Subcutaneous adipose tissue metabolism and pharmacology: a new investigative technique.

According to the Fick principle, any metabolic or hormonal exchange through a given tissue depends on the product of blood flow by arteriovenous difference. Because adipose tissue plays dual storage and endocrine roles, regulation of adipose tissue blood flow (ATBF) is of pivotal importance. Monitoring ATBF in humans can be achieved through different methodologies, such as the (133)Xe washout technique, considered to be the "gold standard", as well as microdialysis and other methods that are not well validated as of yet. This report describes a new method, called "adipose tissue microinfusion" or "ATM", which simultaneously quantifies ATBF by combining the (133)Xe washout technique together with variations of ATBF induced by local infusion of vasoactive agents. The most appropriate site for ATM investigation is the subcutaneous adipose tissue of the anterior abdominal wall. This innovative method conveniently enables the direct comparison of the effects on ATBF of any vasoactive compound, drug, or hormone against a contralateral saline control. The ATM method improves the accuracy and feasibility of physiological and pharmacological studies on the regulation of ATBF in vivo in humans.

Human adipose tissue blood flow and micromanipulation of human subcutaneous blood flow.

Regulation of blood flow in tissues such as skeletal muscle, liver, and adipose tissue is needed to meet the changing local metabolic and physiological demands under varying conditions. In healthy individuals, adipose tissue blood flow (ATBF) is remarkably responsive to meal ingestion, but changes in ATBF in response to other physiological stimuli, such as stress and physical exercise, have also been noted. The ATBF response to nutrient intake may be of particular importance in the regulation of metabolism by facilitating transport of nutrients as well as signaling between adipose tissue and other metabolically active tissues. A reduction in both fasting and postprandial ATBF has been observed in obesity; this impairment is associated with insulin resistance. A better understanding of the physiological basis for (nutritional) regulation of ATBF may therefore give insight to the relationship between disturbances in ATBF and the metabolic disturbances observed in response to insulin resistance. In this chapter, we describe some different approaches to quantify human ATBF, with a particular emphasis on the 133xenon wash-out technique and a method by which regulatory properties of subcutaneous ATBF can be studied by pharmacological micromanipulation (microinfusion).

Extracellular cyclic AMP and adenosine appearance in adipose tissue of Sus scrofa: effects of exercise.

Cyclic AMP (cAMP) appears extracellularly in a variety of tissues including brain, liver, and kidney; whether it appears in adipose tissue and responds to physiological perturbation is unknown. The purpose of this study was to examine adipose tissue extracellular cAMP appearance and metabolism in situ and in vitro in physiologically challenged animals. Littermate swine were either sedentary or exercise trained on a treadmill for 3 months and subjected to acute exercise on experiment day. In situ, microdialysis probes in subcutaneous back fat were perfused before, during, and after animals performed 20 mins of acute exercise, and dialysate was analyzed for cAMP and adenosine. In vitro, isolated adipocytes were hormonally stimulated to provoke cAMP synthesis and efflux, and plasma membrane phosphodiesterase and 5'-nucleotidase activities were measured. Extracellular cAMP and adenosine levels in adipose tissue of sedentary swine averaged 5.2 +/- 1.7 and 863 +/- 278 nM, respectively. Exercise training tended to increase extracellular cAMP (11.3 +/- 1.7 nM) and reduce extracellular adenosine (438 +/- 303 nM), although neither change was statistically significant. Acute exercise caused a significant 3-fold and 16-fold increase in extracellular cAMP and adenosine, respectively, compared to rest. These changes occurred despite a 2- to 3-fold increase in adipose tissue blood flow during acute exercise. In vitro, cAMP efflux from exercise-trained swine was 42% greater than that from adipocytes of sedentary swine, yet adipocyte plasma membranes from exercise-trained and sedentary swine did not differ in maximal phosphodiesterase and 5'-nucleotidase activities. We conclude that cAMP appears extracellularly in swine adipose tissue and that the levels of extracellular cAMP and adenosine in intact swine adipose tissue are influenced by both acute and chronic exercise. The subsequent impact of the changes in these biochemicals on local cellular metabolism and growth remains to be determined.

Functional evaluation of lung by Xe-133 lung ventilation scintigraphy before and after lung volume reduction surgery (LVRS) in patients with pulmonary emphysema.

We evaluated the respiratory functions of patients with pulmonary emphysema who underwent lung volume reduction surgery (LVRS) by the mean transit time (MTT) with Xe-133 lung ventilation scintigraphy, forced expiration volume in 1 sec (FEV1.0), residual volume (RV), distance walked in 6 min (6-min walk), and the Hugh-Jones classification (H-J classification) before and after LVRS. In 69 patients with pulmonary emphysema (62 men, 7 women; age range, 47-75 years; mean age, 65.4 years +/- 6.1, preoperative H-J classification, III (two were II)-V) who underwent LVRS, all preoperative and postoperative parameters (MTT 3 weeks after LVRS and the others 3 months after LVRS) were judged statistically by the Wilcoxon signed-ranks test and Odds ratio. Every postoperative parameter was improved with a significant difference (P < 0.05) compared to preoperative parameters. MTT at 3 weeks after LVRS was not associated with %FEV1.0 and the H-J classification at 3 months after LVRS, but was associated with RV and a 6-min walk at 3 months after LVRS. MTT was useful for the clinical evalution of aerobic capability after LVRS.

[The effect of age on the regional blood flow of neck of femur in normal Chinese].

This study aimed to assess the effect of age on the regional blood flow of neck of femur in normal Chinese. 133Xe washout method was adopted. The result showed that the femur regional blood flow of normal Chinese decreased slowly with the increase of their age. The relation between age and femur blood flow might be expressed as F = -0.042Y + 9.01(F refers to blood flow, Y refers to age). The average femur regional blood flow of normal Chinese was 7.37 +/- 0.62 ml.(100 g.min)-1 for 21-40 years old. The authors suggest this result can be considered to stand for regional femur blood flow of young healthy adults.

Muscle blood flow during intermittent exercise: comparison of the microdialysis ethanol technique and 133Xe clearance.

1. Local skeletal muscle blood flow was monitored using the microdialysis ethanol technique and 133Xe clearance during intermittent isometric contractions (5 s on/10 s off) of the thigh at 0-60% of the maximal voluntary isometric contraction force. 2. A linear increase in blood flow over a 25-fold range was detected using both 133Xe clearance and the microdialysis ethanol technique. 3. The median correlation coefficient between percentage maximal voluntary isometric contraction force and the ethanol outflow/inflow ratio, a marker of blood flow, was r = -0.98 (-0.94 to -0.99) (median and range, n = 6). The corresponding correlation coefficient for 133Xe clearance was r = 0.97 (0.92-0.98), the correlation coefficient between the ethanol outflow/inflow ratio and 133Xe clearance being r = -0.92 (-0.89 to -0.94). 4. Dialysate glucose concentration, although affected by blood flow, was not always significantly correlated with blood flow changes (r = 0.70; 0.51-0.95). 5. It may be concluded that the ethanol technique provides a valid measure of changes in local skeletal muscle blood flow. The data furthermore show that a linear increase in thigh skeletal muscle blood flow exists during the studied protocol of intermittent isometric contractions.

Estimation of error limits for cerebral blood flow values obtained from xenon-133 clearance curves.

I provide the theoretical basis for an error calculus for measurements of cerebral blood flow using a freely diffusible tracer substance such as xenon-133. The use of the error calculus is exemplified by a study of the effect on the error margins in measurements of gray matter blood flow from flow level, relative weight of the gray matter compartment, and use of the earliest parts of the clearance curves. The clinical value of the error calculus is illustrated by its ability to separate different sources of measurement error. As a consequence, it is possible to optimize the method for blood flow calculation from the clearance curves, depending on the type of cerebral blood flow measurement. I show that if a true picture of the regional gray matter blood flow distribution is sought, the earliest part of the clearance curves should be used. This does, however, increase the error in the estimate of the average cerebral blood flow value.

Skin blood flow from gas transport: helium xenon and laser Doppler compared.

A study was designed to compare three independent measures of cutaneous blood flow in normal healthy volunteers: xenon-133 washout, helium flux, and laser velocimetry. All measurements were confined to the volar aspect of the forearm. In a large group of subjects we found that helium flux through intact skin changes nonlinearly with the controlled local skin temperature whereas helium flux through stripped skin, which is directly proportional to skin blood flow, changes linearly with cutaneous temperature over the range 33 degrees to 42 degrees. In a second group of six volunteers we compared helium flux through stripped skin to xenon-133 washout (intact skin) at a skin temperature of 33 degrees, and we found an essentially linear relationship between helium flux and xenon measured blood flow. In a third group of subjects we compared helium flux blood flow (stripped skin) to laser doppler velocimetric (LDV) measurements (intact skin) at adjacent skin sites and found a nonlinear increase in the LDV skin blood flow compared to that determined by helium over the same temperature range. A possible explanation for the nonlinear increases of helium flux through intact skin and of LDV output with increasing local skin temperature is that they reflect more than a change in blood flow. They may also reflect physical changes in the stratum corneum, which alters its diffusional resistance to gas flux and its optical characteristics.

A comparison of the rate of clearance of xenon (133Xe) and pertechnetate ion (99mTcO4-) in murine tumors and normal leg muscles.

Xenon-133 is a widely used tracer for clearance measurements to estimate nutritive blood flow. There are however certain difficulties in handling and obtaining the radionuclide. Pertechnetate ion, 99mTcO4-, is used extensively as a radiotracer in nuclear medicine studies and is easily available. To assess the usefulness of pertechnetate ion for blood flow studies, we have measured the rate of clearance following a local injection of pertechnetate ion and xenon simultaneously in normal mouse leg muscle and in murine tumors (fibrosarcoma and mammary carcinoma) of different sizes. The partition coefficients for the two tracers were also measured. The results indicated that the clearance halftime for pertechnetate ion was consistently greater than that for xenon in the tumors but similar in normal resting muscle. The estimated blood flow values assessed using the two tracers were similar in all cases except for large tumors (greater than 1.5 g). In this case, pertechnetate ion gave lower values than xenon. The difference could be explained by the large intracapillary distances which occur in poorly vascularized regions of large tumors since pertechnetate is a charged ion and hence might be expected to have limited diffusion capacity. The results indicate that caution is warranted when pertechnetate ion is used as a tracer for measurements of blood flow and for other biological determinations which depend on tissue transport for properties.

Focal ischaemia caused by instability of cerebrovascular tone during attacks of hemiplegic migraine. A regional cerebral blood flow study.

During the course of hemiplegic migraine in 3 patients, changes in regional cerebral blood flow (rCBF) were recorded by the intracarotid 133Xe method and a 254 multidetector camera covering one hemisphere. The rCBF measurements were performed in conjunction with cerebral angiography. During repeated rCBF measurements all 3 patients developed focal hypoperfusion originating in the frontal lobe, subsequently spreading posteriorly to involve the precentral and postcentral regions. In 2 cases focal hyperperfusion appeared to precede the hypoperfusion. In association with the rCBF changes the patients developed transient motor and/or sensory deficits and subsequently severe headache. No signs of arterial occlusion were found. In the over and underperfused regions blood flow fluctuated rapidly because of instability of cerebrovascular tone, defined as transient constriction of the smallest cerebral vessels (arterioles) alternating with a normal calibre for these vessels and/or short periods of vasodilatation. It is considered to be a primary pathological condition of the vessels. When vasoconstriction was present the blood flow decreased to values consistent with ischaemia, which was probably the cause of the neurological deficits. On the basis of these observations and previous work from our laboratory we conclude that instability of cerebrovascular tone may cause focal ischaemia during the course of attacks of classical and hemiplegic migraine.

Tissue/blood partition coefficients for xenon in various adipose tissue depots in man.

Tissue/blood partition coefficients (lambda) for xenon were calculated for subcutaneous adipose tissue from the abdominal wall and the thigh, and for the perirenal adipose tissue after chemical analysis of the tissues for lipid, water and protein content. The lambda in the perirenal tissue was found to correlate linearly to the relative body weight (RBW) in per cent with the regression equation lambda = 0.045 . RBW + 0.99. The subcutaneous lambda on the abdomen correlated linearly to the local skinfold thickness (SFT) with the equation lambda = 0.22 SFT + 2.99. Similarly lambda on the thigh correlated to SFT with the equation lambda = 0.20 . SFT + 4.63. It is concluded that the previously accepted lambda value of 10 is generally too high in perirenal as well as in subcutaneous tissue. Thus, by application of the present regression equations, it is possible to obtain more exact estimates of the adipose tissue blood flow measured with the 133Xe wash-out method.

Multicompartmental analysis of tracer clearance and its application to cerebral blood flow measurement.

A new algorithm for computing cerebral blood flow is shown to overcome compartmental slippage and unstability problems associated with the conventional bicompartmental analysis. The tracer clearance curve is decomposed into a nonnegative linear combination of predetermined flow components. A weighted average of flows above (below) a fixed threshold yields the gray (white) matter flow. The accuracy and the stability of the new algorithm are analyzed by Monte-Carlo simulations, determining the effect of factors such as random error in tracer concentration, gray-white flow difference, tissue content of gray matter, and end-fit time. While the new algorithm requires 50-100% more CPU time and memory space than the bicompartmental method, its accuracy and stability is superior, especially as the conditions of the measurement deteriorate. The results suggest that more than twofold error reduction in measuring the blood flow in pathological brain tissue is possible.

Decreased 133Xe clearance in the proximal femur in acromegaly.

Using the 133Xe tissue clearance method, the blood flow in the greater trochanter of the femur was studied in 30 patients with acromegaly. Both the washout rate constant (k2) and blood flow (P2) values are significantly decreased in acromegaly (p less than 0.01). There is a significant negative correlation (r = -0.42, p less than 0.05) between the flow values and mean daily concentrations of growth hormone in serum of acromegalic patients. The observations is presented as a preliminary evidence of a possible influence of growth hormone on the blood flow in bone.

Comparison of external lung monitoring with end-tidal air detection using the 133xenon inhalation method.

When the 133Xe inhalation method is employed for measuring regional cerebral blood flow, the arterial 133Xe concentration is usually approximated by the end-tidal air concentration. However, this approximation may be invalid in the presence of certain lung pathologies or when the breathing pattern is irregular. Jaggi and Obrist, using an intravenous injection of 133Xe, suggested that the counts detected by an external lung probe could provide an alternative estimate for arterial blood concentration once the noise produced by 133Xe in superficial tissues is removed from the signal. A mathematical model, based on hypotheses similar to theirs is presented here together with a new computational procedure for removing the noise. Results from normal rest studies on ten healthy young males indicate that the approximations for arterial blood concentration obtained from end-tidal air and from corrected lung counts are not equivalent when 133Xe is administered by inhalation. The concentration-time curves have different shapes, and these differences are reflected in blood flow values computed by head channel. However, there is no effect on comparisons between homologous regions of the left and right hemispheres.

Dynamic SPECT of the brain using a lipophilic technetium-99m complex, PnAO.

The lipophilic 99mTc-labeled oxime propylene amine oxime (PnAO) should, according to recent reports behave like 133Xe in the human brain. This study compares SPECT images of the two tracers in six subjects: four stroke cases, one transitory ischemic attack case and one normal subject. Technetium-99m PnAO was injected i.v. as a bolus of 15 to 25 mCi. The distribution was followed over 10-sec intervals using a highly sensitive, rapidly rotating SPECT (Tomomatic 64) and compared to 133Xe flow maps. Upon arrival of the PnAO bolus to the brain, a high uptake was found in brain tissue with high cerebral blood flow followed by rapid washout. In the stroke cases, low flow areas were equally well visualized by both tracers. Two dissimilarities were seen in the initial pictures: PnAO visualized the cerebral veins and showed a lesser contrast of gray:white matter uptake. The results suggest that PnAO has a high yet incomplete brain extraction yielding a flow dominated initial distribution with limitations mentioned.

The double isotope technique for in vivo determination of the tissue-to-blood partition coefficient for xenon in human subcutaneous adipose tissue--an evaluation.

Local subcutaneous 133xenon (133Xe) elimination was registered in the human forefoot in 34 patients. The tissue/blood partition coefficient for Xe was estimated individually by simultaneous registration of 133Xe and [131I]antipyrine ([131I]AP) washout from the same local depot. When measured in this way, an average partition coefficient for Xe was found to be 4.3 +/- 1.23 ml g-1. This value is significantly lower than the partition coefficient found in a previous in vitro study in which a Xe partition coefficient of 7.5 +/- 1.57 ml g-1 was found. Thus, if the local blood flow is calculated using the partition coefficient found by the double isotope technique, significantly lower values are obtained than if the in vitro determined coefficient is used. This difference is explained mainly by local dilution when injecting xenon subcutaneously. In short-term studies, utilization of the double isotope technique reduces the coefficient of variation on average flow determinations, thus an improvement in accuracy of local blood flow estimation can be obtained compared to the method in which an average partition coefficient is used. For long-term studies a partition coefficient of 7.5 ml g-1 seems valid.

Clearance of radioactive xenon in evaluating microcirculation in patients with ischaemic heart disease.

In 158 patients with chronic ischaemic heart disease (IHD), the state of microcirculation was investigated, using local 133Xe clearance. With growing severity of the disease, muscular blood flow decreased. Local disturbances were accompanied also by changes in central haemodynamics (decrease in cardiac index, slow-down of blood flow in the pulmonary and systemic circulations, increased peripheral resistance), which were established by radiocardiography using 131I-labelled albumin. A distinct relationship was found between muscular blood flow and the intensity of intravascular microcirculatory disturbances (microthrombosis, intravascular thrombocyte aggregation, arteriolo-venular anastomoses). The authors recommend the use of 133Xe clearance for microcirculation study in patients with chronic IHD.

The effect of lung volume on regional gas transport during high frequency oscillations.

Boluses of 133Xenon entrained in oscillating flow during high frequency oscillation (HFO) were used to study the effects of lung volume on the distribution of gas to topographical lung regions. The distribution of the tracer was measured with scintillation detectors positioned 5, 15 and 25 cm below each lung apex of subjects seated in a plethysmograph, while they maintained a patent glottis and constant breathhold. An index of HFO gas transport was obtained by measuring the 'wash-in' time for Xenon count rates to attain 0.25-0.75 of their final magnitude within each region. The distribution of gas was determined at 5 Hz with similar oscillatory volumes at FRC + 1 L, FRC, FRC - 1 L, and RV. Above FRC wash-in times were similar from apex to base. From FRC to RV, wash-in times at the bases became progressively longer. The results indicate that the regional distribution of HFO gas transport is dependent on lung volume.