Positive interplay between FFAR4/GPR120, DPP-IV inhibition and GLP-1 in beta cell proliferation and glucose homeostasis in obese high fat fed mice

G-protein coupled receptor-120 (GPR120; FFAR4) is a free fatty acid receptor, widely researched for its glucoregulatory and insulin release activities. This study aimed to investigate the metabolic advantage of FFAR4/GPR120 activation using combination therapy. C57BL/6 mice, fed a High Fat Diet (HFD) for 120 days to induce obesity-diabetes, were subsequently treated with a single daily oral dose of FFAR4/GPR120 agonist Compound A (CpdA) (0.1μmol/kg) alone or in combination with sitagliptin (50 mg/kg) for 21 days. After 21-days, glucose homeostasis, islet morphology, plasma hormones and lipids, tissue genes (qPCR) and protein expression (immunocytochemistry) were assessed. Oral administration of CpdA improved glucose tolerance (34% p<0.001) and increased circulating insulin (38% p<0.001). Addition of CpdA with the dipeptidyl peptidase-IV (DPP-IV) inhibitor, sitagliptin, further improved insulin release (44%) compared to sitagliptin alone and reduced fat mass (p<0.05). CpdA alone (50%) and in combination with sitagliptin (89%) induced marked reductions in LDL-cholesterol, with greater effects in combination (p<0.05). All treatment regimens restored pancreatic islet and beta-cell area and mass, complemented with significantly elevated beta-cell proliferation rates. A marked increase in circulating GLP-1 (53%) was observed, with further increases in combination (38%). With treatment, mice presented with increased Gcg (proglucagon) gene expression in the jejunum (130% increase) and ileum (120% increase), indicative of GLP-1 synthesis and secretion. These data highlight the therapeutic promise of FFAR4/GPR120 activation and the potential for combined benefit with incretin enhancing DPP-IV inhibitors in the regulation of beta cell proliferation and diabetes.

CRISPR/Cas9 gene editing demonstrates metabolic importance of GPR55 in the modulation of GIP release and pancreatic beta cell function

G-protein coupled receptor-55 (GPR55), an endocannabinoid receptor, is a novel anti-diabetic target. This study aimed to assess the metabolic functionality of GPR55 ligands using CRISPR/Cas9 gene editing to determine their regulatory role in beta cell function and incretin-secreting enteroendocrine cells. A clonal Gpr55 knockout beta cell line was generated by CRISPR/Cas9 gene editing to investigate insulin secretion and Gpr55 signalling. Acute effects of GPR55 agonists were investigated in high fat fed (HFD) diabetic HsdOla:TO (Swiss TO) mice. Atypical and endogenous endocannabinoid ligands (10^-7-10^-4M) stimulated insulin secretion (p < 0.05-0.001) in rodent (BRIN-BD11) and human (1.1B4) beta cells, with 2-2.7-fold (p < 0.001) increase demonstrated in BRIN-BD11 cells (10^-4M). The insulinotropic effect of Abn-CBD (42 %), AM251 (30 %) and PEA (53 %) were impaired (p < 0.05) in Gpr55 knockout BRIN-BD11 cells, with the secretory effect of O-1602 completely abolished (p < 0.001). Gpr55 ablation abolished the release of intracellular Ca²⁺ upon treatment with O-1602, Abn-CBD and PEA. Upregulation of insulin mRNA by Abn-CBD and AM251 (1.7-3-fold; p < 0.01) was greatly diminished (p < 0.001) in Gpr55 null cells. Orally administered Abn-CBD and AM251 (0.1 μmol/kgBW) improved GIP (p < 0.05-p < 0.01), GLP-1 (p < 0.05-p < 0.001), glucose tolerance (p < 0.001) and circulating insulin (p < 0.05-p < 0.001) in HFD diabetic mice. Abn-CBD in combination therapy with DPP-IV inhibitor (Sitagliptin) resulted in greater improvement in glucose tolerance (p < 0.05) and insulin release (p < 0.05). Antagonism of Gpr55 in-vivo attenuated the glucoregulatory effects of Abn-CBD (p < 0.05). Conclusively, GPR55 agonists enhance insulin, GIP and GLP-1 release, thereby promoting GPR55 agonist monotherapy and combinational therapy as a novel approach for the treatment of type-2-diabetes.

Evaluation of the insulin-releasing and glucose-lowering effects of GPR120 activation in pancreatic β-cells

AIMS

To assess the potency and selectivity of various GPR120 agonists and to determine the cellular localization of GPR120 in clonal β-cells and pancreatic islets.

METHODS

Insulin secretion and alterations in intracellular Ca(2+) and cAMP response to glucose and GPR120 agonists, including endogenous agonists α-linolenic acid (ALA), docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and a synthetic analogue (GW-9508), were examined using clonal pancreatic BRIN-BD11 cells, mouse pancreatic islets and in vivo studies using NIH Swiss mice. Cytotoxicity was assessed by lactate dehydrogenase release. Cellular localization of GPR120 was explored by double-staining immunohistochemistry.

RESULTS

The most potent and selective GPR120 agonist tested was ALA (half maximum effective concentration 1.2 × 10(-8) mol/l) with a maximum stimulation of insulin secretion of 53% at 10(-4) mol/l (p < 0.001) in BRIN-BD11 cells. Stimulation of insulin secretion was also observed with GW-9508 (6.4 × 10(-8) mol/l; 47%), EPA (7.9 × 10(-8) mol/l; 36%) and DHA (1.0 × 10(-7) mol/l; 50%). Results were corroborated by islet studies, with no evidence of cytotoxic effects. Dose-dependent insulin secretion by GPR120 agonists was glucose-sensitive and accompanied by significant elevations of intracellular Ca(2+) and cAMP. Immunocytochemistry showed GPR120 expression on BRIN-BD11 cells and was confined to islet β-cells with no distribution on α-cells. Administration of GPR120 agonists (0.1 µmol/kg body weight) in glucose tolerance studies significantly reduced plasma glucose and augmented insulin release in mice.

CONCLUSIONS

These results indicate that GPR120 is expressed on pancreatic β-cells and that agonists at this receptor are potent insulin secretagogues with therapeutic potential for type 2 diabetes.

Evidence for inhibitory autocrine effects of proinsulin C-peptide on pancreatic β-cell function and insulin secretion

AIMS

Autocrine and paracrine regulatory mechanisms ensure integrated secretion of islet hormones that respond efficiently to changes in metabolic need. As proinsulin C-peptide exerts various biological effects and binds to cell membranes including insulin-secreting β cells, its physiological role in insulin release was examined.

METHODS

Insulin releasing activity of human and rat C-peptides were studied in the clonal pancreatic cell line, BRIN-BD11, with findings substantiated using isolated islets and in vivo studies employing SWISS TO mice.

RESULTS

Acute exposure of clonal β cells to human C-peptide resulted in concentration-dependent inhibitory effects on insulin secretion at 5.6 mM (p < 0.05-p < 0.001) and 16.7 mM (p < 0.01-p < 0.001) glucose. At physiologically relevant intra-islet concentrations (10(-9) -10(-6) M), C-peptide suppressed the insulin-secretory responses to a range of secretagogues acting at different points in the β cell stimulus-secretion coupling pathway including alanine (p < 0.05), Ca(2+) (p < 0.001), arginine (p < 0.05), tolbutamide (p < 0.001), glucagon-like peptide 1 (GLP-1) (p < 0.001), isobutylmethylxanthine (IBMX) (p < 0.01) and KCl (p < 0.05). Similar results were obtained using isolated mouse pancreatic islets. Human C-peptide (3 × 10(-7) M, p < 0.001), somatostatin-14 (3 × 10(-7) M, p < 0.01) and diazoxide (300 µM, p < 0.001) reduced both alanine and glucose-stimulated insulin release by 43, 25 and 48%, respectively. The effects of human C-peptide were reproduced using rat C-peptide I and II. C-peptide also inhibited in vivo glucose-stimulated insulin release and impaired glucose tolerance in mice.

CONCLUSIONS

C-peptide is a biologically active endogenous peptide hormone that exerts inhibitory autocrine effects on pancreatic β-cell function. Mechanisms involving the activation of K(+) channels and a distal effect downstream of increased cytoplasmic Ca(2+) appear to be implicated in the inhibition of insulin secretion.

Evaluation of the insulin releasing and antihyperglycaemic activities of GPR55 lipid agonists using clonal beta-cells, isolated pancreatic islets and mice

BACKGROUND AND PURPOSE

G-protein coupled receptor (GPR)55 is a novel lipid sensing receptor activated by both cannabinoid endogenous ligands (endocannabinoids) and other non-cannabinoid lipid transmitters. This study assessed the effects of various GPR55 agonists on glucose homeostasis.

EXPERIMENTAL APPROACH

Insulin secretion and changes in intracellular Ca(2) (+) and cAMP in response to glucose and a range of GPR55 agonists [endogenous ligands (OEA, PEA), chemically synthetic cannabidiol (CBD) analogues (Abn-CBD, 0-1602), an analogue of rimonabant (AM-251) and antagonist (CBD)] were investigated in clonal BRIN-BD11 cells and mouse pancreatic islets. Cytotoxicity was assessed by LDH release, cellular localization by double-staining immunohistochemistry and in vivo effects assessed in mice.

KEY RESULTS

The most potent and selective GPR55 agonist was the synthetic CBD analogue, Abn-CBD (pEC50 10.33), maximum stimulation of 67% at 10(-4)  mol·L(-1) (P < 0.001) in BRIN-BD11 cells. AM-251 (pEC50 7.0), OEA (pEC50 7.0), 0-1602 (pEC50 7.3) and PEA (pEC50 6.0) stimulated insulin secretion. Results were corroborated by islet studies, with no cytotoxic effects. Concentration-dependent insulin secretion by GPR55 agonists was glucose-sensitive and accompanied by elevations of [Ca(2) (+) ]i (P < 0.01-P < 0.001) and cAMP (P < 0.05-P < 0.01). GPR55 agonists exhibited insulinotropic and glucose lowering activity in vivo. GPR55 was expressed on BRIN-BD11 cells and confined to islet beta cells with no distribution on alpha cells.

CONCLUSION AND IMPLICATIONS

These results demonstrate GPR55 is distributed in pancreatic beta cells and is a strong activator of insulin secretion, with glucose-lowering effects in vivo. Development of agents agonizing the GPR55 receptor may have therapeutic potential in the treatment of type 2 diabetes.

Effect of RU486 on hepatic and adipocyte gene expression improves diabetes control in obesity-type 2 diabetes

Cortisol has wide-ranging actions, namely in gluconeogenesis and glycogenesis and exerts its effects through the glucocorticoid receptor. In the present study, we examined effects of glucocorticoid receptor blockade on type 2 diabetes control using the antagonist, RU486. Obese diabetic mice received daily injections of vehicle or RU486 over 28 days. Food intake, body weight, and plasma glucose were measured frequently. At 28 days, glucose tolerance, insulin sensitivity, and plasma triglycerides were assessed. Epididymal white adipose tissue and liver were excised for measurement of gene expression. Daily administration of RU486 had no effect on body weight or food intake, but plasma glucose concentrations were significantly lowered (1.4-1.6-fold; p<0.05 to p<0.001). Glucose concentrations were also significantly reduced (2.2-fold; p<0.001) following a glucose challenge. Similarly, exogenous insulin evoked a significantly greater reduction in plasma glucose (3.6-fold; p<0.01). Gene expression analysis revealed a significant reduction in hepatic mRNA of key enzymes, namely PEPCK-C (25%; p<0.01) and G6 Pase (32%; p<0.01) and also 11beta-HSD1 (18%; p<0.05). Investigation of adipose tissue gene expression also demonstrated reduced expression in 11beta-HSD1 (47%; p<0.05) and LPL (47%; p<0.001). These data demonstrate wide-ranging effects of glucocorticoid receptor antagonism on gene expression and metabolism, illustrating the therapeutic potential of specific glucocorticoid receptor antagonists in obesity-related diabetes.

Meal-dependent regulation of circulating glycated insulin in type 2 diabetic subjects

There is mounting evidence that elevated circulating concentrations of glycated insulin play a role in insulin resistance in type 2 diabetes. This study evaluated the secretion of glycated insulin in response to enteral stimulation in type 2 diabetic subjects. Following a mixed meal (450 kcal; 44 % carbohydrate; 40 % fat; 16 % protein), glycated insulin rose 10-fold to peak (60 min) at 104.5 +/- 25.0 pmol/l (p < 0.001), representing 22 % total circulating insulin. The response paralleled early rises in insulin and C-peptide, which peaked at 90 min and were more protracted. Maximum glucose concentrations were observed at 50 min. These data indicate that type 2 diabetic subjects exhibit a rapid meal-induced release of glycated insulin from readily releasable pancreatic beta-cell stores, which might contribute to impaired glucose homeostasis following enteral nutrition.

Inhibition of dipeptidyl peptidase IV activity by oral metformin in Type 2 diabetes

AIMS

Glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) are important insulinotropic hormones that enhance the insulin secretory response to feeding. Their potential for treating Type 2 diabetes is limited by short biological half-life owing to degradation by dipeptidyl peptidase IV (DPP IV). We investigated the acute effects of metformin on DPP IV activity in Type 2 diabetes to elucidate inhibition of DPP IV as a possible mechanism of action.

METHODS

Eight fasting subjects with Type 2 diabetes (5M/3F, age 53.1+/-4.2 years, BMI 36.8+/-1.8 kg/m2, glucose 8.9+/-1.2 mmol/l, HbA1c 7.8+/-0.6%) received placebo or metformin 1 g orally 1 week apart in a random, crossover design.

RESULTS

Following metformin, DPP IV activity was suppressed compared with placebo (AUC0-6 h 3230+/-373 vs. 5764+/-504 nmol ml/l, respectively, P=0.001). Circulating glucose, insulin and total GLP-1 were unchanged. Metformin also concentration-dependently inhibited endogenous DPP IV activity in vitro in plasma from Type 2 diabetic subjects.

CONCLUSION

Oral metformin effectively inhibits DPP IV activity in Type 2 diabetic patients, suggesting that the drug may have potential for future combination therapy with incretin hormones.

Characterization of the C-terminal region of molecular forms of human milk bile salt-stimulated lipase

Bile salt-stimulated lipase (BSSL) in human milk exists in multiple molecular forms and it has been shown that approximately one-third of lactating mothers secrete two forms.

AIM

to determine the structural features of BSSL that may give rise to this heterogeneity.

METHODS

Oligosaccharides present in the proline-rich region in the C-terminus of BSSL were investigated using deglycosylating enzymes and lectin affinity probing to determine the origin of the multiple molecular forms.

RESULTS

It was found that the variability in the molecular mass of BSSL is due predominantly to glycosylation. The molecular forms contain similar sugar chains; all forms possess the core disaccharide Galbeta1-3GalNAc and beta-D-galactose, fucose linked at alpha1-6 and sialic acid linkage alpha2-3 to galactose.

CONCLUSION

The molecular mass difference in the BSSL molecular forms cannot be attributed to the type of carbohydrate moiety in the sugar chains of the N- and O-linked sites suggesting that the differences arise from the extent or quantity of glycosylation. The oligosaccharides in the C-terminal region contain Lewis x and b and, less prominently, Lewis a antigenic structures. Owing to the presence of these blood-group-related antigenic determinants, the C-terminal region of BSSL may have an adhesive function in cell-cell interactions.

Effects of nateglinide on the secretion of glycated insulin and glucose tolerance in type 2 diabetes

AIMS

Glycation of insulin has been demonstrated within pancreatic beta-cells and the resulting impaired bioactivity may contribute to insulin resistance in diabetes. We used a novel radioimmunoassay to evaluate the effect of nateglinide on plasma concentrations of glycated insulin and glucose tolerance in type 2 diabetes.

METHODS

Ten patients (5 M/5 F, age 57.8+/-1.9 years, HbA(1c) 7.6+/-0.5%, fasting plasma glucose 9.4+/-1.2 mmol/l, creatinine 81.6+/-4.5 microM/l) received oral nateglinide 120 mg or placebo, 10 min prior to 75 g oral glucose in a random, single blind, crossover design, 1 week apart. Blood samples were taken for glycated insulin, glucose, insulin and C-peptide over 225 min.

RESULTS

Plasma glucose and glycated insulin responses were reduced by 9% (P=0.005) and 38% (P=0.047), respectively, following nateglinide compared with placebo. Corresponding AUC measures for insulin and C-peptide were enhanced by 36% (P=0.005) and 25% (P=0.007) by nateglinide.

CONCLUSIONS

Glycated insulin in type 2 diabetes is reduced in response to the insulin secretagogue nateglinide, resulting in preferential release of native insulin. Since glycated insulin exhibits impaired biological activity, reduced glycated insulin release may contribute to the antihyperglycaemic action of nateglinide.

Demonstration of increased concentrations of circulating glycated insulin in human Type 2 diabetes using a novel and specific radioimmunoassay

AIMS/HYPOTHESIS

Glycation of insulin, resulting in impaired bioactivity, has been shown within pancreatic beta cells. We have used a novel and specific radioimmunoassay to detect glycated insulin in plasma of Type 2 diabetic subjects.

METHODS

Blood samples were collected from 102 Type 2 diabetic patients in three main categories: those with good glycaemic control with a HbA(1c) less than 7%, moderate glycaemic control (HbA(1c) 7-9%) and poor glycaemic control (HBA(1c) greater than 9%). We used 75 age- and sex-matched non-diabetic subjects as controls. Samples were analysed for HbA(1c), glucose and plasma concentrations of glycated insulin and insulin.

RESULTS

Glycated insulin was readily detected in control and Type 2 diabetic subjects. The mean circulating concentration of glycated insulin in control subjects was 12.6+/-0.9 pmol/l ( n=75). Glycated insulin in the good, moderate and poorly controlled diabetic groups was increased 2.4-fold ( p<0.001, n=44), 2.2-fold ( p<0.001, n=41) and 1.1-fold ( n=17) corresponding to 29.8+/-5.4, 27.3+/-5.7 and 13.5+/-2.9 pmol/l, respectively.

CONCLUSION/INTERPRETATION

Glycated insulin circulates at noticeably increased concentrations in Type 2 diabetic subjects.

Secretion of glycated insulin from pancreatic beta-cells in diabetes represents a novel aspect of beta-cell dysfunction and glucose toxicity

Hyperglycaemia, a significant pathophysiological state in diabetes mellitus, may contribute to defective pancreatic beta-cell function, secretion and action of insulin through glycation of important regulatory proteins. This paper highlights recent data supporting the concept that pancreatic beta-cell dysfunction is associated with increased glycation of functional proteins. The pancreatic beta-cell provides a highly favourable environment for the intracellular glycation of insulin which is a relatively rapid, glucose-dependent process. Using a novel radioimmunoassay and immunocytochemical techniques, glycated insulin has been shown to be stored and secreted from pancreatic beta-cells in both human and animal models of diabetes. Glycated insulin represents a significant proportion of total circulating insulin in type 2 diabetes and may have impaired metabolic clearance compared with native insulin. Since glycation of insulin disturbs normal cellular function and results in decreased biological activity, it may play a significant contributory role in the insulin resistance and glucose intolerance of type 2 diabetes. Further studies are necessary to evaluate the possible significance of glycated insulin in both the pathophysiology of diabetes and future therapeutic approaches.

Evaluation of glycated insulin in diabetic animals using immunocytochemistry and radioimmunoassay

Glycated insulin was evaluated in plasma and biological tissues of diabetic animal models by immunocytochemistry (ICC) and a novel radioimmunoassay. Glycated insulin circulated at 0.10 +/- 0.04 ng/ml and 2.20 +/- 0.14 ng/ml in lean and diabetic obese (ob/ob) mice, corresponding to 12.5 and 9.8% total plasma insulin, respectively. The concentration of glycated insulin was elevated 22-fold in obese mice compared to controls (P < 0.001). In the pancreas, glycated insulin was 48 +/- 10 and 83 +/- 4 ng/g wt (P < 0.05) in lean and obese mice, respectively, representing approximately 2% total insulin in the diabetic pancreas (4.60 +/- 0.17 microg/g wt). ICC revealed fluorescent positively stained cells in pancreatic islets from hydrocortisone (HC)-treated diabetic rats. Fasting of HC-treated rats, resulted in 3-fold and 15-fold reductions in plasma glycated insulin (P < 0.01) and insulin (P < 0.001), respectively. Following a 30 min feeding period in these insulin resistant rats, plasma glucose, insulin, and glycated insulin increased (P < 0.001) rapidly with 1.4-, 1.6-, and 2.9-fold elevations, respectively. Injection of HC-treated rats with insulin (50 U/kg) resulted in a rapid 33% decrease of plasma glucose (P < 0.001) and a marked 4-fold increase in plasma insulin (P < 0.01), whereas glycated insulin concentrations remained unchanged. Since glycation of insulin impairs biological activity, physiologically regulated secretion of glycated insulin into the circulation in diabetic animal models suggests a role in the pathogenesis of diabetes.

Degradation and glycemic effects of His(7)-glucitol glucagon-like peptide-1(7-36)amide in obese diabetic ob/ob mice

Glucagon-like peptide-1(7-36)amide (tGLP-1) has attracted considerable potential as a possible therapeutic agent for type 2 diabetes. However, tGLP-1 is rapidly inactivated in vivo by the exopeptidase dipeptidyl peptidase IV (DPP IV), thereby terminating its insulin releasing activity. The present study has examined the ability of a novel analogue, His(7)-glucitol tGLP-1 to resist plasma degradation and enhance the insulin-releasing and antihyperglycemic activity of the peptide in 20-25-week-old obese diabetic ob/ob mice. Degradation of native tGLP-1 by incubation at 37 degrees C with obese mouse plasma was clearly evident after 3 h (35% intact). After 6 h, more than 87% of tGLP-1 was converted to GLP-1(9-36)amide and two further N-terminal fragments, GLP-1(7-28) and GLP-1(9-28). In contrast, His(7)-glucitol tGLP-1 was completely resistant to N-terminal degradation. The formation of GLP-1(9-36)amide from native tGLP-1 was almost totally abolished by addition of diprotin A, a specific inhibitor of DPP IV. Effects of tGLP-1 and His(7)-glucitol tGLP-1 were examined in overnight fasted obese mice following i.p. injection of either peptide (30 nmol/kg) together with glucose (18 mmol/kg) or in association with feeding. Plasma glucose was significantly lower and insulin response greater following administration of His(7)-glucitol tGLP-1 as compared to glucose alone. Native tGLP-1 lacked antidiabetic effects under the conditions employed, and neither peptide influenced the glucose-lowering action of exogenous insulin (50 units/kg). Twice daily s.c. injection of ob/ob mice with His(7)-glucitol tGLP-1 (10 nmol/kg) for 7 days reduced fasting hyperglycemia and greatly augmented the plasma insulin response to the peptides given in association with feeding. These data demonstrate that His(7)-glucitol tGLP-1 displays resistance to plasma DPP IV degradation and exhibits antihyperglycemic activity and substantially enhanced insulin-releasing action in a commonly used animal model of type 2 diabetes.

Structure, antihyperglycemic activity and cellular actions of a novel diglycated human insulin

Human insulin was glycated under hyperglycemic reducing conditions and a novel diglycated form (M(r) 6135.1 Da) was purified by RP-HPLC. Endoproteinase Glu-C digestion combined with mass spectrometry and automated Edman degradation localized glycation to Gly(1) and Phe(1) of the insulin A- and B-chains, respectively. Intraperitoneal (i.p.) administration of diglycated insulin to mice alone or in combination with glucose (7 nmol/kg) resulted in a 43-61% and 11-34% reduction in glucose lowering activity, respectively, compared with native insulin. Consistent with these findings, diglycated insulin (10(-9) to 10(-7) mol/liter) was 22-38% less effective (P < 0.001) than native insulin in stimulating glucose uptake, glucose oxidation and glycogen production in isolated mouse abdominal muscle.

Production and characterization of specific antibodies for evaluation of glycated insulin in plasma and biological tissues

Previous studies have shown that glycation of insulin occurs in pancreatic beta-cells under conditions of hyperglycaemia and that the site of glycation is the N-terminal Phe(1) of the insulin B-chain. To enable evaluation of glycated insulin in diabetes, specific antibodies were raised in rabbits and guinea-pigs by using two synthetic peptides (A: Phe-Val-Asn-Gln-His-Leu-Cys-Tyr, and B: Phe-Val-Asn-Gln-His-Leu-Tyr-Lys) modified by N-terminal glycation and corresponding closely to the N-terminal sequence of the glycated human insulin B-chain. For immunization, the glycated peptides were conjugated either to keyhole limpet haemocyanin or ovalbumin using glutaraldehyde, m-maleimidobenzoyl-N-hydroxysuccinimide ester or 1-ethyl-3-(3-dimethylamino propyl) carbodiimide hydrochloride. Antibody titration curves, obtained using I(125)-tyrosylated tracer prepared from glycated peptide A, revealed high-titre antisera in five groups of animals immunized for 8-28 weeks. The highest titres were observed in rabbits and guinea-pigs immunized with peptide B coupled to ovalbumin using glutaraldehyde. Under radioimmunoassay conditions, these antisera exhibited effective dose (median) (ED(50)) values for glycated insulin of 0.3-15 ng/ml and 0.9-2.5 ng/ml respectively, with negligible cross-reactivity against insulin or other islet peptides. The degree of cross-reaction with glycated proinsulin was approximately 50%. Glycated insulin in plasma of control and hydrocortisone-treated diabetic rats measured using rabbit 3 antiserum (1:10 000 dilution; sensitivity <19 pg/ml) was 0. 08+/-0.01 and 1.5+/-0.6 ng/ml (P<0.01), corresponding to 4 and 16% of total circulating insulin concentration respectively. Immunocytochemistry studies of the pancreas of streptozotocin-treated diabetic rats using a 1:1000 dilution of guinea-pig 2 antiserum revealed clusters of fluorescent positively stained cells in islets. These studies document the successful production of polyclonal antisera specific for glycated insulin and their usefulness in radioimmunoassays and immunocytochemistry. The demonstration of glycated insulin in plasma and islets of animal models of diabetes supports the view that glycation of insulin is involved in the pathogenesis of this disease.

Impaired ability of glycated insulin to regulate plasma glucose and stimulate glucose transport and metabolism in mouse abdominal muscle

Previous studies have shown that glycated insulin is secreted from pancreatic beta-cells under conditions of hyperglycaemia. This study has investigated the effects of monoglycated insulin on plasma glucose homeostasis and in vitro cellular glucose transport and metabolism by isolated abdominal muscle of mice. Monoglycated insulin was prepared under hyperglycaemic reducing conditions, purified by RP-HPLC and identified by electrospray ionisation mass spectrometry (5971.1 Da). When administered to mice at an intraperitoneal dose of 7 nmoles/kg body weight, insulin (non-glycated) decreased plasma glucose concentrations and substantially reduced the glycaemic excursion induced by conjoint intraperitoneal injection of 2 g glucose/kg body weight. In comparison, the same dose of monoglycated insulin decreased plasma glucose concentrations to a lesser extent (P < 0.05), corresponding to an approx. 20% reduction of glucose lowering potency. Using isolated abdominal muscle, insulin (10(-9)-10(-7) M) stimulated dose-dependent increases in cellular 2-deoxy-D-[1-3H]glucose uptake, D-[U-14C]glucose oxidation and glycogen production. Monoglycated insulin was approx. 20% less effective than native insulin in stimulating glucose uptake and both indices of metabolism, generally requiring 10-fold greater concentrations to achieve significant stimulatory effects. These data indicate that the impaired biological activity of glycated insulin may contribute to glucose intolerance of diabetes.

Incidence of molecular forms of bile salt-stimulated lipase in preterm and term human milk

Preterm and term human milk samples obtained at various times after delivery were analyzed for the presence of molecular forms of the human milk enzyme, bile salt-stimulated lipase (BSSL). Thirty-five percent of both the preterm and term milk samples contained two molecular forms of BSSL, of variable molecular mass. The remainder contained only one molecular species of either 115 kD (50%) or 120 kD (15%). The number of molecular forms present was not related to length of lactation, maternal age, gestation, or maternal blood group. The specific activity of BSSL purified from term milk was similar to that purified from preterm milk, and there was no difference in specific activity whether one or two molecular forms were present. This study demonstrates heterogeneity of both molecular mass and molecular forms. We conclude that preterm babies fed their own mother's milk are unlikely to be disadvantaged with respect to fat digestion as BSSL secreted in preterm milk appears to be very similar to that produced in term milk, although we cannot exclude other functional differences.