Jens Bülow

Monitoring tissue oxygen availability with near infrared spectroscopy (NIRS) in health and disease.

Near infrared spectroscopy (NIRS) is becoming a widely used research instrument to measure tissue oxygen (O2) status non‐invasively. Continuous‐wave spectrometers are the most commonly used devices, which provide semi‐quantitative changes in oxygenated and deoxygenated hemoglobin in small blood vessels (arterioles, capillaries and venules). Refinement of NIRS hardware and the algorithms used to deconvolute the light absorption signal have improved the resolution and validity of cytochrome oxidase measurements. NIRS has been applied to measure oxygenation in a variety of tissues including muscle, brain and connective tissue, and more recently it has been used in the clinical setting to assess circulatory and metabolic abnormalities. Quantitative measures of blood flow are also possible using NIRS and a light‐absorbing tracer, which can be applied to evaluate circulatory responses to exercise along with the assessment of tissue O2 saturation. The venular O2 saturation can be estimated with NIRS by applying venous occlusion and measuring changes in oxygenated vs. total hemoglobin. These various measurements provide the opportunity to evaluate several important metabolic and circulatory patterns in very localized regions of tissue and may be fruitful in the study of occupational syndromes and a variety of diseases.

Type I collagen synthesis and degradation in peritendinous tissue after exercise determined by microdialysis in humans

1 Physical activity is known to increase type I collagen synthesis measured as the concentration of biomarkers in plasma. By the use of microdialysis catheters with a very high molecular mass cut‐off value (3000 kDa) we aimed to determine local type I collagen synthesis and degradation in the peritendinous region by measuring interstitial concentrations of a collagen propeptide (PICP; 100 kDa) and a collagen degradation product (ICTP; 9 kDa) as well as an inflammatory mediator (PGE2). 2 Seven trained human runners were studied before and after (2 and 72 h) 3 h of running (36 km). Two microdialysis catheters were placed in the peritendinous space ventral to the Achilles’ tendon under ultrasound guidance and perfused with a Ringer‐acetate solution containing 3H‐labelled human type IV collagen and [15‐3H(N)]PGE2 for in vivo recovery determination. Relative recovery was 37–59% (range of the s.e.m. values) for both radioactively labelled substances. 3 PICP concentration decreased in both interstitial peritendinous tissue and arterial blood immediately after exercise, but rose 3‐fold from basal 72 h after exercise in the peritendinous tissue (55 ± 10 μg l−1, mean ± s.e.m. (rest) to 165 ± 40 μg l−1 (72 h), P < 0·05) and by 25% in circulating blood (160 ± 10 μg l−1 (rest) to 200 ± 12 μg l−1 (72 h), P < 0·05). ICTP concentration did not change in blood, but decreased transiently in tendon‐related tissue during early recovery after exercise only. PGE2 concentration increased in blood during running, and returned to baseline in the recovery period, whereas interstitial PGE2 concentration was elevated in the early recovery phase. 4 The findings of the present study indicate that acute exercise induces increased formation of type I collagen in peritendinous tissue as determined with microdialysis and using dialysate fibre with a very high molecular mass cut‐off. This suggests an adaptation to acute physical loading also in non‐bone‐related collagen in humans.

Persistent Resetting of the Cerebral Oxygen/Glucose Uptake Ratio by Brain Activation: Evidence Obtained with the Kety—Schmidt Technique:

Global cerebral blood flow (CBF), global cerebral metabolic rates for oxygen (CMRO2), and for glucose (CMRglc), and lactate efflux were measured during rest and during cerebral activation induced by the Wisconsin card sorting test. Measurements were performed in healthy volunteers using the Kety–Schmidt technique. Global CMRO2 was unchanged during cerebral activation, whereas global CBF and global CMRglc both increased by 12%, reducing the molar ratio of oxygen to glucose consumption from 6.0 during baseline conditions to 5.4 during activation. Data obtained in the period following cerebral activation showed that the activation-induced resetting of the relation between CMRglc and CMRO2 persisted virtually unaltered for ≥40 min after the mental activation task was terminated. The activation-induced increase in cerebral lactate efflux measured over the same time period accounted for only a small fraction of the activation-induced excess glucose uptake. These data confirm earlier reports that brain activation can induce resetting of the cerebral oxygen/glucose consumption ratio, and indicate that the resetting persists for a long period after cerebral activation has been terminated and physiologic stress indicators returned to baseline values. Activation-induced resetting of the cerebral oxygen/glucose uptake ratio is not necessarily accounted for by increased lactate production from nonoxidative glucose metabolism.

Metabolism and inflammatory mediators in the peritendinous space measured by microdialysis during intermittent isometric exercise in humans

1 The metabolic processes that occur around the tendon during mechanical loading and exercise are undescribed in man. These processes are important for understanding the development of overuse inflammation and injury. 2 A microdialysis technique was used to determine interstitial concentrations of glycerol, glucose, lactate, prostaglandin E2 (PGE2) and thromboxane B2 (TXB2) as well as to calculate tissue substrate balance in the peritendinous region of the human Achilles tendon. Recovery of 48–62% (range) at rest and 70–77% during exercise were obtained for glycerol, glucose and PGE2. 3 Six young healthy humans were studied at rest, during 30 min of intermittent static plantar flexion of the ankle at a workload corresponding to individual body weight, and during 60 min of recovery. Microdialysis was performed in both legs with simultaneous determination of blood flow by 133Xe washout in the same area, and blood sampling from the radial artery. 4 With exercise, the net release of lactate as well as of glycerol from the peritendinous space of the Achilles tendon increased 2‐fold (P < 0.05). Furthermore a 100% increase in interstitial concentration of PGE2 and TXB2 was found, but it was only significant for TXB2(P < 0.05). As peritendinous blood flow increased 2‐ to 3‐fold during intermittent static contractions, this indicates also that the output of these substances from the tissue increased during exercise. 5 This study indicates that both lipid and carbohydrate metabolism as well as inflammatory activity is accelerated in the peritendinous region of the human Achilles tendon with dynamic loading.

Metabolic effects of interleukin‐6 in human splanchnic and adipose tissue

Interleukin‐6 (IL‐6) was infused intravenously for 2.5 h in seven healthy human volunteers at a dose giving rise to a circulating IL‐6 concentration of ≈35 ng l−1. The metabolic effects of this infusion were studied in subcutaneous adipose tissue on the anterior abdominal wall and in the splanchnic tissues by the Fick principle after catheterizations of an artery, a subcutaneous vein draining adipose tissue, and a hepatic vein, and measurements of regional adipose tissue and splanchnic blood flows. In control studies without IL‐6 infusion subcutaneous adipose tissue metabolism was studied by the same technique in eight healthy subjects. The net release of glycerol and fatty acids from the subcutaneous abdominal adipose tissue remained constant in the control experiment. IL‐6 infusion gave rise to increase in net glycerol release in subcutaneous adipose tissue while the net release of fatty acids did not change significantly. In the splanchnic region IL‐6 elicited a pronounced vasodilatation, and the uptake of fatty acids and the gluconeogenic precursors glycerol and lactate increased significantly. The splanchnic net output of glucose and triacylglycerol did not change during the IL‐6 infusion. It is concluded that IL‐6 elicits lipolytic effects in human adipose tissue in vivo, and that IL‐6 also has effects on the splanchnic lipid and carbohydrate metabolism.

Interleukin-6 production in human subcutaneous abdominal adipose tissue: the effect of exercise

The interleukin‐6 (IL‐6) output from subcutaneous, abdominal adipose tissue was studied in nine healthy subjects before, during and for 3 h after 1 h two‐legged bicycle exercise at 60 % maximal oxygen consumption. Seven subjects were studied in control experiments without exercise. The adipose tissue IL‐6 output was measured by direct Fick technique. An artery and a subcutaneous vein on the anterior abdominal wall were catheterized. Adipose tissue blood flow was measured using the 133Xe‐washout method. In both studies there was a significant IL‐6 output in the basal state and no significant change was observed during exercise. Post‐exercise the IL‐6 output began to increase after 30 min. Three hours post‐exercise it was 58.6 ± 22.2 pg (100 g)−1 min−1. In the control experiments the IL‐6 output also increased, but it only reached a level of 3.5 ± 0.8 pg (100 g)−1 min−1. The temporal profile of the post‐exercise change in the IL‐6 output closely resembles the changes in the outputs of glycerol and fatty acids, which we have described previously in the same adipose tissue depot. The difference is that it begins to increase ≈30 min before the glycerol and fatty acid outputs begin to increase. Thus, we suggest that the enhanced IL‐6 production post‐exercise in abdominal, subcutaneous adipose tissue may act locally via autocrine/paracrine mechanisms influencing lipolysis and fatty acid mobilization rate from this lipid depot.

Blood flow and oxygenation in peritendinous tissue and calf muscle during dynamic exercise in humans

1 Circulation around tendons may act as a shunt for muscle during exercise. The perfusion and oxygenation of Achilles’ peritendinous tissue was measured in parallel with that of calf muscle during exercise to determine (1) whether blood flow is restricted in peritendinous tissue during exercise, and (2) whether blood flow is coupled to oxidative metabolism. 2 Seven individuals performed dynamic plantar flexion from 1 to 9 W. Radial artery and popliteal venous blood were sampled for O2, peritendinous blood flow was determined by 133Xe‐washout, calf blood flow by plethysmography, cardiac output by dye dilution, arterial pressure by an arterial catheter‐transducer, and muscle and peritendinous O2 saturation by spatially resolved spectroscopy (SRS). 3 Calf blood flow rose 20‐fold with exercise, reaching 44 ± 7 ml (100 g)−1 min−1 (mean ± s.e.m.) at 9 W, while Achilles’ peritendinous flow increased (7‐fold) to 14 ± 4 ml (100 g)−1 min−1, which was 18 % of the maximal flow established during reactive hyperaemia. SRS‐O2 saturation fell both in muscle (from 66 ± 2 % at rest to 57 ± 3 %, P < 0.05) and in peritendinous regions (58 ± 4 to 52 ± 4 %, P < 0.05) during exercise along with a rise in leg vascular conductance and microvascular haemoglobin volume, despite elevated systemic vascular resistance. 4 The parallel rise in calf muscle and peritendinous blood flow and fall in O2 saturation during exercise indicate that blood flow is coupled to oxidative metabolism in both tissue regions. Increased leg vascular conductance accompanied by elevated microvascular haemoglobin volume reflect vasodilatation in both muscle and peritendinous regions. However, peak exercise peritendinous blood flow reaches only ≈20 % of its maximal blood flow capacity.

Acute effects of nicotine and smoking on blood flow, tissue oxygen, and aerobe metabolism of the skin and subcutis.

BACKGROUND Nicotine released from tobacco smoke causing reduction in blood flow has been suggested as causative for postoperative wound complications in smokers, but the mechanism remains unknown. MATERIALS AND METHODS In eight healthy male smokers and eight ex-smokers, the cutaneous and subcutaneous blood flow (QBF, SqBF) was assessed by Laser Doppler and 133Xe clearance. Tissue oxygen tension (TO(2)) was measured by a LICOX O(2)-electrode. Tissue glucose and lactate (Tgluc, Tlact) were assessed by microdialysis. The parameters were studied after intravenous infusion of 1.0 mg nicotine, smoking of one cigarette, arterial occlusion, and reperfusion. RESULTS Nicotine infusion decreased SqBF from 4.2 +/- 2.0 to 3.1 +/- 1.2 mL/100 g tissue/min (P < 0.01), whereas QBF was 21.7 +/- 8.6 and 22.7 +/- 9.6 Arbitrary Units (AU), respectively (P = 0.21). TO(2) increased from 49.3 +/- 12.0 to 53.9 +/- 12.0 mm Hg (P = 0.01). Tgluc and Tlact remained unaffected. Smoking decreased SqBF from 4.2 +/- 2.0 to 2.7 +/- 1.2 mL/100 g tissue/min (P < 0.01). QBF decreased from 23.4 +/- 9.2 to 20.3 +/- 7.4 AU (P < 0.01), and TO(2) decreased from 53.9 +/- 12.0 to 48.4 +/- 11.1 mm Hg (P < 0.01). Following smoking, Tgluc decreased from 0.7 +/- 0.1 to 0.6 +/- 0.1 ng/mL (P < 0.01), and Tlact increased from 0.2 +/- 0.1 to 0.3 +/- 0.2 ng/mL (P < 0.01). The observed alterations were similar in smokers and ex-smokers. CONCLUSIONS Nicotine has a limited vasoactive effect in the skin and subcutis unlikely to be explained by smoking, which distinctly decreases tissue blood flow, oxygen tension, and aerobe metabolism independent of smoking status.

Effects of a physiological GH pulse on interstitial glycerol in abdominal and femoral adipose tissue

Physiologically, growth hormone (GH) is secreted in pulses with episodic bursts shortly after the onset of sleep and postprandially. Such pulses increase circulating levels of free fatty acid and glycerol. We tested whether small GH pulses have detectable effects on intercellular glycerol concentrations in adipose tissue, and whether there would be regional differences between femoral and abdominal subcutaneous fat, by employing microdialysis for 6 h after administration of GH (200 μg) or saline intravenously. Subcutaneous adipose tissue blood flow (ATBF) was measured by the local Xenon washout method. Baseline of interstitial glycerol was higher in adipose tissue than in blood [220 ± 12 (abdominal) vs. 38 ± 2 (blood) μmol/l, P < 0.0005; 149 ± 9 (femoral) vs. 38 ± 2 (blood) μmol/l, P < 0.0005] and higher in abdominal adipose tissue compared with femoral adipose tissue ( P < 0.0005). Administration of GH induced an increase in interstitial glycerol in both abdominal and femoral adipose tissue (ANOVA: abdominal, P = 0.04; femoral, P = 0.03). There was no overall difference in the response to GH in the two regions during the study period as a whole (ANOVA: P = 0.5), but during peak stimulation of lipolysis abdominal adipose tissue was, in absolute but not in relative terms, stimulated more markedly than femoral adipose tissue (ANOVA: P = 0.03 from 45 to 225 min). Peak interstitial glycerol values of 253 ± 37 and 336 ± 74 μmol/l were seen after 135 and 165 min in femoral and abdominal adipose tissue, respectively. ATBF was not statistically different in the two situations (ANOVA: P = 0.7). In conclusion, we have shown that a physiological pulse of GH increases interstitial glycerol concentrations in both femoral and abdominal adipose tissue, indicating activated lipolysis. The peak glycerol increments after GH were higher in abdominal adipose tissue, perhaps due to a higher basal rate of lipolysis in this region.Physiologically, growth hormone (GH) is secreted in pulses with episodic bursts shortly after the onset of sleep and postprandially. Such pulses increase circulating levels of free fatty acid and glycerol. We tested whether small GH pulses have detectable effects on intercellular glycerol concentrations in adipose tissue, and whether there would be regional differences between femoral and abdominal subcutaneous fat, by employing microdialysis for 6 h after administration of GH (200 microgram) or saline intravenously. Subcutaneous adipose tissue blood flow (ATBF) was measured by the local Xenon washout method. Baseline of interstitial glycerol was higher in adipose tissue than in blood [220 +/- 12 (abdominal) vs. 38 +/- 2 (blood) micromol/l, P < 0.0005; 149 +/- 9 (femoral) vs. 38 +/- 2 (blood) micromol/l, P < 0.0005] and higher in abdominal adipose tissue compared with femoral adipose tissue (P < 0.0005). Administration of GH induced an increase in interstitial glycerol in both abdominal and femoral adipose tissue (ANOVA: abdominal, P = 0. 04; femoral, P = 0.03). There was no overall difference in the response to GH in the two regions during the study period as a whole (ANOVA: P = 0.5), but during peak stimulation of lipolysis abdominal adipose tissue was, in absolute but not in relative terms, stimulated more markedly than femoral adipose tissue (ANOVA: P = 0. 03 from 45 to 225 min). Peak interstitial glycerol values of 253 +/- 37 and 336 +/- 74 micromol/l were seen after 135 and 165 min in femoral and abdominal adipose tissue, respectively. ATBF was not statistically different in the two situations (ANOVA: P = 0.7). In conclusion, we have shown that a physiological pulse of GH increases interstitial glycerol concentrations in both femoral and abdominal adipose tissue, indicating activated lipolysis. The peak glycerol increments after GH were higher in abdominal adipose tissue, perhaps due to a higher basal rate of lipolysis in this region.

Fat metabolism in formerly obese women

An impaired fat oxidation has been implicated to play a role in the etiology of obesity, but it is unclear to what extent impaired fat mobilization from adipose tissue or oxidation of fat is responsible. The present study aimed to examine fat mobilization from adipose tissue and whole body fat oxidation stimulated by exercise in seven formerly obese women (FO) and eight matched controls (C). Lipolysis in the periumbilical subcutaneous adipose tissue, whole body energy expenditure (EE), and substrate oxidation rates were measured before, during, and after a 60-min bicycle exercise bout of moderate intensity. Lipolysis was assessed by glycerol release using microdialysis and blood flow measurement by 133Xe clearance technique. The FO women had lower resting EE than C (3.77 +/- 1.01 vs. 4.88 +/- 0.74 kJ/min, P < 0.05) but responded similarly to exercise. Adipose tissue glycerol release was twice as high in FO than in C at rest (0.455 +/- 0.299 vs. 0.206 +/- 0.102 mumol.100 g-1.min-1, P < 0.05) but increased similarly in FO and C in response to exercise. Despite higher plasma nonesterified fatty acids (NEFA) in FO (P < 0.001), fat oxidation rates during rest and recovery were lower in FO than in C (1.32 +/- 0.84 vs. 3.70 +/- 0.57 kJ/min, P < 0.02) and fat oxidation for a given plasma NEFA concentration was lower at rest (P < 0.001) and during exercise (P = 0.01) in the formerly obese group. In conclusion, fat mobilization both at rest and during exercise is intact in FO, whereas fat oxidation is subnormal despite higher circulation NEFA levels. The lower resting EE and the failure to use fat as fuel contribute to a positive fat balance and weight gain in FO subjects.