Nutritive blood flow improves interstitial glucose and lactate exchange in perfused rat hindlimb

Microdialysis was used to assess the interstitial concentrations of glucose and lactate in the constant-flow-perfused rat hindlimb under varying levels of nutritive flow controlled by vasoconstrictors. Increased nutritive flow was achieved by norepinephrine (NE) or angiotensin II (ANG II) and decreased nutritive flow by serotonin (5-HT). NE and ANG II increased oxygen and glucose uptake as well as hindlimb lactate release by 50%. 5-HT decreased oxygen uptake by 15% but had no significant effect on glucose uptake or hindlimb lactate release. Microdialysis recovery of glucose and lactate was significantly elevated by NE and ANG II and decreased by 5-HT. The calculated interstitial concentration of glucose was increased by NE and ANG II but decreased by 5-HT. The interstitial concentration of lactate was decreased by NE and ANG II but increased by 5-HT. In all cases, nitroprusside reversed the effects of the vasoconstrictors. These data indicate that increased nutritive blood flow enhances the exchange of glucose and lactate by improving the supply of glucose to and the removal of lactate from the interstitium.     

Metabolic alterations in adipose tissue during the early phase of experimental endotoxemia in humans

Adipose tissue plays an important role in energy homeostasis; however, there is only little knowledge about its metabolic activity during critical illness or sepsis. We assessed adipose tissue metabolic activity and local blood flow during experimental endotoxemia in otherwise healthy humans. In a prospective, placebo controlled and randomized experiment we measured changes in lactate, glycerol, and pyruvate concentrations in microdialysate samples of femoral adipose tissue after an intravenous bolus of lipopolysaccharide (LPS, 4 ng/kg). Intravenous endotoxin caused an early and constant increase in interstitial pyruvate, while formation of lactate in adipose tissue was not affected. In contrast, lactate levels in serum were elevated significantly after 90 min (p<0.05) and likewise, serum glycerol concentrations rose 90 min after LPS treatment (p<0.05) and 60 min earlier than in adipose tissue. Subcutaneous adipose tissue blood perfusion increased 2-fold while there was a strong decline in skin blood flow. Pyruvate accumulation in subcutaneous adipose tissue is an early marker of endotoxemia. While adipose tissue is a major source of serum glycerol and lactate in humans during physiological conditions, it contributes only little to increased serum lactate and glycerol levels during endotoxemia.     

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.     

Studies of phosphodiesterase effects on adipose tissue metabolism in obese subjects by the microdialysis technique.

The effect of non-selective (theophylline) inhibition of cyclic AMP breakdown on norepinephrine stimulated lipolysis rate was investigated in subcutaneous adipose tissue of obese subjects. In addition, changes in interstitial glucose and lactate concentration were assessed by means of the microdialysis technique. The interaction of endogenous released insulin and theophylline on adipocyte metabolism was determined. Theophylline and norepinephrine alone increased glycerol outflow significantly. When both agents were perfused in combination, interstitial glycerol concentration increased further. The enhanced glycerol level due to theophylline application was slightly decreased by insulin. In the presence of theophylline, extracellular glucose concentration increased, in contrast to the catecholamine. Norepinephrine decreased interstitial glucose level. When both drugs were added in combination, the level of interstitial glucose increased to about 1 mM, greater than with theophylline alone. With each intervention, lactate was synthesized. Local adipose tissue blood flow was increased by theophylline and theophylline plus norepinephrine. In conclusion, post-receptor mechanisms increased norepinephrine maximal stimulated lipolysis rate in subcutaneous adipose tissue. Glucose uptake was inhibited by the non-specific inhibitor of phosphodiesterase. The effect of insulin on inhibition of lipolysis was modest but sustained in the presence of high theophylline (10(-4) M) concentration. Phosphodiesterase activity may be relatively low in obese subjects in comparison with lean subjects. In lean subjects theophylline caused a transient reversal of the antilipolytic effect of insulin.     

Gender-specific response to interstitial angiotensin II in human white adipose tissue.

Angiotensin II is synthesized locally in various tissues. However, the role of interstitial angiotensin II in the regulation of regional metabolism and tissue perfusion has not as yet been clearly defined. We characterized the effect of interstially applied angiotensin II in abdominal subcutaneous adipose tissue of young, normal-weight, healthy men (n = 8) and women (n = 6) using the microdialysis technique. Adipose tissue was perfused with 0.01, 0.1, and 1 micro M angiotensin II. Dialysate concentrations of ethanol, glycerol, glucose, and lactate were measured to assess changes in blood flow (ethanol dilution technique), lipolysis, and glycolysis, respectively. Baseline ethanol ratio and dialysate lactate were both significantly higher, whereas dialysate glucose was significantly lower in men vs. women. In men, ethanol ratio and dialysate glucose, lactate and glycerol did not change significantly during perfusion with angiotensin II. In women, however, angiotensin II induced a significant increase in ethanol ratio and dialysate lactate and a decrease in dialysate glucose close to values found for men and this response was almost maximal at the lowest angiotensin II concentration used. Dialysate glycerol did not change significantly. We conclude that baseline blood flow and glucose supply and metabolism is significantly higher in women than in men. In men, interstitial Ang II has only a minimal effect on adipose tissue blood flow and metabolism. In women, however, a high physiological concentration of interstitial angiotensin II can reduce blood flow down to values found in men. This is associated with an impaired glucose supply and metabolism. Additionally, Ang II inhibits lipolysis.     

Changes in blood metabolic parameters during the development of Walker-256 tumour-induced cachexia in rats are not caused by decreased food intake

Blood metabolic parameters of Walker-256 tumour-bearing rats, on days 5, 8, 11 and 14 after implantation of tumour, were compared with those of rats without tumour fed ad libitum (free-fed control) or with reduced feeding (pair-fed control), similar to the anorexic tumour-bearing rats. Cachexia parameters and tumour mass also were investigated. In general, especially on day 14 after implantation of tumour, there was reduction of body mass, gastrocnemius muscle mass, food intake and glycemia and increase of blood triacylglycerol, free fatty acids, lactate and urea, compared with free-fed controls rats. These changes did not occur in pair-fed control, except a slight reduction of glycemia. Pair-fed control showed no significant changes in blood cholesterol and glycerol in comparison with free-fed control, although there was reduction of cholesterol and increase of blood glycerol on day 14 after tumour implantation compared with pair-fed control. The results demonstrate that, besides the characteristic signs of the cachexia syndrome such as anorexia, weight loss and muscle catabolism, Walker-256 tumour-bearing rats show several blood metabolic alterations, some of which begin as early as day 5 after implantation of tumour, and are accentuated during the development of cachexia. Evidence that the alterations of blood metabolic parameters of tumour-bearing rats were not found in pair-fed control indicate that they were not caused by decreased food intake. These changes were probably mediated by factors produced by tumour or host tissue in response to the presence of tumour.     

HO-1 expression in primary cultured astroglial cells

Microdialysis of glucose, lactate and glycerol was performed to monitor brain insults and to predict brain injury in a rat model using the mitochondrial toxin malonate (5–100 m m ). Striatal dialysates were analyzed off-line using a CMA 600 microdialysis analyzer or on-line using flow-injection analysis and biosensors for glucose and lactate. Histological damage was evaluated using stereological principles. Lactate (baseline ca . 1 m m ) was dose-dependently increased, reaching a maximum of five- to six-fold increase, whereas glucose (baseline 1–2 m m ) was decreased (>50%) by malonate >20 m m . These changes were reversible upon perfusion with normal Ringer's. Transient increases in glycerol (four- to eight-fold) were only observed in some rats, and were not dose-dependent. Histological damage was related to the perfused malonate concentration, but was not significantly correlated with lactate or glycerol changes.     

On‐line glucose and lactate monitoring in rat striatum: effect of malonate and correlation with histological damage

Microdialysis of glucose, lactate and glycerol was performed to monitor brain insults and to predict brain injury in a rat model using the mitochondrial toxin malonate (5–100 mm ). Striatal dialysates were analyzed off‐line using a CMA 600 microdialysis analyzer or on‐line using flow‐injection analysis and biosensors for glucose and lactate. Histological damage was evaluated using stereological principles. Lactate (baseline ca. 1 mm ) was dose‐dependently increased, reaching a maximum of five‐ to six‐fold increase, whereas glucose (baseline 1–2 mm ) was decreased (>50%) by malonate >20 mm . These changes were reversible upon perfusion with normal Ringer's. Transient increases in glycerol (four‐ to eight‐fold) were only observed in some rats, and were not dose‐dependent. Histological damage was related to the perfused malonate concentration, but was not significantly correlated with lactate or glycerol changes.     

Effects of a Single Therapeutic Dose of Glycerol on Cerebral Metabolism in the Brains of Young Mice: Possible Increase in Brain Glucose Transport and Glucose Utilization

Abstract: This is a study of the effects of a single “therapeutic” dose of glycerol [2 g(22 mmol)/kg i.p.] on brain carbohydrate and energy metabolism in normal nursing weanling mice. Findings were correlated with brain water and electrolyte content and with metabolite changes in plasma, red blood cells, and liver. Plasma glycerol levels peaked at 21 mM 7.5 min after injection and returned to the control value, 0.16 mM, by 2 h. Plasma Na⁺ concentration decreased and plasma protein increased for as long as 2 h after injection. Although red blood cells were freely permeable to glycerol, there was no evidence for glycerol metabolism in these cells. Glycerol levels in liver paralleled those in plasma. Glycerol injection increased liver glucose concentration 23% and doubled hepatic glycerol-1-phosphate levels. Liver ATP levels were reduced 24% after glycerol injection. Brain water concentration was significantly reduced from 7.5 min to 30 min after glycerol injection; brain Na⁺ and K⁺ levels were unchanged. There was no evidence for glycerol entry into brain (the amount detected in brain tissue could be explained by the glycerol content in the blood of the brain). While plasma glucose increased 33%, brain glucose increased 87%. Concomitantly there were statistically significant increases in fructose-1,6-diphosphate, lactate, α-ketoglutarate, and malate levels. The disproportionately high brain glucose value suggests increased transport of glucose from the blood to the brain. Increases in fructose-1,6-diphosphate, lactate, α-ketoglutarate, and malate are compatible with an increased metabolic flux in the glycolytic pathway and Krebs citric acid cycle. As has been previously shown for urea and/or mannitol, these changes may result from the effects of the hyperosmolar glycerol solution on the blood-brain barrier and on cerebral glucose utilization. The sustained lowering of plasma Na⁺ concentration after a single “therapeutic” glycerol injection suggests a need for monitoring plasma Na⁺ levels in the clinical situation. Possible lowering of hepatic ATP levels by the use of glycerol in humans is another concern.     

Inhibitory effect of tumor necrosis factor α on gluconeogenesis in perfused rat liver

Tumor necrosis factor α (TNFα) is a cytokine involved in many metabolic responses in both normal and pathological states. Considering that the effects of TNFα on hepatic gluconeogenesis are inconclusive, we investigated the influence of this cytokine in gluconeogenesis from various glucose precursors. TNFα (10 μg/kg) was intravenously injected in rats; 6 h later, gluconeogenesis from alanine, lactate, glutamine, glycerol, and several related metabolic parameters were evaluated in situ perfused liver. TNFα reduced the hepatic glucose production (p < 0.001), increased the pyruvate production (p < 0.01), and had no effect on the lactate and urea production from alanine. TNFα also reduced the glucose production (p < 0.01), but had no effect on the pyruvate production from lactate. In addition, TNFα did not alter the hepatic glucose production from glutamine nor from glycerol. It can be concluded that the TNFα inhibited hepatic gluconeogenesis from alanine and lactate, which enter in gluconeogenic pathway before the pyruvate carboxylase step, but not from glutamine and glycerol, which enter in this pathway after the pyruvate carboxylase step, suggesting an important role of this metabolic step in the changes mediated by TNFα.     

Muscle interstitial glucose and lactate levels during dynamic exercise in humans determined by microdialysis.

The purpose of the present study was to use the microdialysis technique to determine skeletal muscle interstitial glucose and lactate concentrations during dynamic incremental exercise in humans. Microdialysis probes were inserted into the vastus lateralis muscle, and subjects performed knee extensor exercise at workloads of 10, 20, 30, 40, and 50 W. The in vivo probe recoveries determined at rest by the internal reference method for glucose and lactate were 28.7 +/- 2.5 and 32.0 +/- 2.7%, respectively. As exercise intensity increased, probe recovery also increased, and at the highest workload probe recovery for glucose (61.0 +/- 3.9%) and lactate (66. 3 +/- 3.6%) had more than doubled. At rest the interstitial glucose concentration (3.5 +/- 0.2 mM) was lower than both the arterial (5.6 +/- 0.2 mM) and venous (5.3 +/- 0.3 mM) plasma water glucose levels. The interstitial glucose levels remained lower (P < 0.05) than the arterial and venous plasma water glucose concentrations during exercise at all intensities and at 10, 20, 30, and 50 W, respectively. At rest the interstitial lactate concentration (2.5 +/- 0.2 mM) was higher (P < 0.05) than both the arterial (0.9 +/- 0. 2 mM) and venous (1.1 +/- 0.2 mM) plasma water lactate levels. This relationship was maintained (P < 0.05) during exercise at workloads of 10, 20, and 30 W. These data suggest that interstitial glucose delivery at rest is flow limited and that during exercise changes in the interstitial concentrations of glucose and lactate mirror the changes observed in the venous plasma water compartments. Furthermore, skeletal muscle contraction results in an increase in the diffusion coefficient of glucose and lactate within the interstitial space as reflected by an elevation in probe recovery during exercise.     

Metabolism during anaesthesia and recovery in colic and healthy horses: a microdialysis study

Background  

Muscle metabolism in horses has been studied mainly by analysis of substances in blood or plasma and muscle biopsy specimens. By using microdialysis, real-time monitoring of the metabolic events in local tissue with a minimum of trauma is possible. There is limited information about muscle metabolism in the early recovery period after anaesthesia in horses and especially in the colic horse. The aims were to evaluate the microdialysis technique as a complement to plasma analysis and to study the concentration changes in lactate, pyruvate, glucose, glycerol, and urea during anaesthesia and in the recovery period in colic horses undergoing abdominal surgery and in healthy horses not subjected to surgery.     

The lipid-mobilizing effect of atrial natriuretic peptide is unrelated to sympathetic nervous system activation or obesity in young men

We recently demonstrated that natriuretic peptides and especially the atrial natriuretic peptide (ANP) are powerful lipolytic agents on isolated human fat cells. To search for a possible influence of obesity on ANP responsiveness, we compared the lipolytic effects of human ANP (h-ANP) on isolated subcutaneous abdominal adipose tissue (SCAAT) fat cells from young healthy lean and obese men. The lipid-mobilizing effects of an intravenous infusion of h-ANP was studied, as well as various metabolic and cardiovascular parameters that were compared in the same subjects. h-ANP (50 ng/min/kg) was infused iv for 60 min. Microdialysis probes were inserted in SCAAT to measure modifications of the extracellular glycerol concentrations during h-ANP infusion. Spectral analysis of blood pressure and heart rate oscillations that were recorded using digital photoplethysmography were used to assess changes in autonomic nervous system activity. h-ANP induced a marked and similar increase in glycerol and nonesterified fatty acids, and a weak increase in insulin plasma levels in lean and obese men. Plasma norepinephrine concentrations rose similarly during h-ANP infusion in lean and obese men. The effects of h-ANP infusion on the autonomic nervous system were similar in both groups, with an increase in the spectral energy of the low-frequency band of systolic blood pressure variability and a decrease in the spectral energy of the high-frequency band of heart rate. In SCAAT, h-ANP infusion increased extracellular glycerol concentration and decreased blood flow similarly in both groups. The increase in extracellular glycerol observed during h-ANP infusion was not modified when 0.1 mM propranolol was added to the microdialysis probe perfusate to prevent beta-adrenoceptor activation. These data show that ANP is a potent lipolytic hormone independent of the activation of the sympathetic nervous system, and that obesity did not modify the lipid-mobilizing effect of ANP in young obese subjects.     

FT-IR spectrometry analysis of plasma fatty acyl moieties selective mobilization during endurance exercise

This study was designed to describe changes in plasma fatty acyl moieties during a 2-h endurance exercise. Sixteen endurance-trained subjects cycled 2 h at 55% of maximal power output and capillary blood was sampled every 15 min. Fourier-transform infrared (FT-IR) spectrometry was used to determine correlated changes between plasma fatty acyl moieties (FAM) structural characteristics and metabolic parameters (oxygen consumption, respiratory exchange ratio, glucose, lactate, TG, glycerol, and albumin). Opposite changes were found between carbohydrate and fatty acid metabolism during the second hour of exercise, i.e., a decrease of glucose and lactate concentrations while albumin, FAM, and TG increased. For fatty acid metabolism, FAM and TG did not showed the same pattern of changes at the end of exercise, i.e., TG remained constant after 90 min while FAM continued to increase. This late FAM concentration increase was correlated to the changes in albumin concentration and the nu C=C-H/nu(as) CH3 and nu(as) CH2/nu(as) CH3 ratios. These ratios clearly showed that FAM unsaturation increased while chain length decreased. It was hypothesized that PUFA from TG adipose lipolysis ketone bodies (beta-hydroxybutyric acid) from liver may have been released in higher amounts as glycogen stores became depleted after 90 min of exercise.     

Extracellular N-acetylaspartate depletion in traumatic brain injury

N-Acetylaspartate (NAA) is almost exclusively localized in neurons in the adult brain and is present in high concentration in the CNS. It can be measured by proton magnetic resonance spectroscopy and is seen as a marker of neuronal damage and death. NMR spectroscopy and animal models have shown NAA depletion to occur in various types of chronic and acute brain injury. We investigated 19 patients with traumatic brain injury (TBI). Microdialysis was utilized to recover NAA, lactate, pyruvate, glycerol and glutamate, at 12-h intervals. These markers were correlated with survival and a 6-month Glasgow Outcome Score. Eleven patients died and eight survived. A linear mixed model analysis showed a significant effect of outcome and of the interaction between time of injury and outcome on NAA levels (p = 0.009 and p = 0.004, respectively). Overall, extracellular NAA was 34% lower in non-survivors. A significant non-recoverable fall was observed in this group from day 4 onwards, with a concomitant rise in lactate-pyruvate ratio and glycerol. These results suggest that mitochondrial dysfunction is a significant contributor to poor outcome following TBI and propose extracellular NAA as a potential marker for monitoring interventions aimed at preserving mitochondrial function.     

Effects of epinephrine on the net hepatic uptake of lactate, pyruvate, and glycerol in sheep

Epinephrine caused hyperglycemia in part by increasing gluconeogenesis. However, the mechanism of its gluconeogenic effects has not been studied in ruminants. This study was undertaken to examine the effect of epinephrine on the net hepatic uptake of selected glucose precursors in sheep. The major abdominal blood vessels of the sheep were catheterized in normal and alloxan diabetic sheep. Glucose production, metabolic clearance of glucose, and the hepatic removal of certain glucose precursors were determined before, during, and after epinephrine infusion. Epinephrine increased the hepatic glucose output, the concentrations of lactate and glycerol in plasma, and the net hepatic uptake and fractional hepatic extraction of lactate and glycerol. These effects were independent of changes in the concentrations of insulin and glucagon in plasma. These results show that epinephrine directly stimulates hepatic gluconeogenesis in sheep.     

A Dual-Probe Microdialysis Study in Simultaneously Monitoring Extracellular Pyruvate, Lactate, and Biogenic Amines in Gerbil Striata during Unilateral Cerebral Ischemia

A dual-probe microdialysis technique coupled with liquid chromatographic assays was developed for the simultaneous monitoring of neurochemicals in gerbil striata during cerebral ischemia. Isocratic separation of lactate and pyruvate was achieved within 5 min whereas the separation of biogenic amines was completed within 30 min. An unilateral ligation was produced by occlusion of the right common carotid artery for 30 mins in anethetized gerbils to perform a typical focal cerebral ischemia. Microdialysis probes were inserted in both sides of the striata to simultaneously monitor biogenic amines, lactate and pyruvate during cerebral ischemia. Dynamic and comparative changes of these analytes in ipsilateral and contralateral sides of the brain can be simultaneously measured by the assay. The present assay can be used as a research tool to explore neurochemical substances and their relationships during cerebral ischemia.     

In vivo nitric oxide suppression of lipolysis in subcutaneous abdominal adipose tissue is greater in obese than lean women

Mounting evidence suggests there is a reduced mobilization of stored fat in obese compared to lean women. It has been suggested that this decreased lipid mobilization may lead to, or perpetuate, the obese state; however, there may be a beneficial effect of reduced lipolysis, either by allowing for a sink of excess fatty acids, or by limiting a potentially harmful rise in interstitial and circulating fatty acid concentration. Nitric oxide (NO) may be responsible for a portion of the reduced in vivo rates of lipolysis in obese women because NO reduces adipose tissue lipolysis and adipose tissue nitric oxide synthase (NOS) mRNA is higher in obese than lean individuals. The purpose of this study was to determine if the inhibition of NOS by L-N(g)-monomethyl-L-arginine (L-NMMA) in the absence and presence of lipolytic stimulation would result in a larger increase in lipolytic rate in obese (OB) than lean (LN) women. Microdialysis probes were inserted into the subcutaneous abdominal adipose tissue of seven obese and six lean women to monitor lipolysis. Dialysate glycerol concentration increased in response to L-NMMA in OB (basal 125 ± 26 μmol/l; L-NMMA 225 ± 35 μmol/l) to a greater extent than in LN (basal 70 ± 18 μmol/l; L-NMMA 84 ± 20 μmol/l) women (P < 0.05). Dialysate glycerol increased to a similar extent in OB and LN in response to adrenergic stimulation by isoprenaline or norepinephrine in the presence of L-NMMA. The differential glycerol responses to L-NMMA between obese and lean could not be explained by differential blood flow responses. It can be concluded that NO suppresses basal lipolysis in obese women to a greater extent than in lean women.     

Hollow microneedle-based sensor for multiplexed transdermal electrochemical sensing

The development of a minimally invasive multiplexed monitoring system for rapid analysis of biologically-relevant molecules could offer individuals suffering from chronic medical conditions facile assessment of their immediate physiological state. Furthermore, it could serve as a research tool for analysis of complex, multifactorial medical conditions. In order for such a multianalyte sensor to be realized, it must be minimally invasive, sampling of interstitial fluid must occur without pain or harm to the user, and analysis must be rapid as well as selective. Initially developed for pain-free drug delivery, microneedles have been used to deliver vaccines and pharmacologic agents (e.g., insulin) through the skin. Since these devices access the interstitial space, microneedles that are integrated with microelectrodes can be used as transdermal electrochemical sensors. Selective detection of glucose, glutamate, lactate, hydrogen peroxide, and ascorbic acid has been demonstrated using integrated microneedle-electrode devices with carbon fibers, modified carbon pastes, and platinum-coated polymer microneedles serving as transducing elements. This microneedle sensor technology has enabled a novel and sophisticated analytical approach for in situ and simultaneous detection of multiple analytes. Multiplexing offers the possibility of monitoring complex microenvironments, which are otherwise difficult to characterize in a rapid and minimally invasive manner. For example, this technology could be utilized for simultaneous monitoring of extracellular levels of, glucose, lactate and pH, which are important metabolic indicators of disease states (e.g., cancer proliferation) and exercise-induced acidosis.     

A microdialysis method for the in situ investigation of the action of large peptide molecules in human skeletal muscle: detection of local metabolic effects of insulin

The possibility of using microdialysis catheters with a large pore size dialysis membrane (100 kDa) to investigate the action of macromolecules perfused into the interstitial space of peripheral tissues was explored. This was made possible by increasing the colloid osmotic pressure of the perfusate with 40 g/l of dextran-70 to prevent perfusate loss across the dialysis membranes. Microdialysis catheters were inserted into the quadriceps femoris muscle of 13 human subjects. With different perfusion flow rates (1. 33, 0.66, 0.33 and 0.16 microl/min) the recorded concentrations of glucose, lactate, and urea were in agreement with values previously obtained using a conventional membrane with a smaller pore size (20 kDa) [Rosdahl H, Hamrin K, Ungerstedt U, Henriksson. J Am J Physiol 1998;274:E936-45.]. When insulin was added to the perfusate, the concentration of glucose was significantly reduced, indicating that insulin diffuses across the dialysis membrane and has cellular effects that can be simultaneously recorded. The present findings are the first documentation on the use of microdialysis to study the local metabolic action of large peptide molecules in human tissues and may open new avenues for in-vivo metabolic research.