To evaluate the usefulness of the tissue-microdialysis technique in humans, the glucose concentration in the intercellular water space was measured in the abdominal subcutaneous region in healthy subjects. A 30 X 0.3 mm dialysis fiber with a 3,000 MW cutoff was used. The dialysis catheter was calibrated in vivo by perfusing it with isotonic saline and four to five different glucose concentrations (0-5 mM). The perfusate was collected in 6-min fractions. Regression analysis of the results of the calibration yielded the perfusate glucose concentration, which was in equilibrium with the surrounding tissue. Validation experiments showed that this value could be precisely measured and represented the intercellular glucose concentration. The recovery of glucose in the dialysate (dialysate glucose concentration/medium) during the calibrations in vivo was only approximately one-half of that in vitro (recovery factors 0.28 vs. 0.44, respectively). Under steady-state conditions, the intercellular glucose concentration was similar to the glucose levels in the cubital vein. It is concluded that this microdialysis technique is a useful tool allowing measurements of metabolically active substances in the intercellular water space in vivo provided that the calibrations are properly performed.

The large docking protein IRS-1 is a major substrate for the insulin receptor and other tyrosine kinases. It plays a key role in eliciting many of insulin's actions, including binding and activation of phosphatidylinositol (PI) 3-kinase and the subsequent increase in glucose transport. Gene disruption of IRS-1 in mice is associated with an impaired insulin-stimulated glucose disposal in vivo and glucose transport in vitro, but the survival of the animals and residual insulin sensitivity is dependent on the presence of the alternative docking protein IRS-2. We examined the expression and function of IRS-1 and IRS-2 in adipocytes from healthy and diabetic individuals. Cells from subjects with non-insulin-dependent diabetes mellitus (NIDDM), but not with insulin-dependent diabetes mellitus, had an impaired insulin effect and a marked reduction (70 ؎ 6%) in the expression of IRS-1 protein, whereas IRS-2 was unchanged. In normal cells, IRS-1 was the main docking protein for the binding and activation of insulin-stimulated PI 3-kinase; IRS-2 was also functional but required a higher insulin concentration for a similar binding and activation of PI 3-kinase. In contrast in NIDDM cells with a low IRS-1 content, IRS-2 became the main docking protein. These findings may provide important reasons for the insulin resistance in NIDDM.

Adipose tissue lipolysis and lipoprotein lipase (LPL) activity were studied in biopsies from the femoral and abdominal depots in healthy women during early or late menstrual cycle, pregnancy, and the lactation period.When the differences in cell size were taken into account, basal lipolysis was similar in both regions in nonpregnant women. During lactation, however, lipolysis was significantly higher in the femoral region. The lipolytic effect of noradrenaline (10-' M) was significantly less in the femoral region in the nonpregnant women and during early pregnancy. However, the lipolytic response was the same in both regions in lactating women. LPL activity was higher in the femoral than in the abdominal region except during lactation when a marked decrease in the LPL activity was seen in the femoral region. The LPL activity in the abdominal region remained unchanged in all patient groups.The results imply that in both nonpregnant and pregnant women lipid assimilation is favored in the femoral depot. During lactation, however, the metabolic pattern changes; the LPL activity decreases and lipid mobilization increases in this depot. These changes are much less pronounced in the abdominal region. Thus, fat cells from different regions show a differential response during pregnancy and lactation. These results suggest that the adipose tissue in different regions may have specialized functions.

To estimate the regional subcutaneous glycerol production rate in normal and obese humans, the venous arterialized plasma glycerol, interstitial glycerol in the subcutaneous adipose tissue together with adipose tissue blood flow (ATBF, ml/100 g* min) were measured in the postabsorptive state and for 2 h after ingestion of 100 g of oral glucose. Eight lean and eight obese men with normal oral glucose tolerance tests were investigated with the subcutaneous microdialysis technique and "AXe clearance. In the postabsorptive state, the interstitial glycerol concentrations in lean and obese subjects were 170±21 vs. 282±28 MM (P < 0.01) and 156±23 vs. 225±12 MM (P < 0.05) in the abdominal and femoral subcutaneous adipose tissue, respectively. The corresponding arterial glycerol levels were 54±4 vs. 75±14 MM (NS). Abdominal ATBF was greater in lean subjects (3.2±0.6 vs.

Insulin-resistant subjects have a slow onset of insulin action, and the underlying mechanism has not been determined. To evaluate whether a delayed transcapillary transport is part of the peripheral insulin resistance, we followed the kinetics of infused insulin and inulin in plasma and muscle interstitial fluid in obese insulin-resistant patients and control subjects. A total of 10 lean and 10 obese men (BMI 24 ؎ 0.8 vs. 32 ؎ 0.8 kg/m 2 , P < 0.001) was evaluated during a hyperinsulinemic-euglycemic clamp (insulin infusion rate 120 mU ⅐ m ؊2 ⅐ min ؊1 ) combined with an inulin infusion. Measurements of insulin and inulin in plasma were taken by means of arterial-venous catheterization of the forearm and microdialysis in brachioradialis muscle combined with forearm blood flow measurements with vein occlusion pletysmography. The obese subjects had a significantly lower steady-state glucose infusion rate and, moreover, demonstrated a delayed appearance of insulin (time to achieve half-maximal concentration [T 1/2 ] 72 ؎ 6 vs. 46 ؎ 6 min in control subjects, P < 0.05) as well as inulin (T 1/2 83 ؎ 3 vs. 53 ؎ 7 min, P < 0.01) in the interstitial fluid. Also, the obese subjects had a delayed onset of insulin action (T 1/2 70 ؎ 9 vs. 45 ؎ 5 min in control subjects, P < 0.05), and their forearm blood flow rate was significantly lower. These results demonstrate a delayed transcapillary transport of insulin and inulin from plasma to the muscle interstitial fluid and a delayed onset of insulin action in insulin-resistant obese subjects.

We evaluated the effects of rosiglitazone (4 mg b.i.d.) and metformin (1 g b.i.d.) monotherapy for 26 weeks on adipose tissue insulin-stimulated glucose uptake in patients (n ‫؍‬ 41) with type 2 diabetes. Before and after the treatment, glucose uptake was measured using 2-[18 F]fluoro-2-deoxyglucose and positron emission tomography and adipose tissue masses were quantified using magnetic resonance imaging. Rosiglitazone improved insulin-stimulated whole-body glucose uptake by 44% (P < 0.01 vs. placebo). Mean body weight was unchanged in the rosiglitazone group, while it decreased by 2.0 kg in the metformin group (P < 0.05 vs. placebo). In visceral adipose tissue, glucose uptake increased by 29% (from 17.8 ؎ 2.0 to 23.0 ؎ 2.6 mol ⅐ kg ؊1 ⅐ min

Lactate concentration in the subcutaneous interstitial fluid and adipose tissue blood flow (ATBF, ml/100 g.min) were simultaneously measured with the microdialysis technique combined with 133Xe clearance in the abdominal and femoral subcutaneous adipose tissue in nine lean and nine obese men. The studies were performed both in the postabsorptive state and 2 h after an oral glucose load and the results compared to the lactate levels in arterialized venous plasma. After an overnight's fast, arterial lactate was 738 +/- 49 and 894 +/- 69 microM (mean +/- SE) (P < 0.05) in the lean and obese subjects, respectively. The interstitial lactate levels were significantly higher than blood lactate in both subject groups without any regional differences. Abdominal and femoral ATBF was 3.2 +/- 0.6 vs. 2.8 +/- 0.4 and 1.7 +/- 0.3 vs. 2.4 +/- 0.4 ml/100 g.min (P < 0.05) in lean and obese subjects, respectively. Mean apparent lactate release from the abdominal vs. femoral adipose tissue in the fasting state was 10.5 +/- 3.1 vs. 8.6 +/- 2.3 and 6.0 +/- 2.3 vs. 8.5 +/- 2.3 mumol/kg.min (NS) in lean and obese subjects, respectively. Both plasma and interstitial lactate levels increased significantly after an oral glucose load in both subject groups. However, apparent lactate release increased significantly only in the lean group. It is concluded that subcutaneous adipose tissue is a significant source of whole-body lactate release in the postabsorptive state and that this is further enhanced in obese subjects due to their large adipose mass.

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