Ronald M Lynch

The scarcity of human cadaveric pancreata limits large-scale application of islet transplantation for patients with diabetes. Islets isolated from pathogen-free pigs provide an economical and abundant alternative source assuming immunologic barriers are appropriate. Membrane receptors involved in insulin secretion that also have potential as imaging targets were investigated in isolated porcine islets. Quantitative (q)PCR revealed that porcine islets express mRNA transcripts for sulfonylurea receptor 1 (Sur1), inward rectifying potassium channel (Kir6.2, associated with Sur1), glucagon-like peptide 1 receptor (GLP1R), and adrenergic receptor alpha 2A (ADRα2A). Receptor function was assessed in static incubations with stimulatory glucose concentrations, and in the presence of receptor agonists. Glibenclamide, an anti-diabetic sulfonylurea, and exendin-4, a GLP-1 mimetic, potentiated glucose-stimulated insulin secretion >2-fold. Conversely, epinephrine maximally reduced insulin secretion 72 ± 9% (P 0.05) and had a half maximal inhibitory concentration of 60 nm in porcine islets (95% confidence interval of 45-830 nm). The epinephrine action was inhibited by the ADRα2A antagonist yohimbine. Our findings demonstrate that porcine islets express and are responsive to both stimulatory and inhibitory membrane localized receptors, which can be used as imaging targets after transplantation or to modify insulin secretion.

Complications in pregnancy elevate fetal norepinephrine (NE) concentrations. Previous studies in NE-infused sheep fetuses revealed that sustained exposure to high NE resulted in lower expression of α2-adrenergic receptors in islets and increased insulin secretion responsiveness after acutely terminating the NE infusion. In this study, we determined if the compensatory increase in insulin secretion after chronic elevation of NE is independent of hyperglycemia in sheep fetuses and whether it is persistent in conjunction with islet desensitization to NE. After an initial assessment of glucose-stimulated insulin secretion (GSIS) at 129 ± 1 days of gestation, fetuses were continuously infused for seven days with NE and maintained at euglycemia with a maternal insulin infusion. Fetal GSIS studies were performed again on days 8 and 12. Adrenergic sensitivity was determined in pancreatic islets collected at day 12. NE infusion increased (P  0.01) fetal plasma NE concentrations and lowered (P  0.01) basal insulin concentrations compared to vehicle-infused controls. GSIS was 1.8-fold greater (P  0.05) in NE-infused fetuses compared to controls at both one and five days after discontinuing the infusion. Glucose-potentiated arginine-induced insulin secretion was also enhanced (P  0.01) in NE-infused fetuses. Maximum GSIS in islets isolated from NE-infused fetuses was 1.6-fold greater (P  0.05) than controls, but islet insulin content and intracellular calcium signaling were not different between treatments. The half-maximal inhibitory concentration for NE was 2.6-fold greater (P  0.05) in NE-infused islets compared to controls. These findings show that chronic NE exposure and not hyperglycemia produce persistent adaptations in pancreatic islets that augment β-cell responsiveness in part through decreased adrenergic sensitivity.

Encapsulation devices have the potential to enable cell-based insulin replacement therapies (such as human islet or stem cell-derived β cell transplantation) without immunosuppression. However, reasonably sized encapsulation devices promote ischemia due to high β cell densities creating prohibitively large diffusional distances for nutrients. It is hypothesized that even acute ischemic exposure will compromise the therapeutic potential of cell-based insulin replacement. In this study, the acute effects of high-density ischemia were investigated in human islets to develop a detailed profile of early ischemia induced changes and targets for intervention.

β Cell specificity for a heterobivalent ligand composed of glucagon-like peptide-1 (GLP-1) linked to yohimbine (GLP-1/Yhb) was evaluated to determine its utility as a noninvasive imaging agent.

Children from diabetic pregnancies have a greater incidence of type 2 diabetes. Our objective was to determine if exposure to mild-moderate hyperglycemia, by modeling managed diabetic pregnancies, affects fetal β-cell function. In sheep fetuses, β-cell responsiveness was examined after 2 weeks of sustained hyperglycemia with 3 pulses/day, mimicking postprandial excursions, and compared to saline-infused controls (n = 10). Two pulsatile hyperglycemia (PHG) treatments were studied: mild (mPHG, n = 5) with +15% sustained and +55% pulse; and moderate (PHG, n = 10) with +20% sustained and +100% pulse. Fetal glucose-stimulated insulin secretion and glucose-potentiated arginine insulin secretion were lower (P 0.05) in PHG (0.86 ± 0.13 and 2.91 ± 0.39  ng/ml plasma insulin) but not in mPHG fetuses (1.21 ± 0.08 and 4.25 ± 0.56  ng/ml) compared to controls (1.58 ± 0.25 and 4.51 ± 0.56  ng/ml). Islet insulin content was 35% lower in PHG and 35% higher in mPHG vs controls (P 0.01). Insulin secretion and maximally stimulated insulin release were also reduced (P 0.05) in PHG islets due to lower islet insulin content. Isolated PHG islets also had 63% greater (P 0.01) reactive oxygen species (ROS) accumulation at 11.1  mmol/l glucose than controls (P 0.01), but oxidative damage was not detected in islet proteins. PHG fetuses showed evidence of oxidative damage to skeletal muscle proteins (P 0.05) but not insulin resistance. Our findings show that PHG induced dysregulation of islet ROS handling and decreased islet insulin content, but these outcomes are independent. The β-cell outcomes were dependent on the severity of hyperglycemia because mPHG fetuses had no distinguishable impairments in ROS handling or insulin secretion but greater insulin content.