The welcome resurgence of the α-cell: a pro glucagon commentary

A microfluidic platform integrating pressure-driven and electroosmotic-driven flow with inline filters for affinity separations

Pancreatic islets of Langerhans release glucagon to maintain blood glucose levels, and release of this peptide is dysregulated in diabetes mellitus. Although the importance of proper secretion of this peptide has been shown, no measurement of its release at the single islet level has been reported. In previous work, a non-competitive assay for glucagon was developed with a 6 pM limit of detection, low enough to measure from a single islet. To incorporate this method in an online assay, a microfluidic system with several distinct features was developed. To maintain appropriate flow rates in the presence of the high concentration of salt that was required for the assay, a piezo-actuated pressure transducer with in-line flow sensors was used to drive sample flow through 80 × 50 μm (width × depth) channels, while electroosmotic flow was used to gate the sample away from 15 × 5 μm separation channel. Flow rates tested with this system were 50 - 200 nL min-1 with relative standard deviations (RSDs) ranging from 1 - 4 %. Use of the pressure-driven flow was found to increase the amount of clogs in the system, so a method to incorporate in-line filters into the channels was developed. A total of 4 low resistance, in-line microfabricated filters were evaluated, with all designs prolonging the operation time of the microfluidic device to more than 4 hours without clogs observed. Use of this system enabled highly reproducible injections (3-6% RSD). During initial incorporation of the noncompetitive assay for glucagon, it was determined that Joule heating was problematic and temperature measurements revealed the separation channel increased to more than 50°C during operation. A 3D-printed manifold was used to hold a Peltier cooler in place on the microfluidic device which produced a 2.6-fold improvement in the amount of the noncovalent glucagon complex that was detected compared to without cooling. These features are expected to be useful for not only long-term monitoring of the glucagon release from islets of Langerhans, but has the potential to be applied to a number of other microfluidic separation-based assays as well.

Exenatide treatment for 6 months improves insulin sensitivity in adults with type 1 diabetes

OBJECTIVE

Exenatide treatment improves glycemia in adults with type 2 diabetes and has been shown to reduce postprandial hyperglycemia in adolescents with type 1 diabetes. We studied the effects of exenatide on glucose homeostasis in adults with long-standing type 1 diabetes.

RESEARCH DESIGN AND METHODS

Fourteen patients with type 1 diabetes participated in a crossover study of 6 months' duration on exenatide (10 μg four times a day) and 6 months off exenatide. We assessed changes in fasting and postprandial blood glucose and changes in insulin sensitivity before and after each study period.

RESULTS

High-dose exenatide therapy reduced postprandial blood glucose but was associated with higher fasting glucose concentrations without net changes in hemoglobin A1c. Exenatide increased insulin sensitivity beyond the effects expected as a result of weight reduction.

CONCLUSIONS

Exenatide is a promising adjunctive agent to insulin therapy because of its beneficial effects on postprandial blood glucose and insulin sensitivity in patients with type 1 diabetes.

A role for zinc in pancreatic islet β-cell cross-talk with the α-cell during hypoglycaemia

Signalling by intraislet β-cells to neighbouring α-cells was recognized almost 40 years ago, leading to the hypothesis that this is an essential mechanism to regulate the glucagon counterregulatory response to hypoglycaemia. The thesis was that during normoglycaemia or hyperglycaemia insulin secretion from β-cells would enter the islet periportal circulation and travel downstream to α-cells to dampen glucagon secretion. As a corollary, during hypoglycaemia β-cells would stop secreting insulin, which would permit α-cells to release glucagon into the hepatic portal circulation so it could travel to the liver to increase glucose production and thereby correct hypoglycaemia. This mini-review briefly mentions the early work that established this hypothesis and more extensively examines more recent work that has provided direct evidence supporting the hypothesis. A new twist has been introduced based on the fact that zinc is bound to insulin within β-cells and co-secreted with insulin. Zinc is released from insulin when it reaches the higher pH of blood, and zinc has recently been shown to negatively regulate α-cell secretion. It is now suggested that a switch-off signal provided by a sudden cessation of zinc secretion from β-cells during hypoglycaemia may play a critical role in stimulating glucagon secretion that is independent of the effect of insulin.

Simultaneous capillary electrophoresis competitive immunoassay for insulin, glucagon, and islet amyloid polypeptide secretion from mouse islets of Langerhans

A capillary electrophoresis competitive immunoassay was developed for the simultaneous quantitation of insulin, glucagon, and islet amyloid polypeptide (IAPP) secretion from islets of Langerhans. Separation buffers and conditions were optimized for the resolution of fluorescein isothiocyanate (FITC)-labeled glucagon and IAPP immunoassay reagents, which were excited with the 488 nm line of an Ar(+) laser and detected at 520 nm with a photomultiplier tube (PMT). Cy5-labeled insulin immunoassay reagents were excited by a 635 nm laser diode module and detected at 700 nm with a separate PMT. Optimum resolution was achieved with a 20mM carbonate separation buffer at pH 9.0 using a 20 cm effective separation length with an electric field of 500 V/cm. Limits of detection for insulin, glucagon, and IAPP were 2, 3, and 3 nM, respectively. This method was used to monitor the simultaneous secretion of these peptides from as few as 14 islets after incubation in 4, 11, and 20 mM glucose for 6h. For insulin and IAPP, a statistically significant increase in secretion levels was observed, while glucagon levels were significantly reduced in the 4 and 11 mM glucose conditions. To further demonstrate the utility of the assay, the Ca(2+)-dependent secretion of these peptides was demonstrated which agreed with published reports. The ability to examine the secretion of multiple peptides may allow for the determination of regulation of secretory processes within islets of Langerhans.