Original Article | Published:

Association of adipose tissue blood flow with fat depot sizes and adipokines in women

International Journal of Obesity volume 36, pages 783789 (2012) | Download Citation

Abstract

Objective:

To explore possible associations between adipose tissue (AT) blood flow (ATBF), AT depot sizes and adipocyte-derived hormones (adipokines) in women.

Subjects:

In all, 43 healthy women were divided into four groups: normal-weight (n=11) and obese (n=11) pre-menopausal women and normal-weight (n=10) and obese (n=11) post-menopausal women.

Methods:

Fasting levels of adipokines were obtained, and a single-slice computed tomography scan at the level of L4–L5 was used to estimate fat depot sizes. ATBF was assessed by xenon washout while in a fasting state and after oral glucose load. We also measured glucose, insulin and non-esterified fatty acids.

Results:

Total, subcutaneous and visceral AT areas strongly correlated with ATBF (all P<0.001). Circulating leptin levels strongly and inversely correlated with ATBF (P=0.001), but this association did not remain after adjustment for body mass index. Adiponectin was not associated with blood flow.

Conclusion:

ATBF is closely linked to subcutaneous and visceral AT size. Further analyses are needed to determine possible mediators of this association, including mechanistic studies to assess a putative role for leptin as a significant modulator of blood flow.

Introduction

AT blood flow (ATBF) is an important regulator of AT metabolism, because this tissue is highly vascularised and variations in blood flow facilitate storage and removal of lipids when needed (for example, stress, exercise and fasting).1, 2 Furthermore, post-prandial ATBF may facilitate signalling between AT and other tissues, such as the liver and skeletal muscle.3 The dynamics of ATBF is related to the degree of obesity, and obese subjects respond less to a mixed meal than lean subjects.4, 5 Notably, an attenuated post-prandial ATBF may decrease glucose and triglyceride (TG) uptake, leading to post-prandial hyperglycaemia, hyperinsulinemia and hyperlipidemia. In line with this, McQuaid et al.6 recently demonstrated that obese subjects have a post-prandial reduction in chylomicron-TG-derived fatty acid storage. This provides a possible pathophysiological basis for ectopic fat deposition and lipotoxicity and support for ATBF being a key player in the metabolic disturbances seen in obesity and obesity-related diseases. A putative regulator of ATBF is AT distribution. Regional metabolic differences in lipolysis and fatty acid uptake between upper- and lower-body fat have been described, as well as differences between subcutaneous and visceral fat depots.7, 8 It has been suggested that subcutaneous AT may act as a buffer for dietary lipids, that is a ‘metabolic sink’, protecting other tissues from a lipid overflow with associated lipotoxicity.7 Visceral adipocytes are phenotypically different from subcutaneous adipocytes,9 and increased metabolic/cardiovascular risk is linked to visceral AT accumulation.

We recently found that ATBF is influenced by nitric oxide activity and autonomic nerve balance.10 Putative modulators of a link between obesity, ATBF, nitric oxide activity and autonomic activity include the adipokines leptin and adiponectin, which are secreted into the peripheral circulation from AT.2, 11, 12, 13 These adipokines predict risk for type 2 diabetes and cardiovascular disease,14, 15, 16 and in vitro studies have demonstrated the nitric oxide-mediated vasorelaxation effects of both leptin and adiponectin.2, 11, 12, 13

With menopause, women change their fat distribution to a more central (android) location.17 On the basis of the metabolic differences between subcutaneous and visceral fat, and our recent demonstration of dysregulation of ABTF in post-menopausal overweight women,10 we hypothesised that ATBF is closely related to the size of the abdominal fat depot. We also hypothesised that adipokines are potential independent regulators of ATBF.

Subjects and methods

Subjects

Details of the subjects included in this study were described previously.10 Briefly, we recruited 43 healthy women and divided them into four groups: normal-weight (n=11) and obese (n=11) pre-menopausal women, and normal-weight (n=10) and obese (n=11) post-menopausal women. Post-menopausal status was defined as the lack of menstrual periods for at least 12 continuous months. Basal characteristics are shown in Table 1. Exclusion criteria were pregnancy, thyroid disease, hypertension, known diabetes, cancer, previous stroke, known heart disease, psychiatric disorder, alcohol abuse, electrolyte disturbances and treatment with glucocorticoids, systemic estrogens or lipid-lowering agents. The pre-menopausal women were all studied in the follicular phase. The regional ethics committee for Northern Sweden approved the study, and written informed consent was obtained from all participants.

Table 1: Age, anthropometric data, adipose tissue distribution and selected laboratory parameters of the study population

Clinical protocol

Clinical measurements were made at rest between 08:00 and 13:00 hours after an overnight fast. Baseline anthropometry (weight, waist, hip and blood pressure) was measured and an oral glucose tolerance test was performed as previously described.10 A computed tomography scan was performed on a separate day, within a month from the other measurements.

ATBF

ATBF was measured by 133Xe washout.18 A dose of 1–2 MBq 133Xe was injected subcutaneously into the para-umbilical area. Calculation of fasting ATBF was based on two independent readings taken 15 and 10 min before the oral glucose tolerance test, and post-prandial blood flow was calculated at 15, 30, 45, 60, 90 and 120 min after glucose loading, as previously described.19, 20

Blood sampling and analyses

A cannula (Optiva 2 18–20; G Johnson & Johnson, New Brunswick, Canada) was inserted retrogradely into a distal forearm vein and kept patent by a continuous slow infusion of saline. The lower part of the forearm was heated in a chamber (Biomedical Engineering Department, Huddinge University Hospital, Stockholm, Sweden) to provide arterialised blood samples. A 10 ml aliquot of arterialised venous blood was collected twice before and then 15, 30, 45, 60, 90 and 120 min after glucose intake (Figure 1). At each time point, 1.5 ml of blood was placed in an ethylenediaminetetraacetic acid tube, immediately put on ice and centrifuged at 4 °C. The plasma was collected and stored at −80 °C. The remaining blood volume was centrifuged and the serum was stored at −80 °C for later analyses.

Figure 1
Figure 1

Study design.

Leptin and adiponectin levels were analysed with double-antibody radioimmunoassays (Linco Research, St Louis, MO, USA). The total coefficient of variation was 4.7% at both low (2–4 ng ml−1) and high (10–15 ng ml−1) levels for leptin, and for adiponectin it was 15.2% at low levels (2–4 μg ml−1) and 8.8% at high levels (26–54 μg ml−1). Non-esterified fatty acids were analysed using reagent non-esterified fatty acids C (Wako Chemicals GmbH, Neuss, Germany) on Hitachi 912 (Roche, Basel, Switzerland). Insulin was analysed by a direct chemiluminiscent technique using an insulin reagent kit with a Modular E 170 immunoanalyzer (Roche). High-sensitivity C-reactive protein and IL-6 solid chemiluminescent immunometric phase assays were performed using Immulite (Siemens, Munich, Germany). All other parameters were analysed by dry chemistry on a Vitros 950 (Ortho-Clinical Diagnostics, Raritan, NJ, USA).

AT distribution

All participants underwent a computed tomography examination with a General Electric LightSpeed 4 Channel computed tomography scanner (GE, Milwaukee, WI, USA). A previously described method was used.21, 22 Image data were acquired as 5 mm slices at the L4–L5 level (120 mAs, 50 cm field of view). Anterior and lateral scanograms were used to identify the correct slice positions. The image data were used for calculation of total, subcutaneous and visceral AT areas by using defined levels of Hounsfield units.

Statistical methods

Blood flow calculations were performed in an excel sheet for Windows 2007 (Microsoft, Redmond, WA, USA). The remaining statistical analyses were performed using SPSS Statistics 18 (SPSS, Chicago, IL, USA). P<0.05 was considered to be statistically significant. The Kruskal–Wallis non-parametric test was used to test for differences between groups, followed by the Mann–Whitney post-hoc test if significant differences were found. For bivariate correlation analyses, we used natural logarithm values to achieve approximate normality. Partial correlation analyses were used to assess the relation of ATBF to other parameters after adjustments were made as described in the text. Area under the curve (AUC) responses after glucose loads were estimated according to the trapezoid rule. Tertiles of AT areas were calculated using group-specific cut-off values for pre- and post-menopausal women separately. The homoeostatic model assessment index was calculated as (fasting glucose (mmol l−1) × fasting insulin (μU ml−1))/22.5.23

Results

Basal characteristics

Pre- and post-menopausal obese women had significantly higher body mass index (BMI), waist circumferences, waist-to-hip ratios and intra-abdominal AT areas compared with pre-menopausal normal-weight women (Table 1). Anthropometric data were reported previously for this cohort.10 Notably, both post-menopausal normal-weight and post-menopausal obese women had an approximately twofold larger intra-abdominal AT area compared with their pre-menopausal counterparts, despite similar BMI. The post-menopausal obese group had higher levels of glucose (AUC) and free fatty acids (AUC) compared with the pre-menopausal groups. Fasting levels of glucose, insulin and free fatty acids were published previously.10 Insulin resistance, expressed as a homoeostatic model assessment index, was highest in the post-menopausal obese group. Five individuals (two post-menopausal normal-weight and three post-menopausal obese) had 2 h glucose levels 11.1, thus fulfilling the criteria of diabetes mellitus, despite normal fasting glucose levels.

The obese groups had higher leptin levels, and levels of adiponectin were lowest in obese pre-menopausal women. The ATBF (AUC) response to glucose was significantly lower in both obese groups compared with normal-weight pre-menopausal women. Post-menopausal obese women had significantly lower ATBF (AUC) than their normal weight counterparts.

ATBF and association with measures of obesity, biomarkers and adipokines

ATBF (AUC) was significantly associated with all measures of obesity (BMI, waist, waist-to-hip ratio, total AT area, subcutaneous AT area and intra-abdominal AT area). These associations remained after adjustment for menopausal status and age (Table 2). ATBF was not associated with glucose and insulin levels. Associations were found between ATBF and free fatty acids, and between ATBF and homoeostatic model assessment index; however, only the homoeostatic model assessment index association remained significant after adjustment for menopausal status and age. Leptin was strongly inversely associated with ATBF; the association remained after adjustment for menopausal status and age, but not after adjustment for BMI. By contrast, no association was seen between blood flow and adiponectin. Because baseline levels for ATBF differed among the groups, we also recalculated all data using centred cumulative response, but this did not significantly change the results. The highly significant negative association between leptin and ATBF, and the non-significant association between adiponectin and ATBF are visualised with scatter plots in Figures 2a and b.

Table 2: Associations between adipose tissue blood flow and measures of adipose tissue, biomarkers, and adipokines
Figure 2
Figure 2

Associations between circulating levels of leptin (a), adiponectin (b) and ATBF (area under curve).

AT blood during oral glucose tolerance test

ATBF was significantly lower at baseline in both obese groups compared with normal-weight pre- and post-menopausal women (P<0.05) (Figure 3). After glucose loading, at all time points, post-menopausal obese women had lower ATBF than pre-menopausal normal-weight women (P<0.05). The same pattern was seen between the two post-menopausal groups, except that the difference was non-significant at 60 and 90 min. Obese pre-menopausal women had significantly lower ATBF than their normal weight counterparts throughout the test (P<0.05). There were no significant differences in blood flow between the two obese groups. The post-menopausal normal-weight women had lower ATBF than pre-menopausal normal-weight women at 60 and 120 min (P<0.05).

Figure 3
Figure 3

ATBF in the four groups of women during oral glucose tolerance test. Data presented as mean±standard error of the mean (s.e.m.). Pre-NW, pre-menopausal normal weight; Post-NW, post-menopausal normal weight; Pre-obese, pre-menopausal obese; Post-obese, post-menopausal obese.

ATBF and association with total, subcutaneous and visceral AT

We studied ATBF according to the amount of AT in each depot (expressed as tertiles with group-specific cut-offs) (Figures 4a–c). In each depot, we found highly significant differences in ATBF between the lowest and medium tertiles (P<0.01), and between the lowest and highest tertiles (P<0.01). We also found significant differences in ATBF between the medium and highest tertiles (P<0.05) for total and intra-abdominal AT. In the subcutaneous depot, there was a trend (non-significant) towards a blood flow difference between the medium and highest tertile of the AT area.

Figure 4
Figure 4

Box plots of ATBF according to tertiles of total (a), subcutaneous (b) and intra-abdominal (c) AT areas with separate group-specific cut-off values for pre- and post-menopausal women. The non-parametric Kruskal–Wallis test was used to determine whether means differed, followed by the Mann–Whitney post-hoc test if differences were significant. *P<0.05; **P<0.01; n.s., non-significant.

Discussion

We found a strong association between ATBF and subcutaneous, as well as visceral, AT areas. This suggests that total AT mass, rather than specific sites of fat accumulation, is linked to dysregulation of ATBF. It was previously shown that both fasting ATBF and ATBF responsiveness to nutrients5 are reduced in obesity, but the relationship of ATBF with different depots of abdominal fat had, to our knowledge, not been explored. The importance of our findings is underscored by recent data from obese study participants showing a reduced ability to store fat in AT after meals.6 The absence of a difference between AT accumulation and ATBF in different fat depots suggests that subcutaneous AT accumulation has a relatively greater role than visceral AT in ectopic fat deposition and lipotoxicity, due to its larger volume. Our data therefore support the hypothesis of subcutaneous fat acting as the primary metabolic ‘sink,’ protecting other organs from excessive post-prandial lipid levels.

Visceral fat depot size has been strongly linked to metabolic and cardiovascular diseases;24 differences in insulin action, gene expression, metabolic responses and adipokine secretion25, 26 among fat depots could explain this difference. It has also been proposed that extrinsic factors, including depot-specific blood flow and/or innervation,27 could contribute to distinct gene expression patterns and metabolic profiles in adipocytes in different anatomical regions. It is possible that lipid and glucose uptake in AT depots is regulated differently in people with obesity. Studies using positron emission tomography have elegantly demonstrated differences in perfusion and insulin-stimulated glucose uptake between visceral and abdominal subcutaneous AT in obese people.28, 29 Women are more protected from cardiovascular events than men, until their body fat distribution changes with menopause and takes on a more android (male) distribution.30, 31 If the menopausal transition also has differential effects on lipid and glucose uptake in different fat depots, this would clearly be an area of interest for further studies.

Notably, total perfusion through both abdominal subcutaneous and visceral depots can be up to 900 ml min−1 (18% of average cardiac output) in obese people.28 Thus, ATBF not only has an important impact on cardiac work load, but is also a powerful regulator of AT metabolism. Although the ATBF response to nutrient intake is thought to be of importance in the regulation of metabolism by facilitating signalling between AT and other tissues, such as skeletal muscle and liver,3 the physiological significance of and mechanisms behind post-prandial hyperaemia are not fully understood. Importantly, the rapid post-prandial increase in ATBF and the rapid insulin-mediated suppression of adipocyte lipolysis change the direction of free fatty acids flux in tissues.32 Related to this, we found an inverse correlation between insulin resistance and ATBF.

We also found a strong inverse association between circulating levels of leptin and ATBF. Hyperleptinemia has been linked to an increased risk of later development of type 2 diabetes and cardiovascular disease, including myocardial infarction and stroke.33, 34, 35 Several mechanisms have been suggested to be behind these associations, including mediation of platelet aggregation, increased sympathetic activity and stimulation of inflammation.36, 37 The regulatory effects of leptin on vascular endothelial cells in rodents are also well established,2, 11, 38 but less is known about putative effects on human endothelial function and blood flow regulation in vivo. The non-significant results after adjustments for BMI in this study should be interpreted with caution, because leptin and measures of obesity are strongly correlated and it is debatable whether models exploring the effect of leptin should be adjusted for obesity. On the other hand we cannot rule out that the association between fat mass and ATBF could be determined by other factors than leptin. Furthermore, the links between leptin and later risk of diabetes, as well as cardiovascular outcomes, have been found mainly in males.33, 34, 35 Studies are thus needed on possible differences between males and females regarding the vascular effects of leptin. This includes detailed mechanistic studies.

We found no association between adiponectin and ATBF. This suggests that adiponectin does not have a major role in ATBF regulation. In contrast, hypoadiponectinemia is linked to endothelial dysfunction in peripheral arteries,39 and plasma total adiponectin concentrations are inversely related to the risk of myocardial infarction.

Our study has some limitations. We found a very high correlation between the size of subcutaneous and intra-abdominal depots (R=0.77, P<0.001), in line with previous studies,40, 41 making it difficult to separate the effects of the different depots. Another weakness is the cross-sectional design, which makes it impossible to explore causality.

In summary, we found highly significant correlations between ATBF and total, visceral and subcutaneous AT areas. We also show a strong inverse association between circulating levels of leptin and ATBF, although this was non-significant after adjustment for BMI. Further mechanistic studies are needed to assess the clinical significance of our findings.

References

  1. 1.

    , , , , , et al. Perivascular adipose tissue and mesenteric vascular function in spontaneously hypertensive rats. Arterioscler Thromb Vasc Biol 2006; 26: 1297–1302.

  2. 2.

    , , , , , et al. Leptin induces direct vasodilation through distinct endothelial mechanisms. Diabetes 2000; 49: 293–297.

  3. 3.

    , , , , . Integrative physiology of human adipose tissue. Int J Obes Relat Metab Disord 2003; 27: 875–888.

  4. 4.

    , , , , , et al. Postprandial substrate deposition in human forearm and adipose tissues in vivo. Clin Sci (Lond) 1990; 79: 339–348.

  5. 5.

    , , , , . Subcutaneous abdominal adipose tissue blood flow: variation within and between subjects and relationship to obesity. Clin Sci (Lond) 1996; 91: 679–683.

  6. 6.

    , , , , , et al. Downregulation of adipose tissue fatty acid trafficking in obesity: a driver for ectopic fat deposition? Diabetes 2011; 60: 47–55.

  7. 7.

    . Adipose tissue as a buffer for daily lipid flux. Diabetologia 2002; 45: 1201–1210.

  8. 8.

    , , , , , et al. Regional variation in adipose tissue lipoprotein lipase activity: association with plasma high density lipoprotein levels. Eur J Clin Invest 1991; 21: 398–405.

  9. 9.

    , , , , , et al. Fat depot-related differences in gene expression, adiponectin secretion, and insulin action and signalling in human adipocytes differentiated in vitro from precursor stromal cells. Diabetologia 2008; 51: 155–164.

  10. 10.

    , , , , , et al. Dysregulation of subcutaneous adipose tissue blood flow in overweight postmenopausal women. Menopause 2010; 17: 365–371.

  11. 11.

    , , , , , et al. Involvement of nitric oxide in endothelium-dependent arterial relaxation by leptin. Biochem Biophys Res Commun 2000; 273: 745–749.

  12. 12.

    , , , , , et al. Adiponectin improves endothelial function in hyperlipidemic rats by reducing oxidative/nitrative stress and differential regulation of eNOS/iNOS activity. Am J Physiol Endocrinol Metab 2007; 293: E1703–E1708.

  13. 13.

    , , , , , et al. Leptin effect on endothelial nitric oxide is mediated through Akt-endothelial nitric oxide synthase phosphorylation pathway. Diabetes 2002; 51: 168–173.

  14. 14.

    , , , , , et al. Total and high-molecular-weight adiponectin and resistin in relation to the risk for type 2 diabetes in women. Ann Intern Med 2008; 149: 307–316.

  15. 15.

    , , , , , et al. Leptin is associated with increased risk of myocardial infarction. J Intern Med 1999; 246: 409–418.

  16. 16.

    , , , , , et al. Leptin is a risk marker for first-ever hemorrhagic stroke in a population-based cohort. Stroke 1999; 30: 328–337.

  17. 17.

    , , , , , . Visceral adipose tissue is an independent correlate of glucose disposal in older obese postmenopausal women. J Clin Endocrinol Metab 2000; 85: 2378–2384.

  18. 18.

    , , . Blood flow through human adipose tissue determined with radioactive xenon. Acta Physiol Scand 1966; 66: 337–345.

  19. 19.

    , , , , , . Effects of insulin on adipose tissue blood flow in man. J Physiol 2002; 540 (Part 3): 1087–1093.

  20. 20.

    , , , , , . Impaired postprandial adipose tissue blood flow response is related to aspects of insulin sensitivity. Diabetes 2002; 51: 2467–2473.

  21. 21.

    , , , , , . A multicompartment body composition technique based on computerized tomography. Int J Obes Relat Metab Disord 1994; 18: 219–234.

  22. 22.

    , , , , , . A method to obtain the same levels of CT image noise for patients of various sizes, to minimize radiation dose. Br J Radiol 2002; 75: 140–150.

  23. 23.

    , , , , , . Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia 1985; 28: 412–419.

  24. 24.

    . Health consequences of visceral obesity. Ann Med 2001; 33: 534–541.

  25. 25.

    , , . Regional differences of insulin action in adipose tissue: insights from in vivo and in vitro studies. Acta Physiol Scand 2005; 183: 13–30.

  26. 26.

    . Subcutaneous and visceral adipose tissue: their relation to the metabolic syndrome. Endocr Rev 2000; 21: 697–738.

  27. 27.

    , . Search for the preadipocyte progenitor cell. J Clin Invest 2006; 116: 3103–3106.

  28. 28.

    , , , , , et al. Effects of weight loss on visceral and abdominal subcutaneous adipose tissue blood-flow and insulin-mediated glucose uptake in healthy obese subjects. Ann Med 2009; 41: 152–160.

  29. 29.

    , , , , , et al. Glucose uptake and perfusion in subcutaneous and visceral adipose tissue during insulin stimulation in nonobese and obese humans. J Clin Endocrinol Metab 2002; 87: 3902–3910.

  30. 30.

    , , . Sex- and menopause-associated changes in body-fat distribution. Am J Clin Nutr 1992; 55: 950–954.

  31. 31.

    , , , , , et al. Regional body fat distribution and metabolic profile in postmenopausal women. Metabolism 2008; 57: 1101–1107.

  32. 32.

    , , , , , et al. Preferential uptake of dietary fatty acids in adipose tissue and muscle in the postprandial period. Diabetes 2007; 56: 168–176.

  33. 33.

    , , , , , et al. Leptin and coronary heart disease: prospective study and systematic review. J Am Coll Cardiol 2009; 53: 167–175.

  34. 34.

    , , , , , et al. Leptin, but not adiponectin, predicts stroke in males. J Intern Med 2004; 256: 128–136.

  35. 35.

    , , , , , et al. Leptin predicts the development of diabetes in Mauritian men, but not women: a population-based study. Int J Obes (Lond) 2007; 31: 1126–1133.

  36. 36.

    . Leptin and atherosclerosis. Atherosclerosis 2006; 189: 47–60.

  37. 37.

    . Role of leptin in blood pressure regulation and arterial hypertension. J Hypertens 2006; 24: 789–801.

  38. 38.

    . Regulation of vascular tone: the fat connection. Hypertension 2004; 44: 255–256.

  39. 39.

    , , , , , et al. Association of hypoadiponectinemia with impaired vasoreactivity. Hypertension 2003; 42: 231–234.

  40. 40.

    , , , , . Relationships of generalized and regional adiposity to insulin sensitivity in men. J Clin Invest 1995; 96: 88–98.

  41. 41.

    , , , , , et al. Magnetic resonance imaging of total body fat. J Appl Physiol 1998; 85: 1778–1785.

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Acknowledgements

This study was supported by the Swedish Research Council, the Swedish Heart and Lung Foundation, the Heart Foundation of Northern Sweden, Northern Sweden County Council, Västerbotten County Council, and the Faculty of Medicine, Umeå University. We gratefully acknowledge nurses Inger Arnesjö and Veronica Sjöberg for their technical assistance in this study.

Author information

Affiliations

  1. Department of Public Health and Clinical Medicine, Medicine, Umeå University, Umeå, Sweden

    • J Andersson
    • , L-G Sjöström
    • , S Söderberg
    •  & T Olsson
  2. Heart Center, Umeå University Hospital, Umeå, Sweden

    • J Andersson
    •  & S Söderberg
  3. NIHR Oxford Biomedical Research Centre, Churchill Hospital, Oxford, UK

    • F Karpe
  4. Department of Radiation Sciences, Diagnostic Radiology, Umeå University, Umeå, Sweden

    • K Riklund

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Competing interests

The authors declare no conflict of interest.

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Correspondence to J Andersson.

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DOI

https://doi.org/10.1038/ijo.2011.152

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