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PubMed2005

Voltammetric study of extracellular dopamine near microdialysis probes acutely implanted in the striatum of the anesthetized rat.

Establishing in vivo microdialysis methods for the quantitative determination of dopamine concentrations in the extracellular space of the brain is an important yet challenging objective. The source of the challenge is the difficulty in directly measuring the microdialysis recovery of dopamine during an in vivo experiment. The recovery value is needed for quantitative microdialysis, regardless of whether conventional or no-net-flux methods are used. Numerical models of microdialysis that incorporate both diffusion and active transport processes suggest that dopamine recovery is strongly affected by processes occurring in the tissue closest to the probe. Some evidence suggests that the tissue adjacent to the probe becomes disrupted during probe implantation. Hence, the objective of the present study was to further identify whether the tissue adjacent to the probe is disrupted and, if so, whether that disruption might affect dopamine recovery. The experiments were conducted with microdialysis probes implanted acutely in the striatum of rats anesthetized with chloral hydrate. Carbon fiber voltammetric microelectrodes were used to monitor extracellular dopamine at three sites near the probes; immediately adjacent to the probe, 220-250 microm from the probe, and 1 mm from the probe. Probes were lowered slowly over a 30 min period, so that dialysate dopamine levels were stable, in the low nanomolar range, and partially TTX-sensitive by the time experiments began. Starting 2h after probe implantation, dopamine was monitored by fast-scan cyclic voltammetry during electrical stimulation of the medial forebrain bundle and during administration of the dopamine uptake inhibitor, nomifensine. The findings of this study show that a gradient of dopamine release and uptake activity extends at least 220 microm from microdialysis probes implanted acutely in the striatum of the anesthetized rat.

Anesthetics, Intravenous↗

PubMed2005

Measurement of neurotransmitters from extracellular fluid in brain by in vivo microdialysis and chromatography-mass spectrometry.

During the last three decades, a great deal of information has been discovered about chemical neurotransmission. However, the most important processes, namely the complex nature of neuronal circuitry, the "cross talk" between multiple neurotransmitter systems, and the varying effects neurochemicals have at different receptors, are still being explored. Techniques such as microdialysis are routinely employed to measure neurotransmitter levels in living tissue systems. Moreover, microdialysis studies have proven to be valuable in the investigation of neurodegenerative and psychiatric disease pathology, as well as in identifying novel drugs to treat such disorders. One particular challenge in performing these experiments is the requirement to couple microdialysis to sophisticated analytical equipment. Recently, considerable attention has been focused on the development of chromatographic-mass spectrometric techniques to provide more sensitive and accurate measurements of neurochemicals collected from in vivo microdialysis experiments. This review will provide a brief overview of the microdialysis technique, as well as how microdialysis and chromatography-mass spectrometry are being used to measure extracellular levels of neurotransmitters. The primary emphasis of this review will be on how these applications are used to measure levels of acetylcholine (ACh), dopamine, norepinephrine and gamma-aminobutyric acid (GABA).

Animals↗

PubMed2006

Microdialysis sampling of cytokines.

Microdialysis sampling is a well-known method for collection of low molecular weight hydrophilic analytes. Due to the success of this sampling technique for these analytes, many researchers have wanted to extend the use of this method to a wider range of analytes-particularly proteins and peptides. These analytes pose unique challenges during microdialysis sampling. The primary challenges are the reduced recovery across the dialysis semi-permeable membrane and the volume limitations/requirements for the typical immunoassay methods used for detection of proteins. This review covers the practical and theoretical aspects needed for in vivo microdialysis sampling of cytokines, which are a vitally important class of signaling proteins. In addition to the basics of the microdialysis method for sampling cytokines, the use of the microdialysis device as a localized cytokine delivery method is also described. Since relative recovery of cytokines is often low during microdialysis sampling, methods to improve the membrane recovery are discussed for in vitro and in vivo applications.

Animals↗

PubMed2000

Microdialysis in mice for drug delivery research.

Intracerebral microdialysis was first performed in the mouse at the end of the 1980s. Most microdialysis studies on mice were confined to neuropharmacology and changes in neurotransmitter concentrations up to 1995, although pharmacological studies were done on other tissues like the skin, kidney and implanted tumors. The use of microdialysis in mice for pharmacokinetic and drug delivery studies owes much to the recent availability of genetically engineered mice, such as mice in which the genes encoding multiple drug resistance have been knocked out. The quantitative microdialysis of blood and various tissue fluids of the mouse is now feasible and the recent development of specific microdialysis devices for use in mice should facilitate its use in these small animals. This review covers the technical aspects of microdialysis in the mouse and includes references to many of the published studies on pharmacokinetics and drug delivery.

Animals↗

PubMed2000

Effects of glucose and insulin levels on adipose tissue glucose measurement by microdialysis probes retained for three weeks in Type 1 diabetic patients.

BACKGROUND: To evaluate the effects of acute hyperglycaemia and hyperinsulinaemia on adipose tissue glucose measurements by microdialysis probes inserted for a 3-week period. METHODS: Microdialysis probes were implanted pairwise in abdominal adipose tissue in seven Type 1 diabetic patients and remained in situ during the complete study. Stepped hyperglycaemic hyperinsulinaemic clamps were performed at weekly intervals at which the probes were prepared for microdialysis. Adipose tissue glucose by microdialysis was compared to venous and capillary blood glucose concentrations. RESULTS: The mean time after which the acute rise in blood glucose was first detected was 11.3 min, which corresponds to the system delay of the microdialysis probe. The increase of the glucose concentration in dialysate was completed during the following 16 min. Hyperglycaemia and hyperinsulinaemia did not influence recovery compared to venous blood glucose concentrations, while recovery values compared to capillary blood glucose levels increased slightly under hyperinsulinaemic conditions (P<0.01). CONCLUSIONS: In Type 1 diabetic patients, recovery of glucose in adipose tissue compared to venous and capillary blood does not decrease during acute hyperglycaemia and hyperinsulinaemia. Although there is still a relevant time-delay to monitor a rise in blood glucose, these results show that microdialysis may offer an opportunity for future glucose monitoring over a prolonged time-period.

Adipose Tissue↗

PubMed1998

Considerations in the determination by microdialysis of resting extracellular amino acid concentrations and release upon spinal cord injury.

The following issues are further addressed: (1) Is there considerable leakage of amino acids from the circulation into the space around microdialysis probes, or are amino acid concentrations naturally much higher in the interstitial space than is generally thought? (2) Do observed high interstitial concentrations or depletion of substances in the intracellular space by microdialysis affect release measurements upon spinal cord injury? Amino acid concentrations around microdialysis fibres in the spinal cord of rats were found to approach those in the circulation and to be much higher than interstitial concentrations previously estimated in the CNS. However, much lower concentrations of amino acids were derived in the hippocampus by analogous experiments. Considerable Evans Blue/albumin leaked from the circulation into the interstitial space in the spinal cord immediately after fibre insertion. However, this movement diminished considerably by 4 h later, demonstrating substantial resealing of the blood-brain barrier, at least to large molecules. There is either substantial damage-induced movement of amino acids from the circulation into the dialysis zone after insertion of a microdialysis probe, or there is much less impediment to movement of amino acids across the blood-brain barrier in the spinal cord than in the brain. At low flow rates through the fibre, adding concentrations of amino acids to the inside of the fibre equal to the concentrations around the fibre to prevent their depletion by removal through the microdialysis fibre did not affect increases in concentrations of amino acids in microdialysates following injury. Thus the high concentrations of amino acids present around microdialysis fibres following their insertion do not seem to disturb measurements of amino acid release upon spinal cord injury.

Amino Acids↗

PubMed2000

Subcutaneous microdialysis in rats correlates with carbamazepine concentrations in plasma and brain.

Microdialysis simplifies the measurement of pharmacokinetics in pharmacodynamically relevant tissues. Microdialysis permits serial measurements of unbound drug concentration. The objective of the present work was to study rats to correlate plasma carbamazepine pharmacokinetics with subcutaneous and brain tissues. Microdialysis probes were inserted into the jugular vein, the brain, and subcutaneous tissue in Sprague-Dawley rats. After receiving single doses of carbamazepine, 12 mg/kg i.p., pharmacokinetic sampling occurred simultaneously from three microdialysis sites. Microdialysis sampled unbound carbamazepine and carbamazepine-10, 11-epoxide concentrations. Concentrations measured in brain, subcutaneous, and plasma correlated with each other. Except where differences were anticipated, pharmacokinetic parameters, including half-life and time to maximum concentration, were the same regardless of measurement site. The present study suggests microdialysis may allow pharmacokinetic measurements in peripheral physiological spaces that are surrogates for the pharmacologically relevant tissue.

Animals↗

PubMed2004

Interstitial glycerol increase in microdialysis after glycerol enema.

Microdialysis allows the measurement of extracellular concentrations of various endogenous substances, such as excitotoxic amino acids or metabolic end products. Recent advances in microdialysis techniques have led to widespread use in patients with brain disorders. Microdialysis has proved to be a useful tool for monitoring cerebral biochemical metabolism and secondary brain damage in severe head injury, subarachnoid haemorrhage, stroke, and epilepsy. In our neurosurgical intensive care unit, microdialysis was performed on 42 patients. Four patients received a glycerol enema for therapy of a paralytic ileus. A glycerol peak was observed in both intracerebral and subcutaneous microdialysis occurring three to four hours after the glycerol enema in all four patients. The highest glycerol value was 1187micromol/l cerebral and 2997micromol/l in the subcutaneous tissue. Our study indicates that besides the measurement of serum osmolality and serum glycerol level, microdialysis may be an additional valuable tool to control glycerol therapy in patients with cerebral oedema and elevated intracranial pressure.

Adult↗

PubMed1997

The design and validation of a novel intravenous microdialysis probe: application to fluconazole pharmacokinetics in the freely-moving rat model.

PURPOSE: The purpose of this study was to design and validate a concentric, flexible intravenous microdialysis probe to determine drug concentrations in blood from the inferior vena cava of a freely-moving animal model. METHODS: An intravenous microdialysis probe was constructed using fused-silica tubing and an acrylonitrile/sodium methallyl sulfonate copolymer hollow fiber. The probe was tested in vitro for the recovery of fluconazole and UK-54,373, a fluconazole analog used for probe calibration by retrodialysis. Subsequent in vivo validation was done in rats (n = 7) that had a microdialysis probe inserted into the inferior vena cava via the femoral vein, and the femoral artery was cannulated for simultaneous blood sampling. Comparisons of fluconazole pharmacokinetic parameters resulting from the two sampling methods were performed at 2 and 10 days after probe implantation. RESULTS: There were no statistical differences between the microdialysis sampling and conventional blood sampling methods for the T1/2, Cl, Vdss, and dose-normalized AUC by paired t-test (p > 0.05) for repeated dosing at day 2 and day 10 after probe placement. The probe recovery, as determined by retrodialysis, significantly decreased over the ten day period. This finding indicates the necessity for frequent recovery determinations during a long-term blood microdialysis experiment. CONCLUSIONS: These results show that microdialysis sampling in the inferior vena cava using this unique and robust probe design provides an accurate method of determining blood pharmacokinetics in the freely-moving rat for extended experimental periods. The probe design allows for a simple surgical placement into the inferior vena cava which results in a more stable animal preparation for long-term sampling and repeated-measures experimental designs.

Animals↗

PubMed1994

Dermal microdialysis sampling in vivo.

Microdialysis sampling of the dermis in vivo was accomplished using a linear microdialysis probe. In contrast to previous studies using a commercial cannula-style microdialysis probe, the linear probe had no effect on the flux of drug through the skin in vitro. The extent of tissue damage in vivo due to probe implantation was evaluated by histological examination and microdialysis delivery studies. Tissue damage due to implantation of the linear probe was minimal with no bleeding or edema observed. Infiltration of lymphocytes into the tissue was observed beginning 6 hours after probe implantation with scar tissue beginning to form after approximately 32 hours. The infiltration of lymphocytes had no effect on the behavior of implanted microdialysis probes. Delivery of 5-fluorouracil was between 20 and 25% for six different probes implanted in six different animals demonstrating good probe-to-probe and implantation-to-implantation reproducibility. Constant delivery was maintained for at least 24 hours in all cases indicating that experiments of at least 24 hour duration are feasible. The dermal concentration of topically applied 5-FU cream, Efudex, was continuously monitored by an implanted microdialysis probe demonstrating the feasibility of this technique as for monitoring skin drug levels in vivo. The dermal concentration of 5-FU following topical application was approximately 40-fold higher for in vitro excised skin than for in vivo intact skin.

Animals↗

PubMed2002

Optimisation of microdialysis sampling recovery by varying inner cannula geometry.

In vitro microdialysis recoveries of glucose were evaluated and optimised by varying the inner cannula dimensions for a concentric type microdialysis probe. The recovery was shown to be significantly dependent on the outer radius of the inner cannula (r alpha). Increasing r alpha by 50% while holding all other microdialysis variables constant caused a 26% increase in recovery under the conditions tested. Mass transport modeling of microdialysis sampling showed that the magnitude of the recovery change in response to changes in r alpha may depend on other experimental variables such as effective dialysis length, membrane dimensions, membrane type and medium composition. These data indicate that microdialysis recovery may be substantially improved by optimising the outer diameter of the inner cannula. These data also suggest that in order to fully characterise and compare data from microdialysis experiments, r alpha should become one of the reported variables.

Glucose↗

PubMed2003

Validity of the microdialysis technique for experimental in vivo studies of myocardial energy metabolism.

OBJECTIVES: The validity of the microdialysis technique for experimental in vivo studies of myocardial energy metabolism is not known. To address this question interstitial levels of energy-related metabolites (lactate, adenosine, inosine and hypoxanthine) obtained by the microdialysis technique were compared with corresponding metabolites from myocardial biopsies at given intervals in a porcine heart model using different protocols of ischaemia and reperfusion. METHODS: In an open chest porcine heart model, interstitial levels of energy-related metabolites were monitored using the microdialysis technique. All animals (n = 23) were subjected to 120-min pretreatment followed by 40 min of regional ischaemia and 120 min of reperfusion. Tissue biopsies were obtained in the beginning, middle and at the end of the 40-min ischaemic period and at the end of the reperfusion period. Pretreatment consisted of either rest (group 1, n = 7), or rest for 90 min and one ischaemia/reperfusion (10 + 20 min) cycle (group 2, n = 9), or four ischaemia/reperfusion cycles (10 + 20 min each) (group 3, n = 7). RESULTS: Interstitial levels of energy-related metabolites monitored by the microdialysis technique correlated with tissue biopsy levels of lactate (r = 0.90, P < 0.001), adenosine (r = 0.89, P < 0.001), inosine (r = 0.88, P < 0.001) and hypoxanthine (r = 0.91, P < 0.001), respectively, which were obtained by tissue biopsies at given time intervals. These significant correlations were valid regardless of the functional state of the myocardium. CONCLUSION: We observed significant correlations between microdialysis probe levels and tissue biopsy levels of energy-related metabolites in both ischaemic and non-ischaemic tissue. These data assess the validity of the microdialysis technique (in the current setting) for studying dynamic changes of myocardial energy metabolism.

Adenosine↗

PubMed1998

Cutaneous microdialysis in the rat: insertion trauma studied by ultrasound imaging.

Microdialysis is a method by which compounds can be studied in the extracellular space in skin, in vivo. The microdialysis probe is inserted in the dermis using a guide cannula. It is expected that intradermal oedema associated with insertion trauma as well as the probe depth can influence the results of microdialysis studies. The aim of the present study was to assess the effects of insertion trauma and, additionally, the probe depth by ultrasound. High-frequency (20 MHz) ultrasound is a noninvasive method for measuring the thickness and echostructure of skin. Hairless rats were anaesthetized with either halothane (n = 7) or pentobarbital sodium (n = 6). The insertion of the microdialysis probe resulted in a 38% relative increase in skin thickness. At 120 min the skin thickness had not reached the pre-insertion value. Thus, significant skin thickening, representing traumatic oedema, developed due to insertion of the microdialysis probe in the skin. The microdialysis probe could be inserted reproducibly in the lower dermis.

Administration, Cutaneous↗

PubMedDate not supplied

Determination of markers for collagen type I turnover in peritendinous human tissue by microdialysis: effect of catheter types and insertion trauma.

OBJECTIVES: Previous results from our group have shown that loading of human tendon elevates tendinous type I collagen production measured by microdialysis. However, exclusion of the observed elevation as a response to trauma from inserting the microdialysis catheters or a possible influence from the collagen production in skin was not determined. METHODS: Using the microdialysis method we measured the tissue levels of type I collagen metabolism markers [procollagen I COOH-terminal propeptide (PICP) and COOH-terminal telopeptide of type I collagen (ICTP)] in peritendinous tissue of the Achilles tendon in volunteers at two time points, 0 and 72 h. Using two different catheter types, an investigation of the contribution from the skin in the collagen results obtained was also examined. RESULTS: The data showed no significant changes in the dialysate levels for PICP or ICTP (p>0.05) in either of the catheters. CONCLUSION: Inserting microdialysis fibres around the Achilles tendon twice does not increase the collagen type I metabolism determined 3 days after the initial trauma, and when using microdialysis for measuring peritendinous collagen turnover the skin contribution can be regarded as negligible. These findings support microdialysis as a valid method for the determination of collagen metabolism in peritendinous tissue.

Achilles Tendon↗

PubMed2005

Cerebral microdialysis methodology--evaluation of 20 kDa and 100 kDa catheters.

Microdialysis monitoring of cerebral metabolism is now performed in several neuro-intensive care units. Conventional microdialysis utilizes CMA70 catheters with 20 kDa molecular weight cut-off membranes enabling the measurement of small molecules such as glucose, lactate, pyruvate and glutamate. The CMA71 100 kDa molecular weight cut-off microdialysis catheter has recently been introduced to allow detection of larger molecules such as cytokines. The objective of this study was to perform in vitro and in vivo testing of the CMA71 microdialysis catheter, comparing its performance with the CMA70. In vitro comparison studies of three of each catheter using reference analyte solutions, demonstrated equivalent recovery for glucose, lactate, pyruvate and glutamate (range 94-97% for CMA70 and 88-103% for CMA71). In vivo comparison involved intracranial placement of paired CMA70 and CMA71 catheters (through the same cranial access device) in six patients with severe traumatic brain injury. Both catheters were perfused with CNS Perfusion Fluid without dextran at 0.3 microl min-1 with hourly sampling and bedside analysis on a CMA600 microdialysis analyser. The two catheters yielded equivalent results for glucose, lactate, pyruvate, glutamate and lactate/pyruvate ratio. CMA71 microdialysis catheters can, therefore, be used for routine clinical monitoring of extracellular substances, as well as for their intended research role of larger molecular weight protein sampling.

Adolescent↗

PubMed2002

Certain forms of matrix metalloproteinase-9 accumulate in the extracellular space after microdialysis probe implantation and middle cerebral artery occlusion/reperfusion.

Matrix metalloproteinases (MMPs) are activated in focal cerebral ischemia. The activation of MMP-9 is involved in blood-brain barrier breakdown and tissue remodeling. The MMPs are released to the extracellular space, but the form and fate of secreted enzymes in brain are unknown. Using microdialysis in vivo, the authors studied whether ischemia-induced MMP-9 in brain tissue was related to free MMP-9 in the extracellular fluid. A microdialysis probe was placed into the right striatum and microdialysis was initiated 24 hours later in controls (n = 7). One hour prior to microdialysis, a group of rats (n = 7) was subjected to 1-hour occlusion of the right middle cerebral artery, followed by reperfusion. Dialysates were collected at discrete time points up to 24 hours, and subjected to zymography and Western blot analysis. The MMP-9 was released after ischemia and accumulated in the extracellular space at 24 hours (P < 0.05). Free MMP-9 forms include mainly the 95-kd proform, and, to a lesser extent, dimers and cleaved active forms (70 kd), but not the 88-kd form found in tissue. Probe implantation and microdialysis increased free MMP-9 in the dialysate. This increase was concomitant with neutrophil infiltration after the mechanical lesion, as myeloperoxidase was found by means of Western blot analysis in the brain hemisphere subjected to microdialysis (P < 0.005), and immunohistochemistry revealed the presence of myeloperoxidase stain surrounding the site of probe implantation. The results suggest that certain forms of MMP-9 are released and accumulate in the extracellular space after brain injury, and that vascular alterations and neutrophil recruitment elicit MMP-9 activation in the brain after focal ischemia and trauma.

Animals↗

PubMed2004

Microdialysis methods for measuring human metabolism.

PURPOSE OF REVIEW: To discuss the advantages and limitations of the microdialysis technique as a diagnostic and research tool using recent findings on human metabolism. RECENT FINDINGS: Results from several studies have supported the potential of microdialysis as a diagnostic tool for metabolic monitoring in difficult accessible tissues (brain, liver, intestine). However, despite promising results, no clear diagnostic measures have yet emerged. Several studies combining microdialysis with stable isotope tracers have shown that this approach has great potential for studying human metabolism non-invasively in specific tissue beds in a more dynamic way. SUMMARY: Bearing in mind the limitations and assumptions, the microdialysis technique is a useful tool in investigations of human metabolism. Its main advantages are that it can be used safely with low-grade invasiveness in humans, and thereby allows continuous sampling over prolonged periods of time from specific tissues without taking any biopsies. At present, microdialysis would seem to be useful as a diagnostic tool when integrated in the total clinical, physiological and pharmacological evaluation. Within human metabolic research, microdialysis has been and will be a very useful technique.

Brain↗

PubMed2006

Microdialysis for monitoring inflammation: efficient recovery of cytokines and anaphylotoxins provided optimal catheter pore size and fluid velocity conditions.

Microdialysis emerges as a useful tool to evaluate tissue inflammation in a number of clinical conditions, like sepsis and transplant rejection, but systematic methodological studies are missing. This study was undertaken to determine the recovery of relevant inflammatory mediators using the microdialysis system, comparing microdialysis membranes with two different molecular weight cut-offs at different flow rates. Twenty and 100 kDa pore sizes CMA microdialysis catheters were investigated using velocities of 0.3, 1.0 and 5.0 microl/min. Reference preparations for cytokines [tumour necrosis factor (TNF)-alpha, interleukin (IL)-1beta, IL-6 and IL-10; m.w. 17-28 kDa] and chemokines (IL-8, MCP-1, IP-10 and MIG; m.w. 7-11 kDa) were prepared from plasma after incubating human whole blood with lipopolysaccharide. Reference preparation for complement anaphylatoxins (C3a, C4a, C5a; m.w. 9-11 kDa) was prepared by incubating human plasma with heat-aggregated immunoglobulin G. The reference preparations were quantified for the respective inflammatory molecules and used as medium for the microdialysis procedure. Through the 20 kDa filter only the four chemokines passed, but with low recovery (3-7%) and limited to the 1.0 microl/min velocity. The recovery with the 100 kDa filter was as follows: IL-1beta = 75%, MCP-1 = 55%, MIG = 50%, IL-8 = 38%, C4a = 28%, IP-10 = 22%, C5a = 20%, C3a = 16%, IL-6 = 11, IL-10 = 8% and TNF-alpha = 4%. The highest recovery for all chemokines and anaphylatoxins were consistently at velocity 1.0 microl/min, whereas IL-1beta and IL-10 recovered most efficiently at 0.3 microl/min. Thus, microdialysis using catheters with a cut-off of 100 kDa is a reliable method to detect inflammation as judged by a defined panel of inflammatory markers. These findings may have important implications for future clinical studies.

Anaphylatoxins↗