Rapid and sustained stimulation of ␤-cells with glucose induces biphasic insulin secretion. The two phases appear to reflect a characteristic of stimulus-secretion coupling in each ␤-cell rather than heterogeneity in the time-course of the response between ␤-cells or islets. There is no evidence indicating that biphasic secretion can be attributed to an intrinsically biphasic metabolic signal. In contrast, the biphasic rise in cytoplasmic Ca

Glucose increases insulin secretion by raising cytoTight control of insulin secretion by pancreatic ␤-cells is critical for glucose homeostasis. This control is exerted by a number of physiological agents, among which circulating nutrients, in particular glucose, play a central role. Glucose regulation of insulin secretion involves two major signaling pathways leading to the production of triggering and amplifying signals respectively (1). The triggering pathway consists in a now well accepted cascade of events. The metabolism of glucose by oxidative glycolysis causes a rise in the ATP/ADP ratio, which closes ATP-sensitive K ϩ (K ϩ -ATP) 1 channels. The resulting decrease in K ϩ conductance leads to membrane depolarization, with subsequent opening of voltage-dependent Ca 2ϩ channels, Ca 2ϩ influx from the extracellular space, and rise in the concentration of free cytoplasmic Ca 2ϩ ([Ca 2ϩ ] i ), which triggers the exocytosis of insulin granules (2-7). However, the triggering action of Ca 2ϩ does not completely explain the stimulation of insulin secretion by glucose. Amplifying signals are also produced in ␤-cells, and these augment the magnitude of the secretory response, in particular during the sustained phase of stimulation (8 -10). Whereas the importance of this pathway is now undisputed (1), the underlying mechanisms remain controversial (1,(11)(12)(13)(14)(15)(16).The present study was prompted by the current controversy surrounding the hypothesis that intracellular glutamate, formed through amination of ␣-ketoglutarate by glutamate dehydrogenase (GDH), may serve as second messenger in this amplifying pathway (17). The hypothesis was originally based on the observations that glutamate increased insulin release from permeabilized INS-1 cells perifused with elevated fixed concentrations of Ca 2ϩ and ATP, that glucose increased insulinoma and islet cell glutamate content, and that a membranepermeant ester of glutamate increased insulin secretion from intact cells (17). On the other hand, the hypothesis was contradicted by reports suggesting that glucose does not affect glutamate levels in islets from ob/ob mice (18, 19) or rats (20) and in insulinoma cells (15). In addition, the ability of glutamate dimethyl ester to increase insulin secretion has been attributed to its use as a nutrient by ␤-cells (21).Manipulations of the genes coding for glutamate decarboxylase (GAD) and for GDH have also yielded conflicting results. Overexpression of GAD65 in INS-1E cells increased the enzyme activity 26-fold, lowered cell glutamate content by ϳ40%, but inhibited insulin secretion (40%) at 15 mM glucose only, being without effect at 2.5 and 7.5 mM glucose (22). Overexpression of GAD65 in rat islet cells did not affect insulin secretion in response to 8.3 mM glucose but inhibited the sustained response to 16.7 mM glucose (22). These results were considered to support the role of glutamate in glucose-induced insulin secretion. In contrast, transgenic mice overexpressing GAD65 in ␤-cells, normally released insulin in resp...

Objective Emerging studies have focused on the association between non-alcoholic fatty liver disease (NAFLD) and the risk of type 2 diabetes mellitus (T2DM). We aimed to investigate whether NAFLD diagnosed by ultrasonography could predict the risk of future T2DM in a Japanese middle-aged health check population.Methods We conducted a 10-year observational study in a health checkup population of middle-aged Japanese men and women at Hidaka Hospital from 2004 to 2013. We excluded cases with an alcohol intake exceeding 20 g/day and those with impaired glucose tolerance. The remaining 1,544 men and 864 women were classified into fatty liver and non-fatty liver groups based on the findings of abdominal ultrasonography. Both groups were followed for the development of diabetes. A multiple regression analysis was performed for each variable to predict the risk of future diabetes.Results The median age of the participants was 46.0 years at the entry, and the follow-up period was 10 years. The incidence of diabetes in the fatty liver group was 12.5% (29/232) in men and 26.3% (10/38) in women, whereas the incidence of diabetes in the non-fatty liver group was 2.5% (34/1,312) in men and 1.8% (15/826) in women. The relative risk of diabetes associated with fatty liver was 4.8 [95% confidence interval (CI) 3.0 -7.8, p<0.0001] in men and 14.5 (95% CI 7.0-30.1, p<0.0001) in women.Conclusion NAFLD was a significant predictor for future diabetes in a Japanese middle-aged health check population, especially in women.

Betacellulin (BTC) is a polypeptide growth factor isolated from conditioned medium of b-TC-3 insulinoma cells [1]. Mouse BTC comprises 80 amino acids and has three potential glycosylation sites. Fifty amino acid residues of the carboxy terminal region of BTC have a strong homology with transforming growth factor a (TGF-a) and heparin-binding epidermal growth factor-like growth factor (HB-EGF) [1]. Six cystein residues found in members of the epidermal growth factor (EGF) family are conserved in BTC. BTC is thus considered to belong to the EGF family. BTC stimulates cell growth in fibroblasts and vascular smooth muscle cells [1,2], and its potency is nearly identical to that of EGF. Consistent with this notion, radioactive iodine labelled BTC binds to the EGF receptor, and the growth promoting action of BTC is inhibited by a blocking antibody against the EGF receptor [2]. These results indicate that BTC stimulates DNA synthesis by acting on the EGF receptor in fibroblasts [2]. Receptors for the EGF family have been characterised extensively with four Diabetologia (1998) Summary Betacellulin is a member of the epidermal growth factor family and converts pancreatic AR42J cells into insulin-producing cells. This study was conducted to characterise the receptor for betacellulin in AR42J cells. AR42J cells expressed two classes of binding sites for radioactive iodine labelled betacellulin, with Kd values of 4.6 10 11 mol/l and 3.0 10 10 mol/l. The binding of [ 125 I]betacellulin was inhibited by unlabelled betacellulin in a dose-dependent manner, but epidermal growth factor was 50 fold less effective than betacellulin. Affinity cross-linking showed a [ 125 I]betacellulin-binding protein with a molecular weight of approximately 180 KDa. When this protein was immunoprecipitated with antibody against epidermal growth factor receptors ErbB-1, ErbB-2, ErbB-3 or ErbB-4, it was immunoprecipitated only by the anti-ErbB-1 antibody. When the [ 125 I]betacellulin-labelled proteins were immunoprecipitated with a combination of the four ErbB antibodies, and the unprecipitated proteins were then immunoprecipitated with anti-phosphotyrosine antibody, a 190 KDa protein was observed. Betacellulin induced the tyrosine phosphorylation of ErbB-1, ErbB-2 and ErbB-4. Finally, while 100 pmol/l betacellulin converted all of the AR42J into insulin-producing cells in the presence of activin A, 10 nmol/l epidermal growth factor induced differentiation in only about 30 % of the cells. Higher concentrations of epidermal growth factor were less effective. Neu differentiation factor in the presence or absence of epidermal growth factor was ineffective. These results indicate that betacellulin binds to ErbB-1 and possibly another protein with a molecular weight of 190 KDa. The latter betacellulin-binding protein may be involved in the differentiation-inducing activity of betacellulin.

]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 mol/l tolbutamide. In G3, arginine induced small increases in [Ca 2ϩ ]i but no IS. G7 alone only slightly increased [Ca 2ϩ ]i and IS but markedly potentiated arginine effects on [Ca 2ϩ ]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 2ϩ ]i. At all glucose concentrations, tolbutamide dose dependently increased [Ca 2ϩ ]i and IS with thresholds of 25 mol/l for [Ca 2ϩ ]i and 100 mol/l for IS at G3 and of 5 mol/l for both at G7 and above. Between G7 and G15, the effect of tolbutamide on [Ca 2ϩ ]i increased only slightly, whereas that on IS was strongly potentiated. The linear relationship between IS and [Ca 2ϩ ]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 2ϩ ]i. In conclusion, glucose potentiation of arginine-or tolbutamide-induced IS involves increases in both the rise of [Ca 2ϩ ]i and the action of Ca 2ϩ