Microdialysis--a novel technique for clinical investigations

Use of the Microdialysis Technique in the Monitoring of Subcutaneous Tissue Glucose Concentration

For some time the subcutaneous (s.c.) tissue has been the target for continuous glucose measurement. The microdialysis technique permits an extracellular region approach, which has been used for about two decades for measuring various metabolites in dialysates obtained from different body regions. By connecting a s.c. implanted microdialysis probe to a flow chamber of an amperometric glucose sensor, the procedure of glucose sensing was transferred to ex vivo. Using this device it was possible to obtain, for up to 24 hours, s.c. tissue glucose profiles of healthy and diabetic people. The microdialysis theory, the calibration process and other microdialysis technique applications are discussed in this paper. Although the combination of the microdialysis technique and amperometric glucose sensing requires certain technical equipment, the combination of microdialysis and glucose sensor seems to be a promising approach to a continuously functioning glucose sensing system.

Sympathetic nervous system activity and anti-lipolytic response to iv-glucose load in subcutaneous adipose tissue of obese and obese type 2 diabetic subjects

The study aim was to investigate the effect of endogenous insulin release on lipolysis in subcutaneous adipose tissue after adrenergic stimulation in obese subjects diagnosed with type 2 diabetes (T2D). In 14 obese female T2D subjects, or 14 obese non-T2D controls, glycerol concentration was measured in response to the α1,2,ß-agonist norepinephrine, the α1-agonist norfenefrine and the ß2-agonist terbutaline (each 10-4 M), using the microdialysis technique. After 60 minutes of stimulation, an intravenous glucose load (0.5 g/kg lean body mass) was given. Local blood flow was monitored by means of the ethanol technique. Norepinephrine and norfenefrine induced a four and three fold rise in glycerol dialysate concentration (p<0.001, each), with a similar pattern in adipose tissue. Following agonist stimulation and glucose infusion, endogenous insulin release inhibited lipolysis in the presence of norepinephrine, which was more rapid and pronounced in healthy obese controls than in T2D subjects (p = 0.024 obese vs T2D subjects). Insulin-induced inhibition of lipolysis in the presence of norfenefrine was similar in all study participants. In the presence of terbutaline the lipolysis rate increased two fold until the effect of endogenous insulin (p<0.001). A similar insulin-induced decrease in lipolysis was observed for each of the norfenefrine groups and the terbutaline groups, respectively. Adipose tissue blood flow remained unchanged after the iv-glucose load. Both norepinephrine and norfenefrine diminished blood flow slightly, but insulin reversed this response (p<0.001 over the entire time). Terbutaline alone and terbutaline plus increased endogenous insulin augmented local blood flow (p<0.001 over the entire time). In conclusion, a difference in insulin-induced inhibition of lipolysis was observed in obese T2D subjects compared to obese healthy controls following modulation of sympathetic nervous system activity and is assumed to be due to ß1-adrenoceptor mediated stimulation by norepinephrine.

Changes in metabolism and blood flow in peripheral tissue (skeletal muscle) during cardiac surgery with cardiopulmonary bypass: the biochemical microdialysis study

The aim of this study was to monitor the metabolism and blood flow in the interstitium of the skeletal muscle during cardiac surgery with cardiopulmonary bypass (CPB) and in the early postoperative period by means of microdialysis and to compare metabolic changes during CPB at normothermia (NT) and hypothermia (HT).

Surgical revascularization using CPB was performed in 50 patients, 25 patients (group HT) were operated using hypothermic CPB, 25 (group NT) using normothermic CPB. Interstitial microdialysis was performed by two CMA 60 probes (CMA Microdialysis AB, Solna, Sweden) inserted into the patient’s deltoid muscle. Constituents analysed in the obtained dialysates, collected at intervals, were glucose, urea, glycerol and lactate. Tissue blood flow was monitored by dynamic microdialysis with gentamicin as a marker.

In both groups, NT versus HT, similar dynamics of concentrations were found. Low initial concentrations were followed by gradual increases during CPB and in the following phase of the operation. Concentrations were higher in the NT group. Immediately after the operation, the decrease in values continued, with a gradual increase in the succeeding postoperative period in both groups. Similar dynamic changes in the lactate concentration were found in both groups. The gentamicin concentrations were lower in the NT group (versus the HT group).

The results showed dynamic changes in the interstitial concentrations of glucose, urea, glycerol and lactate, which depend on the phase of the surgery in the CPB and early postoperative phase in the both groups of patients. Higher tissue perfusion of the skeletal muscle was noted in those patients operated on in normothermia. The dynamics of the concentration changes of these substances in the interstitium of the skeletal muscle has been proven to be caused by both the metabolic activity of the tissue and by the blood flow through the interstitium of the muscle.

Sympathetic innervation does not contribute to glycerol release in ischemic flaps

BACKGROUND

Extracellular glycerol as detected by microdialysis has been used as a surrogate marker for (ischemic) tissue damage and cellular membrane breakdown in the monitoring of free microvascular musculocutaneous flaps. One confounding factor for glycerol as a marker of ischemic cell damage is the effect of lipolysis and associated glycerol release as induced by sympathetic signalling alone. We hypothesized that extracellular glycerol concentrations in a microvascular flap with sympathetic innervation would be confounded by intact innervation per se as compared to denervated flap. Clinical relevance is related to the use of both free and pedicled flaps in reconstructive surgery. We tested the hypothesis in an experimental model of microvascular musculocutaneal flaps.

METHODS

Twelve pigs were anesthetized and mechanically ventilated. Two identical rectus abdominis musculocutaneal flaps were raised for the investigation. In the A-flaps the adventitia of the artery and accompanying innervation was carefully stripped, while in the B-flaps it was left untouched. Flap ischemia was induced by clamping both vessels for 60 minutes. The ischemia was confirmed by measuring tissue oxygen pressure, while extracellular lactate to pyruvate ratio indicated the accompanying anaerobic metabolism locally.

RESULTS

Intramuscular and subcutaneal extracellular glycerol concentrations were measured by microdialysate analyzer. Contrary to our hypothesis, glycerol concentrations were comparable between the two ischemia groups at 60 minutes (p = 0.089, T-test).

CONCLUSIONS

In this experimental model of vascular flap ischemia, intact innervation of the flap did not confound ischemia detection by glycerol. Extrapolation of the results to clinical setting warrants further studies.

β-adrenoreceptor activation in brain, lung and adipose tissue, measured by microdialysis in pig

PURPOSE

The aim of this study is to investigate the effect of local activation of β-adrenoreceptor by Isoprenaline on metabolism in brain, fat and lung measured by microdialysis.

METHODS

We used 8 healthy pigs under general anaesthesia and placed microdialysis catheters in brain, fat, lung and artery. We performed a direct measurement of glucose, lactate, pyruvate and glycerol. The stimulation was performed by one-hour infusion of Isoprenaline, a β-adrenoreceptor agonist.

RESULTS

The infusion of isoprenaline did not affect the glucose in any tissue. The levels of lactate (p=0.008) and pyruvate (p=0.011) decreased significantly in lung after isoprenaline infusion. There was a significant increase in L/P ratio in fat tissue (p=0.001) while no significant changes could be found in brain (p=0.086) and lung (p=0.679). The most pronounced and significant change was observed in glycerol in fat (p<0.001) that increased by 95%.

CONCLUSION

The prominent increase in glycerol in fat proved to be a good measure of β-adrenoreceptor activation and a measure of lipolysis. This can be used to online monitor β-adrenoreceptor activation by glycerol measurement in patients.

Antimicrobial chemotherapy and lung microdialysis: a review

Pneumonia is a form of lung infection that may be caused by various micro-organisms. The predominant site of infection in pneumonia is debatable. Advances in the fields of diagnostic and therapeutic medicine have had a less than optimal effect on the outcome of pneumonia and one of the many causes is likely to be inadequate antimicrobial concentrations at the site of infection in lung tissue. Traditional antimicrobial therapy guidelines are based on indirect modelling from blood antimicrobial levels. However, studies both in humans and animals have shown the fallacy of this concept in various tissues. Many different methods have been employed to study lung tissue antimicrobial levels with limited success, and each has limitations that diminish their utility. An emerging technique being used to study the pharmacokinetics of antimicrobial agents in lung tissue is microdialysis. Development of microdialysis catheters, along with improvement in analytical techniques, has improved the accuracy of the data. Unfortunately, very few studies have reported the use of microdialysis in lung tissue, and even fewer antimicrobial classes have been studied. These studies generally suggest that this technique is a safe and effective way of assessing the pharmacokinetics of antimicrobial agents in lung tissue. Further descriptive studies need to be conducted to study the pharmacokinetics and pharmacodynamics of different antimicrobial classes in lung tissue. Data emanating from these studies could inform decisions for appropriate dosing schedules of antimicrobial agents in pneumonia.

Microdialysis as a method for biochemical and physiological studies of the porcine and human disc

At present, no minimally invasive technique exists for the continuous evaluation of the biochemistry of animal and human intervertebral discs, but for research purposes microdialysis may be such a new technique. Thus, the aims of this study were to (1) evaluate if microdialysis can be used successfully in lumbar porcine disc and (2) develop a suitable procedure for the application of microdialysis in the human disc. Conventional specific pathogen-free pigs were used to evaluate cannulation (n = 2) and then insertion of a 10 mm microdialysis membrane, including recovery of a dialysate from the nucleus pulposus (n = 6). The procedure was performed immediately after euthanasia and aided by fluoroscopy to guide the needle and confirm catheter placement. Access to lumbar porcine disc was obtained with an 18 G 2 in. needle applied at a 35-45 degrees angle from the sagittal plane, and took less than 8 min to perform. At a 0.5 microL/min flush rate, dialysates could be recovered and analysable amounts of glucose, lactate and pyruvate were obtained. In one human cadaver, the L4-L5 disc was accessed by a 19 G 3 in. needle inserted at a 35-40 degrees angle. It was possible to apply 10 mm as well as 30 mm microdialysis membranes in the nucleus. In both species the position of the membranes was verified by direct fluoroscopy and with contrast fluid. The results obtained from porcine and human cadavers are promising, and encourage further in vivo studies using microdialysis technique on intervertebral discs.

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.

Impact of cardiopulmonary bypass on peripheral tissue metabolism and microvascular blood flow

The aim of this study was to monitor and compare the changes in metabolism and blood flow in the skeletal muscles during cardiac operations performed with cardiopulmonary bypass (CPB) and operations without CPB (off-pump) by means of interstitial microdialysis (Figure 1). Surgical revascularization, coronary artery bypass grafting (CABG), was performed in 40 patients randomized to two groups. Twenty patients (On-Pump Group) were operated on using CPB, 20 patients (Off-Pump Group) were operated on without CPB. Interstitial microdialysis was performed by 2 probes of a CMA 60 (CMA Microdialysis AB, Solna, Sweden) inserted into the patient's deltoid muscle. Microdialysis measurements were performed at 30-minute intervals. Glucose, lactate, pyruvate and glycerol as markers of basic metabolism and tissue perfusion were measured in samples from the first probe, using a CMA 600 Analyzer (CMA Microdialysis AB). Blood flow through the interstitium was monitored by means of dynamic microdialysis of ethanol as a flow-marker in the dialysates taken from the second probe (ethanol dilution technique). Results in both the groups were statistically processed and compared. Both the groups were similar in respect of preoperative characteristics. Dynamic changes of interstitial concentrations of the measured analytes were found in both the patient groups (on-pump vs. off-pump) during the operation. There was no significant difference in dialysate concentrations of glucose and lactate between the groups. Significant differences were detected in pyruvate and glycerol interstitial concentrations, lactate/pyruvate ratio and lactate/glucose ratio between the on-pump vs. off-pump patients. In the Off-Pump Group, pyruvate concentrations were higher and the values of concentrations of glycerol lower. The lactate/pyruvate ratio and the lactate/glucose ratio, indicating the aerobic and anaerobic tissue metabolism status, were lower in the Off-Pump Group. There was no significant difference in dialysate concentrations of ethanol as a flow-marker during the surgery in either of the groups. There was no statistically significant difference between the groups (On-Pump Group vs. Off-Pump Group) comparing the postoperative clinical outcome (ICU stay, ventilation duration, length of hospital stay). The dynamic changes in the interstitial concentrations of the glucose, glycerol, pyruvate and lactate were found in both the groups of patients (On-Pump Group and Off-Pump Group), but there was no difference in local blood flow when the ethanol dilution technique was used. These results showed significantly higher aerobic metabolic activity of the peripheral tissue of patients in the Off-Pump Group vs. the On-Pump Group during the course of cardiac revascularization surgery. Results suggest that extracorporeal circulation, cardiopulmonary bypass, compromises peripheral tissue (skeletal muscles) energy metabolism. These changes have no impact on the postoperative clinical outcome; no significant difference between the groups was found.

In situ profiling of adipokines in subcutaneous microdialysates from lean and obese individuals

Adipose tissue (AT) had emerged as an endocrine organ and a key regulator of the metabolically triggered inflammation. The aims of this study were 1) to investigate the usefulness of a multiplexed bioassay in characterizing a panel of adipokines in subcutaneous (sc) microdialysate samples and 2) to determine whether lean and obese individuals differ in their interstitial adipokines levels following microdialysis (MD) probe insertion. Ultrafiltrating MD membranes were inserted in opposite sites of the sc abdominal AT of six lean (L) and six obese (OB) males at the beginning (M1) and during the last 120 min (M2) of the study. Interstitial and serum concentrations of adipokines were quantified using the Luminex technique and ELISA at 60-min intervals for 5 h. In comparison with L subjects, OB subjects exhibited elevated interstitial leptin (P < 0.001), IL-8 (P < 0.05), and IL-18 levels (P = 0.05), as well as higher serum concentrations of leptin (P < 0.0001), IL-6 (P < 0.0001), tumor necrosis factor-alpha (P < 0.001), IL-8 (P = 0.01) and interferon-gamma-inducible protein 10 (P < 0.05). In samples from the M1 membranes, leptin decreased and IL-1alpha, IL-18, and RANTES (regulated on activation, normal T-cell expressed and secreted) remained relatively stable, whereas IL-6, IL-8, and monocyte chemoattractant protein-1 significantly increased after the first hour (P < 0.0001 vs. baseline). Notably, either the magnitude of increase from the initial values or the time pattern of all the adipokines in M1 and M2 dialysates were similar between the groups. In conclusion, the current work provides valuable information on the optimal time frame to collect in situ AT microdialysate samples. Further studies are needed, however, to unravel the intricate interplay of cytokines in AT interstitial fluid.

Deep sedation with dexmedetomidine in a porcine model does not compromise the viability of free microvascular flap as depicted by microdialysis and tissue oxygen tension

BACKGROUND

Deep sedation is often necessary after major reconstructive plastic surgery in the face and neck regions to prevent sudden spontaneous movements capable of inflicting mechanical injury to the transplanted musculocutaneous flap(s). An adequate positioning may help to optimize oxygenation and perfusion of the transplanted tissues. We hypothesized that dexmedetomidine, a central alpha2-agonist and otherwise potentially ideal postoperative sedative drug, may induce vasoconstriction in denervated flaps, and thus increase the risk of tissue deterioration.

METHODS

Two symmetrical myocutaneous flaps were raised on each side of the upper abdomen in 12 anesthetized pigs. The sympathetic nerve fibers were stripped from the arteries in one of the flaps (denervated flap), while nerve fibers were kept untouched in the other (innervated flap). After simulation of ischemia and reperfusion periods, the animals were randomized to deep postoperative sedation with either propofol (n = 6) or dexmedetomidine (n = 6). Flap tissue metabolism was monitored by microdialysis and tissue-oxygen partial pressure. Glucose, lactate, and pyruvate concentrations were analyzed from the dialysate every 30 min for 4 h.

RESULTS

Mean arterial blood pressure was higher in the dexmedetomidine group (P = 0.036). Flap tissue metabolism remained stable throughout the experiment as measured by lactate-pyruvate and lactate-glucose ratios (median ranges 14.3-24.5 for lactate-pyruvate and 0.3-0.6 for lactate-glucose) and by tissue-oxygen partial pressure, and no differences were found between groups.

CONCLUSIONS

Our data suggest that dexmedetomidine, even if used for deep sedation, does not have deleterious effects on local perfusion or tissue metabolism in denervated musculocutaneous flaps.

Monitoring of metabolite gradients in tissue-engineered constructs

At present, the assessment of developing tissue-engineered constructs is almost always carried out destructively using biochemical or histological methods to determine cell number, viability and tissue growth throughout the construct. Since many of these experiments are long, taking weeks or even months to complete, simple and readily applicable non-destructive methods of monitoring changes in cell metabolism, viability and tissue deposition within the construct would be invaluable; such methods could point out adverse responses during the early stages of culture. Here, we describe the use of microdialysis for detecting local changes in cellular metabolism within a tissue-engineered construct. Three-dimensional constructs consisting of bovine articular chondrocytes entrapped in an alginate gel were cultured in a bioreactor for two weeks. Glucose and lactate were monitored by microdialysis, as the major nutrient and metabolite, respectively. Concentration gradients within the construct were evident, with the highest lactate concentrations in the construct centre. The local lactate concentration was a measure of cellular metabolic activity, decreasing as cellular activity fell and increasing as cellular activity was stimulated. Nutrient starvation and cell death in the construct centre could be readily detected in constructs deliberately cultured under adverse conditions. The results show that probe measurements can give an early warning of inappropriate local metabolic changes. Such information during the growth of tissue-engineered constructs would allow either corrective action or else an early end to an unsuccessful test.

Bench-to-bedside review: microdialysis in intensive care medicine

Microdialysis is a technique used to measure the concentrations of various compounds in the extracellular fluid of an organ or in a body fluid. It is a form of metabolic monitoring that provides real-time, continuous information on pathophysiological processes in target organs. It was introduced in the early 1970s, mainly to measure concentrations of neurotransmitters in animal experiments and clinical settings. Using commercial equipment it is now possible to conduct analyses at the bedside by collecting interstitial fluid for measurement of carbohydrate and lipid metabolites. Important research has been reported in the field of neurosurgery in recent decades, but use of metabolic monitoring in critical care medicine is not yet routine. The present review provides an overview of findings from clinical studies using microdialysis in critical care medicine, focusing on possible indications for clinical biochemical monitoring. An important message from the review is that sequential and tissue-specific metabolic monitoring, in vivo, is now available.

Assessment of adipose tissue metabolism by means of subcutaneous microdialysis in patients with sepsis or circulatory failure

To evaluate the role of adipose tissue in the metabolic stress response of critically ill patients, the release of glycerol and lactate by subcutaneous adipose tissue was assessed by means of microdialysis in patients with sepsis or circulatory failure and in healthy subjects. Patients with sepsis had lower plasma free fatty acid concentrations and non-significant elevations of plasma glycerol concentrations, but higher adipose-systemic glycerol concentrations gradients than healthy subjects or patients with circulatory failure, indicating a stimulation of subcutaneous adipose lipolysis. They also had a higher lipid oxidation. Lipid metabolism (adipose-systemic glycerol gradients, lipid oxidation) was not altered in patients with circulatory failure. These observations highlight major differences in lipolysis and lipid utilization between patients with sepsis and circulatory failure. Hyperlactataemia was present in both groups of patients, but the adipose-systemic lactate concentration gradient was not increased, indicating that lactate production by adipose tissue was not involved. This speaks against a role of adipose tissue in the development of hyperlactataemia in critically ill patients.

Continuous glucose monitoring by means of the microdialysis technique: underlying fundamental aspects

The microdialysis technique allows extraction of substances (e.g., glucose) from fluids in the human body for quantitative measurements ex vivo. The microdialysis catheter can be inserted in many different tissues; for continuous glucose monitoring it is most often implanted in the subcutaneous fat tissue in the abdominal region. Perfusion of the thin catheter with an isotonic solution without glucose leads to a diffusion of glucose available in the interstitial fluid along the concentration gradient across the semipermeable membrane into the catheter. The glucose levels in the dialysate are measured quantitatively outside the body by means of specific sensors. A number of factors have a profound impact on the amount of glucose extracted (i.e., the glucose levels in the dialysate can be considerably lower than that in the interstitial fluid). However, as long as this proportion remains constant (independent of the prevailing glucose level), the sensor signal, which is related to the glucose level in the interstitial fluid, can be calibrated to the blood glucose level by means of a conventional blood glucose measurement. The microdialysis systems that are commercially available or in clinical development allow (after a run-in phase of some hours) continuous glucose monitoring with a good reliability over several days. Insertion of the microdialysis catheters cannot be performed by the patients themselves but requires professional help. From a technological point of view the microdialysis technique is demanding; consequently the costs of continuous glucose monitoring using this approach are considerable. However, further developments probably will allow development of cheaper patient self-care systems that can be used for longer periods of time.

Local lactate and histamine changes in small bowel circulation measured by microdialysis in pig hemorrhagic shock

Hemorrhagic shock results in inadequate tissue oxygenation. Plasma lactate (L) can characterize the degree of systemic oxygen debt (OD), but gives no information on local changes. The aim of this study was to characterize different degrees of hemorrhagic shock by microdialysis measurement of L and histamine (H) in small bowel circulation. Thirty-eight pigs were randomized to five groups of increasing OD (< 50 --> 120 ml/kg). The OD was accrued by hemorrhage over 60 min and was followed by retransfusion and observation for 3 days. In parallel to plasma probes, subserosa(ss)-, submucosa(sm)-, and intraluminal(il)-L- and H-probes were obtained by small bowel microdialysis every 30 min for 210 min. Ss- and sm-L increased during hemorrhage from 1.2 +/- 0.06 and 1.18 +/- 0.06 to 2.57 +/- 0.15 and 2.96 +/- 0.27 mmol/L. Highest mean L > 3.5mmol/L resulted 90 and 120 min after induction of hemorrhage. Although ss- and sm- levels hardly differed, il-L was significantly decreased with 0.27 +/- 0.02 mmol/L at 0 min and highest mean il-L at 120 min: 2.45 +/- 0.51 mmol/L. Sm-L was significantly increased after 60, 90, 120, and 150 min of highest hemorrhage severity (OD > 100 mL/kg). In parallel, systemic L increased significantly during hemorrhage and correlated well with the severity of shock. Although systemic H increased significantly during hemorrhage (from 1.3 +/- 0.31 to 15.2 +/- 0.67 ng/mL), H-dialysates showed no effect either over time nor with the degree of hemorrhage. In conclusion, microdialysis allows evaluation of local L changes in small bowel circulation in pig hemorrhagic shock. Sm-L levels appear to correlate with the degree of shock. Local H changes were not observed during hemorrhagic shock in this study.

A microdialysis method for the recovery of IL-1beta, IL-6 and nerve growth factor from human brain in vivo

Intracerebral microdialysis is used extensively as a research tool in the investigation of the neurochemical and metabolic changes that occur following acute brain injury. Microdialysis has enabled elucidation of intra-cerebral levels of substances such as lactate, pyruvate and glycerol but, as yet, has not been used effectively to recover macromolecules from the human brain. Traumatic brain injury is known to result in the generation of cytokines and neurotrophins into extracellular fluid compartment of the brain, with effects on neuronal damage and repair. We have developed a technique of in vivo sampling of the interstitial fluid of the brain of patients with severe head injuries which has allowed the measurement of IL-1beta, IL-6 and nerve growth factor. This report confirms the safety and effectiveness of this modified microdialysis method in the clinical setting of a neurological intensive care unit. The technique provides a timely addition to the armamentarium of the clinical scientist and will potentially lead to a greater understanding of neuroinflammation following acute traumatic brain injury.

In vivo response to alpha(1)-adrenoreceptor stimulation in human white adipose tissue

OBJECTIVE

Recent studies in rats suggest an important effect of alpha(1)-adrenoreceptor stimulation on glucose uptake in white adipocytes. It is not known if alpha(1)-adrenoreceptor stimulation elicits similar metabolic effects in humans.

RESEARCH METHODS AND PROCEDURES

Three microdialysis catheters in abdominal subcutaneous adipose tissue were perfused with 0.00, 0.01, 0.10, 1.00, and 10.00 microM isoproterenol, phenylephrine, or phenylephrine plus 100 microM propranolol. Dialysate concentrations of ethanol, glycerol, glucose, and lactate were measured for estimating blood flow (ethanol-dilution technique), lipolysis, and glycolysis, respectively.

RESULTS

Phenylephrine, with or without propranolol, did not elicit a change in ethanol ratio. In contrast, the ethanol ratio decreased markedly with isoproterenol. Dialysate glucose concentration decreased with phenylephrine with and without propranolol and increased with isoproterenol. Phenylephrine caused a dose-dependent increase in dialysate glycerol concentration, with a maximal effect similar to that of isoproterenol. The effect was attenuated with propranolol.

DISCUSSION

Our findings suggest that alpha(1)-adrenoreceptor stimulation by phenylephrine increases glucose uptake and metabolism in human abdominal adipose tissue. Furthermore, phenylephrine elicits a marked increase in lipolytic activity in white adipose tissue through beta-adrenoreceptor activation.

How can we measure substrate, metabolite and neurotransmitter concentrations in the human brain?

Cerebral injury and disease is associated with fundamental derangements in metabolism, with changes in the concentration of important substrates (e.g. glucose), metabolites (e.g. lactate) and neurotransmitters (e.g. glutamate and y-aminobutyric acid) in addition to changes in oxygen utilization. The ability to measure these substances in the human brain is increasing our understanding of the pathophysiology of trauma, stroke, epilepsy and tumours. There are several techniques in clinical practice already in use and new methods are under evaluation. Such techniques include the use of cerebral probes (e.g. microdialysis. voltammetry and spectrophotometry) and functional imaging (e.g. positron emission tomography and magnetic resonance spectroscopy). This review describes these techniques in terms of their principles and clinical applications.

Microdialysis in the intensive care unit: a novel tool for clinical investigation or monitoring?

Microdialysis is a minimally invasive tool that allows us to gain insight into metabolism at the tissue level. In human investigations, it can be safely performed in the brain (neurosurgical patients), skeletal muscle and adipose tissue. Basically, the technique allows interstitial concentrations of small solutes to be evaluated. Several limitations of the method and possible ways to circumvent them are indicated. Recent technical developments are reviewed. At present, this method is rarely used in metabolic monitoring of critically ill patients, but its potential applications are highlighted.

Metabolism in pedicled and free TRAM flaps: a comparison using the microdialysis technique

The most common complication in flap surgery is of a circulatory nature. Impeded blood flow leads to altered metabolism in the tissue. Possible metabolic differences between different zones of the transverse rectus abdominis muscle (TRAM) flap were studied and the metabolism of pedicled and free TRAM flaps was compared intraoperatively and postoperatively. The method used was microdialysis, which is a useful technique for following local metabolic changes continuously in various tissues.Twenty-two patients with a pedicled or free TRAM flap were monitored using the microdialysis technique. Two microdialysis catheters were placed subcutaneously in the flap (zone I and zone II), and a third one was placed subcutaneously in the flank to serve as a control. The flaps were monitored intraoperatively and postoperatively for 3 days with repeated analyses of extracellular glucose, lactate, and glycerol concentrations. An additional analysis of pyruvate was performed in some patients to calculate the lactate-to-pyruvate ratio. This study showed that glucose, lactate, and glycerol change in a characteristic way when complete ischemia (i.e., complete inhibition of the blood circulation) is present. A slower stabilization with prolonged metabolic signs of ischemia, such as lower glucose and higher lactate and glycerol concentrations, was seen in zone II compared with zone I, and more pronounced metabolic signs of ischemia, but with a faster recovery, were detected in the free TRAM flap group than in the pedicled TRAM flap group. The fact that the metabolites returned to normal earlier in free flaps than in pedicled flaps may indicate that free TRAM flaps sustain less ischemic damage because of better and more vigorous perfusion.

Metabolic and hemodynamic response of adipose tissue to angiotensin II

OBJECTIVE

Recent studies have revealed the presence of a local renin-angiotensin system in adipose tissue. To examine the possible role of this system in adipose tissue, we performed microdialysis studies on the effect of angiotensin II (Ang II) on blood flow and metabolism in abdominal subcutaneous adipose tissue (aSAT) and femoral subcutaneous adipose tissue (fSAT) in young healthy men.

RESEARCH METHODS AND PROCEDURES

Using the microdialysis technique, two different protocols were run perfusion with Ringer's solution + 50 mM ethanol with the subsequent addition of 125, 250, and 500 microg/liter Ang II (n = 8) and Ringers's solution + 50 mM ethanol with the subsequent addition of isoproterenol (1 microM) alone and in combination with 500 microg/liter Ang II (n = 6). Dialysate concentrations of ethanol, glycerol, glucose, and lactate were measured for estimating blood flow (ethanol dilution technique), lipolysis, and glycolysis, respectively.

RESULTS

Perfusion with Ang II resulted in a dose-dependent decrease in blood flow (fSAT > aSAT), lipolysis (fSAT > aSAT), and glucose uptake (fSAT = aSAT). Isoproterenol increased blood flow and lipolysis at both sites and those effects could be returned to baseline values by the addition of Ang II in aSAT but not fSAT.

DISCUSSION

In conclusion, our data indicate that in addition to its well-known vasoconstricting effect, Ang II inhibits lipolysis in adipose tissue, whereby femoral fat depots seem to be more sensitive to this effect than abdominal depots.

Adenosine, a metabolic trigger of the exercise pressor reflex in humans

There is substantial evidence that adenosine activates muscle afferent nerve fibers leading to sympathetic stimulation, but the issue remains controversial. To further test this hypothesis, we used local injections of adenosine into the brachial artery while monitoring systemic muscle sympathetic nerve activity (MSNA) with peroneal microneurography. The increase in MSNA induced by 3 mg intrabrachial adenosine (106+/-32%) was abolished if forearm afferent traffic was interrupted by axillary ganglionic blockade (21+/-19%, n=5, P:<0.05). Furthermore, the increase in MSNA induced by intravenous adenosine was 3.7-fold lower and later (onset latency 20.9+/-4.8 seconds versus 8.5+/-1 seconds) than intrabrachial adenosine. Finally, we used forearm exercise (dynamic handgrip at 50% and 15% maximal voluntary contraction, MVC), with or without superimposed ischemia, to modulate interstitial levels of adenosine (estimated with microdialysis) while monitoring MSNA. Fifteen minutes of intense (50% MVC) and moderate (15% MVC) exercise increased adenosine dialysate concentrations from 0.31+/-0.1 to 1.24+/-0.4 micromol/L (528+/-292%) and from 0.1+/-0.02 to 0.419+/-0.16 micromol/L (303+/-99%), respectively (n=7, P:<0.01). MSNA increased 88+/-25% and 38+/-28%, respectively. Five minutes of moderate exercise increased adenosine from 0.095+/-0.02 to 0.25+/-0.12 micromol/L, and from 0.095+/-0.02 to 0.48+/-0.19 micromol/L when ischemia was superimposed on exercise (n=7, P:=0.01). The percent increase in MSNA induced by the various interventions correlated with the percent increase in dialysate adenosine levels (r=0.96). We conclude that adenosine activates muscle afferent nerves, triggering reflex sympathetic activation.

Microdialysis in peripheral tissues

The objective of this review is to survey the recent literature regarding the applications of microdialysis in pharmacokinetic studies and facilitating many other studies in peripheral tissues such as muscle, subcutaneous adipose tissue, heart, lung, etc. It has been reported extensively that microdialysis is a useful technique for monitoring free concentrations of compounds in extracellular fluid (ECF), and it is gaining popularity in pharmacokinetic and pharmacodynamic studies, both in experimental animals and humans. The first part of this review discusses the use of microdialysis technique for ECF sampling in peripheral tissues in animal studies. The second part of the review describes the use of microdialysis for ECF sampling in peripheral tissues in human studies. Microdialysis has been applied extensively to measure both endogenous and exogenous compounds in ECF. Of particular benefit is the fact that microdialysis measures the unbound concentrations in the peripheral tissue fluid which have been shown to be responsible for the pharmacological effects. With the increasing number of applications of microdialysis, it is obvious that this method will have an important place in studying drug pharmacokinetics and pharmacodynamics.

Activation of alpha(2)-adrenergic receptors impairs exercise-induced lipolysis in SCAT of obese subjects

With the use of the microdialysis method, exercise-induced lipolysis was investigated in subcutaneous adipose tissue (SCAT) in obese subjects and compared with lean ones, and the effect of blockade of alpha(2)-adrenergic receptors (ARs) on lipolysis during exercise was explored. Changes in extracellular glycerol concentrations and blood flow were measured in SCAT in a control microdialysis probe at rest and during 60-min exercise bouts (50% of heart rate reserve) and in a probe supplemented with the alpha(2)-AR antagonist phentolamine. At rest and during exercise, plasma norepinephrine and epinephrine concentrations were not different in obese compared with lean men. In the basal state, plasma and extracellular glycerol concentrations were higher, whereas blood flow was lower in SCAT of obese subjects. During exercise, the increase of plasma glycerol was higher in obese subjects (115 +/- 35 vs. 65 +/- 21 micromol/l). Oppositely, the exercise-induced increase in extracellular glycerol concentrations in SCAT was five- to sixfold lower in obese than in lean subjects (50 +/- 14 vs. 318 +/- 53 micromol/l). The exercise-induced increase in extracellular glycerol concentration was not significantly modified by phentolamine infusion in lean subjects but was strongly enhanced in the obese subjects and reached the concentrations found in lean sujects (297 +/- 46 micromol/l). These findings demonstrate that the physiological stimulation of SCAT adipocyte alpha(2)-ARs during exercice-induced sympathetic nervous system activation contributes to the blunted lipolysis noted in obese men.

Estimation of skeletal muscle interstitial adenosine during forearm dynamic exercise in humans

It has been proposed that adenosine is a metabolic signal that triggers activation of muscle afferents involved in the exercise pressor reflex. Furthermore, exogenous adenosine induces sympathetic activation that mimics the exercise pressor reflex, and blockade of adenosine receptors inhibits sympathetic activation induced by exercise. Thus, we hypothesize that adenosine is released locally by the muscle during exercise. We used microdialysis probes, placed in the flexor digitorium superficialis muscle, to estimate muscle interstitial adenosine levels in humans. We estimated resting in vivo muscle interstitial adenosine concentrations (0.292+/-0.058 micromol/L, n=4) by perfusing increasing concentrations of adenosine to determine the gradient produced in the dialysate. Muscle interstitial adenosine concentrations increased from 0.23+/-0.04 to 0.82+/-0.14 micromol/L (n=14, P<0.001) during intermittent dynamic exercise at 50% of maximal voluntary contraction. Lactate increased from 0.8+/-0.1 to 2.3+/-0.3 mmol/L (P<0.001). Lower intensity (15% maximal voluntary contraction) intermittent dynamic exercise increased adenosine concentrations from 0.104+/-0.02 to 0.42+/-0.16 micromol/L (n=7). The addition of ischemia to this low level of exercise produced a greater increase in adenosine (from 0.095+/-0.02 to 0.48+/-0.2 micromol/L) compared with nonischemic exercise (0. 095+/-0.02 to 0.25+/-0.12 micromol/L). These results indicate that microdialysis is useful in estimating adenosine concentrations and in reflecting changes in muscle interstitial adenosine during dynamic exercise in humans.

Microdialysis study of bromocriptine and its metabolites in rat pituitary and striatum

Bromocriptine, a D2 receptor agonist, was administered intravenously (1mg/kg) to anesthetized rats. Microdialysis probes were implanted in the pituitary and the striatum, known sites of D2 agonist action. Bromocriptine and its metabolites were monitored in plasma and tissue dialysates for 4 h. Drug analyses were performed using two different enzyme immunoassays specific for untransformed bromocriptine or a pool of parent drug plus hydroxylated metabolites. The metabolites/parent drug ratio for areas under the curve was 5.5 in plasma and 1 in the pituitary. No metabolites could be detected in the striatum. Bromocriptine penetration was at least 10-fold greater in the pituitary than in the striatum. The kinetics of bromocriptine in the pituitary and striatum did not parallel those in plasma, indicating that the prolonged action of bromocriptine reported by other authors may be due to slow dissociation from receptors.

Amino acid concentration in the interstitium of human skeletal muscle: a microdialysis study

BACKGROUND

The microdialysis technique has been widely used for in vivo monitoring of the interstitial composition of several tissues. Remarkably high concentrations of taurine and glycerol were reported in a recent human study. As taurine and glycerol are predominantly present in the intracellular space, cellular trauma after probe insertion may have resulted in elevated interstitial concentrations. With the present study we wanted to investigate the impact of the initial trauma on the interstitial concentrations of amino acids and glycerol.

METHODS

Microdialysis probes were inserted into the vastus lateralis muscle in eight subjects. Using a slow perfusion rate of 0.3 muL min-1, dialysate samples were collected in five 75-min periods. Simultaneously, plasma samples were taken from a peripheral vein for amino acid determination.

RESULTS

During the first collection period, the dialysate concentration for 21 measured amino acids was on average 180% +/- 51% higher than the concentration in plasma water. This difference decreased to 52% +/- 15%, 32% +/- 8%, 37% +/- 8% and 31% +/- 7% during periods 2, 3, 4 and 5 respectively. Carnosine, which is not present in plasma, was detected in high concentrations in the interstitium during the first collection period and decreased subsequently.

CONCLUSION

In the post-absorptive phase, the concentrations of most amino acids in muscle interstitium are slightly higher than in venous plasma water. The leakage of intracellular amino acids, because of probe insertion, will initially lead to an overestimation of the actual interstitial concentration of amino acids. Therefore, reliable baseline values of amino acids cannot be obtained until 120-150 min after probe insertion. The dialysate concentration of carnosine may be used as a marker of cellular leakage.

In vivo evaluation of glucose permeability of an immunoisolation device intended for islet transplantation: a novel application of the microdialysis technique

Immunoisolation devices consist of semipermeable membranes chosen to protect the islets from the immune system but still allow sufficient passage of nutrients, oxygen, and the therapeutic products, insulin. The exchange between the device and the microcirculation will influence the survival of the graft as well as the metabolic efficacy of the islet implant. Glucose is the important trigger factor for insulin secretion. In this study, we evaluate the in vivo glucose permeability of the Theracyte immunoisolation device at various times after implantation. Empty devices were implanted s.c. in rats. The glucose kinetics in the device was compared to that in the SC tissue during i.v. glucose tolerance tests (IVGTTs), using the microdialysis technique. In rats studied on day 1, or 1, 2, and 4 weeks after implantation, the peak glucose levels (Cmax) were significantly lower, the times-to-peak (TTP) were significantly longer, and the areas under the curve during the first 40 min (AUC(0-40)) were significantly smaller in the device than in the SC fat. However, at 3 months all parameters improved and Cmax, TTP, and AUC(0-40) in the device did not differ significantly from those measured in the SC fat. Thus, during the first 4 weeks the device constitutes a significant diffusion barrier, but at 3 months the exchange between the lumen of devices and the blood stream improves. Our data indicate that implantation of the device several months before transplantation of the cellular graft would improve the exchange across the membrane during the early posttransplant period. This should have positive effects on graft survival and function. We also suggest that microdialysis is a useful tool for evaluating the in vivo performance of macroencapsulation devices.

Evaluation of transdermal iontophoresis of enoxacin from polymer formulations: in vitro skin permeation and in vivo microdialysis using Wistar rat as an animal model

Polymers were used in vehicles to form hydrogel matrices in this study to evaluate the in vitro permeation and in vivo microdialysis of enoxacin. The highest transdermal delivery determined by area under flux-time curve (AUC) and intracutaneous enoxacin concentration were observed in methylcellulose (MC) and polyvinylpyrrolidone (PVP) hydrogels, respectively. To avoid the pH shift in vehicles during iontophoresis, buffer species were added to formulations to increase the buffer capacity. As expected, the permeability of enoxacin of anodal iontophoresis was larger than that of cathodal iontophoresis. Combination of benzalkonium chloride, a cationic surfactant as an enhancer, and iontophoresis exerted an enhancing effect for anionic enoxacin at pH 10.0. However, no effect or a negative effect was detected for cationic enoxacin in deionized water or pH 5.0 buffer, due to the shielding of the negative charge in the skin. The skin residue of enoxacin was slightly increased after the incorporation of Azone in PVP hydrogel. The result of in vivo microdialysis was in accordance with that of in vitro study. The effect of Azone on the intracutaneous enoxacin was more significant for in vivo microdialysis than in the in vitro study indicating the clinical feasibility of Azone for iontophoretic delivery. Microdialysis can be considered as a useful technique to investigate the pharmacokinetics of transdermal iontophoresis in vivo.

Tissue hypoxanthine reflects gut vulnerability in porcine endotoxin shock

OBJECTIVE

To study differences in organ sensitivity during progressive endotoxin shock tissue levels of hypoxanthine, used as an indicator of adenosine triphosphate depletion and cellular energy failure, were monitored simultaneously in several organs by in vivo microdialysis.

DESIGN

Prospective, controlled animal study.

SETTING

University research laboratory.

SUBJECTS

Seventeen landrace pigs.

MEASUREMENTS AND MAIN RESULTS

Tissue levels of hypoxanthine, assessed by in vivo microdialysis, were monitored (in the ileum, liver, lung, skeletal muscle, subcutaneous fat, and arterial blood) simultaneously in addition to central hemodynamics during endotoxin shock in ten pigs. Seven sham animals not receiving endotoxin served as controls. Marked changes were seen in central hemodynamic parameters in response to endotoxemia. Very prominent increases were seen in the ileum and liver, followed by the lung, whereas only limited changes were observed in subcutaneous fat. These results indicate a differentiated development of cellular energy failure in response to endotoxemia in different organs. By considering the high amounts of xanthine oxidase seen in the gut, the increases in hypoxanthine may provide an important substrate for reactive oxygen species formation in this organ. The limited changes seen in subcutaneous fat suggest that this tissue may provide limited sensitivity when monitoring the septic patient.

CONCLUSIONS

These findings support the concept of specific vulnerability of the gut during endotoxemia.

In vivo monitoring of gabapentin in rats: a microdialysis study coupled to capillary electrophoresis and laser-induced fluorescence detection

Gabapentin (GP) is a new anticonvulsant used in refractory epilepsy. Few studies have monitored the drug in vivo. We report the combination of capillary electrophoresis and laser-induced fluorescence detection (CZE-LIFD) with brain microdialysis and plasma ultrafiltration in an attempt to measure GP and offer an alternative technique for pharmacokinetic studies. We found that CZE-LIFD is capable of linearly measuring 10(-7)-10(-9) M GP in a 1 nL volume. It was also demonstrated that it is possible to monitor GP in prefrontal cortex dialysates and plasma in rats. It is concluded that the method permits in vivo monitoring of the drug in pharmacological as well as in clinical studies.

In situ detection of myocardial infarction in pig by measurements of aspartate aminotransferase (ASAT) activity in the interstitial fluid

Microdialysis probes permeable to large molecules (m.w. cut-off > 200 kD) were introduced into the myocardium of anaesthetized pigs in order to evaluate their potential for early detection of myocardial ischaemia and enzyme markers for infarction. The left anterior descending coronary artery was occluded for 30 min and the myocardium was reperfused for 3 h. The concentrations of aspartate aminotransferase (ASAT), lactate, glucose and selected free amino acids were measured. The levels in the interstitium of ischaemic and non-ischaemic myocardium were compared with those in plasma from the coronary sinus as well as from a peripheral vein. Twelve probes were inserted in six pigs and withdrawn after 8-72 hours of sampling. No complications occurred. Simultaneous 100% increase of ASAT and lactate was found in myocardial dialysates after 30 min of ischaemia. ASAT activity remained at that level until the end of reperfusion. The plasma peak ASAT level was not attained until after 3 h. Glutamate was the only amino acid which increased significantly in the myocardial interstitium during ischaemia, peaking after 30 min of reperfusion. Dialysates from the unaffected myocardium showed no effects on lactate, ASAT or glutamate. The use of myocardial microdialysis for pre- and postoperative recordings in man is discussed.

New approaches in clinical chemistry: on-line analyte concentration and microreaction capillary electrophoresis for the determination of drugs, metabolic intermediates, and biopolymers in biological fluids

The use of capillary electrophoresis (CE) for clinically relevant assays is attractive since it often presents many advantages over contemporary methods. The small-diameter tubing that holds the separation medium has led to the development of multicapillary instruments, and simultaneous sample analysis. Furthermore, CE is compatible with a wide range of detectors, including UV-Vis, fluorescence, laser-induced fluorescence, electrochemistry, mass spectrometry, radiometric, and more recently nuclear magnetic resonance, and laser-induced circular dichroism systems. Selection of an appropriate detector can yield highly specific analyte detection with good mass sensitivity. Another attractive feature of CE is the low consumption of sample and reagents. However, it is paradoxical that this advantage also leads to severe limitation, namely poor concentration sensitivity. Often high analyte concentrations are required in order to have injection of sufficient material for detection. In this regard, a series of devices that are broadly termed 'analyte concentrators' have been developed for analyte preconcentration on-line with the CE capillary. These devices have been used primarily for non-specific analyte preconcentration using packing material of the C18 type. Alternatively, the use of very specific antibody-containing cartridges and enzyme-immobilized microreactors have been demonstrated. In the current report, we review the likely impact of the technology of capillary electrophoresis and the role of the CE analyte concentrator-microreactor on the analysis of biomolecules, present on complex matrices, in a clinical laboratory. Specific examples of the direct analysis of physiologically-derived fluids and microdialysates are presented, and a personal view of the future of CE in the clinical environment is given.

Rat adipose tissue amino acid metabolism in vivo as assessed by microdialysis and arteriovenous techniques

In fed, anesthetized rats, microdialysis demonstrated a net release of glycerol, glutamine, serine, tyrosine, and taurine and a net uptake of glutamate, aspartate, glycine, and arginine across the inguinal adipose depot. However, the results also indicated excessive proteolysis associated with implantation of the microdialysis probe, and a novel arteriovenous difference technique was developed. Arteriovenous difference across the inguinal fat pat demonstrated a net uptake of glucose and a net release of lactate and glycerol. Starvation (48 h) resulted in higher rates of glycerol and lactate release with lower rates of glucose uptake. A net uptake of triacylglycerol was seen in starved-refed animals. Net glutamine, tyrosine, and taurine release was seen in fed and starved animals, but in starved-refed animals taurine and serine were the only amino acids showing significant release. No significant net uptake or release of ammonia, pyruvate, or alanine was observed. These experiments confirm that adipose tissue is a site of glutamine synthesis and suggest that the principal substrates are derived from intracellular proteolysis. The results also demonstrate the viability of an arteriovenous difference technique for the study of adipose tissue in the rat.

In situ microdialysis in bone tissue. Stimulation of prostaglandin E2 release by weight-bearing mechanical loading

In this study we describe, to our knowledge, the first experiment using the microdialysis technique for studying the release of prostaglandin E2 (PGE2) in the proximal tibia metaphysis secondary to mechanical loading. Nine healthy females, six in the loading group and three controls, mean age 34+/-2 (years+/-SEM), participated. A standard microdialysis catheter was inserted into the tibia metaphyseal bone under local anesthesia. Samplings were done every 15 min under a 2-h equilibration period. Thereafter, heel-drops (one impact per second) with as hard impact of the heels into the floor as possible, were done for 5 min by the subjects in the loading group. The control group performed no exercise. Sampling continued after this for another 2-h period. Basal levels of PGE2 in the proximal tibial metaphysis were determined to a mean of 45+/-10 pg/ml (mean+/-SEM, n = 6). The major finding was a 2.5-3.5-fold increase in the release of PGE2 after mechanical loading. The increase was statistically significant (P < 0.05 compared with basal levels) 1 h after the mechanical loading and persisted for the rest of the experimental period. No major alterations were observed in the control group. In conclusion, our data demonstrate that in situ microdialysis is a useful method for studying the PGE2 production in human bone. Furthermore, a rapid and significant increase in PGE2 levels was noticed in response to dynamic mechanical loading.

Interstitial insulin during euglycemic-hyperinsulinemic clamp in obese and lean individuals

Transcapillary insulin transport has been considered a rate-limiting step of insulin action. However, direct measurement of interstitial insulin levels during physiologic levels of insulinemia have not been performed. We determined changes in interstitial insulin in eight healthy non-obese men and seven healthy obese men by microdialysis during a euglycemic-hyperinsulinemic clamp. Interstitial insulin was determined in the subcutaneous tissue of the abdomen and thigh. Steady-state insulin concentrations were reached approximately 10 minutes after the start of insulin infusion in the subcutaneous tissue of the abdomen and thigh and returned to basal levels approximately 10 minutes after the infusion was discontinued. There was no difference in the rapidity of change in interstitial insulin between obese and lean individuals at either site studied, irrespective of the pattern of fat distribution. The relative change in dialysate insulin concentration during the euglycemic clamp did not differ between obese and lean individuals at either site studied. It was also unaffected by the waist to hip ratio. The rapid change in interstitial insulin concentration could be of physiologic significance in determining the effects of changes in circulating insulin concentration. We conclude that transcapillary insulin transport in adipose tissue is unaffected by obesity and the pattern of fat distribution in healthy men. It is also concluded that when interstitial insulin is determined directly, transcapillary insulin transport is rapid and does not demonstrate a significant lag phase.

Ultrafiltrate sampling device for continuous monitoring

A light, portable sampling device for the continuous collection and storage of subcutaneous fluid is described. It consists of a hollow-fibre probe for ultrafiltration, a long tube for sample storage and a vacuum tube as driving force. Introduction of a restriction in the flow path allows a constant pulse-free flow as low as 100 nl min-1. The contents of the storage tube reflect the metabolic events that have happened during the 24 h collection period. The ultrafiltrate fluid collected from the subcutaneous tissue is free from large molecules and can be continuously analysed. An application in combination with a bionsensor for the analysis of glucose is described, but the device also allows monitoring of other subcutaneous constituents such as ions, metabolites, drugs and their metabolites.

Simultaneous brain and blood microdialysis study with a new removable venous probe. Serotonin and 5-hydroxyindolacetic acid changes after D-norfenfluramine or fluoxetine

A removable intravenous microdialysis probe was developed and simultaneously used with a removable microdialysis probe placed in the lateral hypothalamus (LH). Serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) changes in blood and brain dialysates were measured by HPLC-EC after an i.p. injection of 5 mg/kg d-norfenfluramine (dNF) or 10 mg/kg fluoxetine (FLU) in freely moving rats. 5-HT in the LH significantly increased after both drugs, but the rise was larger and faster with dNF [F(7,28)=4.0 p<0.05] than with FLU [F(5,20)=5.0 p<0.01]. By contrast, in venous blood 5-HT increased after FLU [F(5,20)=2.96 p<0.05] but not after dNF. 5-HIAA after both drugs continued decreasing significantly in the LH [dNF F(7,28)=11.4 p<0.01; FLU F(5,20)=22.8 p<0.01], but it did not change in blood. Simultaneous dialysis in brain and blood allowed evaluation of the differential effects of dNF and FLU on 5-HT and 5-HIAA in the two places. Removable venous probes prevented the inflammatory reaction that may occur around permanently implanted probes, and the dialysis could be more efficient and with less risk of clogging.

Allergen-induced histamine release in intact human skin in vivo assessed by skin microdialysis technique: characterization of factors influencing histamine releasability

BACKGROUND

The purposes of the study were to characterize allergen-induced histamine release in intact human skin in vivo by using a novel microdialysis technique and to study covariates influencing histamine releasability.

METHODS

Hollow microdialysis fibers were inserted into the upper dermis in 15 timothy-sensitivity subjects. Up to 12 fibers were inserted in each subject. Each fiber was perfused with Krebs-Ringer's solution at a rate of 3.0 microliters/min. Three to four serial dilutions of allergen were applied to the skin by intracutaneous injections or skin prick test above individual fibers. Samples were collected in two 2-minute fractions before skin challenge and in 10 consecutive samples for 20 minutes after skin challenge. Histamine was assayed spectrofluorometrically.

RESULTS

A significant dose-response relationship for histamine release was demonstrated with intracutaneous tests and skin prick tests. The time to reach peak histamine release after an intracutaneous test was 4 to 8 minutes, compared with 12 to 14 minutes for a skin prick test. Histamine release correlated significantly with wheal size. Intrasubject coefficient of variation on histamine release was about 20%. A substantial intersubject variation in histamine releasability was observed. Seventy to seventy-five percent of the variation could be accounted for by a combination of gender, total and allergen-specific IgE, and an in vitro basophil histamine release test.

CONCLUSIONS

Using a skin microdialysis technique, we have described in detail histamine release in intact human skin by allergen. The microdialysis method proved to be a reproducible technique for monitoring histamine release in allergic skin reactions and for studying histamine releasability of skin mast cells in vivo.

Continuous monitoring of lactate during exercise in humans using subcutaneous and transcutaneous microdialysis

We have evaluated the possibility of monitoring the plasma lactate concentration in human volunteers during cycle ergometer exercise using subcutaneous and transcutaneous microdialysis. In transcutaneous microdialysis, the relative increase in dialysate lactate concentration exceeded that of plasma lactate concentration by a factor of 6 during exercise due to exercise-induced lactate secretion in sweat. During exercise the subcutaneous microdialysis dialysate lactate concentration underestimated the plasma lactate concentration possibly due to diffusion limitation or adipose tissue lactate production. While it was demonstrated that microdialysis can be used for on-line lactate monitoring, neither subcutaneous nor transcutaneous dialysate lactate concentration were linearly related to the plasma lactate concentration during exercise, and it was found therefore that it was not possible to monitor directly plasma lactate concentration during exercise.