Measurement of glucose and metabolites in subcutaneous adipose tissue during hyperglycemia with microdialysis at various perfusion flow rates

Quick and In Situ Detection of Different Polar Allelochemicals in Taxus Soil by Microdialysis Combined with UPLC-MS/MS

The action of allelopathy need that allelochemicals exist in the soil and reach a certain concentration. Also, the detection of allelochemicals in the soil is one of the most important research topics in the process of exploring allelopathy. To solve the problem of the simultaneous detection of allelochemicals with low concentrations and different polarities, a novel strategy for the quick detection of the allelochemicals in Taxus soil by microdialysis combined with UPLC-MS/MS on the basis of in situ detection without destroying the original structure of soil was developed for the first time in the work. The dialysis conditions were optimized by the Box-Behnken design (BBD): 70% methanol, 3 μL/min flow rate, and 3 cm long membrane tube. A reliable UPLC-MS/MS program was systematically optimized for the simultaneous detection of nine allelochemicals with different polarities. The results proved the differences in the contents and distributions of nine allelochemicals in three different Taxus soils.

Clinical evaluation of extracellular ADMA concentrations in human blood and adipose tissue

Circulating asymmetrical dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthesis, has been proposed as a biomarker for clinical outcome. Dimethylarginine dimethylaminohydrolase (DDAH) is the main enzyme responsible for ADMA metabolism and elimination. Adipose tissue ADMA concentrations and DDAH activity and their role in diabetes and obesity have not yet been investigated. In this study, we evaluated clinical microdialysis in combination with a sensitive analytical method (GC-MS/MS) to measure ADMA concentrations in extracellular fluid. Adipose tissue ADMA concentrations were assessed before and during an oral glucose tolerance test in lean healthy subjects and subjects with diabetes (n = 4 each), and in morbidly obese subjects before and after weight loss of 30 kg (n = 7). DDAH activity was determined in subcutaneous and visceral adipose tissue obtained during laparoscopic surgery (n = 5 paired samples). Mean interstitial ADMA concentrations did not differ between study populations (healthy 0.17 ± 0.03 μM; diabetic 0.21 ± 0.03 μM; morbidly obese 0.16 ± 0.01 and 0.17 ± 0.01 μM before and after weight loss, respectively). We did not observe any response of interstitial ADMA concentrations to the oral glucose challenge. Adipose tissue DDAH activity was negligible compared to liver tissue. Thus, adipose tissue ADMA plays a minor role in NO-dependent regulation of adipose tissue blood flow and metabolism.

Diurnal variation of phenylalanine and tyrosine concentrations in adult patients with phenylketonuria: subcutaneous microdialysis is no adequate tool for the determination of amino acid concentrations

BACKGROUND

Metabolic control and dietary management of patients with phenylketonuria (PKU) are based on single blood samples obtained at variable intervals. Sampling conditions are often not well-specified and intermittent variation of phenylalanine concentrations between two measurements remains unknown. We determined phenylalanine and tyrosine concentrations in blood over 24 hours. Additionally, the impact of food intake and physical exercise on phenylalanine and tyrosine concentrations was examined. Subcutaneous microdialysis was evaluated as a tool for monitoring phenylalanine and tyrosine concentrations in PKU patients.

METHODS

Phenylalanine and tyrosine concentrations of eight adult patients with PKU were determined at 60 minute intervals in serum, dried blood and subcutaneous microdialysate and additionally every 30 minutes postprandially in subcutaneous microdialysate. During the study period of 24 hours individually tailored meals with defined phenylalanine and tyrosine contents were served at fixed times and 20 min bicycle-ergometry was performed.

RESULTS

Serum phenylalanine concentrations showed only minor variations while tyrosine concentrations varied significantly more over the 24-hour period. Food intake within the patients' individual diet had no consistent effect on the mean phenylalanine concentration but the tyrosine concentration increased up to 300% individually. Mean phenylalanine concentration remained stable after short-term bicycle-exercise whereas mean tyrosine concentration declined significantly. Phenylalanine and tyrosine concentrations in dried blood were significantly lower than serum concentrations. No close correlation has been found between serum and microdialysis fluid for phenylalanine and tyrosine concentrations.

CONCLUSIONS

Slight diurnal variation of phenylalanine concentrations in serum implicates that a single blood sample does reliably reflect the metabolic control in this group of adult patients. Phenylalanine concentrations determined by subcutaneous microdialysis do not correlate with the patients' phenylalanine concentrations in serum/blood.

Enhanced human tissue microdialysis using hydroxypropyl-ß-cyclodextrin as molecular carrier

Microdialysis sampling of lipophilic molecules in human tissues is challenging because protein binding and adhesion to the membrane limit recovery. Hydroxypropyl-ß-cyclodextrin (HP-ß-CD) forms complexes with hydrophobic molecules thereby improving microdialysis recovery of lipophilic molecules in vitro and in rodents. We tested the approach in human subjects. First, we determined HP-ß-CD influences on metabolite stability, delivery, and recovery in vitro. Then, we evaluated HP-ß-CD as microdialysis perfusion fluid supplement in 20 healthy volunteers. We placed 20 kDa microdialysis catheters in subcutaneous abdominal adipose tissue and in the vastus lateralis muscle. We perfused catheters with lactate free Ringer solution with or without 10% HP-ß-CD at flow rates of 0.3–2.0 µl/min. We assessed tissue metabolites, ultrafiltration effects, and blood flow. In both tissues, metabolite concentrations with Ringer+HP-ß-CD perfusate were equal or higher compared to Ringer alone. Addition of HP-ß-CD increased dialysate volume by 10%. Adverse local or systemic reactions to HP-ß-CD did not occur and analytical methods were not disturbed. HP-ß-CD addition allowed to measure interstitial anandamide concentrations, a highly lipophilic endogenous molecule. Our findings suggest that HP-ß-CD is a suitable supplement in clinical microdialysis to enhance recovery of lipophilic molecules from human interstitial fluid.

Improved Insulin Sensitivity during Pioglitazone Treatment Is Associated with Changes in IGF-I and Cortisol Secretion in Type 2 Diabetes and Impaired Glucose Tolerance

Background. Hypercortisolism and type 2 diabetes (T2D) share clinical characteristics. We examined pioglitazone's effects on the GH-IGF-I and HPA axes in men with varying glucose intolerance. Methods. 10 men with T2D and 10 with IGT received pioglitazone 30–45 mg for 12 weeks. OGTT with microdialysis in subcutaneous adipose tissue and 1 μg ACTH-stimulation test were performed before and after. Glucose, insulin, IGF-I, IGFBP1, and interstitial measurements were analyzed during the OGTT. Insulin sensitivity was estimated using HOMA-IR. Results. HOMA-IR improved in both groups. IGF-I was initially lower in T2D subjects (P = 0.004) and increased during treatment (−1.4 ± 0.5 to −0.5 ± 0.4 SD; P = 0.007); no change was seen in IGT (0.4 ± 39 SD before and during treatment). Fasting glycerol decreased in T2D (P = 0.038), indicating reduced lipolysis. Fasting cortisol decreased in T2D (400 ± 30 to 312 ± 25 nmol/L; P = 0.041) but increased in IGT (402 ± 21 to 461 ± 35 nmol/L; P = 0.044). Peak cortisol was lower in T2D during treatment (599 ± 32 to 511 ± 43, versus 643 ± 0.3 to 713 ± 37 nmol/L in IGT; P = 0.007). Conclusions. Pioglitazone improved adipose tissue and liver insulin sensitivity in both groups. This may explain increased IGF-I in T2D. Pioglitazone affected cortisol levels in both groups but differently, suggesting different mechanisms for improving insulin sensitivity between T2D and IGT.

Monitoring of glucose in brain, adipose tissue, and peripheral blood in patients with traumatic brain injury: a microdialysis study

BACKGROUND

Episodes of hyperglycemia are considered to be a secondary insult in traumatically brain-injured patients and have been shown to be associated with impaired outcome. Intensive insulin therapy to maintain a strict glucose level has been suggested to decrease morbidity and mortality in critically ill patients but this aggressive insulin treatment has been challenged. One aspect of strict glucose control is the risk of developing hypoglycemia. Extracellular intracerebral hypoglycemia monitored by intracerebral microdialysis has been shown to correlate with poor outcome. Monitoring of blood glucose during neurointensive care is important because adequate glucose supply from the systemic circulation is crucial to maintain the brain's glucose demand after brain injury. This study investigates the correlation of glucose levels in peripheral blood, subcutaneous (SC) fat, and extracellular intracerebral tissue in patients with severe traumatic brain injury during neurointensive care.

METHODS

In this study, we included 12 patients with severe traumatic brain injury. All patients received one microdialysis catheter each, with a membrane length of 10 mm (CMA 70, CMA Microdialysis AB) in the injured hemisphere of the brain and in the noninjured hemisphere of the brain. An additional microdialysis catheter with a membrane length of 30 mm (CMA 60, CMA Microdialysis AB) was placed in the periumbilical subcutaneous adipose tissue. We studied the correlation among levels of glucose measured in peripheral blood, adipose tissue, and the noninjured hemisphere of the brain during the first 12 hours and during 3 consecutive days in neurointensive care.

RESULTS

We found a significant positive correlation between levels of glucose in peripheral blood, SC fat, and the noninjured brain during the initial 12 hours but not in injured brain. However, the result varied between the patients during the 3-day measurements. In 7 patients, there was a significant positive correlation between glucose in blood and noninjured brain, while in 4 patients this correlation was poor. In 4 patients, there was a significant positive correlation in injured brain and blood. Furthermore, there was a significant correlation between brain and adipose tissue glucose during the 3-day measurements in 11 out of 12 patients.

CONCLUSION

This study indicates that there is a good correlation between blood glucose and adipose tissue during initial and later time points in the neurointensive care unit whereas the correlation between blood and brain seems to be more individualized among patients. This emphasizes the importance of using intracerebral microdialysis to ensure adequate intracerebral levels of glucose in patients suffering from severe traumatic brain injury and to detect hypoglycemia in the brain despite normal levels of blood glucose.

Analyte flux at a biomaterial-tissue interface over time: implications for sensors for type 1 and 2 diabetes mellitus

OBJECTIVE

The very presence of an implanted sensor (a foreign body) causes changes in the adjacent tissue that may alter the analytes being sensed. The objective of this study was to investigate changes in glucose availability and local tissue metabolism at the sensor-tissue interface in patients with type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM).

METHOD

Microdialysis was used to model implanted sensors. Capillary glucose and subcutaneous (sc) microdialysate analytes were monitored in five T1DM and five T2DM patients. Analytes included glucose, glycolysis metabolites (lactate, pyruvate), a lipolysis metabolite (glycerol), and a protein degradation byproduct (urea). On eight consecutive days, four measurements were taken during a period of steady state blood glucose.

RESULTS

Microdialysate glucose and microdialysate-to-blood-glucose ratio increased over the first several days in all patients. Although glucose recovery eventually stabilized, the lactate levels continued to rise. These trends were explained by local inflammatory and microvascular changes observed in histological analysis of biopsy samples. Urea concentrations mirrored glucose trends. Urea is neither produced nor consumed in sc tissue, and so the initially increasing urea trend is explained by increased local capillary presence during the inflammatory process. Pyruvate in T2DM microdialysate was significantly higher than in T1DM, an observation that is possibly explained by mitochondrial dysfunction in T2DM. Glycerol in T2DM microdialysate (but not in T1DM) was higher than in healthy volunteers, which is likely explained by sc insulin resistance (insulin is a potent antilipolytic hormone). Urea was also higher in microdialysate of patients with diabetes mellitus compared to healthy volunteers. Urea is a byproduct of protein degradation, which is known to be inhibited by insulin. Therefore, insulin deficiency or resistance may explain the higher urea levels. To our knowledge, this is the first histological evaluation of a human tissue biopsy containing an implanted glucose monitoring device.

CONCLUSIONS

Monitoring metabolic changes at a material-tissue interface combined with biopsy histology helped to formulate an understanding of physiological changes adjacent to implanted glucose sensors. Microdialysate glucose trends were similar over 1-week in T1DM and T2DM; however, differences in other analytes indicated wound healing and metabolic activities in the two patient groups differ. We propose explanations for the specific observed differences based on differential insulin insufficiency/resistance and mitochondrial dysfunction in T1DM versus T2DM.

Microdialysis based device for continuous extravascular monitoring of blood glucose

Glycemic control of intensive care patients can be beneficial for this patient group but the continuous determination of their glucose concentration is challenging. Current continuous glucose monitoring systems based on the measurement of interstitial fluid glucose concentration struggle with sensitivity losses, resulting from biofouling or inflammation reactions. Their use as decision support systems for the therapeutic treatment is moreover hampered by physiological time delays as well as gradients in glucose concentration between plasma and interstitial fluid. To overcome these drawbacks, we developed and clinically evaluated a system based on microdialysis of whole blood. Venous blood is heparinised at the tip of a double lumen catheter and pumped through a membrane based micro-fluidic device where protein-free microdialysate samples are extracted. Glucose recovery as an indicator of long term stability was studied in vitro with heparinised bovine blood and remained highly stable for 72 h. Clinical performance was tested in a clinical trial in eight healthy volunteers undergoing an oral glucose tolerance test. Glucose concentrations of the new system and the reference method correlated at a level of 0.96 and their mean relative difference was 1.9 +/- 11.2%. Clinical evaluation using Clark's Error Grid analysis revealed that the obtained glucose concentrations were accurate and clinically acceptable in 99.6% of all cases. In conclusion, results of the technical and clinical evaluation suggest that the presented device delivers microdialysate samples suitable for accurate and long term stable continuous glucose monitoring in blood.

Imaging the tissue distribution of glucose in livers using a PARACEST sensor

Noninvasive imaging of glucose in tissues could provide important insights about glucose gradients in tissue, the origins of gluconeogenesis, or perhaps differences in tissue glucose utilization in vivo. Direct spectral detection of glucose in vivo by (1)H NMR is complicated by interfering signals from other metabolites and the much larger water signal. One potential way to overcome these problems is to use an exogenous glucose sensor that reports glucose concentrations indirectly through the water signal by chemical exchange saturation transfer (CEST). Such a method is demonstrated here in mouse liver perfused with a Eu(3+)-based glucose sensor containing two phenylboronate moieties as the recognition site. Activation of the sensor by applying a frequency-selective presaturation pulse at 42 ppm resulted in a 17% decrease in water signal in livers perfused with 10 mM sensor and 10 mM glucose compared with livers with the same amount of sensor but without glucose. It was shown that livers perfused with 5 mM sensor but no glucose can detect glucose exported from hepatocytes after hormonal stimulation of glycogenolysis. CEST images of livers perfused in the magnet responded to changes in glucose concentrations demonstrating that the method has potential for imaging the tissue distribution of glucose in vivo.

Serial lactate measurements using microdialysis of interstitial fluid do not correlate with plasma lactate in children after cardiac surgery

OBJECTIVES

Serial postoperative blood lactate (BL) concentrations have been shown to predict outcome of children after congenital heart surgery (CHS), and interventions aimed at lowering lactate can improve the outcome of these children. The cumulative blood loss for diagnostic purposes, such as repetitive arterial blood sampling in the intensive care unit, contributes, especially in small children, to anemia. Techniques to limit blood loss can therefore be of use. Microdialysis is a technique to monitor tissue chemistry in various clinical settings, and we hypothesized that it may be a valuable alternative for frequent blood sampling to monitor lactate in children after CHS.

METHODS

Fifteen children with a mean age of 40 months (range, 4-118 months) were prospectively enrolled after CHS. A CMA double lumen microdialysis catheter was inserted into the subcutaneous adipose tissue of the abdominal wall and infused with an isotone mannitol 5% solution at 1 microL/min via the inlet tubing. Microdialysate fluid was collected every hour for 48 hrs and stored at -80 degrees C for lactate determination (interstitial fluid lactate, IFL). Every hour arterial blood was taken for lactate determination. Individual profiles, correlation coefficient, and Bland-Altman analysis were used to compare BL and IFL results.

RESULTS

There were no complications with the microdialysis technique. All patients were discharged alive from hospital. Six hundred twenty paired samples were analyzed. Mean recovery of microdialysate fluid was 84%. Median (interquartile range) was 0.95 (0.70-1.15) mmol/L for BL and 1.13 (0.86-1.48) mmol/L for IFL (p < 0.0001). Individual profiles showed that IFL follows changes in BL in some, but not all children. With this study, we could not explain this discrepancy. The correlation between BL and IFL was poor (r = .77 (p < 0.0001) r = .59). Bland-Altman analysis confirmed the insufficient performance of the current microdialysis-based procedure compared with BL.

CONCLUSION

Serial lactate measurements in microdialysis fluid of subcutaneous adipose tissue are feasible, but cannot be used as a reliable interchangeable method for plasma lactate analysis in children after CHS at this time. Whether this technique has its own place in the assessment of the overall hemodynamic status and tissue perfusion in children after CHS needs to be addressed in future studies.

Reverse iontophoresis of L‐lactate: In vitro and in vivo studies

This work investigates the reverse iontophoretic extraction of lactate, a widely used marker of tissue distress in critically ill patients and of sports performance. In vitro experiments were performed to establish the relationship between subdermal lactate levels and lactate iontophoretic extraction fluxes. Subsequently, the iontophoretic extraction of lactate was performed in vivo in healthy volunteers. Lactate was quickly and easily extracted by iontophoresis both in vitro and in vivo. During a short initial phase, iontophoresis extracts the lactate present in the skin reservoir, providing information of relevance, perhaps, for dermatological and cosmetic applications. In a second step, lactate is extracted from the interstitial subdermal fluid allowing local lactate kinetics to be followed in a completely non-invasive way. The simultaneous in vivo extraction of chloride, and its possible role as an internal standard to calibrate lactate reverse iontophoretic fluxes, was also demonstrated. Despite these positive findings, however, considerably more research is necessary to eliminate potential artefacts and to facilitate interpretation of the data.

On-chip microdialysis system with flow-through glucose sensing capabilities

BACKGROUND

Microdialysis is a sampling technique based on controlling the mass transfer rate of different-sized molecules across a semipermeable membrane. Because the dialysis process has minimal effects on the surrounding fluid, it is viewed as a tool for continuous monitoring of human metabolites. In diabetes treatment, microdialysis probes have been used as sampling systems coupled to a glucose biosensor but may struggle to obtain high recoveries of analytes, as the sampling housing, probes, and glucose sensors are fabricated as separate pieces and then assembled, resulting in a large dead volume, which limits sensing frequency. An in situ combination of a miniaturized microdialysis probe with an integrated glucose sensor could help solve some of these problems.

METHOD

The system was fabricated by bonding a 6-mum-thick polycarbonate track-etch membrane with 100-nm-diameter pores onto microfluidic channels with the electrochemical glucose sensing electrodes patterned within the microchannels.

RESULTS

In vitro experiments demonstrating glucose microdialysis with continuous sensing were conducted. The permeability of glucose to the polycarbonate membrane with a 100-nm-diameter pore size was obtained to be 5.44 mum/s. Glucose recovery of 99% was observed using this microdialysis system at a perfusion flow rate of 0.5 microl/min. Experiments monitoring fluctuating glucose concentrations in the time domain at 99% recovery were also performed. The lag time was measured to be 210 seconds with 45 seconds contributed by mass transfer limitations and the rest from dead volume within the experimental setup.

CONCLUSION

The electrochemical sensing component was able to continuously track concentration changes in the reservoir. This system is expected to have the proper sensitivity to track physiologically relevant concentration changes of glucose with a lag time of less than 1 minute and minimal amplitude reduction for continuous glucose monitoring for diabetes treatment.

Recent progress in analytical instrumentation for glycemic control in diabetic and critically ill patients

Implementing strict glycemic control can reduce the risk of serious complications in both diabetic and critically ill patients. For this reason, many different analytical, mainly electrochemical and optical sensor approaches for glucose measurements have been developed. Self-monitoring of blood glucose (SMBG) has been recognised as being an indispensable tool for intensive diabetes therapy. Recent progress in analytical instrumentation, allowing submicroliter samples of blood, alternative site testing, reduced test time, autocalibration, and improved precision, is comprehensively described in this review. Continuous blood glucose monitoring techniques and insulin infusion strategies, developmental steps towards the realization of the dream of an artificial pancreas under closed loop control, are presented. Progress in glucose sensing and glycemic control for both patient groups is discussed by assessing recent published literature (up to 2006). The state-of-the-art and trends in analytical techniques (either episodic, intermittent or continuous, minimal-invasive, or noninvasive) detailed in this review will provide researchers, health professionals and the diabetic community with a comprehensive overview of the potential of next-generation instrumentation suited to either short- and long-term implantation or ex vivo measurement in combination with appropriate body interfaces such as microdialysis catheters.

Optical sensor for interstitial pH measurements

An optical fiber sensor for measuring the pH in interstitial fluid is described. Microdialysis is the approach followed for extracting the sample from the subcutaneous adipose tissue. The interstitial fluid drawn flows through a microfluidic circuit formed by a microdialysis catheter in series with a pH glass capillary. The pH indicator (phenol red) is covalently immobilized on the internal wall of the glass capillary. An optoelectronic unit that makes use of LEDs and photodetectors is connected to the sensing capillary by means of optical fibers. Optical fibers are used to connect the interrogating unit to the sensing capillary. A resolution of 0.03 pH units and an accuracy of 0.07 pH units are obtained. Preliminary in vivo tests are carried out in pigs with altered respiratory function.

In vivo and in vitro microdialysis sampling of free fatty acids

Microdialysis is a technique that allows continuous sampling of compounds from the interstitial fluid of different tissues with minimal influence on surrounding tissues and/or whole body function. In the present study, the feasibility of using microdialysis as a technique to sample free fatty acids (FFA) was investigated both in vitro and in vivo, by use of a high molecular weight (MW) cut-off membrane (3 MDa) and a push-pull system to avoid loss of perfusion fluid through the dialysis membrane. The relative recovery was examined in vitro for three different concentrations of radiolabelled oleic acid-BSA solutions (oleic acid:BSA molar ratio 1:1) and for various temperatures and flow rates. The recovery of oleic acid was found to be dependent on the concentration of analyte in the medium surrounding the membrane (17.3%, 29.0% and 30.6% for 50, 100 and 200 microM oleic acid-BSA solutions, respectively). Addition of 0.25% BSA to the perfusion fluid resulted, however, in a concentration-independent recovery of 31.4%, 28.1% and 28.1% for the 50, 100 and 200 microM solutions, respectively. The capability of the method to measure FFA together with glycerol was investigated in vivo in visceral adipose tissue of rats, before and after lipolytic treatment with the beta3-adrenergic agent, BRL37344. BRL37344 caused an increase in both dialysate FFA and glycerol, although the increase was markedly higher for glycerol, amounting to 24.5% and 329.2% increase from baseline, respectively. Subsequent in vitro test of probe performance revealed a decrease in the dialysing properties with regard to FFA, but not glycerol. This suggests that clogging of the membrane pores after 110 min prevented the measurement of the full FFA response in vivo.

Transport within the interstitial space, rather than membrane permeability, determines norepinephrine recovery in microdialysis

Microdialysis is a sampling method that permits measurement of hormones, drugs, and other lower molecular weight compounds present in interstitial fluid. We developed a straightforward mathematical model that predicts a linear relationship between the reciprocal dialysate concentration of the analyte from the interstitium and perfusion rate, permitting estimation of the interstitial concentration by extrapolation to zero perfusion rate. Conversely, linearity between the reciprocal dialysate concentration of internal standard added to the perfusion medium (retrodialysis), and the reciprocal perfusion rate, is predicted. In nine healthy volunteers, interstitial norepinephrine (NE) was estimated by NE measurements in microdialysates obtained from skeletal muscle and adipose subcutaneous tissue, using sodium salicylate (Sal) in the perfusion buffer as internal standard, at perfusion rates of 2 and 5 mul/min. Comparison with microdialysis in vitro by immersing the probe in a large volume of buffer containing NE showed that the in vivo (retro)recovery of NE and Sal is almost exclusively determined by transport of NE through the interstitial space toward and Sal from the membrane and that membrane permeability itself plays a negligible role. This was supported by the observation that applying lower body negative pressure, a measure that is unlikely to affect membrane permeability, resulted in a significant (p < 0.05) decrease of Sal retrorecovery from muscle interstitium. This validated new model significantly adds insight into the factors determining recovery of substances from the interstitium in microdialysis and provides a simpler alternative to previous approaches for estimation of interstitial concentrations.