Glucose gradient differences in subcutaneous tissue of healthy volunteers assessed with ultraslow microdialysis and a nanolitre glucose sensor

Real-time clinical monitoring of biomolecules

Continuous monitoring of clinical biomarkers offers the exciting possibility of new therapies that use biomarker levels to guide treatment in real time. This review explores recent progress toward this goal. We initially consider measurements in body fluids by a range of analytical methods. We then discuss direct tissue measurements performed by implanted sensors; sampling techniques, including microdialysis and ultrafiltration; and noninvasive methods. A future directions section considers analytical methods at the cusp of clinical use.

A computational model of adipose tissue metabolism: evidence for intracellular compartmentation and differential activation of lipases

Regulation of lipolysis in adipose tissue is critical to whole body fuel homeostasis and to the development of insulin resistance. Due to the challenging nature of laboratory investigations of regulatory mechanisms in adipose tissue, mathematical models could provide a valuable adjunct to such experimental work. We have developed a computational model to analyze key components of adipose tissue metabolism in vivo in human in the fasting state. The various key components included triglyceride-fatty acid cycling, regulation of lipolytic reactions, and glyceroneogenesis. The model, consisting of spatially lumped blood and cellular compartments, included essential transport processes and biochemical reactions. Concentration dynamics for major substrates were described by mass balance equations. Model equations were solved numerically to simulate dynamic responses to intravenous epinephrine infusion. Model simulations were compared with the corresponding experimental measurements of the arteriovenous difference across the abdominal subcutaneous fat bed in humans. The model can simulate physiological responses arising from the different expression levels of lipases. Key findings of this study are as follows: (1) Distinguishing the active metabolic subdomain ( approximately 3% of total tissue volume) is critical for simulating data. (2) During epinephrine infusion, lipases are differentially activated such that diglyceride breakdown is approximately four times faster than triglyceride breakdown. (3) Glyceroneogenesis contributes more to glycerol-3-phosphate synthesis during epinephrine infusion when pyruvate oxidation is inhibited by a high acetyl-CoA/free-CoA ratio.

Continuous glucose profiles in healthy subjects under everyday life conditions and after different meals

BACKGROUND

This study investigated continuous glucose profiles in nondiabetic subjects.

METHODS

Continuous interstitial glucose measurement was performed under everyday life conditions (2 days) and after ingestion of four meals with standardized carbohydrate content (50 grams), but with different types of carbohydrates and variable protein and fat content. Twenty-four healthy volunteers (12 female, 12 male, age 27.1 +/- 3.6 years) participated in the study. Each subject wore two microdialysis devices (SCGM1, Roche Diagnostics) simultaneously.

RESULTS

The mean 24-hour interstitial glucose concentration under everyday life conditions was 89.3 +/- 6.2 mg/dl (mean +/- SD, n = 21), and mean interstitial glucose concentrations at daytime and during the night were 93.0 +/- 7.0 and 81.8 +/- 6.3 mg/dl, respectively. The highest postprandial glucose concentrations were observed after breakfast: 132.3 +/- 16.7 mg/dl (range 101-168 mg/dl); peak concentrations after lunch and dinner were 118.2 +/- 13.4 and 123.0 +/- 16.9 mg/dl, respectively. Mean time to peak glucose concentration was between 46 and 50 minutes. After ingestion of standardized meals with fast absorption characteristics, peak interstitial glucose concentrations were 133.2 +/- 14.4 and 137.2 +/- 21.1 mg/dl, respectively. Meals with a higher fiber, protein, and fat content induced a smaller increase and a slower decrease of postprandial glucose concentrations with peak values of 99.2 +/- 10.5 and 122.1 +/- 20.4 mg/dl, respectively.

CONCLUSIONS

This study provided continuous glucose profiles in nondiabetic subjects and demonstrated that differences in meal composition are reflected in postprandial interstitial glucose concentrations. Regarding the increasing application of continuous glucose monitoring in diabetic patients, these data suggest that detailed information about the ingested meals is important for adequate interpretation of postprandial glucose profiles.

Multi-scale computational model of fuel homeostasis during exercise: effect of hormonal control

A mathematical model of the whole-body metabolism is developed to predict fuel homeostasis during exercise by using hormonal control over cellular metabolic processes. The whole body model is composed of seven tissue compartments: brain, heart, liver, GI (gastrointestinal) tract, skeletal muscle, adipose tissue, and "other tissues". Each tissue compartment is described by dynamic mass balances and major cellular metabolic reactions. The glucagon-insulin controller is incorporated into the whole body model to predict hormonal changes during exercise. Moderate [150 W power output at 60% of peak oxygen consumption (VO(2max))] exercise for 60 min was implemented by increasing ATP utilization rates in heart and skeletal muscle. Arterial epinephrine level was given as an input function, which directly affects heart and skeletal muscle metabolism and indirectly other tissues via glucagon-insulin controller. Model simulations were validated with experimental data from human exercise studies. The exercise induced changes in hormonal signals modulated metabolic flux rates of different tissues in a coordinated way to achieve glucose homeostasis, demonstrating the efficacy of hormonal control over cellular metabolic processes. From experimental measurements of whole body glucose balance and arterial substrate concentrations, this model could predict the dynamic changes of hepatic glycogenolysis and gluconeogenesis, which are not easy to measure experimentally, suggesting the higher contribution of glycogenolysis ( approximately 75%). In addition, it could provide dynamic information on the relative contribution of carbohydrates and lipids for fuel oxidation in skeletal muscle. Model simulations indicate that external fuel supplies from other tissue/organ systems to skeletal muscle become important for prolonged exercise emphasizing the significance of interaction among tissues. In conclusion, this model can be used as a valuable complement to experimental studies due to its ability to predict what is difficult to measure directly, and usefulness to provide information about dynamic behaviors.

In vivo glucose monitoring: the clinical reality and the promise

Glucose monitoring is an essential component of modern diabetes management. Three in vivo glucose sensors are now available for clinical use: a subcutaneously implanted amperometric enzyme electrode, a reverse iontophoresis system and a microdialysis-based device. Improvements in glucose-sensing technology continue to be sought, e.g. wired enzyme technology, viscometric affinity sensing and totally implanted glucose sensors. Non-invasive glucose sensing is the ultimate goal of glucose monitoring, but the most investigated approach, near-infrared (NIR) spectroscopy, is presently too imprecise for clinical application. Fluorescence-based glucose sensing offers several advantages and we are investigating strategies which include NIR-based fluorescence resonance energy transfer using concanavalin A/dextran; changes in the intrinsic fluorescence of hexokinase encapsulated in sol-gel; and non-invasive glucose monitoring of cells by measuring glucose-related changes in NADP(H).

Navigation within host tissues: Schistosoma mansoni and Trichobilharzia ocellata schistosomula respond to chemical gradients

After penetration of human or duck host's skin schistosomula of Schistosoma mansoni and Trichobilharzia ocellata migrate parallel to the surface in the epidermis, then they enter the dermis and venules prior to further migration. This study focuses on potential behavioural mechanisms and host cues which may enable this navigation within host tissues. We stimulated cercariae to penetrate into agar substrates and to transform to schistosomula, and analysed their orientation behaviour within chemical concentration gradients. Both species were chemotactically attracted by low molecular weight fractions of their host's serum (human, duck) and D-glucose and L-arginine were identified as attractive components in serum. They responded to gradients, which established after addition of very low concentrations of D-glucose (1 microM in T. ocellata and 2 microM in S. mansoni) and L-arginine (0.025 microM in T. ocellata and 1.0 microM in S. mansoni). The response to D-glucose was specific as other saccharides had no stimulatory activity. L-Arginine stimulated chemotactic orientation both when free and bound in peptides. However, the two species responded differently to the position of L-arginine within the peptide (terminal or subterminal), and only S. mansoni, not T. ocellata, responded to peptides occurring in serum and endothelial cells: fibronectin (1 microM), bradykinin (25 pM) and its fragment 1-5 (2.5 microM). Both species adjusted their body axis with the ventral side towards the higher concentrations of D-glucose and of L-arginine. We argue that the chemotactic orientation and the alignment of the body axis enable the parasites (i) to orientate towards deeper skin layers and avoid accidental perforation of the covering skin surface layers, (ii) to determine their position during their surface-parallel migration within the epidermis, (iii) to locate blood vessels.

Complexation of polysaccharide and monosaccharide with thiolate boronic acid capped on silver nanoparticle

Synthesized thiolate boronic acid was found to complex with both a polysaccharide (dextran) and a monosaccharide (glucose) with similar affinities but displayed more affinity with dextran than with glucose when capped as a ligand on silver nanoparticle. Coupling on multiple sites of dextran, the silver particles were aggregated. The aggregated particles displayed a decrease of absorbance at 397 nm and an increase at 640 nm. Luminescence intensity displayed an upward deviation increase with the concentration of dextran. The luminescence spectral change was ascribed to surface-enhanced fluorescence by the enhanced field from the aggregated metallic particles.

Aggregation of Silver Nanoparticle-Dextran Adducts with Concanavalin A and Competitive Complexation with Glucose

Tiopronin-protected silver nanoparticles (average diameter = 5 nm) were partially displaced by (2-mercapto-propionylamino) acetic acid 2,5-dioxo-pyrrolidin-1-ylesters via ligand exchange, and the succinimide-terminated silver particles were bound to amine-labeled Dextran 3000 (1 amine/per chain) or Dextran 10 000 (2.5 amine/per chain), respectively. The particle-Dextran 10 000 adducts were self-aggregated by interactions of multiple amines on Dextran and multi-functionalized ligands on the particle. The transverse plasmon band was blue shifted while the longitudinal plasmon at 575 nm increased, corresponding to the compact aggregation of particles. The particle-Dextran 3000 adducts, which were not aggregated, were coupled to Concanavalin A (Con A) to facilitate the aggregation of particles. The aggregated particles displayed an absorbance spectral change depending on the mole ratio of Con A/particle-Dextran 3000. The particle-Dextran 3000 adduct was released by a competitive complexation of glucose. This process was monitored by both the change in plasmon absorbance and wavelength, with the glucose concentration. The aggregation and dissociation of Con A/particle-Dextran complexes were also verified by TEM images.

Baseline levels of glucose metabolites, glutamate and glycerol in malignant glioma assessed by stereotactic microdialysis

The metabolism of high grade astrocytoma was studied in 15 patients using intra-tumoural microdialysis. Two catheters were implanted during a stereotactic biopsy procedure: one in the tumour and one in the peri-tumoural tissue. The patients were fully mobilized the same day as the operation. Microdialysis samples were collected the next day and subsequently analysed for glucose, lactate, pyruvate, glutamate and glycerol. The main objective was to establish base-line levels of the studied substances. In addition, an in vitro experiment was performed in order to estimate recovery for the flow rates and catheters used. Glucose showed a tendency to be lower in tumour than in peri-tumoural tissue, indicating a high energy demand of the tumour. Lactate was significantly higher in tumour tissue. This supports previous reports that high grade astrocytomas use glycolysis rather than respiration to meet their energy demand. The tumours were also classified into necrotic and non-necrotic, according to the radiological finding. The necrotic tumours showed significantly higher levels of glutamate. They also presented a tendency to higher levels of glycerol than the non-necrotic tumours. These findings might be explained by the release of intracellular glutamate and of cell-membrane glycerol by cell destruction. We believe that microdialysis in awake and mobilized patients will prove to be a valuable tool in investigating metabolic events in malignant brain tumours especially during therapy.