Arginine-stimulated Insulin Release Research Articles

First-Phase Insulin and Amylin after Bariatric Surgery: A Prospective Randomized Trial on Patients with Insulin Resistance or Diabetes after Gastric Bypass or Sleeve Gastrectomy

Background: Most patients with severe obesity show glucose intolerance. Early after sleeve gastrectomy (LSG) or gastric bypass (LRYGB), a marked amelioration in glycemic control occurs. The underlying mechanism is not yet clear. Objective: To determine whether the improvement in glycemic control on the level of endocrine pancreatic function is due to an increased first-phase insulin secretion comparing LRYGB to LSG. Setting: University of Basel Hospital and St. Clara Research Ltd., Basel, Switzerland. Methods: Sixteen morbidly obese patients with severe obesity and different degrees of insulin resistance were randomized to LSG or LRYGB, and islet cell functions were tested by intravenous glucose and intravenous arginine administration before and 4 weeks after surgery. Results: Fasting insulin and glucose levels and homeostasis model assessment insulin resistance were significantly lower in both groups after surgery compared to baseline, while no change was seen in fasting C-peptide, amylin, and glucagon. After intravenous glucose stimulation, no statistically significant pre- to postoperative change in area under the curve (AUC 0–60 min) was seen for insulin, glucagon, amylin, and C-peptide. No statistically significant pre- to postoperative change in incremental AUC for first-phase insulin release (AUC 0–10 min), second-phase insulin secretion (AUC 10–60 min), and insulin/glucose ratio could be shown in either group. Arginine-stimulated insulin and glucagon release showed no pre- to postoperative change. Conclusion: Intravenous glucose and arginine administrations show no pre- to postoperative changes of insulin release, amylin, glucagon, or C-peptide concentrations, and no differences between LRYGB and LSG were found. The postoperative improvement in glycemic control is not caused by changes in endocrine pancreatic hormone secretion.

Assessment of beta‐cell function in young patients with type 2 diabetes: arginine‐stimulated insulin secretion may reflect beta‐cell reserve

Simple methods for the evaluation of dynamic b-cell function in epidemiological and clinical studies of patients with type 2 diabetes (T2D) are needed. The aim of this study was to evaluate the dynamic beta-cell function in young patients with T2D with different disease durations and treatments. Overall, 54 subjects with T2D from the Diabetes Incidence Study in Sweden (DISS) and 23 healthy control participants were included in this cross-sectional study. Beta-cell function was assessed by intravenous (i.v.) administration of arginine followed by i.v. glucose. The acute insulin and C-peptide responses to arginine (AIRarg and Ac-pepRarg, respectively) and to glucose (AIRglu and Ac-pepRglu, respectively)were estimated.Homeostasis model assessment of b-cell function(HOMA-b) andCpeptide assessments were also used for comparisons between patients with T2D and control participants. AIRarg and Ac-pepRarg, but not AIRglu and Ac-pepRglu, could differentiate between patients with different disease durations. AIRglu values were 89% (P < 0.001) lower and AIRarg values were 29% (P < 0.01) lower in patients with T2D compared with control participants. HOMA-b and fasting plasma C-peptide levels did not differ between the T2D and control groups. In young patients with T2D, the insulin secretory response to i.v. glucose is markedly attenuated, whereas i.v. arginine-stimulated insulin release is better preserved and can distinguish between patients with different disease duration and antidiabetic therapies. This suggests that the i.v. arginine stimulation test may provide an estimate of functional beta-cell reserve.

Transamination Is Required for α-Ketoisocaproate but Not Leucine to Stimulate Insulin Secretion*

It remains unclear how α-ketoisocaproate (KIC) and leucine are metabolized to stimulate insulin secretion. Mitochondrial BCATm (branched-chain aminotransferase) catalyzes reversible transamination of leucine and α-ketoglutarate to KIC and glutamate, the first step of leucine catabolism. We investigated the biochemical mechanisms of KIC and leucine-stimulated insulin secretion (KICSIS and LSIS, respectively) using BCATm(-/-) mice. In static incubation, BCATm disruption abolished insulin secretion by KIC, D,L-α-keto-β-methylvalerate, and α-ketocaproate without altering stimulation by glucose, leucine, or α-ketoglutarate. Similarly, during pancreas perfusions in BCATm(-/-) mice, glucose and arginine stimulated insulin release, whereas KICSIS was largely abolished. During islet perifusions, KIC and 2 mM glutamine caused robust dose-dependent insulin secretion in BCATm(+/+) not BCATm(-/-) islets, whereas LSIS was unaffected. Consistently, in contrast to BCATm(+/+) islets, the increases of the ATP concentration and NADPH/NADP(+) ratio in response to KIC were largely blunted in BCATm(-/-) islets. Compared with nontreated islets, the combination of KIC/glutamine (10/2 mM) did not influence α-ketoglutarate concentrations but caused 120 and 33% increases in malate in BCATm(+/+) and BCATm(-/-) islets, respectively. Although leucine oxidation and KIC transamination were blocked in BCATm(-/-) islets, KIC oxidation was unaltered. These data indicate that KICSIS requires transamination of KIC and glutamate to leucine and α-ketoglutarate, respectively. LSIS does not require leucine catabolism and may be through leucine activation of glutamate dehydrogenase. Thus, KICSIS and LSIS occur by enhancing the metabolism of glutamine/glutamate to α-ketoglutarate, which, in turn, is metabolized to produce the intracellular signals such as ATP and NADPH for insulin secretion.

Combined Risk Allele Score of Eight Type 2 Diabetes Genes Is Associated With Reduced First-Phase Glucose-Stimulated Insulin Secretion During Hyperglycemic Clamps

OBJECTIVEAt least 20 type 2 diabetes loci have now been identified, and several of these are associated with altered β-cell function. In this study, we have investigated the combined effects of eight known β-cell loci on insulin secretion stimulated by three different secretagogues during hyperglycemic clamps.RESEARCH DESIGN AND METHODSA total of 447 subjects originating from four independent studies in the Netherlands and Germany (256 with normal glucose tolerance [NGT]/191 with impaired glucose tolerance [IGT]) underwent a hyperglycemic clamp. A subset had an extended clamp with additional glucagon-like peptide (GLP)-1 and arginine (n = 224). We next genotyped single nucleotide polymorphisms in TCF7L2, KCNJ11, CDKAL1, IGF2BP2, HHEX/IDE, CDKN2A/B, SLC30A8, and MTNR1B and calculated a risk allele score by risk allele counting.RESULTSThe risk allele score was associated with lower first-phase glucose-stimulated insulin secretion (GSIS) (P = 7.1 × 10−6). The effect size was equal in subjects with NGT and IGT. We also noted an inverse correlation with the disposition index (P = 1.6 × 10−3). When we stratified the study population according to the number of risk alleles into three groups, those with a medium- or high-risk allele score had 9 and 23% lower first-phase GSIS. Second-phase GSIS, insulin sensitivity index and GLP-1, or arginine-stimulated insulin release were not significantly different.CONCLUSIONSA combined risk allele score for eight known β-cell genes is associated with the rapid first-phase GSIS and the disposition index. The slower second-phase GSIS, GLP-1, and arginine-stimulated insulin secretion are not associated, suggesting that especially processes involved in rapid granule recruitment and exocytosis are affected in the majority of risk loci.

Calpain-10 Expression Is Elevated in Pancreatic Islets from Patients with Type 2 Diabetes

BackgroundCalpain-10 was the first gene to be identified influencing the risk of type 2 diabetes (T2D) by positioning cloning. Studies in β-cell lines and rodent islets suggest that calpain-10 may act as a regulator of insulin secretion. However, its role in human pancreatic islets remains unclear. The aim of this study was to examine if calpain-10 expression is altered in islets from patients with T2D and if the transcript level correlates with insulin release. We also tested if polymorphisms in the CAPN10 gene are associated with gene expression and insulin secretion in vitro.Methodology/Principal FindingsCalpain-10 mRNA expression was analysed in human pancreatic islets from 34 non-diabetic and 10 T2D multi-organ donors. CAPN10 SNP-43 and SNP-44 were genotyped and related to gene expression and insulin release in response to glucose, arginine and glibenclamide. The mRNA level of calpain-10 was elevated by 64% in pancreatic islets from patients with T2D compared with non-diabetic donors (P = 0.01). Moreover, the calpain-10 expression correlated positively with arginine-stimulated insulin release in islets from non-diabetic donors (r = 0.45, P = 0.015). However, this correlation was lost in islets from patients with T2D (r = 0.09; P = 0.8). The G/G variant of SNP-43 was associated with reduced insulin release in response to glucose (P≤0.04) in non-diabetic donors.ConclusionsWhile calpain-10 expression correlates with insulin release in non-diabetic human islets, this correlation is lost in T2D suggesting that a stimulatory effect of calpain-10 could be lost in patients with T2D.

Alteration of $beta;-cell constitutive NO synthase activity is involved in the abnormal insulin response to arginine in a new rat model of type 2 diabetes

We have previously obtained a new type 2 diabetic syndrome in adult rats given streptozotocin and nicotinamide, characterized by reduced β-cell mass, partially preserved insulin response to glucose and tolbutamide and excessive responsiveness to arginine. We have also established that the neuronal isoform of constitutive NO synthase (nNOS) is expressed in β-cells and modulates insulin secretion. In this study, we explored the kinetics of glucose- and arginine-stimulated insulin release in perifused isolated islets as well as the effect of N-ω-nitro-l-arginine methyl ester (l-NAME), a NOS inhibitor, to get insight into the possible mechanisms responsible for the arginine hypersensitivity observed in vitro in this and other models of type 2 diabetes. A reduced first phase and a blunted second phase of insulin secretion were observed upon glucose stimulation of diabetic islets, confirming previous data in the isolated perfused rat pancreas. Exposure of diabetic islets to 10 mM arginine, in the presence of 2.8 mM glucose, elicited a remarkable monophasic increment in insulin release, which peaked at 639±31 pg/islet/min as compared to 49±18 pg/islet/min in control islets (P⪡0.01). The addition of l-NAME to control islets markedly enhanced the insulin response to arginine, as expected from the documented inhibitory effect exerted by nNOS activity in normal β-cells, whereas it did not further modify the insulin secretion in diabetic islets, thus implying the occurrence of a defective nNOS activity in these islets. A reduced expression of nNOS mRNA was found in the majority but not in all diabetic islet preparations and therefore cannot totally account for the absence of l-NAME effect, that might also be ascribed to post-transcriptional mechanisms impairing nNOS catalytic activity. In conclusion, our results provide for the first time evidence that functional abnormalities of type 2 experimental diabetes, such as the insulin hyper-responsiveness to arginine, could be due to an impairment of nNOS expression and/or activity in β-cells.

Alteration of β-cell constitutive NO synthase activity is involved in the abnormal insulin response to arginine in a new rat model of type 2 diabetes

We have previously obtained a new type 2 diabetic syndrome in adult rats given streptozotocin and nicotinamide, characterized by reduced β-cell mass, partially preserved insulin response to glucose and tolbutamide and excessive responsiveness to arginine. We have also established that the neuronal isoform of constitutive NO synthase (nNOS) is expressed in β-cells and modulates insulin secretion. In this study, we explored the kinetics of glucose- and arginine-stimulated insulin release in perifused isolated islets as well as the effect of N-ω-nitro- l-arginine methyl ester ( l-NAME), a NOS inhibitor, to get insight into the possible mechanisms responsible for the arginine hypersensitivity observed in vitro in this and other models of type 2 diabetes. A reduced first phase and a blunted second phase of insulin secretion were observed upon glucose stimulation of diabetic islets, confirming previous data in the isolated perfused rat pancreas. Exposure of diabetic islets to 10 mM arginine, in the presence of 2.8 mM glucose, elicited a remarkable monophasic increment in insulin release, which peaked at 639±31 pg/islet/min as compared to 49±18 pg/islet/min in control islets ( P⪡0.01). The addition of l-NAME to control islets markedly enhanced the insulin response to arginine, as expected from the documented inhibitory effect exerted by nNOS activity in normal β-cells, whereas it did not further modify the insulin secretion in diabetic islets, thus implying the occurrence of a defective nNOS activity in these islets. A reduced expression of nNOS mRNA was found in the majority but not in all diabetic islet preparations and therefore cannot totally account for the absence of l-NAME effect, that might also be ascribed to post-transcriptional mechanisms impairing nNOS catalytic activity. In conclusion, our results provide for the first time evidence that functional abnormalities of type 2 experimental diabetes, such as the insulin hyper-responsiveness to arginine, could be due to an impairment of nNOS expression and/or activity in β-cells.

Gliclazide directly inhibits arginine-induced glucagon release.

Arginine-stimulated insulin and somatostatin release is enhanced by the sulfonylurea gliclazide. In contrast, gliclazide inhibits the glucagon response. The aim of the present study was to investigate whether this inhibition of glucagon release was mediated by a direct suppressive effect of gliclazide or was secondary to the paracrine effect of released somatostatin. To eliminate the paracrine effects of somatostatin, we first perfused isolated rat pancreata with a medium supplemented with 23% of the standard calcium content. Second, we perifused isolated rat islets with a novel and highly specific antagonist of type 2 somatostatin receptor, DC-41-33 (2 micro mol/l), which fully antagonizes the suppressive somatostatin effect on rat A cells. Gliclazide (30 micro mol/l) inhibited glucagon release by 54% in the perfusion experiments, whereas the somatostatin response was nearly abolished. In islet perifusions with DC-41-33, arginine-induced glucagon release was inhibited by 66%. We therefore concluded that gliclazide inhibits glucagon release by a direct action on the pancreatic A cell.

Effects of Acute or Prolonged Exposure to Human Leptin on Isolated Human Islet Function

The adipocyte-derived hormone leptin has been reported to inhibit, have no effect, or potentiate insulin secretion in-vitro; these effects mainly depend on the species considered, the concentrations used, and the length of exposure. We investigated the direct effects of recombinant human leptin (HL) on human pancreatic beta cell function by studying insulin secretion (IS), hexokinase and glucokinase activity and Km, and potassium channel permeability in purified human islets (HI). In acute experiments, no effect of 1, 5, 20, or 50 ng/ml HL on glucose or arginine stimulated insulin release was found, whereas 500 ng/ml HL caused a significant decrease of glucose induced IS. After 24h pre-culture with either 20 or 500 ng/ml HL, a significant reduction of glucose (but not arginine) stimulated IS was observed. Exposure to leptin caused a significant increase of potassium channel permeability, whereas hexokinase and glucokinase activity and Km remained unchanged. These results suggest that physiological human leptin concentration is able to importantly affect glucose (but not arginine) stimulated insulin release from human islets only after prolonged exposure. This effect is probably mediated by changes of potassium channel permeability, and is not accompanied by modifications of glucose phosphorylating enzymes properties.

Effects of Imidazoline Derivative RX871024 on Insulin, Glucagon, and Somatostatin Secretion from Isolated Perfused Rat Pancreas

The effects of the imidazoline compound RX871024 on arginine-induced insulin, glucagon, and somatostatin secretion in the isolated perfused rat pancreas have been investigated. Arginine induced biphasic insulin, glucagon, and somatostatin release when infused for 20 min at 20 mM concentration and 3.3 mM glucose in the medium. RX871024, at 10 μM, did not influence basal hormone secretion but enhanced arginine-stimulated insulin and somatostatin release. In contrast, glucagon secretion was markedly inhibited by 10 μM imidazoline. RX871024 (1 μM) did not significantly affect arginine-induced insulin and somatostatin secretion but had an inhibitory effect on the second phase of glucagon release. In conclusion, RX871024 exerts a complex effect on the endocrine pancreas challenged by arginine, comprising stimulation of insulin and somatostatin release and inhibition of glucagon release. These effects on hormone release probably constitute the main mechanism of the antidiabetogenic action of the imidazolines.

Postprandial stimulation of insulin release by glucose-dependent insulinotropic polypeptide (GIP). Effect of a specific glucose-dependent insulinotropic polypeptide receptor antagonist in the rat.

Glucose-dependent insulinotropic polypeptide (GIP) is a 42-amino acid peptide produced by K cells of the mammalian proximal small intestine and is a potent stimulant of insulin release in the presence of hyperglycemia. However, its relative physiological importance as a postprandial insulinotropic agent is unknown. Using LGIPR2 cells stably transfected with rat GIP receptor cDNA, GIP (1-42) stimulation of cyclic adenosine monophosphate (cAMP) production was inhibited in a concentration-dependent manner by GIP (7-30)-NH2. Competition binding assays using stably transfected L293 cells demonstrated an IC50 for GIP receptor binding of 7 nmol/liter for GIP (1-42) and 200 nmol/liter for GIP (7-30)-NH2, whereas glucagonlike peptide-1 (GLP-1) binding to its receptor on ++betaTC3 cells was minimally displaced by GIP (7-30)-NH2. In fasted anesthetized rats, GIP (1-42) stimulated insulin release in a concentration-dependent manner, an effect abolished by the concomitant intraperitoneal administration of GIP (7-30)-NH2 (100 nmol/ kg). In contrast, glucose-, GLP-1-, and arginine-stimulated insulin release were not affected by GIP (7-30)-NH2. In separate experiments, GIP (7-30)-NH2 (100 nmol/kg) reduced postprandial insulin release in conscious rats by 72%. It is concluded that GIP (7-30)-NH2 is a GIP-specific receptor antagonist and that GIP plays a dominant role in mediating postprandial insulin release.

Arginine stimulated glucagon and insulin secretion by islets of Langerhans of pregnant and lactating rats.

Glucagon secretion by isolated pancreatic rat islets was not affected by an increase of the glucose concentration from 2.5 to 5.0 mM, but was stimulated by 25 mM arginine. This stimulation was only slightly increased by pregnancy and lactation. Insulin secretion increased, when the glucose concentration was raised from 2.5 to 5.0 mM, especially in pregnant islets and was also stimulated by arginine. The arginine stimulated insulin release was greatly augmented during late pregnancy and lactation. Islet content did not reflect the changes in secretion: glucagon content increased in the course of pregnancy and lactation, while insulin content was increased during late pregnancy, but not during lactation. It is concluded that, unlike insulin, glucagon is only marginally involved in the increase in anabolism during pregnancy and lactation.

Physiological Characteris tics of Spontaneously Developed Diabetes in Male WBN/Kob Rat and Prevention of Development of Diabetes by Chronic Oral Administration of Synthetic Trypsin Inhibitor (FOY-305)

The male WBN/Kob rats spontaneously develop diabetes mellitus with age. In this study, we examined how glucose tolerance, potency of insulin release, and histology of the pancreas were changed with age in this model. Furthermore, we examined the effect of FOY-305, a synthetic trypsin inhibitor, on this model. Male WBN/Kob rats were divided into two groups: one group fed on standard pellet diet (STD group) and the other on pellet containing 0.1% FOY-305 (FOY group) for 56 weeks after age 4 weeks. Oral glucose (2 g/kg) tolerance test, histology of the pancreas, and glucose (8.3 mM)- and arginine (10 mM)-stimulated insulin release from the isolated perfused pancreas were examined at 8, 20, 40, and 60 weeks of age in both groups. Pancreatic insulin content was examined at 60 weeks. In the STD group, impairment of glucose tolerance and destruction and fibrosis of pancreatic tissues progressed with age. Glucose-stimulated insulin release was remarkably reduced with age, while arginine-stimulated insulin release was preserved. By contrast, in the FOY group, development of glucose intolerance was delayed and the pancreas showed fewer pathologic changes compared with the STD group. Insulin releases in response to both glucose and arginine were preserved at all ages examined. Total pancreatic insulin content at 60 weeks of age was significantly greater than that of the STD group. The male WBN/Kob rat is a new type of diabetic model that shows a similar pattern of insulin release to that in rat with non-insulin-dependent diabetes mellitus and also shows unique histopathological changes in exocrine pancreas. FOY-305 was effective in preventing the development of diabetes in this model, although its mechanism is still unknown.

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

Interleukin-1β (IL-1β) is known to inhibit glucose-induced insulin release by pancreatic islets. We studied the effect of nicotinamide, an inhibitor of poly[adenosine diphosphate (ADP)-ribose] synthetase and a free-radical scavenger, on this IL-1β-induced inhibition using rat pancreatic islets. In static experiments, groups of five islets were incubated for 24 hours in culture medium CMRL-1066, with or without 50 U/mL IL-1β, in the presence or absence of nicotinamide (dose range, 0 to 50 mmol/L), and then exposed for 1 hour to either 1.4 or 19.4 mmol/L glucose, 10 mmol/L arginine, or 10 μmol/L glyburide. Basal insulin secretion was 183 ± 32 pg/islet/h (mean ± SE, n = 7) and 176 ± 39 (n = 7) in control islets and in islets exposed to 50 U/mL IL-1β, respectively. Glucose-stimulated insulin secretion was significantly reduced (185 ± 41) in IL-1β-exposed islets in comparison to control islets (2,037 ± 363). In parallel, arginine-stimulated insulin release was inhibited by IL-1β exposure (166 ± 31 pg/islet/h, mean ± SE, n = 3) in comparison to control islets (1,679 ± 307). In contrast, IL-1β exposure did not significantly reduced glyburide-induced insulin secretion (1,516 ± 231 and 1,236 ± 214 in control and IL-1β-exposed islets, respectively; mean ± SE, n = 3). When islets were simultaneously exposed to IL-1β and increasing concentrations of nicotinamide, a dose-dependent recovery of glucose-induced insulin secretion was observed, with the maximum effect at 25 mmol/L nicotinamide (1,007 ± 123, P < .001). Arginine-stimulated insulin release was also partially restored by the addition of 25 mmol/L nicotinamide. To study the phases of insulin release, groups of 100 islets exposed to IL-1β in the presence or absence of nicotinamide (as in static experiments) were perifused for 20 minutes at 37°C in phosphate buffer with glucose 1.4 mmol/L (basal secretion) and then stimulated with glucose 19.4 mmol/L for 35 minutes. In control islets, glucose-stimulated insulin secretion followed the characteristic biphasic pattern: the first peak was 54.9 ± 9.7 and the second one was 70.8 ± 12.5 pg/islet/min (mean ± SE, n = 7). In islets exposed to IL-1β, both phases were greatly reduced (4.9 ± 1.1 and 7.8 ± 2.2, respectively; P < .001). In the presence of 25 mmol/L nicotinamide, glucose-stimulated insulin secretion was partially restored (19.3 ± 3.9, P < .01 in the first phase, and 28.1 ± 6.7, P < .01 in the second phase). Nicotinamide, therefore, is able to partially prevent the effect of IL-1β on blunting both glucose- and arginine-induced insulin secretion.

Acute insulin response to intravenous glucose, glucagon and arginine in some subjects at risk for Type 1 (insulin-dependent) diabetes mellitus

The relationships between first-phase insulin secretion to i.v. glucagon and i.v. arginine were studied in 19 healthy adult volunteers (Group I) and in 21 subjects at risk for Type 1 (insulin-dependent) diabetes mellitus with either a "normal" (n = 11; Group IIa) or a "low" insulin response to i.v. glucose (n = 10; Group IIb). Groups I and IIa displayed similar insulin responses to the three secretagogues. In contrast, Group IIb demonstrated lower insulin responses to both glucagon and arginine than control subjects (p less than 0.007 and p less than 0.04 respectively) or than "normo-responders" to glucose (p less than 0.007 and p less than 0.04 respectively). In Group IIb however, arginine-stimulated insulin release was increased compared to the response to glucose (p less than 0.006), while glucagon and glucose led to non-statistically different responses. Five "low-responders" developed Type 1 diabetes. As a group, they displayed lower responses to glucagon and to arginine than subjects who up to now have not developed the disease (p less than 0.05 and p less than 0.0003 respectively). In the subjects who progressed to diabetes, the responses to glucose and glucagon were similarly blunted. In the "low-responders" who have not developed the disease, no statistical difference could be detected between mean responses to glucagon and glucose, but four out of these five subjects had a glucagon-stimulated response within the control range and higher than their corresponding response to glucose. Arginine led to a higher stimulation than glucose, in subgroups that either progressed to diabetes (p less than 0.006) or did not (p less than 0.002).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of Lysine Acetyl-Salicylate on Glucose and Arginine Stimulated Insulin Release in Man

In order to determine the influence of acute inhibition of prostaglandin synthesis on insulin release in man, the influence of lysine acetyl-salicylate (0.9 g) on glucose- and arginine-stimulated insulin release was studied in eight volunteers. No significant differences were found in plasma C-peptide levels between the salicylate and the control study days during administration of arginine (0.5 g/kg; 30 min) nor during a hyperglycemic clamp (glucose level 17 mMol/L; 60 minutes). These studies indicate that acute administration of salicylate does not change insulin release in man.

Dose-response characteristics for glucose-stimulated insulin release in man and assessment of influence of glucose on arginine-stimulated insulin release

Glucose potentiates arginine-induced insulin release. We investigated the dose-response characteristics for both phases of glucose-induced insulin release in normal man, and studied the influence of hyperglycemia on arginine-induced insulin secretion. Dose-response curves of plasma C-peptide increments achieved during 60-minute hyperglycemia clamps (7, 11, 17, 24, and 32 mmol/L) with and without a primed continuous infusion of arginine (infusion rate, 15 mg/kg/min) were analyzed with a modified Michaelis-Menten equation. The ED 50 (half-maximally stimulating blood glucose concentration) of first-phase insulin release (determined from plasma C-peptide increments at 5 minutes) was significantly lower than the ED 50 for the second phase (60 minutes; 8.4 ± 0.8 v 14.3 ± 1.3 mmol/L, respectively, P < .002). Combined glucose-arginine stimulation significantly increased insulin release. Vmax of both phases of glucose-arginine-stimulated insulin release were positively correlated ( r = .75, P < .05). The ED 50 of the influence of glucose on first-phase arginine-induced insulin release was significantly lower than the ED 50 for the second phase (9.0 ± 1.1 v 12.7 ± 1.0 mmol/L, respectively, P < .02). For each insulin secretion phase separately, the ED 50 for the influence of hyperglycemia on arginine-induced insulin release were not significantly different from the ED 50 for glucose-induced insulin secretion (without arginine). When dose-response curves of plasma insulin increments were analyzed with the same equation, the ED 50 of second-phase glucose-induced plasma insulin increments was significantly higher than the ED 50 assessed from the plasma C-peptide increments (21.6 ± 2.8 v 14.3 ± 1.3 mmol/L, respectively, P < .05) We conclude (1) that the regulation by glucose of the two phases of insulin release involves (partly) different mechanisms in vivo, (2) that this difference in regulation occurs both in glucose- and in combined glucose-arginine-stimulated insulin release; (3) that combined glucose-arginine stimulation can be used as a tool for the assessment of B-cell glucose sensitivity; (4) that analysis using plasma insulin (instead of plasma C-peptide) increments can give a falsely high assessment for the parameter of the ED 50.

Pancreatic beta-cell somatostatin receptors

The characteristics of somatostatin (SRIF) receptors in rat pancreatic beta-cells were investigated using rat islets and the beta-cell line HIT-T15 (HIT). The biochemical properties of the SRIF receptors were examined with 125I-labeled des-Ala-1,Gly-2-desamino-Cys-3-[Tyr-11]- dicarba3,14-somatostatin (CGP 23996). 125I-CGP 23996 bound to SRIF receptors in HIT cells with high affinity and in a saturable manner. The binding of 125I-CGP 23996 to SRIF receptors was blocked by SRIF analogues with a rank order of potency of somatostatin 28 (SRIF-28) greater than D-Trp-8-somatostatin greater than somatostatin 14 (SRIF-14). To investigate the physical properties of the HIT cell SRIF receptor, the receptor was covalently labeled with 125I-CGP 23996 using photo-cross-linking techniques. 125I-CGP 23996 specifically labeled a protein of 55 kDa in HIT cell membranes. The size of the SRIF receptor in HIT cells is similar to the size of the SRIF receptor labeled with 125I-CGP 23996 in membranes of freshly isolated islets, suggesting that the physical properties of SRIF receptors in HIT cells and rat islet cells are similar. The binding studies suggest that beta-cells predominantly express a SRIF-28-preferring receptor. In freshly isolated islets, glucose- and arginine-stimulated insulin release was effectively blocked by SRIF-28 but not by SRIF-14. SRIF-14 did inhibit arginine-stimulated glucagon secretion from freshly isolated islets. The dissociation of the inhibitory effects of SRIF-28 and SRIF-14 on insulin and glucagon release from freshly isolated islets suggests that the two peptides act through different receptors in islets to regulate hormone secretion.

Effects of porcine diazepam-binding inhibitor on insulin and glucagon secretion in vitro from the rat endocrine pancreas

Porcine diazepam-binding inhibitor (pDBI) is a novel peptide that has been isolated from the small bowel of the pig, and that occurs also in the islet D-cells. We have studied its effects on hormone release in vitro from the endocrine pancreas of the rat. In isolated islets, pDBI (10 −9–10 −6 M) did not affect basal insulin release at 3.3 mM glucose, whereas stimulated release at 8.3 mM glucose was dose-dependently suppressed by 32–69% ( P < 0.01). Furthermore, insulin secretion stimulated by either 16.7 mM glucose or 1 mM IBMX (3-isobutyl-1-methylxanthine) or 1 μg/ml glibenclamide was suppressed by pDBI at 10 −8 M (by 28–30%, P < 0.05) and 10 −7 M (by 43–47%, P < 0.01). In contrast, islet insulin secretion induced by 20 mM arginine was unaffected by these concentrations of pDBI. In the perfused rat pancreas, pDBI (10 −8 M) enhanced by 30% ( P < 0.05) the first phase (0–5 min) of arginine-stimulated insulin release, whereas the second phase (5–20 min) was unchanged. Moreover, pDBI suppressed by 28% ( P < 0.05) the second phase of arginine-induced glucagon release. Arginine-induced somatostatin release was not significantly affected by the peptide. Since pDBI immunoreactivity has been localized also to islet D-cells, the present results suggest that pDBI may act as a local modulator of islet hormone release.

Islet cell surface antibodies preferentially inhibit glucose-stimulated insulin release in vitro

The effect of islet surface antibodies (ICSA) on in vitro insulin release was studied. Isolated rat islets were incubated in the presence of immunoglobulin preparations from patients with insulin-dependent and non-insulin-dependent diabetes mellitus (IDDM, NIDDM) and healthy subjects, and stimulated with D-glucose, L-arginine or tolbutamide. After incubation, the amount of insulin release from the rat islets was determined. The immunoglobulin preparations from 5 newly diagnosed IDDM patients who were positive for ICSA, and from 5 age-matched healthy subjects were examined. Even in the absence of complement or lymphocytes, immunoglobulin fractions positive for ICSA significantly inhibited low and high concentrations of glucose-stimulated insulin release compared with normal control ( P < 0.02), but had little influence on insulin release after stimulation with tolbutamide. Arginine-stimulated insulin release was almost the same in ICSA-positive immunoglobulin fractions and the control. Immunoglobulin fractions negative for ICSA either from four patients with recently diagnosed IDDM or from four newly diagnosed NIDDM patients had only negligible effect on insulin release after stimulation with glucose. These results suggest that ICSA in IDDM patients, even in the absence of complement or lymphocytes, may preferentially interfere with the mechanisms of glucose-stimulated insulin release in the pancreatic B cells.