduced, with maximum inhibition of 90% by glucose administration and 83% by arginine administration. The insulin secretion was not increased by 2-deoxy-D-glucose administration in GR piglets. The pancreatic juice secretions induced by vagal stimulation and secretagogues in GR piglets were not different from those induced in the control piglets. However, amylase activity in the pancreatic juice and in the pancreas was significantly decreased in GR piglets, although trypsin and chymotrypsin activities were not different. In the immunohistochemical analysis, the numbers of islets and the staining degree for insulin antibody also declined in the pancreases of GR piglets. These results indicated the reduction of insulin and amylase secretions from the pancreas in GR piglets, suggesting that a dysfunction of pancreatic endocrine and exocrine secretion during growth after weaning may be an important factor in the induction of growth retardation in piglets.

Expression of PDX-1 is reduced in pancreatic islets from pups of rat dams fed a low protein diet during gestation and lactation

Intrauterine and early postnatal malnutrition has profound consequences on fetal and postnatal development in both humans and animals. In addition, low birth weight has been reported to be associated with impaired insulin secretion, insulin resistance and diminished area of pancreatic islets. Because the transcription factor pancreatic and duodenal homeobox 1 (PDX-1) is important for the maintenance of B-cell physiology, PDX-1 expression and islet area were assessed in neonatal rats of dams fed low (6%) or normal (17%) protein diets during pregnancy. PDX-1 protein and mRNA levels, as well as insulin secretion and islet area, were measured after 28 d of life in normal, low protein and recovered rats whose dams consumed a normal protein diet after delivery. Insulin secretion by isolated islets in response to 2.8 and 16.7 mmol glucose/L was reduced in 28-d-old low protein rats compared with the control (P < 0.05). At birth and after 28 d of life, the islet area and PDX-1 protein expression were also reduced (P < 0.05). In contrast, PDX-1 mRNA levels in islets from 28-d-old low protein rats were not different from control rats. PDX-1 protein expression in pancreatic islets, the area of islets and insulin secretion were restored in recovered rats, whereas PDX-1 mRNA levels were higher than in normal rats (P < 0.05). These results suggest a link among diminished PDX-1 protein expression, a reduction in islet area and impaired insulin secretion in low protein rats. The reintroduction of a normal diet early in life restored islet area and cell physiology.

Pancreatic islet insulin secretion and metabolism in adult rats malnourished during neonatal life

Pancreatic islets were isolated from rats that had been nursed by dams fed with a control or an 8.7% protein diet during the first 12 d of the lactation period. Glucose-induced insulin secretion from islets in the 8.7% protein group was reduced 50%. The islet insulin and DNA content were similar, whereas the pancreatic insulin content was reduced by 30 % in the rats fed 8.7 % protein. In order to elucidate the mechanism responsible for the attenuation of insulin secretion, measurements were performed of the activity of several islet enzymes that had previously been supposed to be involved in the coupling of glucose stimulation to insulin secretion. Islet glucose oxidation was unaffected, but glucose-stimulated hydrolysis of phosphatidylinositol was reduced by one-third in the islets of rats fed 8.7% protein. The activity of mitochondrial glycerophosphate dehydrogenase was similar in islets of rats fed the 8.7% protein diet and those fed the control diet. The activity of Ca-independent phospholipase A2 was increased fourfold in the islets of rats fed 8.7% protein. It is concluded that impairment of glucose-induced insulin secretion in rats fed a low-protein diet may be caused by attenuation of islet phosphatidylinositol hydrolysis, and it is tentatively suggested that the increased activity of Ca-independent phospholipase A2 in islets of rats fed a low-protein diet may participate in the stimulation of apoptosis.

Low protein diet confers resistance to the inhibitory effects of interleukin 1beta on insulin secretion in pancreatic islets*

High protein content in the diet during childhood and adolescence has been associated to the onset insulin-dependent diabetes mellitus. We investigated the effect of interleukin-1beta (IL-1beta) on insulin secretion, glucose metabolism, and nitrite formation by islets isolated from rats fed with normal protein (NP, 17%) or low protein (LP, 6%) after weaning. Pretreatment of islets with IL-1beta for 1 h or 24 h inhibited the insulin secretion induced by glucose in both groups, but it was less marked in LP than in NP group. Islets from LP rats exhibited a decreased IL-1beta-induced nitric oxide (NO) production, lower inhibition of D-[U(14)C]-glucose oxidation to (14)CO(2) and less pronounced effect of IL-1beta on alpha-ketoisocaproic acid-induced insulin secretion than NP islets. However, when the islets were stimulated by high concentrations of K(+) the inhibitory effect of IL-1beta on insulin secretion was not different between groups. In conclusion, protein restriction protects beta-cells of the deleterious effect of IL-1beta, apparently, by decreasing NO production. The lower NO generation in islets from protein deprived rats may be due to increased free fatty acids oxidation and consequent alteration in Ca(2+) homeostasis.

Protein deficiency and nutritional recovery modulate insulin secretion and the early steps of insulin action in rats

Maternal malnutrition was shown to affect early growth and leads to permanent alterations in insulin secretion and sensitivity of offspring. In addition, epidemiological studies showed an association between low birth weight and glucose intolerance in adult life. To understand these interactions better, we investigated the insulin secretion by isolated islets and the early events related to insulin action in the hind-limb muscle of adult rats fed a diet of 17% protein (control) or 6% protein [low (LP) protein] during fetal life, suckling and after weaning, and in rats receiving 6% protein during fetal life and suckling followed by a 17% protein diet after weaning (recovered). The basal and maximal insulin secretion by islets from rats fed LP diet and the basal release by islets from recovered rats were significantly lower than that of control rats. The dose-response curves to glucose of islets from LP and recovered groups were shifted to the right compared to control islets, with the half-maximal response (EC50) occurring at 16.9 +/- 1.3, 12.4 +/- 0.5 and 8.4 +/- 0.1 mmol/L, respectively. The levels of insulin receptor, as well as insulin receptor substrate-1 and phosphorylation and the association between insulin receptor substrate-1 and phosphatidylinositol 3-kinase were greater in rats fed a LP diet than in control rats. In recovered rats, these variables were not significantly different from those of the other two groups. These results suggest that glucose homeostasis is maintained in LP and recovered rats by an increased sensitivity to insulin as a result of alterations in the early steps of the insulin signal transduction pathway.

Altered activity of the autonomous nervous system as a determinant of the impaired beta-cell secretory response after protein-energy restriction in the rat

Glucose-induced insulin secretion in vivo is known to be severely blunted in the rat as a consequence of protein-energy restriction starting early in life. We have recently reported in such malnourished rats (M rats) that the release of the counterregulatory hormones that defend against hypoglycemia was severely disturbed, and their plasma levels of epinephrine and norepinephrine were prominently increased. Knowing that the autonomic nervous system has the potential to play a major role in the control of insulin secretion in response to glucose in vivo, we therefore determined whether protein-energy restriction starting after weaning could alter sympathetic and/or parasympathetic nerve activities, and whether these changes could be responsible for the lack of response to glucose of their beta-cells in vivo. When tested in the basal postabsorptive state, the malnourished rats exhibited profound alterations of both parasympathetic and sympathetic nerve activities; the firing rates of the vagus nerve and the superior cervical ganglion were dramatically decreased and increased, respectively. Under the same conditions, insulin secretion in vivo in response to a glucose load (deltaI/deltaG) was severely decreased in M rats compared with that in control (C) rats. When evaluated after administration of acetylcholine, deltaI was amplified to the same extent in M rats as in C rats. After administration of the alpha2A-adrenergic agonist oxymetazoline, glucose-induced insulin release in M rats was not significantly affected, whereas it was sharply decreased in C rats. Finally, administration of yohimbine, an alpha2-adrenergic antagonist, partially restored the lack of reactivity of the beta-cells to glucose in the M rats, as deltaI/deltaG was amplified by 6-fold in the M group and by 3.3-fold in the C group. We conclude that protein-energy restriction starting early in life in rats brings about changes in the overall activity of the autonomic nervous system that, in turn, are responsible at least in part for the acquisition/maintenance of decreased beta-cell reactivity to glucose in vivo.

Descrição de dieta purificada para indução de quadro de desnutrição protéica em ratos

No presente trabalho é descrita a composição de dieta hipoprotéica (6% de proteína) purificada para indução de quadro de desnutrição em roedores. A referida dieta foi padronizada em laboratório a partir de modificação da AIN-93 (documento do American Institute of Nutrition que estabelece os padrões nutricionais para roedores de laboratório), visando a obtenção de animais desnutridos para estudar as alterações metabólicas decorrentes da desnutrição protéica associada a situações como exercício físico, gestação e diabetes. A dieta em questão contém os seguintes componentes (g/ kg): amido de milho (480), caseína (71,5), dextrina de milho (159), sacarose (121), óleo de soja (70), microcelulose (50), mistura mineral AIN-93-G-MX (35), mistura de vitaminas AIN-93-G-VX, (10), L-cistina (1), cloridrato de colina (2,5). Ratos alimentados cronicamente com a dieta apresentaram sinais comumente presentes na desnutrição protéica humana e de animais de laboratório: redução do ganho de peso, hipoproteinemia, hipoalbuminemia, elevação dos ácidos graxos livres séricos e do glicogênio hepático.

Glucose-induced insulin secretion is impaired and insulin-induced phosphorylation of the insulin receptor and insulin receptor substrate-1 are increased in protein-deficient rats

Malnutrition is related to diabetes in tropical countries. In experimental animals, protein deficiency may affect insulin secretion. However, the effect of malnutrition on insulin receptor phosphorylation and further intracellular signaling events is not known. Therefore, we decided to evaluate the rate of insulin secretion and the early molecular steps of insulin action in insulin-sensitive tissues of an animal model of protein deficiency. Pancreatic islets isolated from rats fed a standard (17%) or a low (6%) protein diet were studied for their secretory response to increasing concentrations of glucose in the culture medium. Basal as well as maximal rates of insulin secretion were significantly lower in the islets isolated from rats fed a low protein diet. Moreover, the dose-response curve to glucose was significantly shifted to the right in the islets from malnourished rats compared with islets from control rats. During an oral glucose tolerance test, there were significantly lower circulating concentrations of insulin in the serum of rats fed a low protein diet in spite of no difference in serum glucose concentration between the groups, suggesting an increased peripheral insulin sensitivity. Immunoblotting and immunoprecipitation were used to study the phosphorylation of the insulin receptor and the insulin receptor substrate-1 as well as the insulin receptor substrate-1-p85 subunit of phosphatidylinositol 3-kinase association in response to insulin. Values were greater in hind-limb muscle from rats fed a low protein diet compared with controls. No differences were detected in the total amount of protein corresponding to the insulin receptor or insulin receptor substrate-1 between muscle from rats fed the two diets. Therefore, we conclude that a decreased glucose-induced insulin secretion in pancreatic islets from protein-malnourished rats is responsible, at least in part, for an increased phosphorylation of the insulin receptor, insulin receptor substrate-1 and its association with phosphatidylinositol 3-kinase. These might represent some of the factors influencing the equilibrium in glucose concentrations observed in animal models of malnutrition and undernourished subjects.