Pathophysiology of insulin secretion in diabetes mellitus

Review of methods for measuring β-cell function: Design considerations from the Restoring Insulin Secretion (RISE) Consortium

The Restoring Insulin Secretion (RISE) study was initiated to evaluate interventions to slow or reverse the progression of β-cell failure in type 2 diabetes (T2D). To design the RISE study, we undertook an evaluation of methods for measurement of β-cell function and changes in β-cell function in response to interventions. In the present paper, we review approaches for measurement of β-cell function, focusing on methodologic and feasibility considerations. Methodologic considerations included: (1) the utility of each technique for evaluating key aspects of β-cell function (first- and second-phase insulin secretion, maximum insulin secretion, glucose sensitivity, incretin effects) and (2) tactics for incorporating a measurement of insulin sensitivity in order to adjust insulin secretion measures for insulin sensitivity appropriately. Of particular concern were the capacity to measure β-cell function accurately in those with poor function, as is seen in established T2D, and the capacity of each method for demonstrating treatment-induced changes in β-cell function. Feasibility considerations included: staff burden, including time and required methodological expertise; participant burden, including time and number of study visits; and ease of standardizing methods across a multicentre consortium. After this evaluation, we selected a 2-day measurement procedure, combining a 3-hour 75-g oral glucose tolerance test and a 2-stage hyperglycaemic clamp procedure, augmented with arginine.

Paracrinology of islets and the paracrinopathy of diabetes

New results have brought to light the importance of the regulation of glucagon by β-cells in the development of diabetes. In this perspective, we examine the normal paracrinology of α- and β-cells in nondiabetic pancreatic islets. We propose a Sherringtonian model of coordinated reciprocal secretory responses of these juxtaposed cells that secrete glucagon and insulin, hormones with opposing actions on the liver. As insulin is a powerful inhibitor of glucagon, we propose that within-islet inhibition of α-cells by β-cells creates an insulin-to-glucagon ratio that maintains glycemic stability even in extremes of glucose influx or efflux. By contrast, in type 1 diabetes mellitus, α-cells lack constant action of high insulin levels from juxtaposed β-cells. Replacement with exogenous insulin does not approach paracrine levels of secreted insulin except with high doses that "overinsulinize" the peripheral insulin targets, thereby promoting glycemic volatility. Based on the stable normoglycemia of mice with type 1 diabetes during suppression of glucagon with leptin, we conclude that, in the absence of paracrine regulation of α-cells, tonic inhibition of α-cells improves the dysregulated glucose homeostasis. These results have considerable medical implications, as they suggest new approaches to normalize the extreme volatility of glycemia in diabetic patients.

A novel glucokinase activator modulates pancreatic islet and hepatocyte function

The glucose-sensing enzyme glucokinase (GK) plays a key role in glucose metabolism. We report here the effects of a novel glucokinase activator, LY2121260. The activator enhanced GK activity via binding to the allosteric site located in the hinge region of the enzyme. LY2121260 stimulated insulin secretion in a glucose-dependent manner in pancreatic beta-cells and increased glucose use in rat hepatocytes. In addition, incubation of beta-cells with the GK activator resulted in increased GK protein levels, suggesting that enhanced insulin secretion on chronic treatment with a GK activator may be due to not only changed enzyme kinetics but also elevated enzyme levels. Animals treated with LY2121260 showed an improved glucose tolerance after oral glucose challenge. These results support the concept that GK activators represent a new class of compounds that increase both insulin secretion and hepatic glucose use and in doing so may prove to be effective agents for the control of blood glucose levels in patients with type 2 diabetes.

Increased proinsulin levels and decreased acute insulin response independently predict the incidence of type 2 diabetes in the insulin resistance atherosclerosis study

Previous studies have indicated that beta-cell dysfunction predicts the development of diabetes, although it is unknown whether the use of combinations of insulin secretory measures further improves prediction. The Insulin Resistance Atherosclerosis Study is a prospective, multicenter, epidemiological study of the relationship between insulin sensitivity and the risk of diabetes and cardiovascular disease. At baseline, fasting concentrations of insulin, intact proinsulin (PI), and split PI were measured, and acute insulin response (AIR) was determined during a frequently sampled intravenous glucose tolerance test (FSIGTT). Subjects who were nondiabetic at baseline (n = 903) were reexamined after 5 years of follow-up; 148 had developed diabetes. In separate logistic regression models adjusted for age, sex, clinic, and ethnicity, 1 SD differences in measures of beta-cell dysfunction were associated with diabetes incidence (AIR: odds ratio [OR] 0.37, 95% CI 0.27-0.52; intact PI: OR 1.90, 95% CI 1.57-2.30; split PI: OR 1.94, 95% CI 1.63-2.31). After additional adjustment for BMI, impaired glucose tolerance, and insulin sensitivity, these measures continued to be significantly associated with risk of diabetes (all P < 0.0001). Furthermore, in models that included both PI and AIR, each was an independent predictor, and individuals who had combined low AIR and high PI experienced the highest diabetes risk. In conclusion, both low AIR and high PI independently predicted diabetes in a well-characterized multiethnic population. Although fasting PI is simpler to assess, determining AIR from an FSIGTT may further improve prediction. If pharmacological agents to prevent diabetes are proved to be efficacious in ongoing clinical trials, then it may be beneficial to perform FSIGTTs to identify better (for intensive intervention) prediabetic subjects who would ultimately require lifelong pharmacological therapy.

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.