Stimulus-secretion coupling of arginine-induced insulin release. Functional response of islets to L-arginine and L-ornithine

Pancreatic islet α cell function and proliferation requires the arginine transporter SLC7A2

Interrupting glucagon signaling decreases gluconeogenesis and the fractional extraction of amino acids by liver from blood resulting in lower glycemia. The resulting hyperaminoacidemia stimulates α cell proliferation and glucagon secretion via a liver-α cell axis. We hypothesized that α cells detect and respond to circulating amino acids levels via a unique amino acid transporter repertoire. We found that Slc7a2ISLC7A2 is the most highly expressed cationic amino acid transporter in α cells with its expression being three-fold greater in α than β cells in both mouse and human. Employing cell culture, zebrafish, and knockout mouse models, we found that the cationic amino acid arginine and SLC7A2 are required for α cell proliferation in response to interrupted glucagon signaling. Ex vivo and in vivo assessment of islet function in Slc7a2-/- mice showed decreased arginine-stimulated glucagon and insulin secretion. We found that arginine activation of mTOR signaling and induction of the glutamine transporter SLC38A5 was dependent on SLC7A2, showing that both's role in α cell proliferation is dependent on arginine transport and SLC7A2. Finally, we identified single nucleotide polymorphisms in SLC7A2 associated with HbA1c. Together, these data indicate a central role for SLC7A2 in amino acid-stimulated α cell proliferation and islet hormone secretion.

Pancreatic α-cells - The unsung heroes in islet function

The pancreatic islets of Langerhans consist of several hormone-secreting cell types important for blood glucose control. The insulin secreting β-cells are the best studied of these cell types, but less is known about the glucagon secreting α-cells. The α-cells secrete glucagon as a response to low blood glucose. The major function of glucagon is to release glucose from the glycogen stores in the liver. In both type 1 and type 2 diabetes, glucagon secretion is dysregulated further exaggerating the hyperglycaemia, and in type 1 diabetes α-cells fail to counter regulate hypoglycaemia. Although glucagon has been recognized for almost 100 years, the understanding of how glucagon secretion is regulated and how glucagon act within the islet is far from complete. However, α-cell research has taken off lately which is promising for future knowledge. In this review we aim to highlight α-cell regulation and glucagon secretion with a special focus on recent discoveries from human islets. We will present some novel aspects of glucagon function and effects of selected glucose lowering agents on glucagon secretion.

Milk in the prevention and management of type 2 diabetes: The potential role of milk proteins

Globally, diabetes mellitus is not only considered a leading cause of mortality and morbidities but has also created a substantial economic burden. There is growing evidence that foods and their components can be implemented in the prevention and management of type 2 diabetes mellitus (T2DM). Increased dairy consumption has been linked to a lower risk of T2DM. The protective role of dairy foods in the development of T2DM is thought to be largely attributable to dairy nutrients, one of them being dairy protein. There is considerable evidence that milk proteins increase the postprandial insulin response and lower the postprandial blood glucose response in both healthy subjects and patients with T2DM. The exact mechanisms by which milk proteins lower postprandial glucose levels are yet to established; however, the amino acids and bioactive peptides derived from milk proteins are thought to modify a physiological milieu, which includes delayed gastric emptying and the enhancement of incretin and insulin responses, consequently leading to lower postprandial glucose levels. The present review will focus on providing a clear presentation of the potential implementation of milk proteins as a dietary supplement in the prevention and management of T2DM by summarizing the relevant supporting evidence for this particular topic.

Lupin seed hydrolysate promotes G-protein-coupled receptor, intracellular Ca2+ and enhanced glycolytic metabolism-mediated insulin secretion from BRIN-BD11 pancreatic beta cells

Lupin seed proteins have been reported to exhibit hypoglycaemic effects in animals and humans following oral administration, however little is known about its mechanism of action. This study investigated the signalling pathway(s) responsible for the insulinotropic effect of the hydrolysate obtained from lupin (Lupinus angustifolius L.) seed extracts utilizing BRIN-BD11 β-cells. The extract was treated with digestive enzymes to give a hydrolysate rich in biomolecules ≤7 kDa. Cells exhibited hydrolysate induced dose-dependent stimulation of insulin secretion and enhanced intracellular Ca²⁺ and glucose metabolism. The stimulatory effect of the hydrolysate was potentiated by depolarizing concentrations of KCl and was blocked by inhibitors of the ATP sensitive K⁺ channel, Gα_q protein, phospholipase C (PLC) and protein kinase C (PKC). These findings reveal a novel mechanism for lupin hydrolysate stimulated insulin secretion via Gα_q mediated signal transduction (Gα_q/PLC/PKC) in the β-cells. Thus, lupin hydrolysates may have potential for nutraceutical treatment in type 2 diabetes.

Safety of dietary supplementation with arginine in adult humans

Previous studies with animals and humans have shown beneficial effects of dietary supplementation with L-arginine (Arg) on reducing white fat and improving health. At present, a long-term safe level of Arg administration to adult humans is unknown. The objective of this study was to conduct a randomized, placebo-controlled, clinical trial to evaluate the safety and tolerability of oral Arg in overweight or obese but otherwise healthy adults with a body mass index of ≥ 25 kg/m². A total of 142 subjects completed a 7-day wash-in period using a 12 g Arg/day dose. All the remaining eligible 101 subjects who tolerated the wash-in dose (45 men and 56 women) were assigned randomly to ingest 0, 15 or 30 g Arg (as pharmaceutical-grade Arg-HCl) per day for 90 days. Arg was taken daily in at least two divided doses by mixing with a flavored beverage. At Days 0 and 90, blood pressures of study subjects were recorded, their physical examinations were performed, and their blood and 24-h urine samples were obtained to measure: (1) serum concentrations of amino acids, glucose, fatty acids, and related metabolites; and (2) renal, hepatic, endocrine and metabolic parameters. Our results indicate that the serum concentration of Arg in men or women increased (P < 0.05) progressively with increasing oral Arg doses from 0 to 30 g/day. Dietary supplementation with 30 g Arg/day reduced (P < 0.05) systolic blood pressure and serum glucose concentration in females, as well as serum concentrations of free fatty acids in both males and females. Based on physiological and biochemical variables, study subjects tolerated oral administration of 15 and 30 g Arg/day without adverse events. We conclude that a long-term safe level of dietary Arg supplementation is at least 30 g/day in adult humans.

Dietary protein supplementation in the elderly for limiting muscle mass loss

Supplementation with whey and other dietary protein, mainly associated with exercise training, has been proposed to be beneficial for the elderly to gain and maintain lean body mass and improve health parameters. The main objective of this review is to examine the evidence provided by the scientific literature indicating benefit from such supplementation and to define the likely best strategy of protein uptake for optimal objectified results in the elderly. Overall, it appears that an intake of approximately 0.4 g protein/kg BW per meal thus representing 1.2-1.6 g protein/kg BW/day may be recommended taking into account potential anabolic resistance. The losses of the skeletal muscle mass contribute to lower the capacity to perform activities in daily living, emphasizing that an optimal protein consumption may represent an important parameter to preserve independence and contribute to health status. However, it is worth noting that the maximal intake of protein with no adverse effect is not known, and that high levels of protein intake is associated with increased transfer of protein to the colon with potential deleterious effects. Thus, it is important to examine in each individual case the benefit that can be expected from supplementation with whey protein, taking into account the usual protein dietary intake.

Lactobacillus brevis OPK-3 isolated from kimchi inhibits adipogenesis and exerts anti-inflammation in 3T3-L1 adipocyte

BACKGROUND

Kimchi is a traditional fermented food in Korea that contains various unique microorganisms. Diverse bacteria are involved in the process of Kimchi fermentation and the healthful advantages; one of the major species is Lactobacillus. We investigated whether lactic acid bacteria isolated from Kimchi (KLAB) are capable of reducing intracellular lipid accumulation by downregulating the expression of adipogenesis and lipogenesis promoting genes in differentiating 3T3-L1 cells.

RESULTS

KLAB (Lactobacillus brevis OPK-3) mediated dose-dependent inhibition of adipocyte differentiation, intracellular triglyceride accumulation and glycerol-3-phosphate dehydrogenase (GPDH) activity. The expression of transcription factors such as peroxisome proliferator-activated receptor γ and CCAAT/enhancer-binding protein α involved in adipogenesis was markedly decreased by the KLAB treatment. Terminal adipogenic marker, e.g. adipocyte fatty acid binding protein (aP2), lipoprotein lipase, liver X receptor α, leptin and GPDH were significantly downregulated by KLAB treatment compared to untreated control. Moreover, cytokine genes, such as tumor necrosis factor-α and interleukin-6 mRNA expressions level were also decreased, whereas adiponectin mRNA level was upregulated by KLAB.

CONCLUSION

These results suggest that the KLAB inhibits lipid accumulation in the differentiating adipocyte through downregulating the expression of adipogenic transcription factors and other specific genes involved in lipid metabolism.

Role of proteins in insulin secretion and glycemic control

Dietary proteins are essential for the life of all animals and humans at all stages of the life cycle. They serve many structural and biochemical functions and have significant effects on health and wellbeing. Dietary protein consumption has shown an upward trend in developed countries in the past two decades primarily due to greater supply and affordability. Consumption is also on the rise in developing countries as affluence is increasing. Research shows that proteins have a notable impact on glucose homeostasis mechanisms, predominantly through their effects on insulin, incretins, gluconeogenesis, and gastric emptying. Since higher protein consumption and impaired glucose tolerance can be commonly seen in the same population demographics, a thorough understanding of the former's role in glucose homeostasis is crucial both toward the prevention and management of the latter. This chapter reviews the current state of the art on proteins, amino acids, and their effects on blood glucose and insulin secretion.

Insulinotropic and muscle protein synthetic effects of branched-chain amino acids: potential therapy for type 2 diabetes and sarcopenia

The loss of muscle mass and strength with aging (i.e., sarcopenia) has a negative effect on functional independence and overall quality of life. One main contributing factor to sarcopenia is the reduced ability to increase skeletal muscle protein synthesis in response to habitual feeding, possibly due to a reduction in postprandial insulin release and an increase in insulin resistance. Branched-chain amino acids (BCAA), primarily leucine, increases the activation of pathways involved in muscle protein synthesis through insulin-dependent and independent mechanisms, which may help counteract the "anabolic resistance" to feeding in older adults. Leucine exhibits strong insulinotropic characteristics, which may increase amino acid availability for muscle protein synthesis, reduce muscle protein breakdown, and enhance glucose disposal to help maintain blood glucose homeostasis.

The L-α-amino acid receptor GPRC6A is expressed in the islets of Langerhans but is not involved in L-arginine-induced insulin release

GPRC6A is a seven-transmembrane receptor activated by a wide range of L-α-amino acids, most potently by L-arginine and other basic amino acids. The receptor is broadly expressed, but its exact physiological role remains to be elucidated. It is well established that L-arginine stimulates insulin secretion; therefore, the receptor has been hypothesized to have a role in regulating glucose metabolism. In this study, we demonstrate that GPRC6A is expressed in islets of Langerhans, but activation of the receptor by L-arginine did not stimulate insulin secretion. We also investigated central metabolic parameters in GPRC6A knockout mice compared with wildtype littermates and found no difference in glucose metabolism or body fat percentage when mice were administered a standard chow diet. In conclusion, our data do not support a role for GPRC6A in L-arginine-induced insulin release and glucose metabolism under normal physiological conditions.

Functional alterations in endocrine pancreas of rats with different degrees of dexamethasone-induced insulin resistance

OBJECTIVES

We have analyzed the peripheral insulin and glucose sensitivity in vivo, and islet function ex vivo in rats with different degrees of insulin resistance induced by dexamethasone (DEX).

METHODS

Dexamethasone, in the concentrations of 0.1 (DEX 0.1), 0.5 (DEX 0.5), and 1.0 mg/kg body weight (DEX 1.0) was administered daily, intraperitoneally, to adult Wistar rats for 5 days, whereas controls received saline.

RESULTS

Dexamethasone treatment induced peripheral insulin resistance in a dose-dependent manner. At the end of the treatment, only DEX 1.0 rats showed significant increase of postabsorptive blood glucose and serum triglycerides, and nonesterified fatty acids levels. Incubation of pancreatic islets in increasing glucose concentrations (2.8-22 mM) led to an augmented insulin secretion in all DEX-treated rats. Leucine, carbachol, and high KCl concentrations induced the insulin release in DEX 0.5 and DEX 1.0, whereas arginine augmented secretion in all DEX-treated groups.

CONCLUSIONS

We demonstrate that in DEX 0.5 and, especially in DEX 0.1 groups, but not in DEX 1.0, the adaptations that occurred in the endocrine pancreas are able to counteract metabolic disorders (glucose intolerance and dyslipidemia). These animal models seem to be interesting approaches for the study of degrees of subjacent effects that may mediate type 2 diabetes (DEX 1.0) and islet function alterations, without collateral effects (DEX 0.1 and DEX 0.5).

Regulation of insulin gene expression and insulin production in Nile tilapia (Oreochromis niloticus)

Compared to mammals, little is known about insulin gene expression in fish. Using transient transfection experiments and mammalian insulinoma cell lines we demonstrate that transcription of the Nile tilapia (Oreochromis niloticus) insulin gene is (a) regulated in a beta-cell-specific manner; and (b) not sensitive to the glucose stimulations. Deletion analysis of the 1575 bp 5' insulin gene flanking sequence revealed that cooperative interactions between regulatory elements within the proximal (-1 to -396 bp) and the distal (-396 bp to -1575 bp) promoter regions were necessary for induction of the beta-cell-specific transcription. Effects of glucose and arginine on endogenous insulin secretion, translation, and transcription in isolated tilapia Brockmann bodies were determined using Northern hybridization, Western analysis, and quantitative RT-PCR. Similar to the regulation of mammalian insulin, we found that increases of glucose (1-70 mM) and arginine (0.4-25 mM) induced insulin secretion. However, transcription of the insulin gene was activated only by extremely high concentrations of glucose and arginine added simultaneously. When stimulated for 24 h with low concentrations of both inducers or with either of them added separately, tilapia beta-cells were able to replenish secreted insulin and to maintain insulin stores at a constant level without elevations of the insulin mRNA levels. Since the basal level of insulin mRNA was approximately 3.7-fold higher in tilapia beta-cells than it is in mammalian beta-cells, insulin production in tilapia cells probably relies on an enlarged intracellular insulin mRNA pool and does not require the transcriptional activation of the insulin gene.

Pancreatic Insulin Secretion in Rats Fed a Soy Protein High Fat Diet Depends on the Interaction between the Amino Acid Pattern and Isoflavones

Obesity is frequently associated with the consumption of high carbohydrate/fat diets leading to hyperinsulinemia. We have demonstrated that soy protein (SP) reduces hyperinsulinemia, but it is unclear by which mechanism. Thus, the purpose of the present work was to establish whether SP stimulates insulin secretion to a lower extent and/or reduces insulin resistance, and to understand its molecular mechanism of action in pancreatic islets of rats with diet-induced obesity. Long-term consumption of SP in a high fat (HF) diet significantly decreased serum glucose, free fatty acids, leptin, and the insulin:glucagon ratio compared with animals fed a casein HF diet. Hyperglycemic clamps indicated that SP stimulated insulin secretion to a lower extent despite HF consumption. Furthermore, there was lower pancreatic islet area and insulin, SREBP-1, PPARgamma, and GLUT-2 mRNA abundance in comparison with rats fed the casein HF diet. Euglycemic-hyperinsulinemic clamps showed that the SP diet prevented insulin resistance despite consumption of a HF diet. Incubation of pancreatic islets with isoflavones reduced insulin secretion and expression of PPARgamma. Addition of amino acids resembling the plasma concentration of rats fed casein stimulated insulin secretion; a response that was reduced by the presence of isoflavones, whereas the amino acid pattern resembling the plasma concentration of rats fed SP barely stimulated insulin release. Infusion of isoflavones during the hyperglycemic clamps did not stimulate insulin secretion. Therefore, isoflavones as well as the amino acid pattern seen after SP consumption stimulated insulin secretion to a lower extent, decreasing PPARgamma, GLUT-2, and SREBP-1 expression, and ameliorating hyperinsulinemia observed during obesity.

Protein Kinase Cδ Plays a Non-redundant Role in Insulin Secretion in Pancreatic β Cells

Protein kinase C (PKC) is considered to modulate glucose-stimulated insulin secretion. Pancreatic beta cells express multiple isoforms of PKCs; however, the role of each isoform in glucose-stimulated insulin secretion remains controversial. In this study we investigated the role of PKCdelta, a major isoform expressed in pancreatic beta cells on beta cell function. Here, we showed that PKCdelta null mice manifested glucose intolerance with impaired insulin secretion. Insulin tolerance test showed no decrease in insulin sensitivity in PKCdelta null mice. Studies using islets isolated from these mice demonstrated decreased glucose- and KCl-stimulated insulin secretion. Perifusion studies indicated that mainly the second phase of insulin secretion was decreased. On the other hand, glucose-induced influx of Ca2+ into beta cells was not altered. Immunohistochemistry using total internal reflection fluorescence microscopy and electron microscopic analysis showed an increased number of insulin granules close to the plasma membrane in beta cells of PKCdelta null mice. Although PKC is thought to phosphorylate Munc18-1 and facilitate soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors complex formation, the phosphorylation of Munc18-1 by glucose stimulation was decreased in islets of PKCdelta null mice. We conclude that PKCdelta plays a non-redundant role in glucose-stimulated insulin secretion. The impaired insulin secretion in PKCdelta null mice is associated with reduced phosphorylation of Munc18-1.

Dual mechanism of the potentiation by glucose of insulin secretion induced by arginine and tolbutamide in mouse islets

Glucose induces insulin secretion (IS) and also potentiates the insulin-releasing action of secretagogues such as arginine and sulfonylureas. This potentiating effect is known to be impaired in type 2 diabetic patients, but its cellular mechanisms are unclear. IS and cytosolic Ca(2+) concentration ([Ca(2+)](i)) were measured in mouse islets during perifusion with 3-15 mmol/l glucose (G3-G15, respectively) and pulse or stepwise stimulation with 1-10 mmol/l arginine or 5-250 micromol/l tolbutamide. In G3, arginine induced small increases in [Ca(2+)](i) but no IS. G7 alone only slightly increased [Ca(2+)](i) and IS but markedly potentiated arginine effects on [Ca(2+)](i), which resulted in significant IS (already at 1 mmol/l). For each arginine concentration, both responses further increased at G10 and G15, but the relative change was distinctly larger for IS than [Ca(2+)](i). At all glucose concentrations, tolbutamide dose dependently increased [Ca(2+)](i) and IS with thresholds of 25 micromol/l for [Ca(2+)](i) and 100 micromol/l for IS at G3 and of 5 micromol/l for both at G7 and above. Between G7 and G15, the effect of tolbutamide on [Ca(2+)](i) increased only slightly, whereas that on IS was strongly potentiated. The linear relationship between IS and [Ca(2+)](i) at increasing arginine or tolbutamide concentrations became steeper as the glucose concentration was raised. Thus glucose augmented more the effect of each agent on IS than that on [Ca(2+)](i). In conclusion, glucose potentiation of arginine- or tolbutamide-induced IS involves increases in both the rise of [Ca(2+)](i) and the action of Ca(2+) on exocytosis. This dual mechanism must be borne in mind to interpret the alterations of the potentiating action of glucose in type 2 diabetic patients.

Effects of a nutritional supplement containing Salacia oblonga extract and insulinogenic amino acids on postprandial glycemia, insulinemia, and breath hydrogen responses in healthy adults

OBJECTIVE

This study evaluated the postprandial glycemic, insulinemic, and breath hydrogen responses to a liquid nutritional product containing Salacia oblonga extract, an herbal alpha-glucosidase inhibitor, and two insulinogenic amino acids.

METHODS

In a randomized, double-masked, crossover design, 43 healthy subjects were fed the following meals on separate days after overnight fasting: control (C; 480 mL of a study beverage containing 82 g of carbohydrate, 20 g of protein, and 14 g of fat), control plus 3.5 g each of phenylalanine and leucine (AA), control plus 1000 mg of S. oblonga extract (S), and control plus S and AA (SAA). Postprandially, fingerstick capillary plasma glucose and venous serum insulin levels were measured for 180 min, and breath hydrogen excretion was measured for 480 min.

RESULTS

The baseline-adjusted peak glucose response was not different across meals. However, changes in plasma glucose areas under the curve (0 to 120 min and 0 to 180 min, respectively) compared with C were -9% and -11% for AA (P>0.05 each), -27% and -24% for S (P=0.035 and 0.137), and -27% and -29% for SAA (P<0.05 each). Changes in insulin areas under the curve were +5% and +5% for AA (P>0.05 each), -35% and -36% for S (P<0.001 each), and -6% and -7% for SAA (P>0.05 each). Breath hydrogen excretion was 60% greater (P<0.001) in the S-containing meals than in the C- and AA-containing meals and was associated with mild flatulence.

CONCLUSIONS

Salacia oblonga extract is a promising nutraceutical ingredient that decreased glycemia in this study. Supplementation with amino acids had no significant additional effect on glycemia.

Glucose homeostasis and tissue transcript content of insulin signaling intermediates in four inbred strains of mice: C57BL/6, C57BLKS/6, DBA/2, and 129X1

Transgenic mice phenotypes generally depend on the background strains used in their creation. To examine the effects of genetic background on insulin signaling, we analyzed glucose homeostasis in four inbred strains of mice [C57BL/6 (B6), C57BLKS/6 (KLS), DBA/2 (DBA), and 129X1] and quantitated mRNA content of insulin receptor (IR) and its substrates in insulin-responsive tissues. At 2 months, the male B6 mouse is the least glucose-tolerant despite exhibiting similar insulin sensitivity and first-phase insulin secretion as the other strains. The 129X1 male mouse islet contains less insulin and exhibits a higher threshold for glucose-stimulated first-phase insulin secretion than the other strains. Female mice generally manifest better glucose tolerance than males, which is likely due to greater insulin sensitivity in liver and adipose tissue, a robust first-phase insulin secretion in B6 and KLS females, and improved insulin sensitivity in muscle in DBA and 129X1 females. At 6 months, although males exhibit improved first-phase insulin secretion, their physiology was relatively unchanged, whereas female B6 and KLS mice became less insulin sensitive. Gene expression of insulin signaling intermediates in insulin-responsive tissues was generally not strain dependent with the cell content of IR mRNA being highest. IR substrate (IRS)-1 and IRS-2 mRNA are ubiquitously expressed and IRS-3 and IRS-4 mRNA were detected in significant amounts in fat and brain tissues, respectively. These data indicate strain-, gender-, and age-dependent tissue sensitivity to insulin that is generally not associated with transcript content of IR or its substrates and should be taken into consideration during phenotypic characterization of transgenic mice.

Amino acid ingestion strongly enhances insulin secretion in patients with long-term type 2 diabetes

OBJECTIVE

Insulin secretion in response to carbohydrate intake is blunted in type 2 diabetic patients. However, it is not clear whether the insulin response to other stimuli, such as amino acids, is also diminished. Recently, we defined an optimal insulinoptropic mixture containing free leucine, phenylalanine, and a protein hydrolysate that substantially enhances the insulin response in healthy young subjects when coingested with carbohydrate. In this study, we aimed to investigate the insulinotropic capacity of this mixture in long-term type 2 diabetic patients.

RESEARCH DESIGN AND METHODS

Ten type 2 diabetic patients (aged 59.1 +/- 2.0 years, BMI 26.5 +/- 0.7 kg/m(2)) and 10 healthy control subjects (58.8 +/- 2.1 years, 26.5 +/- 0.7 kg/m(2)) visited our lab twice, during which insulin responses were determined following ingestion of carbohydrate only (CHO) or carbohydrate with the free amino acid/protein mixture (CHO+PRO). All subjects received 0.7 g x kg(-1) x h(-1) carbohydrate with or without 0.35 g x kg(-1) x h(-1) of the amino acid/protein mixture.

RESULTS

Insulin responses were dramatically increased in the CHO+PRO trial in both the type 2 diabetic and control groups (189 and 114%, respectively) compared with the CHO trial (P < 0.01). Plasma glucose, glucagon, growth hormone, cortisol, IGF-I, and IGF binding protein 3 responses were not different between trials within the 2-h time frame.

CONCLUSIONS

The insulin secretory capacity in long-term type 2 diabetic patients is substantially underestimated, as the insulin response following carbohydrate intake can be nearly tripled by coingestion of a free amino acid/protein mixture. Future research should be performed to investigate whether such nutritional interventions can improve postprandial glucose disposal.

In vivo induction of insulin secretion by ornithine alpha-ketoglutarate: involvement of nitric oxide and glutamine

We previously demonstrated that ornithine alpha-ketoglutarate (OKG), known for its anabolic properties, induces insulin secretion in vitro. The present study was undertaken to further characterize this effect in vivo and investigate a possible interaction with glucose both in vivo and in vitro. Male Wistar rats received an intravenous bolus of OKG (25 mg/kg) and/or glucose (0.8 g/kg) or saline, and their plasma insulin and glucose levels were monitored for 30 minutes. OKG alone increased plasma insulin to a similar extent to glucose. In combination with glucose, OKG significantly increased glucose-induced insulin secretion in vivo and in vitro, and led to a significant increase in glucose utilization in vivo. The absence of significant variations in plasma arginine and glutamine suggests a direct effect of OKG on the pancreas. To assess the involvement of the synthesis of nitric oxide and glutamine in OKG-induced insulin secretion, the experiments were repeated in the presence of inhibitors of these 2 pathways, respectively L-nitroarginine-methylester (L-NAME) and methionine sulfoximine (MSO). Both inhibitors were able significantly to reduce OKG-induced insulin secretion without affecting either basal or glucose-induced insulin release. Thus OKG acts directly with glucose on islets to induce insulin secretion via mechanisms involving NO and glutamine synthesis. In addition, our results suggest that OKG and glucose act via separate pathways.

Effects of ornithine alpha-ketoglutarate on insulin secretion in rat pancreatic islets: implication of nitric oxide synthase and glutamine synthetase pathways

Ornithine alpha-ketoglutarate (OKG) administration in human subjects elicits insulin secretion. We investigated whether this action was related to an effect of OKG on islets of Langerhans, and addressed the underlying mechanisms of action. For this purpose the influence of OKG on insulin secretion was measured in isolated rat islets of Langerhans under two different conditions. In incubated islets, OKG (0.25 to 2.5 mmol/l) significantly and dose-relatedly increased insulin secretion (1.7- to 4.2-fold; P<0.05 v. basal). To study the kinetics of OKG-stimulated insulin secretion, perifusion experiments were performed, which showed that OKG affected insulin secretion in both initial and later phases. Experiments using alpha-ketoglutarate (alpha-KG) (1 mmol/l) or ornithine (Orn) (2 mmol/l) alone, in concentrations equal to that of OKG, showed that the OKG-induced insulin secretion could not be obtained by either component alone, suggesting that an alpha-KG-Orn interaction is mandatory for the insulin-secreting effect to occur. Since data obtained in vivo suggest that effects of OKG may depend on the synthesis of NO, glutamine and/or polyamines, three metabolic pathways potentially involved in insulin secretion, we then evaluated their contribution by means of their respective inhibitors: l-NG-nitroarginine methyl ester (l-NAME), methionine sulfoximine (MSO) and difluoromethylornithine (DFMO). Both l-NAME and MSO were able significantly to reduce OKG-induced insulin secretion (30 and 40 % respectively; P<0.05), while DFMO was ineffective. Thus OKG is an effective stimulator of insulin secretion, requiring the joint presence of both Orn and alpha-KG, and acting mainly via the synthesis of NO and glutamine. A better understanding of OKG insulino-secretory properties and its mechanisms of action are a prerequisite for its use in insulin-compromised situations.

L-arginine: an ultradian-regulated substrate coupled with insulin oscillations in healthy volunteers

OBJECTIVE

Coupled oscillations of 50-110 min in insulin and glucose have been found previously in healthy men under continuous enteral nutrition. Because L-arginine induces insulin release as glucose does, we tested the hypothesis that L-arginine can also display such an ultradian rhythm.

RESEARCH DESIGN AND METHODS

Seven healthy male subjects participated in one experimental night during which blood was sampled every 10 min from 2300 to 0700. Plasma glucose, C-peptide, and L-arginine levels were measured simultaneously. The insulin secretion rate (ISR) was calculated from plasma C-peptide levels by a deconvolution procedure.

RESULTS

Plasma glucose followed the recognizable profiles, with oscillations closely linked to similar changes in the ISR. Pulse analysis of L-arginine profiles revealed significant oscillations linked to glucose and ISR oscillations, with the highest cross-correlation coefficients at time lag 0 ranging from 0.380 to 0.680 for glucose and L-arginine and from 0.444 to 0.726 for ISR and L-arginine (P < 0.01). The mean period of L-arginine oscillations was 77.2 +/- 6.2 min, and their mean amplitude was 19.9 +/- 1.7%, similar to that of glucose (17.0 +/- 1.9%), when expressed as the percentage of mean overnight levels.

CONCLUSIONS

This newly discovered ultradian rhythm of L-arginine and its coupling with glucose and ISR oscillations sheds new light on the regulation of L-arginine, the substrate of numerous metabolic pathways, including nitric oxide synthesis. These oscillations may be of significance in conditions of hyperinsulinemia or abnormal glucose tolerance.

Plasma glucagon and insulin responses depend on the rate of appearance of amino acids after ingestion of different protein solutions in humans

To find out whether the hormonal response to feeding with protein solutions is influenced by the nature and degree of protein fractionation, we examined insulin and glucagon responses after intake of protein solutions containing the same amount of nitrogen (2.9 g each) in three men and three women. Four test meals (600 mL) [glucose (419 kJ/L), pea (PPH) and whey peptide hydrolysates (WPH) (921 and 963 kJ/L, respectively) and a cow's milk solution (MS) containing complete milk proteins (2763 kJ/L)] were tested. Peptide hydrolysates elicited a faster increase in venous plasma amino acids than did MS (P < 0.05). Despite the higher carbohydrate content of the MS, the peptide hydrolysates elicited a peak insulin response that was two and four times greater than that evoked by the MS and glucose solutions, respectively (P < 0.05). The insulin response was closely related to the increase in plasma amino acids, especially leucine, isoleucine, valine, phenylalanine and arginine, regardless of the rate of gastric emptying. The three protein solutions elicited similar increases of plasma glucagon; however, the response was fastest for both peptide hydrolysates (P < 0.05) and more prolonged for the MS (P < 0.05). The glucagon response was linearly related to the increase in plasma amino acids, regardless of the rate of gastric emptying or meal composition (r = 0.93, r = 0.96 and r = 0.78, all P < 0.05, for the PPH, WPH and MS). Among the plasma amino acids, tyrosine (r = 0.82-0.98, P < 0.05) and methionine (r = 0.98, P < 0.001) were most closely related to the plasma glucagon response. This study shows that the glucagon response to feeding with protein solutions depends on the increase in plasma amino acid concentrations. The combined administration of glucose and peptide hydrolysates stimulates a synergistic release of insulin, regardless of the protein source.

A protein-restricted diet during pregnancy alters in vitro insulin secretion from islets of fetal Wistar rats

Previous studies indicate that insulin secretion from the fetuses of dams fed a low protein (LP) diet is reduced in response to leucine or arginine. The aim of this study was to locate the defect in the insulin secretion pathway induced by a LP diet during gestation. The effects of various secretagogues acting at different levels of the insulin secretion cascade were investigated in vitro in fetal islets from dams fed either a normal or a LP diet during pregnancy. Insulin content, insulin secretion and the cAMP content were then measured. Although insulin content of LP islets did not differ from that of control islets, insulin secretion from LP fetal islets was reduced when challenged by amino acids or cAMP enhancers. This reduction did not appear to be related solely to an altered islet cAMP content. An impairment of insulin secretion remained after stimulation of fetal LP islets with either metabolic or nonmetabolic secretagogues. The insulin secretion by LP islets was restored to normal, however, with barium or cytochalasin-B. These findings demonstrate that an in utero isocaloric LP diet impairs insulin secretion of the fetus. This alteration is located at the exocytosis step in the insulin secretion cascade and not in the insulin pool of the beta cell.

Stimulus-secretion coupling of arginine-induced insulin release: comparison between the cationic amino acid and its methyl ester

The role currently ascribed to the accumulation of L-arginine in the pancreatic islet B-cell as a determinant of its insulinotropic action was reevaluated by comparing the uptake and the metabolic, ionic, electric, and secretory effects of the cationic amino acid with those of its more positively charged methyl ester in rat pancreatic islets. The response to L-arginine methyl ester differed from that evoked by the unesterified amino acid by a lower uptake and oxidation, lack of inhibitory action on D-glucose metabolism, more severe inhibition of the catabolism of endogenous L-glutamine, inhibition of 45Ca net uptake, decrease in both 86Rb outflow from prelabeled islets perifused at normal extracellular Ca2+ concentration and 45Ca efflux from prelabeled islets perifused in the absence of extracellular Ca2+, and delayed and lesser insulinotropic action. These findings reinforce the view that the carrier-mediated entry of L-arginine into the islet B-cells, with resulting depolarization of the plasma membrane, represents the essential mechanism for stimulation of insulin release by this cationic amino acid.

Ingestion of protein hydrolysate and amino acid-carbohydrate mixtures increases postexercise plasma insulin responses in men

To optimize the postexercise insulin response and to increase plasma amino acid availability, we studied postexercise insulin levels after the ingestion of carbohydrate and wheat protein hydrolysate with and without free leucine and phenylalanine. After an overnight fast, eight male cyclists visited our laboratory on five occasions, during which a control drink and two different beverage compositions in two different doses were tested. After they performed a glycogen-depletion protocol, subjects received a beverage (3.5 mL. kg(-1)) every 30 min to ensure an intake of 1.2 g. kg(-1). h(-1) carbohydrate and 0, 0.2 or 0.4 g. kg(-1). h(-1) protein hydrolysate (and amino acid) mixture. After the insulin response was expressed as the area under the curve, only the ingestion of the beverages containing wheat protein hydrolysate, leucine and phenylalanine resulted in a marked increase in insulin response (+52 and + 107% for the 0.2 and 0.4 g. kg(-1). h(-1) mixtures, respectively; P: < 0. 05) compared with the carbohydrate-only trial). A dose-related effect existed because doubling the dose (0.2-0.4 g. kg(-1). h(-1)) led to an additional rise in insulin response (P: < 0.05). Plasma leucine, phenylalanine and tyrosine concentrations showed strong correlations with the insulin response (P: < 0.0001). This study provides a practical tool to markedly elevate insulin levels and plasma amino acid availability through dietary manipulation, which may be of great value in clinical nutrition, (recovery) sports drinks and metabolic research.

Plasma insulin responses after ingestion of different amino acid or protein mixtures with carbohydrate

BACKGROUND

Protein induces an increase in insulin concentrations when ingested in combination with carbohydrate. Increases in plasma insulin concentrations have been observed after the infusion of free amino acids. However, the insulinotropic properties of different amino acids or protein (hydrolysates) when co-ingested with carbohydrate have not been investigated.

OBJECTIVE

The aim of this study was to define an amino acid and protein (hydrolysate) mixture with a maximal insulinotropic effect when co-ingested with carbohydrate.

DESIGN

Eight healthy, nonobese male subjects visited our laboratory, after an overnight fast, on 10 occasions on which different beverage compositions were tested for 2 h. During those trials the subjects ingested 0.8 g*kg(-)(1)*h(-)(1) carbohydrate and 0.4 g*kg(-)(1)*h(-)(1) of an amino acid and protein (hydrolysate) mixture.

RESULTS

A strong initial increase in plasma glucose and insulin concentrations was observed in all trials, after which large differences in insulin response between drinks became apparent. After we expressed the insulin response as area under the curve during the second hour, ingestion of the drinks containing free leucine, phenylalanine, and arginine and the drinks with free leucine, phenylalanine, and wheat protein hydrolysate were followed by the largest insulin response (101% and 103% greater, respectively, than with the carbohydrate-only drink; P < 0.05).

CONCLUSIONS

Insulin responses are positively correlated with plasma leucine, phenylalanine, and tyrosine concentrations. A mixture of wheat protein hydrolysate, free leucine, phenylalanine, and carbohydrate can be applied as a nutritional supplement to strongly elevate insulin concentrations.

Polyamines regulate serine/threonine protein phosphatases in insulin-secreting cells

Reversible protein phosphorylation is an important mechanism by which cells transduce external signals into biologic responses. Levels of protein phosphorylation are determined by the balanced actions of both protein kinases and protein phosphatases (PPases). However, compared with protein kinases, regulation of PPases has been relatively neglected. The insulin secretagogue L-arginine, an immediate metabolic precursor to polyamines, causes a rapid and transient decrease in PPase-1 activity in insulin-secreting RINm5F cells. We here show that polyamines dose-dependently suppress PPase-1-like activity when added to RINm5F cell homogenates at physiologic concentrations (spermine > spermidine > putrescine), while having minor and inconsistent effects on PPase-2A-like activity. The IC50 value for spermine on PPase-1-like activity was approximately 4 mM. The inhibitory effect was reproduced and of comparable magnitude on purified PPases types 1 and 2A. On the other hand, when endogenous polyamine pools were exhausted by 4 days of exposure to the specific L-ornithine decarboxylase inhibitor DL-alpha-difluoromethylornithine, there was an increase in PPase-2A-like activity. Quantitative Western analysis revealed that the amount of PPase-2A protein did not change after this treatment. It is concluded that polyamines cause time-and concentration-dependent inhibitory effects on RINm5F cell PPase activities, which may contribute to the increase in phosphorylation state that occurs after secretory stimulation.

Effects of soybean diet on the beta cells in the streptozotocin treated rats for induction of diabetes

Certain dietary components have been reported to potentially suppress the initiation of experimental insulin-dependent diabetes mellitus (IDDM) in animal models. In the present study, we showed that dietary soybean prevents induction of experimental hyperglycemia by retaining beta cell activity. In rats fed raw soybean, expression of insulin mRNA in pancreatic beta cells was significantly increased compared to those fed with normal diet. In those rats and upon injection of streptozotocin, only few beta cells underwent cell death, most of them demonstrating active viability with enhanced mRNA expression and insulin content. This is consistent with the fact that blood glucose level was normalized (72.51 +/- 1.54 mg/dl) after a transitory hyperglycemic state (> 300 mg/dl). It implies the dietary soybean can prevent beta cell injury by streptozotocin. Moreover, a prolonged hyperglycemia was not observed in rats fed raw soybean even when this was substituted by the normal diet after streptozotocin injection. In contrast, in streptozotocin-treated rats fed normal diet, most beta cells were destroyed and severe hyperglycemia was observed. Although the protective effect was not recorded in the rats fed with heated soybean, some beta cells were found to retain their cell organelles for insulin secretion indicating that some heat-stable components of soybean might prevent the cytotoxic insult. Therefore, we suggest that dietary soybean protected the streptozotocin-induced beta cell damage and restrained the development of hyperglycemia in rats.

Mechanisms for stimulation of rat anterior pituitary cells by arginine and other amino acids

1. Arginine and other amino acids are secretagogues for growth hormone and prolactin in the intact animal, but the mechanism of action is unclear. We have studied the effects of amino acids on cytosolic free calcium concentration ([Ca2+]i) in single rat anterior pituitary (AP) cells. Arginine elicited a large increase of [Ca2+]i) in about 40% of all the AP cells, suggesting that amino acids may modulate hormone secretion by acting directly on the pituitary. 2. Cell typing by immunofluorescence of the hormone the cells store showed that the arginine-sensitive cells are distributed uniformly within all the five AP cell types. The arginine-sensitive cells overlapped closely with the subpopulation of cells sensitive to thyrotrophin-releasing hormone. 3. Other cationic as well as several neutral (dipolar) amino acids had the same effect as arginine. The increase of [Ca2+]i was dependent on extracellular Ca2+ and blocked by dihydropyridine, suggesting that it is due to Ca2+ influx through L-type voltage-gated Ca2+ channels. The [Ca2+]i increase was also blocked by removal of extracellular Na+ but not by tetrodotoxin. The substrate specificity for stimulation of AP cells resembled closely that of the amino acid transport system B0+. We propose that electrogenic amino acid influx through this pathway depolarizes the plasma membrane with the subsequent activation of voltage-gated Ca2+ channels and Ca2+ entry. 4. Amino acids also stimulated prolactin secretion in vitro with a similar substrate specificity to that found for the [Ca2+]i increase. Existing data on the stimulation of secretion of other hormones by amino acids suggest that a similar mechanism could apply to other endocrine glands.

Electrogenic arginine transport mediates stimulus-secretion coupling in mouse pancreatic beta-cells

1. We have investigated the mechanism by which L-arginine stimulates membrane depolarization, an increase of intracellular calcium ([Ca2+]i) and insulin secretion in pancreatic beta-cells. 2. L-Arginine failed to affect beta-cell metabolism, as monitored by NAD(P)H autofluorescence. 3. L-Arginine produced a dose-dependent increase in [Ca2+]i, which was dependent on membrane depolarization and extracellular calcium. 4. The cationic amino acids L-ornithine, L-lysine, L-homoarginine (which is not metabolized) and NG-monomethyl-L-arginine (L-NMMA, a nitric oxide synthase inhibitor) produced [Ca2+]i responses similar to that produced by L-arginine. The neutral nitric oxide synthase inhibitors NG-nitro-L-arginine (L-NNA) and N omega-monomethyl-L-arginine (L-NAME) also increased [Ca2+]i. D-Arginine was ineffective. 5. L-Arginine did not affect whole-cell Ca2+ currents or ATP-sensitive K+ currents, but produced an inward current that was carried by the amino acid. 6. The reverse transcriptase-polymerase chain reaction demonstrated the presence of messenger RNA for the murine cationic amino acid transporters mCAT2A and mCAT2B within the beta-cell. 7. L-Arginine did not affect beta-cell exocytosis as assayed by changes in cell capacitance. 8. Our data suggest that L-arginine elevates [Ca2+]i and stimulates insulin secretion as a consequence of its electrogenic transport into the beta-cell. This uptake is mediated by the mCAT2A transporter.

Mechanisms involved in the effect of nitric oxide synthase inhibition on L-arginine-induced insulin secretion

1. A constitutive nitric oxide synthase (NOSc) pathway negatively controls L-arginine-stimulated insulin release by pancreatic beta cells. We investigated the effect of glucose on this mechanism and whether it could be accounted for by nitric oxide production. 2. NOSc was inhibited by N omega-nitro-L-arginine methyl ester (L-NAME), and sodium nitroprusside (SNP) was used as a palliative NO donor to test whether the effects of L-NAME resulted from decreased NO production. 3. In the rat isolated perfused pancreas, L-NAME (5 mM) strongly potentiated L-arginine (5 mM)-induced insulin secretion at 5 mM glucose, but L-arginine and L-NAME exerted only additive effects at 8.3 mM glucose. At 11 mM glucose, L-NAME significantly inhibited L-arginine-induced insulin secretion. Similar data were obtained in rat isolated islets. 4. At high concentrations (3 and 300 microM), SNP increased the potentiation of arginine-induced insulin output by L-NAME, but not at lower concentrations (3 or 30 nM). 5. L-Arginine (5 mM) and L-ornithine (5 mM) in the presence of 5 mM glucose induced monophasic beta cell responses which were both significantly reduced by SNP at 3 nM but not at 30 nM; in contrast, the L-ornithine effect was significantly increased by SNP at 3 microM. 6. Simultaneous treatment with L-ornithine and L-arginine provoked a biphasic insulin response. 7. At 5 mM glucose, L-NAME (5 mM) did not affect the L-ornithine secretory effect, but the amino acid strongly potentiated the alteration by L-NAME of L-arginine-induced insulin secretion. 8. L-Citrulline (5 mM) significantly reduced the second phase of the insulin response to L-NAME (5 mM) + L-arginine (5 mM) and to L-NAME + L-arginine + SNP 3 microM. 9. The intermediate in NO biosynthesis, NG-hydroxy-L-arginine (150-300 microM) strongly counteracted the potentiation by L-NAME of the secretory effect of L-arginine at 5 mM glucose. 10. We conclude that the potentiation of L-arginine-induced insulin secretion resulting from the blockade of NOSc activity in the presence of a basal glucose concentration (1) is strongly modulated by higher glucose concentrations, (2) is not due to decreased NO production but (3) is probably accounted for by decreased levels of NG-hydroxy-L-arginine or L-citrulline, resulting in the attenuation of an inhibitory effect on arginase activity.

L-arginine and pancreatic islet blood flow in anesthetized rats

The aim of the present study was to see if L-arginine, which induces insulin release and is a precursor of the endothelial-derived relaxing factor nitric oxide, affects whole pancreatic and/or islet blood flow. For this purpose, anesthetized male Sprague-Dawley rats were injected intravenously with either saline or L-arginine (25, 100 or 250 mg/kg body weight). All doses of arginine caused a slight increase in blood glucose concentration, while the highest dose (250 mg/kg body weight) also increased insulin concentration. However, no changes in either mean arterial blood pressure, whole pancreatic or islet blood flow could be discerned with any of the doses of arginine used. It is concluded that insulin release is not necessarily associated with an increased islet blood perfusion.

Arginine enhances glycogen synthesis in response to insulin in 3T3-L1 adipocytes

The present study was undertaken to define the role of L-arginine (L-Arg) in glucose metabolism in differentiated 3T3-L1 adipocytes in culture. L-Arg alone had no effect on 2-deoxyglucose uptake or basal glycogen synthesis, but this amino acid increased by 153 +/- 10% (P < 0.01) the incorporation of glucose into glycogen in insulin-treated cells. L-Glutamate (L-Glu), a major metabolite of L-Arg, also enhanced insulin-stimulated glycogen synthesis. The response to insulin was not altered by L-lysine (L-Lys), but the effect of L-Arg was markedly attenuated by L-Lys. Cell incubation with L-Arg markedly enhanced arginase-mediated urea synthesis, whereas L-Lys abolished this response. The stimulatory effect of L-Arg on insulin-stimulated glycogen synthesis did not appear to be accounted for by the generation of polyamines or the production of nitric oxide, both potentially derived from the enzymatic conversion of L-Arg. In the presence of insulin, cellular ATP levels were significantly increased by L-Arg, L-Glu, and L-Lys as well. These data suggest that metabolic degradation of L-Arg not related to citric acid cycle activity is important in the mechanism by which L-Arg enhances insulin-stimulated glycogen synthesis.

Evidence supporting membrane fusion as the mechanism of myelin basic protein-induced insulin release from rat pancreatic islets

In order to clarify insulinotropic effects of the myelin basic protein (MBP) we studied mode of association and distribution of MBP in the pancreatic islets and tested the insulin-releasing activity of various MBP peptides. Rat pancreatic islets were first stimulated in a static incubation with 10 microM bovine MBP (bMBP) at a substimulatory (3.5 mM) glucose concentration. The islets exposed to MBP released significantly more insulin and glucagon in a second incubation in the absence of added stimulant and in the presence of 11.5 mM arginine than the incubated, non-stimulated islets and islets initially stimulated with 15 mM glucose. Response to stimulation with 15 mM glucose in the second incubation by islets exposed first to MBP was impaired compared to incubated, non-stimulated islets. Immunoelectron microscopy showed that MBP had entered into the islet cells and associated with membranes of intracellular vacuoles, most of which represented enlarged, often fused insulin granules. MBP was also present at the islet edge and in the intercellular spaces. Of the purified MBP peptides of sizes of 4.8-13.6 kDa, produced from the digestion with brain acid proteinase and with pepsin and covering the entire bMBP sequence, only the large peptides (1-88, 9.8 kDa and 43-169, 13.6 kDa) stimulated insulin secretion significantly. Heterogeneous peptide mixtures, obtained from a time-course digestion of bMBP by myelin calcium-activated neutral protease, consisting of peptides of approximate molecular weights of 8-11 kDa and larger, also stimulated insulin release. The glucagon-releasing activity of MBP peptides was low and followed the same pattern as the insulin-releasing activity. The present results suggest that MBP-induced fusion of the membranes of hormone granules is involved in MBP-induced insulin release. The hormone-releasing activity of the large peptides in addition to that of the intact molecule is explained as being due to the ability of these peptides to associate with membranes. MBP-induced hormone release and related effects could be associated with neuropathological conditions such as stroke and multiple sclerosis.

Insulin secretion in rats with chronic nitric oxide synthase blockade

Nitric oxide, which is produced from L-ar-ginine by a nitric oxide-synthase enzyme, has been shown to be a ubiquitous messenger molecule. Recently, it has been suggested that nitric oxide might influence insulin secretion by activating the soluble guanylate cyclase and generating cyclic guanosine monophosphate (cGMP). We have investigated the role of the nitric oxide pathway in insulin secretion by evaluating the insulin response to several secretagogues in rats in which nitric oxide-synthase was chronically inhibited by oral administration of the L-arginine analogue, NG-nitro-L-arginine methyl ester (L-NAME). Blood pressure and aortic wall cGMP content were used as indices of nitric oxide-synthase blockade. Insulin secretion was evaluated after an intravenous bolus of D-glucose, L-arginine or D-arginine. Chronic L-NAME administration induced a 30% increase in blood pressure and a seven-fold drop in arterial cGMP content. Body weight, fasting plasma glucose and insulin were not influenced by L-NAME administration. First-phase insulin secretion (1 + 3 min) in response to glucose was not significantly different in L-NAME and control rats. The areas under the insulin curve were similar in both groups. Insulin secretion in response to D-arginine or L-arginine in L-NAME-treated and control rats were also similar. In conclusion, chronic nitric oxide-synthase blockade increases blood pressure and decreases aortic cGMP content, but does not alter insulin secretion in response to several secretagogues. Chronic oral administration of L-NAME in the rat provides an adequate animal model for studying the L-arginine nitric oxide-pathway.

Role of polyamines in the regulation of proliferation and hormone production by insulin-secreting cells

This paper focuses on the mechanisms regulating proliferation and insulin production by normal and tumoral pancreatic beta-cells. In particular, the evidence for involvement of polyamines is reviewed. Pancreatic islet cells contain high levels of polyamines, and based on findings obtained using enzyme-directed inhibitors, it appears that putrescine and spermidine are necessary for proinsulin biosynthesis, whereas spermine may exert a stimulatory or permissive role in RNA transcription-stabilization and long-term insulin release. Islet polyamine content is not altered by short-term secretory stimulation, nor is the acute secretory response impeded by polyamine synthesis inhibitors, making it unlikely that these amines play any major role in short-term insulin release. Various mitogens increase islet polyamine contents and DNA synthesis, but increases in cytosolic polyamines do not seem to mediate their mitogenicity. Nuclear polyamine content is not altered by the inhibitors, suggesting that maintenance of polyamines within this organelle may be sufficient to sustain elevated DNA synthesis. In tumoral RINm5F cells, polyamine depletion results in decreased proliferation and increased cellular content of insulin and insulin secretory granules without affecting insulin mRNA levels or translation. Moreover, polyamine-depleted RINm5F cells display improved substrate metabolism and sensitivity of the stimulus-secretion coupling. Possible levels of polyamine interaction with Ca2+ metabolism are discussed.

Ornithine supplementation and insulin release in bodybuilders

Ornithine supplementation has gained popularity with athletes because of its alleged potential to release anabolic hormones, factors governing skeletal muscle hypertrophy. Three female and nine male bodybuilders served as subjects in a study to test the effectiveness of oral ornithine in bringing about the release of insulin, an anabolic hormone. After an overnight fast, subjects were administered 40, 100, or 170 mg.kg-1 L-ornithine.HCl by mouth in a random fashion on three consecutive Saturday mornings. Blood samples were drawn at baseline (T = 0), 45, and 90 min afterward. Serum ornithine levels were elevated (p < 0.01) at T = 45 and 90 min for all three dosage levels. However, serum insulin did not change from baseline levels at any dose of ornithine. The present findings show that ornithine is not an insulin secretagogue.

Stimulus-secretion coupling of arginine-induced insulin release: significance of changes in extracellular and intracellular pH

The possible relevance of changes in extracellular and/or intracellular pH to the insulinotropic action of L-arginine and L-homoarginine was investigated in rat pancreatic islets. A rise in extracellular pH from 7.0 to 7.4 and 7.8 augmented the secretory response to these cationic amino acids whilst failing to affect the uptake of L-arginine by islet cells and whilst decreasing the release of insulin evoked by D-glucose. Under these conditions, a qualified dissociation was also observed between secretory data and 45Ca net uptake. Moreover, at high extracellular pH, the homoarginine-induced increase in 86Rb outflow from prelabelled islets rapidly faded out, despite sustained stimulation of insulin release. The cationic amino acids failed to affect the intracellular pH of islet cells, whether in the absence or presence of D-glucose and whether at normal or abnormal extracellular pH. These findings argue against the view that the secretory response to L-arginine would be related to either a change in cytosolic pH or the accumulation of this positively charged amino acid in the beta-cell. Nevertheless, they suggest that the yet unidentified target for L-arginine and its non-metabolized analogue in islet cells displays pH-dependency with optimal responsiveness at alkaline pH.

Stimulus-secretion coupling of arginine-induced insulin release. Resistance of arginine- and ornithine-stimulated glucagon and insulin release to D,L-alpha-difluoromethylornithine

In the isolated perfused rat pancreas, D,L-difluoromethylornithine, tested at a concentration of 3 mmol/L, failed to affect the release of glucagon and insulin caused, over 15 min stimulation, by either L-arginine or L-ornithine (2.0, 5.0 or 10.0 mmol/L) in the presence of either 3.3 or 5.6 mmol/L D-glucose. The inhibition of ornithine decarboxylase also failed to affect the release of glucagon provoked by either L-leucine (2 or 3 mmol/L) or L-glutamine (2 mmol/L) and the secretion of insulin stimulated by a rise in glucose concentration from 5.6 to 10.6 mmol/L. These data are interpreted to suggest that the rapid generation of polyamines from either L-arginine or L-ornithine does not play any significant role in the immediate glucagonotropic and insulinotropic action of these cationic amino acids.

Perturbation of pancreatic islet function in glucose-infused rats

The secretory behavior of insulin- and glucagon-producing cells was found to be perturbed in isolated perfused pancreases removed from rats infused with hypertonic solutions of glucose for 48 hours. The anomalies included a high basal release of insulin and a paradoxical increase in insulin output and decrease in glucagon release in response to a fall in D-glucose concentration. Likewise, in isolated islets prepared from the glucose-infused rats, L-arginine or theophylline stimulated insulin release at a low ambient concentration of D-glucose, at variance with the situation found in islets removed from normal rats. These secretory perturbations could not be attributed to any obvious defect in either the transport of D-glucose into islet cells or its further utilization and oxidation, but coincided with the abnormal accumulation of glycogen in the B-cell. It is proposed that the latter anomaly may play a role in the altered dynamics of insulin release found in animals or patients with long-term hyperglycemia.