Quantifying insulin sensitivity and entero-insular responsiveness to hyper- and hypoglycemia in ferrets

Incretin dysfunction and hyperglycemia in cystic fibrosis: Role of acyl-ghrelin

BACKGROUND

Insulin secretion is insufficient in cystic fibrosis (CF), even before diabetes is present, though the mechanisms involved remain unclear. Acyl-ghrelin (AG) can diminish insulin secretion and is elevated in humans with CF.

METHODS

We tested the hypothesis that elevated AG contributes to reduced insulin secretion and hyperglycemia in CF ferrets.

RESULTS

Fasting AG was elevated in CF versus non-CF ferrets. Similar to its effects in other species, AG administration in non-CF ferrets acutely reduced insulin, increased growth hormone, and induced hyperglycemia. During oral glucose tolerance testing, non-CF ferrets had responsive insulin, glucagon like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) levels and maintained normal glucose levels, whereas CF ferrets had insufficient responses and became hyperglycemic. Interestingly in wild-type ferrets, the acyl-ghrelin receptor antagonist [D-Lys3]-GHRP-6 impaired glucose tolerance, and abolished insulin, GLP-1, and GIP responses during glucose tolerance testing. By contrast, in CF ferrets [D-Lys3]-GHRP-6 improved glucose tolerance, enhanced the insulin-to-glucose ratio, but did not impact the already low GLP-1 and GIP levels.

CONCLUSIONS

These results suggest a mechanism by which elevated AG contributes to CF hyperglycemia through inhibition of insulin secretion, an effect magnified by low GLP-1 and GIP. Interventions that lower ghrelin, ghrelin action, and/or raise GLP-1 or GIP might improve glycemia in CF.

Combined Intravital Microscopy and Contrast-enhanced Ultrasonography of the Mouse Hindlimb to Study Insulin-induced Vasodilation and Muscle Perfusion

It has been demonstrated that insulin's vascular actions contribute to regulation of insulin sensitivity. Insulin's effects on muscle perfusion regulate postprandial delivery of nutrients and hormones to insulin-sensitive tissues. We here describe a technique for combining intravital microscopy (IVM) and contrast-enhanced ultrasonography (CEUS) of the adductor compartment of the mouse hindlimb to simultaneously visualize muscle resistance arteries and perfusion of the microcirculation in vivo. Simultaneously assessing insulin's effect at multiple levels of the vascular tree is important to study relationships between insulin's multiple vasoactive effects and muscle perfusion. Experiments in this study were performed in mice. First, the tail vein cannula is inserted for the infusion of anesthesia, vasoactive compounds and ultrasound contrast agent (lipid-encapsulated microbubbles). Second, a small incision is made in the groin area to expose the arterial tree of the adductor muscle compartment. The ultrasound probe is then positioned at the contralateral upper hindlimb to view the muscles in cross-section. To assess baseline parameters, the arterial diameter is assessed and microbubbles are subsequently infused at a constant rate to estimate muscle blood flow and microvascular blood volume (MBV). When applied before and during a hyperinsulinemic-euglycemic clamp, combined IVM and CEUS allow assessment of insulin-induced changes of arterial diameter, microvascular muscle perfusion and whole-body insulin sensitivity. Moreover, the temporal relationship between responses of the microcirculation and the resistance arteries to insulin can be quantified. It is also possible to follow-up the mice longitudinally in time, making it a valuable tool to study changes in vascular and whole-body insulin sensitivity.

A Transient Metabolic Recovery from Early Life Glucose Intolerance in Cystic Fibrosis Ferrets Occurs During Pancreatic Remodeling

Cystic fibrosis (CF)-related diabetes in humans is intimately related to exocrine pancreatic insufficiency, yet little is known about how these 2 disease processes simultaneously evolve in CF. In this context, we examined CF ferrets during the evolution of exocrine pancreatic disease. At 1 month of age, CF ferrets experienced a glycemic crisis with spontaneous diabetic-level hyperglycemia. This occurred during a spike in pancreatic inflammation that was preceded by pancreatic fibrosis and loss of β-cell mass. Surprisingly, there was spontaneous normalization of glucose levels at 2-3 months, with intermediate hyperglycemia thereafter. Mixed meal tolerance was impaired at all ages, but glucose intolerance was not detected until 4 months. Insulin secretion in response to hyperglycemic clamp and to arginine was impaired. Insulin sensitivity, measured by euglycemic hyperinsulinemic clamp, was normal. Pancreatic inflammation rapidly diminished after 2 months of age during a period where β-cell mass rose and gene expression of islet hormones, peroxisome proliferator-activated receptor-γ, and adiponectin increased. We conclude that active CF exocrine pancreatic inflammation adversely affects β-cells but is followed by islet resurgence. We predict that very young humans with CF may experience a transient glycemic crisis and postulate that pancreatic inflammatory to adipogenic remodeling may facilitate islet adaptation in CF.

PET/CT imaging reveals unrivaled placental avidity for glucose compared to other tissues

INTRODUCTION

The goal of this study was to define the kinetics of glucose transport from maternal blood to placenta to fetus using real time imaging.

METHODS

Positron emission tomography (PET) imaging of the glucose-tracer [(18)F]fluorodeoxyglucose (FDG) was used to temporally and spatially define, in vivo, the kinetics of glucose transport from maternal blood into placentae and fetuses, in the late gestational gravid rat. Computed tomography (CT), with intravenous contrast, co-registered to the PET images allowed anatomic differentiation of placentae from fetal and maternal tissues.

RESULTS

FDG was rapidly taken up by placentae and subsequently appeared in fetuses with minimal temporal lag. FDG standardized uptake values in placentae and fetuses approached that of maternal brain. In both anesthetized and awake dams, one quarter of the administered FDG ultimately was accrued in the collective fetuses and placentae. Accordingly, kinetic modeling demonstrated that the placentae had very high avidity for FDG, 2-fold greater than that of the fetus and maternal brain, when accounting for the fact that fetal FDG necessarily must first be taken up by placentae. Consistent with this, placental expression of glucose transporter 1 exceeded that of all other tissues.

DISCUSSION

Fetal and placental tissues place a substantial glucose metabolic burden on the mother, owing to very high avidity of placentae for glucose coupled with the large relative mass of fetal and placental tissues.

CONCLUSIONS

The placenta has a tremendous capacity to uptake and transport glucose. PET/CT imaging is an ideal means to study metabolite transport kinetics in the fetoplacental unit.