Adipose Tissue Microdialysis Research Articles

Catecholamines in the Periphery

Recent research related to catecholamines in the periphery has dealt with assessment of peripheral catecholaminergic function, catecholamines and stress, catecholamines and pain, catecholamines and neuroimmunology, adrenomedullary secretion and cosecretion, neurocardiology, catecholamines and metabolism, and roles of catecholamines in the brain in regulation of the cardiovascular system. All of these are discussed in this chapter. Peripheral catecholaminergic systems probably play a role in many neurocardiologic disorders, either secondarily as homeostatic effectors or primarily as etiologic factors. The hemodynamic hallmark of neurocardiogenic syncope is systemic vasodilation, with or without bradycardia. The vasodilation results from sudden, marked decreases in sympathoneural outflows. In contrast, plasma epinephrine (Epi) levels generally increase in this setting. Studies show a close association between lymphocyte concentrations of catecholamines and of cyclic adenosine monophosphate (cAMP). The usefulness of lymphocyte norepinephrine (NE) concentrations in evaluating sympathoneural function remains unknown. Studies using microdialysis, specific activity of [ 3 H]normetanephrine, and NE concentrations in lymph is likely to enable estimates of NE concentrations in interstitial fluid and, thereby, estimates of release of NE from sympathetic nerves. More convincing, consistent evidence is required to understand the role of catecholaminergic systems in components of immune responses. Increased use of microdialysis in adipose tissue and skeletal muscle is likely to enhance understanding of catecholaminergic systems in patients with a variety of metabolic disorders, including insulin-dependent diabetes mellitus (IDDM) and obesity.

Modulation of vascular tone and glycerol levels measured by in situ microdialysis in rat adipose tissue.

Changes in blood flow induced by the beta-adrenoceptor agonist isoproterenol (Iso) and a stable analogue of the major metabolite of arachidonic acid in adipose tissue, carbaprostacyclin (cPGI2), have been studied in rat periepididymal fat pad with in situ microdialysis measuring the distribution ratio of 0.2% ethanol in the dialysate (outflow) to that in the perfusate (inflow) (O/I ratio). Local perfusions of 1 microM cPGI2 or 1 microM Iso led to reversible decreases of the O/I ratio that were similar to the decrease induced by the vasodilating reference drug hydralazine (Hydra, 630 microM). Interestingly, a continuous perfusion of Hydra at a submaximal vasodilating concentration (63 microM) was sufficient to prevent further vasodilatation induced by Iso or cPGI2. To take advantage of this observation, experiments were designed to evaluate the influence of the vasodilating effect of Iso or cPGI2 on the ability of either to induce lipolysis in vivo. The results showed that the vasodilating effect of Iso could contribute to glycerol removal from the extracellular fluid and demonstrate that cPGI2 was devoid of lipolytic activity.

Microdialysis of adipose tissue during surgery: effect of local alpha- and beta-adrenoceptor blockade on blood flow and lipolysis

Microdialysis of adipose tissue during surgery: effect of local alpha- and beta-adrenoceptor blockade on blood flow and lipolysis

Effect of training on epinephrine-stimulated lipolysis determined by microdialysis in human adipose tissue

Trained humans (Tr) have a higher fat oxidation during submaximal physical work than sedentary humans (Sed). To investigate whether this reflects a higher adipose tissue lipolytic sensitivity to catecholamines, we infused epinephrine (0.3 nmol.kg-1.min-1) for 65 min in six athletes and six sedentary young men. Glycerol was measured in arterial blood, and intercellular glycerol concentrations in abdominal subcutaneous adipose tissue were measured by microdialysis. Adipose tissue blood flow was measured by 133Xe-washout technique. From these measurements adipose tissue lipolysis was calculated. During epinephrine infusion intercellular glycerol concentrations were lower, but adipose tissue blood flow was higher in trained compared with sedentary subjects (P < 0.05). Glycerol output from subcutaneous tissue (Tr: 604 +/- 322 nmol.100 g-1.min-1; Sed: 689 +/- 203; mean +/- SD) as well as arterial glycerol concentrations (Tr: 129 +/- 36 microM; Sed: 119 +/- 56) did not differ between groups. It is concluded that in intact subcutaneous adipose tissue epinephrine-stimulated blood flow is enhanced, whereas lipolytic sensitivity to epinephrine is the same in trained compared with untrained subjects.

Role of phosphodiesterase III in the antilipolytic effect of insulin in vivo.

The effect of three types of phosphodiesterase (PDE) inhibitors on in vivo antilipolysis was investigated in healthy subjects using a 2-h euglycemic, hyperinsulinemic (40 mU.m-2.min) clamp together with microdialysis of abdominal subcutaneous adipose tissue. During hyperinsulinemia (approximately 330 pmol/l), the circulating glycerol concentration was reduced to approximately 50% of the basal level of 53.2 +/- 3.6 mumol/l, indicating an antilipolytic effect. The decrease in adipose tissue dialysate glycerol, which mirrors the change in interstitial glycerol concentration, was about 40% during hyperinsulinemia when Ringer's solution alone was perfused. Local perfusion with a selective PDE IV inhibitor, rolipram (10(-4) mol/l), did not influence the insulin-induced decrease in dialysate glycerol (F = 0.8 vs. perfusion with Ringer's solution by two-factor analysis of variance [ANOVA]), although rolipram increased the dialysate glycerol level by 144 +/- 7% of the baseline value. However, local perfusion with a selective PDE III inhibitor, amrinone (10(-3) mol/l), or a nonselective PDE inhibitor, theophylline (10(-2) mol/l), abolished the ability of insulin to lower dialysate glycerol (F = 16.5, P < 0.01 and F = 8.5, P < 0.01, respectively, as compared with perfusion with Ringer's solution). The findings could not be explained by changes in the local blood flow (as measured by a microdialysis--ethanol escape technique), which was not affected by hyperinsulinemia in the presence or the absence of PDE inhibitors in the dialysis solvent. We conclude that PDEs play an important role in mediating the antilipolytic effect of insulin in vivo and that PDE III is the dominant isoenzyme modulating this effect.

Microdialysis of rat skeletal muscle and adipose tissue: dynamics of the interstitial glucose pool

Microdialysis was evaluated as a method for studying glucose metabolism in skeletal muscle. Dialysis probes (0.5 x 10 mm) were perfused at 0.5 or 1.0 microliter min-1. Based upon perfusion with glucose, the muscle interstitial glucose concentration was estimated to be 6.9 +/- 0.3 mM (n = 14), which was not significantly different from the blood glucose level. With insulin infusion (1200 mU kg-1 body wt i.v.), the insulin-induced change in the glucose concentration of the interstitial space of muscle was of equal magnitude to that of blood and adipose tissue. In spite of this, when the perfusion medium was not supplemented with glucose, the glucose concentration decreased more in skeletal muscle dialysates (to 36.7 +/- 4.9% of the initial level) than in blood (to 29.7 +/- 5.0%) but less than in adipose tissue (to 17.7 +/- 4.9% of the initial level) (P < 0.05). The results indicate that these differences are due to tissue-specific differences in the dynamic balance between the supply to, and removal from, the interstitial glucose pool. This balance is revealed as a result of the constant glucose drainage by the microdialysis probe. The present results show that, in skeletal muscle, increases in glucose uptake occur with a concomitant increase in tissue blood flow as revealed by the microdialysis ethanol technique, whereas in adipose tissue the glucose uptake increases in the absence of a corresponding increase in blood flow.

Long-term continuous glucose monitoring with microdialysis in ambulatory insulin-dependent diabetic patients

Summary The glucose concentration in the extracellular space of subcutaneous adipose tissue closely mirrors the blood glucose concentration. With a microdialysis technique, we undertook continuous ambulatory monitoring of adipose tissue glucose in 17 insulin-dependent diabetic patients with labile glycaemic control. The aims of the study were to investigate performance of the microdialysis device and to evaluate whether consecutive 24 h glucose profiles could be used to adjust insulin therapy. A microdialysis probe was implanted subcutaneously, perfused by a portable microinfusion pump, and dialysate fractions were collected every 1 or 2 h for 75 h. The mean (SE) tissue dialysate glucose concentration was 93 (3)% of the concentration in venous plasma, and variations in adipose tissue glucose closely paralleled changes in plasma glucose. Mean 24 h tissue glucose concentration correlated significantly with glycosylated haemoglobin (HbA 1c; r=0·62, p<0·01). Most patients had a reproducible daily pattern of glucose swings, and in more than half the patients consecutive ambulatory glucose profiles were almost superimposed. When patients' insulin therapy was adjusted on the basis of ambulatory glucose monitoring, HbA 1c decreased by almost 2% (p < 0·01), and this decrease lasted for at least 9 months. Microdialysis of adipose tissue can be used for continuous long-term monitoring of glucose concentrations in diabetic patients during ordinary daily life. Daily glucose profiles are often reproducible and the recordings may thus be used for individual tailoring of insulin therapy to improve glycaemic control.

Role of vascular alpha-2 adrenoceptors in regulating lipid mobilization from human adipose tissue.

The role of alpha-2 adrenoceptors in lipid mobilization and blood flow was investigated in situ using microdialysis of subcutaneous adipose tissue in nonobese healthy subjects. The alpha-2 agonist clonidine caused dose-dependent biphasic response with increased glycerol levels at low clonidine concentrations and decreased glycerol levels at concentrations > 10(-7) mol/liter. Similar results were observed with epinephrine plus propranolol. Clonidine action was unaffected in the presence of labetalol (beta-/alpha-1 antagonist) but completely blunted by the presence of yohimbine (alpha-2 antagonist). The pseudolipolytic effect of clonidine was significantly more pronounced in gluteal as compared with abdominal adipose tissue. When clonidine was added together with the vasodilating agents nitroprusside or hydralazine, the pseudolipolytic effect was abolished and a dose-dependent decrease in dialysate glycerol was observed at all clonidine concentrations (10(-10)-10(-4) mol/liter). When ethanol was added to the perfusate to monitor blood flow, the escape of alcohol from the dialysate was accelerated by 30% with hydralazine or nitroprusside (P < 0.01) and 30% retarded (P < 0.05) by clonidine (10(-10) mol/liter). Thus, the results demonstrate an important role of blood flow for regulating lipid mobilization from adipose tissue in vivo. Alpha-2 adrenoceptor activation causes marked retention of lipids in adipose tissue due to vasoconstriction in combination with antilipoiysis.

Microdialysis of adipose tissue

Although microdialysis has been available for almost two decades, it has only recently been applied in investigations of adipose tissue. The microdialysis technique enables continuous sampling of metabolites and other small molecules from the extracellular space of subcutaneous adipose tissue from intact animals or man, and the exposure of this compartment locally to metabolically active agents without causing generalized effects. To date, the method has been used to measure the steady-state interstitial levels of metabolites and to investigate the regulation of lipolysis and carbohydrate metabolism in situ in subcutaneous adipose tissue. Apart from a great potential for experimental research, the microdialysis method offers several new possibilities for clinical investigation. Because microdialysis probes are easy to implant and cause little discomfort, they may be used for continuous monitoring of glucose and glycerol (lipolysis index) in the treatment of diabetes, obesity and other disorders characterized by disturbances in lipid and carbohydrate metabolism.

Microdialysis of subcutaneous adipose tissue in vivo for continuous glucose monitoring in man

Subcutaneous adipose tissue extracellular glucose was investigated in vivo in man with a microdialysis technique. A small dialysis probe (4 or 10×0.5 mm) was implanted subcutaneously, and was perfused continuously with a micro-infusion pump. Dialysate samples were collected in 15-min periods. A transient yield of ATP was recorded immediately after insertion of the probe; thereafter ATP was almost undetectable. During steady-state conditions the tissue dialysate glucose concentration remained constant for at least 2 h, which indicated that there was no drainage of glucose from the interstitial fluid to the tissue dialysate. Simultaneous dialysis of venous blood and subcutaneous fat in rats showed that the recovery of glucose from the adipose tissue interstitial fluid to the dialysate was only 20% of that from blood. However, in vivo dialysis of human adipose tissue with glucose-containing solutions produced an equilibrium with the extracellular space at a glucose concentration that was similar to the blood glucose concentration. After oral glucose ingestion and following i. v. insulin and glucose administration the relative variations in subcutaneous glucose closely resembled those in blood glucose. It is concluded that the subcutaneous implantation of the presently used small dialysis device causes only minor and transient traumatic effects. The dialysis recovery of glucose is lower in adipose tissue than in blood. However, the relative kinetics of subcutaneous tissue dialysate glucose are closely related to variations in the blood glucose concentration, and thus may be used for monitoring of glycaemic control in man.

Microdialysis of subcutaneous adipose tissue in vivo for continuous glucose monitoring in man.

Subcutaneous adipose tissue extracellular glucose was investigated in vivo in man with a microdialysis technique. A small dialysis probe (4 or 10 x 0.5 mm) was implanted subcutaneously, and was perfused continuously with a micro-infusion pump. Dialysate samples were collected in 15-min periods. A transient yield of ATP was recorded immediately after insertion of the probe; thereafter ATP was almost undetectable. During steady-state conditions the tissue dialysate glucose concentration remained constant for at least 2 h, which indicated that there was no drainage of glucose from the interstitial fluid to the tissue dialysate. Simultaneous dialysis of venous blood and subcutaneous fat in rats showed that the recovery of glucose from the adipose tissue interstitial fluid to the dialysate was only 20% of that from blood. However, in vivo dialysis of human adipose tissue with glucose-containing solutions produced an equilibrium with the extracellular space at a glucose concentration that was similar to the blood glucose concentration. After oral glucose ingestion and following i.v. insulin and glucose administration the relative variations in subcutaneous glucose closely resembled those in blood glucose. It is concluded that the subcutaneous implantation of the presently used small dialysis device causes only minor and transient traumatic effects. The dialysis recovery of glucose is lower in adipose tissue than in blood. However, the relative kinetics of subcutaneous tissue dialysate glucose are closely related to variations in the blood glucose concentration, and thus may be used for monitoring of glycaemic control in man.

Microdialysis of adipose tissue and blood for in vivo lipolysis studies.

Glycerol levels (lipolysis index) were continuously monitored in intact rats using simultaneous microdialysis of venous blood and subcutaneous adipose tissue. Dialysis probes (0.5 X 4 mm) were implanted and perfused using a microinjection pump. The basal glycerol levels in perfusates of blood and adipose tissue were stable. Intravenous isoproterenol (isoprenaline) increased the glycerol levels in a similar fashion in blood perfusate as compared with plasma and the dialysate of two different adipose tissue probes implanted in the same animal. Isoproterenol included in the adipose tissue dialysis solvent increased the glycerol level in adipose tissue perfusate but not in the blood perfusate; this indicates a local lipolytic effect of catecholamines. Increases in intravenous doses of isoproterenol caused a transient and marked dose-dependent elevation of glycerol in the adipose tissue perfusate and a gradual but less marked rise of glycerol in blood perfusate. This indicates that glycerol kinetics in blood and adipose tissue differ after catecholamine stimulation. In conclusion, microdialysis of blood and subcutaneous adipose tissue offers new and unique possibilities for in vivo lipolysis studies in intact animals.