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J Appl Physiol 88: 881-887, 2000;
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Vol. 88, Issue 3, 881-887, March 2000

Cellular adaptations in fat tissue of exercise-trained miniature swine: role of excess energy intake

Gale B. Carey

Department of Animal and Nutritional Sciences, University of New Hampshire, Durham, New Hampshire 03824


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

This study examined the influence of energy expenditure and energy intake on cellular mechanisms regulating adipose tissue metabolism.1 Twenty-four swine were assigned to restricted-fed sedentary, restricted-fed exercise-trained, full-fed sedentary, or full-fed exercise-trained groups. After 3 mo of treatment, adipocytes were isolated and adipocyte size, adenosine A1 receptor characteristics, and lipolytic sensitivity were measured. Swine were infused with epinephrine during which adipose tissue extracellular adenosine, plasma fatty acids, and plasma glycerol were measured. Results revealed that adipocytes isolated from restricted-fed exercised swine had a smaller diameter, a lower number of A1 receptors, and a greater sensitivity to lipolytic stimulation, compared with adipocytes from full-fed exercised swine. Extracellular adenosine levels were transiently increased on infusion of epinephrine in adipose tissue of restricted-fed exercised but not full-fed exercised swine. These results suggest a role for adenosine in explaining the discrepancy between in vitro and in vivo lipolysis findings and underscore the notion that excess energy intake dampens the lipolytic sensitivity of adipocytes to beta -agonists and adenosine, even if accompanied by exercise training.

lipolysis; adenosine


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

EXERCISE TRAINING IS KNOWN to reduce body fat, reduce adipocyte size, and increase adipocyte lipolytic sensitivity to hormones in humans, rats, and miniature swine (7, 28, 32). Suggested possible mechanisms include an increase in adenylyl cyclase activity (17), an increase in receptor-G protein coupling (18, 46), an increase in hormone-sensitive lipase activity (4), and an increase in calcium signaling (16). More recently, our laboratory has suggested yet another mechanism: a reduced sensitivity to adenosine, an antilipolytic agent, via a decrease in adenosine A1 receptor number (5, 8).

The degree to which exercise training reduces body fatness in humans varies, depending on gender, exercise intensity, training duration, and energy intake. In men, there is a significant inverse correlation between physical activity energy expenditure and percentageof body fat (44). This correlation may not exist in women, who dietarily compensate for expended calories in some (42, 45) but not all studies (41). Female miniature swine are similar to female humans: in a previous study, we demonstrated that body fatness remained high in exercise-trained female swine allowed to eat a high-fat diet to fullness (28). Moreover, the increase in energy intake had the biochemical consequence of obliterating the exercise-induced reduction in adipocyte adenosine sensitivity (28). The role of excess energy intake, as opposed to excess fat intake, in the disappearance of the adenosine effect is unknown.

The goal of this study was to examine how moderate vs. excess energy intake of a low-fat diet by female Yucatan miniature swine influences adipocyte lipolytic sensitivity to beta -adrenergic agonists and adenosine in vitro and in vivo. Our hypothesis was that excess energy intake of a low-fat diet would dampen the lipolytic sensitivity of adipocytes, even if adipocytes were isolated from exercise-trained animals. In vitro, we examined adipocyte lipolytic sensitivity to isoproterenol, epinephrine, and adenosine and adenosine A1 receptor characteristics. In vivo, we examined extracellular adenosine levels in adipose tissue and whole body lipolysis. Our results revealed that in vivo and in vitro exercise-induced changes in adipose tissue lipolytic sensitivity were negated by excess energy intake. The results also suggested that the presence of extracellular adenosine may account, in part, for the discrepancy often reported in the literature between in vitro and in vivo lipolytic responsiveness of adipose tissue.


    MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Pigs and diet. Six sets of female Yucatan miniature swine, four littermates per set at age 10-12 wk, were used for this 3-mo study. Each littermate was assigned to one of four treatments: restricted-fed sedentary, restricted-fed exercised, full-fed sedentary, or full-fed exercised. Adipose tissue from male and female swine respond similarly to adenosine in vitro (5); female swine were used in this study because of their better compliance to the exercise regimen and their greater amount of subcutaneous adipose tissue compared with males.

Swine were housed four to six swine per 18-m2 pen at the Burley-Demeritt swine facility (Lee, NH). All swine were fed a miniature swine ration (Agway, Syracuse, NY) with a caloric density of 10.21 kJ/g, formulated to meet the nutritional needs of growing miniature swine. Full-fed swine were fed 73 g · kg body wt-1 · day-1 in two meals; this was equivalent to the daily caloric intake of swine in our previous study (28). Restricted-fed swine were fed 53 g · kg body wt-1 · day-1 in two meals; this was ~25% less than the full-fed swine and equivalent to the usual daily caloric intake of our swine. Restricted-fed swine were fed individually, but logistical constraints necessitated that full-fed swine be fed as pairs (sedentary and exerciser). Food was preweighed, and any orts present were collected and weighed to determine actual food intake. Swine had free access to water except during the exercise sessions. Swine body weights were recorded at the beginning of each week. All procedures were approved by the University of New Hampshire Animal Care and Use Committee (approval no. 930305).

Exercise. An endurance exercise regimen was employed as described by Carey and Sidmore (5). This regimen gradually adapted the swine to run on motor-driven treadmills with rubberized belts so that, by the end of 2 mo, they ran for 45 min/day, 5 days/wk, at 9 km/h under climate-controlled conditions. They continued to train for an additional month at this intensity.

Jugular catheter implantation. After 3 mo of exercise training or rest, swine were anesthetized with isoflurane and catheters were implanted into the left jugular vein. The surgical procedure of Moritz et al. (29) was followed, and the tubing was kept patent with a 60% polyvinylpyrrolidone solution in saline with 500 U heparin/ml.

Fat biopsy and adipocyte isolation. Concurrent with the jugular vein catheterization, an ~8-g portion of adipose tissue from over the shoulder was removed surgically and transported to the laboratory in warm saline. Tissue was minced into 1-mm3 pieces, rinsed with warm saline, and dissociated with collagenase; adipocytes were then isolated as described previously (5).

Membrane preparation and adenosine A1 receptor binding assay. Approximately 3 ml of packed cells were used to prepare an adipocyte crude plasma membrane fraction, as described by Dong and Carey (8). The fraction was resuspended to 2 mg protein/ml and stored at -80°C until being assayed for A1 receptor binding kinetics (8).

Adipocyte incubations. The remaining adipocytes were resuspended to a 5% solution (vol/vol) and used for in vitro lipolysis measurements. Duplicate aliquots of cells (600 µl) were pipetted into 7-ml polypropylene incubation vials and placed in a 38°C (swine body temperature) shaking water bath at 50 oscillations/min. Lipolysis was initiated with the addition of 1 unit of adenosine deaminase and one of the following agents: 10-5, 10-6, 10-7, 10-8, 10-9, or 10-10 M epinephrine, 10-6 M isoproterenol, or 10-6 M epinephrine plus one of eight levels of phenylisopropyladenosine (PIA, final concentration varied from 10-8 to 5 × 10-10 M). Final incubation volume was 750 µl. Vials were gassed with 95% O2-5% CO2 and capped and shaken at 90-100 oscillations/min at 38°C.

After 90 min, lipolysis was stopped by decanting vial contents into microcentrifuge tubes containing 56 µl of 7 M HClO4. After remaining on ice for 60 min, the precipitated proteins were removed by centrifugation and the supernate was neutralized with 10 N KOH. Samples were adjusted to pH of ~7 and frozen at -80°C until being assayed for glycerol content using an enzyme-coupled spectrophotometric assay (43). Lipolysis was calculated as nanomoles glycerol released per centimeter squared adipocyte surface area per 90 min.

Adipocyte size and number. Two 600-µl aliquots of isolated cells were fixed in osmium tetroxide (10 g/l) for 48 h, filtered, and resuspended for adipocyte sizing and counting (28).

Muscle biopsy and citrate synthase measurement. The left brachialis muscle was biopsied at the time of the jugular vein catheterization. Approximately 200 mg of tissue were removed, frozen in liquid nitrogen, and stored at -80°C until being assayed for citrate synthase using the procedure of Srere (38).

Drug challenge and microdialysis. Three days after insertion of the jugular vein catheter, swine were anesthetized with isoflurane for an in vivo challenge experiment using one hormone and two drugs. Epinephrine was used to stimulate lipolysis (15, 36), dipyridamole was used to block adenosine transport (6, 19, 37), and theophylline was used to antagonize the adenosine A1 receptor (10). The ear vein was readied for infusion of drugs and/or hormones by catheterization. The catheter was kept patent by infusing saline at a rate of 1 ml/min (polystaltic pump, Buchler Instruments, Fort Lee, NJ). A scalpel blade was used to knick the skin so that an 18-gauge needle could be inserted into adipose tissue over the shoulder to a depth of 2.5 cm. After the needle was removed, a 4-mm microdialysis probe (CMA-4, Bioanalytical Systems, West Lafayette, IN) was inserted quickly into the same location. The probe was connected to a CMA/100 microperfusion pump and a CMA/140 microfraction collector (Bioanalytical Systems). The probe was perfused with Ringer solution at a rate of 5 µl/min.

Effluent from the probe was collected at 10-min intervals for the final 30 min of the 60-min saline infusion. At time 0, epinephrine was infused at 1 µg · kg body wt-1 · min-1. At 10-min intervals during the 30-min epinephrine infusion, blood samples were collected from the jugular vein catheter into heparinized tubes and placed on ice. Microdialysis samples also were collected at 10-min intervals and placed on ice. At 30 min, epinephrine plus dipyridamole (8 µg · kg body wt-1 · min-1) was infused into the ear vein. Blood and microdialysis samples were collected at 10-min intervals. At 60 min, a solution of epinephrine plus dipyridamole plus theophylline (150 µg · kg body wt-1 · min-1) was infused into the ear vein; again, blood and microdialysis samples were collected at 10-min intervals for 30 min.

Microdialysis samples were stored at -80°C until they were assayed for adenosine content using an RIA (25). Blood samples were centrifuged at 3,000 g for 10 min, and plasma was stored at -80°C until it was assayed for glycerol and free fatty acids (Sigma Chemical, St. Louis, MO).

Statistical analyses. Data were analyzed using a two-way ANOVA, blocked for litter for the treatment effects of exercise and energy intake. When a significant F test was obtained, pairwise comparison was performed via Fisher's least significant difference test. Student's t-test was used to test for temporal changes in plasma free fatty acids, plasma glycerol, and extracellular adenosine. Significance was set at P < 0.05 unless otherwise indicated. Systat software (version 7.0.1) was used for all statistical analyses.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Food intake, body weight, and muscle enzyme activity. Restricted-fed swine consumed an average of 52 g · kg body wt-1 · day-1, whereas full-fed swine consumed an average of 71 g · kg body wt-1 · day-1 (Table 1). These diet intake values approximated the targeted energy consumption for the restricted-fed and full-fed swine at 53 and 73 g · kg body wt-1 · day-1, respectively. Exercised restricted-fed swine consumed slightly less food than sedentary restricted-fed swine; exercised full-fed swine and sedentary full-fed swine were fed as pairs; therefore, it was not possible to determine the effect of exercise on food consumption in full-fed swine. Average body weights of swine in each group were similar at the start of the study, but, after 4 wk, there was a significant effect of diet on swine body weight (Table 1). Average weight gain for swine in the restricted-fed treatments was 1.46 kg/wk, and there was no difference in the rate of gain for restricted-fed exercised vs. restricted-fed sedentary swine. Full-fed swine gained an average of 2.0 kg/wk (Table 1).

                              
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  		Table 1.   Food intake, body weight, and muscle citrate synthase activity

Muscle citrate synthase activity was 47% greater in restricted-fed exercised vs. restricted-fed sedentary swine (11.18 vs. 7.64 µmol · min-1 · g muscle-1, respectively, P < 0.05) and 99% greater in full-fed exercised vs. full-fed sedentary swine (13.49 vs. 6.79 µmol · min-1 · g muscle-1, respectively, P < 0.05) (Table 1). Exercise caused an overall 72% increase in muscle citrate synthase activity; there was no significant effect of diet on muscle aerobic capacity.

Adipocyte size, lipolytic sensitivity, and A1 receptor characteristics. Exercise and diet treatment significantly influenced adipocyte size (Table 2). Restricted-fed exercised swine had the smallest diameter cells at 89 µm, and full-fed sedentary had the largest diameter cells at 114 µm. Exercise-trained swine within a dietary group had significantly smaller cells than their sedentary littermates, and full-feeding increased adipocyte size, within both the exercise-trained and sedentary groups.

                              
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  		Table 2.   Adipocyte size, lipolytic sensitivity, and adenosine A1 receptor characteristics

There was a significant effect of exercise on adipocyte lipolysis in response to stimulation by isoproterenol (Table 2). However, when adipocytes were lipolytically stimulated with epinephrine, both exercise and diet had significant treatment effects. In contrast to what was seen with isoproterenol, epinephrine-stimulated lipolysis was suppressed by ~50% with full feeding in both the sedentary and exercise-trained groups, compared with their sedentary controls.

The dampening effect of overfeeding on adipocyte lipolytic sensitivity to epinephrine (Table 2) disappeared when the epinephrine concentration was raised from 10-6 to 10-5 M (Fig. 1). Lipolytic sensitivity at 10-5 M epinephrine was similar to that at 10-6 M isoproterenol. At concentrations of 10-7 M epinephrine or lower, there was no significant difference in lipolytic response among the groups.


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  		Fig. 1.   Lipolytic sensitivity of isolated adipocytes. Results are expressed as nmol glycerol released per cm2 adipocyte surface area per 90 min. Statistical analysis by ANOVA revealed the following: at 10-5 M epinephrine, restricted-fed exercised = full-fed exercised > full-fed sedentary, and full-fed sedentary = restricted-fed sedentary and restricted-fed sedentary = restricted-fed exercised = full-fed exercised. At 10-6 M epinephrine, restricted-fed exercised > restricted-fed sedentary = full-fed exercised > full-fed sedentary. Pooled SE values for each epinephrine concentration are 10.25 (10-5 M), 4.96 (10-6 M), 7.94 (10-7 M), 1.15 (10-8 M), 1.36 (10-9 M), and 2.04 (10-10M). R-S, restricted-fed sedentary; R-E, restricted-fed exercised; F-S, full-fed sedentary; F-E, full-fed exercised.

Although there was no significant treatment effect of exercise or diet on adipocyte sensitivity to adenosine (P = 0.205 and 0.231, respectively), there were trends. The concentration of PIA needed for 50% inhibition of lipolysis was greatest for the restricted-fed exercised group, the group that was most lipolytically sensitive to isoproterenol and epinephrine, whereas it was lowest for the full-fed sedentary group, the group that was the least lipolytically sensitive (Table 2). These trends were inversely related to the number of adenosine A1 receptors: the restricted-fed exercised group had the least number of receptors, whereas the full-fed sedentary group had the most. There was no difference in the binding affinity of PIA for the adenosine receptor among the four groups.

Adipose tissue extracellular adenosine levels. For the first 10 min after microdialysis probes were inserted in adipose tissue, extracellular adenosine levels averaged 685 nM (Fig. 2). This was apparently due to tissue trauma because, by 20 min, the levels dropped 78% to an average of 154 nM and, by 40 min, the levels had stabilized at 87 nM. There was no difference in extracellular adenosine levels among the four groups at the end of the 60-min baseline period.


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  		Fig. 2.   Adipose tissue extracellular adenosine level in response to drug infusion. Statistical analysis of the area under the curve (AUC) during epinephrine infusion, corrected for baseline, shows a significant effect of diet, with restricted-fed group exercised significantly greater than full-fed exercised group. AUC analysis during epinephrine plus dipyridamole infusion shows that a main effect of diet approached significance (P = 0.073). There were no significant treatment effects during epinephrine plus dipyridamole plus theophylline infusion. Values are means ± SE (n = 6).

The adenosine area under the curve (AUC) during each infusion period was calculated for each swine, after being corrected for baseline. The adenosine AUC for the epinephrine infusion was significantly influenced by diet, with the restricted-fed exercised group significantly greater than the full-fed exercised group. The AUC for the epinephrine plus dipyridamole infusion was also influenced by diet, with the restricted-fed exercised group gradually rising during the infusion period, although this did not reach statistical significance (P = 0.073). There was no significant effect of exercise or diet on AUC when theophylline was added to the infusate.

Local inflammatory response due to probe. In a separate study of identical design, adipose tissue histology around the microdialysis probe insertion site was examined. At the conclusion of the in vivo challenge, the probe was removed and a 1-cm adipose tissue core surrounding the location of the probe was removed. A control tissue core was removed from a site ~1 cm from the probe insertion site. Samples were placed in formalin and examined histologically using light microscopy. There was evidence of mild perivascular inflammation, with scattered neutrophils and macrophages, in 4 of 10 probe samples and 1 of 10 control samples.

Plasma free fatty acids and glycerol. Analysis of the plasma glycerol levels over the baseline period suggested an effect of diet (P < 0.055): overfed swine averaged 1.60 mg/100 ml glycerol, whereas restricted-fed swine averaged 1.20 mg/100 ml glycerol (Fig. 3). On epinephrine infusion, however, sedentary swine appeared more lipolytically sensitive than exercise-trained swine. Plasma glycerol levels were greater at 10, 20, and 30 min of epinephrine infusion in the restricted-fed sedentary and full-fed sedentary groups compared with their baseline values at 20 and 30 min in the full-fed exercised group compared with baseline and at 20 min in the restricted-fed exercised group compared with baseline. There were no differences among the groups in AUC for plasma glycerol during any of the infusion periods.


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  		Fig. 3.   Plasma glycerol levels in response to drug infusion. Values are means ± SE (n = 6).

Unlike the plasma glycerol data, there was no significant effect of diet or exercise on plasma free fatty acid levels during the baseline period (Fig. 4). However, like the plasma glycerol response during the drug challenge, sedentary swine also showed a greater plasma free fatty acid response than the exercise-trained swine. Free fatty acid levels were greater at 20 and 30 min of epinephrine infusion in the restricted-fed sedentary and full-fed sedentary groups compared with baseline values and at 30 min only in the restricted-fed exercised group compared with baseline. There were no differences between the groups in the AUC for free fatty acids during any of the infusion periods.


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  		Fig. 4.   Plasma free fatty acid levels in response to drug infusion.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

The primary finding of this study confirms our hypothesis that excess energy intake, even if accompanied by exercise training, dampens the lipolytic sensitivity of adipocytes to beta -agonists and adenosine at the cellular level. These cellular adaptations may be responsible for, or a consequence of, larger adipocytes and higher body weight of full-fed exercise-trained swine, compared with restricted-fed exercise-trained controls. In either case, the question arises: Must exercise be accompanied by energy restriction to be an effective mechanism for weight loss or weight maintenance?

It is commonly believed that an increase in energy expenditure will be compensated for by an increase in food intake. This implies that increasing physical activity is a poor strategy for losing weight. However, some researchers believe that energy expenditure and energy intake are weakly coupled (3, 20). Thus exercise may be a useful mechanism to achieve weight loss or weight maintenance, if dietary overindulgence is not practiced. This study illustrated that dietary overindulgence (which comes naturally to swine) attenuates the effects of exercise. For humans, such overindulgences include inappropriate food choices, a desire for self-reward after exercise, and misjudgments about the relative rates at which energy is expended or consumed (3).

Body weight is not necessarily an indicator of body fatness (11). In this study, restricted-fed exercised swine consuming the same amount of energy as their sedentary littermates had similar body weights. However, the smaller adipocyte of the restricted-fed exercised swine suggests that there was a difference in body composition between exercised and sedentary swine. Unfortunately, body composition was not measured in this study. In a study by Kraemer et al. (22), a disparity between body weight and body composition was documented. Overweight men subjected to caloric restriction for 12 wk lost the same amount of body weight as overweight men subjected to caloric restriction and exercise. However, the composition of that loss was significantly different: body fat decreased 3.6% with caloric restriction alone, whereas body fat decreased 8.4% with caloric restriction and exercise. This and other studies support the notion that body weight and body fat are not necessarily synonymous.

A secondary finding of this study is the potential role of extracellular adenosine in regulating lipolysis. Adenosine is a locally produced hormone with a half-life in the blood of <1 s (30). It is known to be taken up rapidly by endothelial cells (30, 32) and may be produced by parenchymal cells directly or from adenine nucleotide precursors either intracellularly (9) or extracellularly (47). In adipocytes, adenosine binds to the adenosine A1 receptor and inhibits adenylyl cyclase and thus lipolytic activity. In vitro and in vivo findings demonstrate that adenosine A1 receptor downregulation can occur in response to elevated extracellular adenosine (12, 14, 26). In the present study, differences in extracellular adenosine between treatments were not observed under basal conditions but only on in vivo infusion of epinephrine or epinephrine plus dipyridamole in restricted-fed exercised swine. These transient rises in adenosine may, over time, be responsible for the observed downregulation of the A1 receptor in the restricted-fed exercised group. The failure of theophylline infusion to further exacerbate the rise in extracellular adenosine may be due to the gradual desensitization to infused epinephrine; theophylline infusion occurred 60 min after the epinephrine infusion had begun. Houseknecht et al. (15) demonstrated that plasma free fatty acids and glycerol returned to baseline by 60 min in cows despite continued infusion of epinephrine; desensitization to epinephrine also has been demonstrated in vivo in humans (39). This limitation can be avoided in future studies by using a bolus injection of epinephrine rather than a continuous infusion or shortening the time frame of the experiment.

The epinephrine-induced rise in extracellular adenosine also may account, in part, for the low-plasma free fatty acid and glycerol levels seen on epinephrine infusion in these swine. The in vivo findings, which suggest low lipolytic sensitivity, contrast with in vitro findings in which adipocytes isolated from exercise-trained swine fed moderate amounts of feed have greater lipolytic sensitivity than adipocytes isolated from sedentary or overfed swine. This discrepancy between lipolysis in vitro and in vivo, noted by others (24), suggests the involvement of other physiological mechanisms present in intact adipose tissue that may regulate extracellular adenosine level. Such mechanisms may involve the rapid uptake of adenosine by endothelial cells, as observed in heart tissue (31, 33), or an increase in epinephrine-stimulated blood flow, as observed with exercise training in humans (40).

The plasma free fatty acid level in response to epinephrine infusion must be interpreted with caution. Free fatty acid levels represent the balance between release (primarily from adipose tissue), esterification (primarily by adipose tissue), and uptake (primarily by muscle); therefore, changes in free fatty acid levels may be the result of a change in release, esterification, uptake, or a combination of the three. The relationship between plasma free fatty acid concentration and lipolysis may be uncoupled by the physiological state, such as exercise, as elegantly demonstrated by Klein et al. (21).

The role of adenosine in regulating in vivo lipolysis in response to short-term fasting in humans was investigated by Peters et al. (34). Theophylline was infused at a dose sufficient to antagonize the adenosine receptor but not interfere with cAMP phosphodiesterase activity in subjects who were fasted for 14 or 86 h. In vivo lipolysis was measured by the rate of appearance of glycerol using D5-glycerol infusion. Theophylline infusion caused a mild increase in lipolysis in the 14-h fasted subjects and a marked increase in lipolysis in the 86-h fasted subjects, suggesting an increase in adenosine "activity" with fasting. However, these data can be viewed from another perspective: adenosine activity can be realized only when adenosine is present, and extracellular adenosine was not measured in this study. It is possible that the reverse adaptation that we observed with exercise was taking place with fasting: adenosine receptors may be upregulated in response to a lowered extracellular adenosine content. However, the greater number of receptors will only be called on if there is sufficient ligand to bind to them. Clearly, more work is warranted in this area.

Microdialysis has proven to be a very useful technique to ascertain adipose tissue extracellular metabolite concentrations (1, 2, 13, 35), including adenosine (27). The concentration of adenosine will be influenced by its rate of uptake and production by adipocytes and endothelial cells and its rate of delivery and removal via the microcirculation (23). One shortcoming of the present study is that adipose tissue blood flow was not measured; therefore, it is uncertain whether changes in adenosine levels reflect changes in adenosine production or removal. Resting adipose tissue blood flow in humans has been shown to increase with exercise training (40), and, conversely, adipose tissue blood flow plays an important role in regulating lipid metabolism (36). A second shortcoming is the mild inflammation around the probe and the possibility of leukocytes contributing to local adenosine production. Although this would not impact differences observed in the present study among groups, it may impact the extracellular adenosine values.

In summary, our results suggest that excess energy intake overrides the benefits of exercise training in adipose tissue of miniature swine. Furthermore, adenosine appears to play a role in regulating lipolysis in vivo. Future studies will be needed to examine the source of extracellular adenosine in adipose tissue and how adenosine production may be regulated.


    ACKNOWLEDGEMENTS

This study represents the collective effort of many people; it would not have been possible without their help. I am most grateful to the following individuals from the University of New Hampshire (unless otherwise noted) for their contributions: Van Gould, animal technician, for adipose tissue biopsy and jugular vein surgery; Tom Oxford, swine barn manager, for care and feeding of the swine; Haven Hayes, technician, for assistance with surgeries, swine transportation, drug infusion studies, glycerol assays, and swine exercise; Jen LeClair, undergraduate student, for swine exercise; Jennifer Schuchman, Maureen Tanguay, and Eli Morse, undergraduate students, for glycerol assays; Terri Ainaire, technician, for glycerol assays; Tony Tagliaferro, Professor, and Anne Ronan, technician, for plasma glycerol and free fatty acid assays; Joel Linden, Professor, and Toni Barbera, technician (University of Virginia), for adenosine assays; Ed Zambraski, Professor (Rutgers University), for advice on implanting jugular vein catheters, and Carroll Jones, veterinary pathologist, for adipose tissue histology.


    FOOTNOTES

1 Original submission in response to a special call for papers on "Molecular and Cellular Basis of Exercise Adaptations."   The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

This work was supported by Grant 94077635 from the American Heart Association and Grant H346 from the New Hampshire Agricultural Experiment Station. Scientific contribution number 2036 was from the New Hampshire Agricultural Experiment Station.

Address for reprint requests and other correspondence: G. B. Carey, Dept. of Animal and Nutritional Sciences, Univ. of New Hampshire, Durham, NH 03824 (E-mail: gbc{at}cisunix.unh.edu).

Received 13 October 1999; accepted in final form 1 December 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

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9.   Finnegan, R. B., and G. B. Carey. Characterization of cyclic AMP efflux from swine adipocytes in vitro. Obesity Res. 6: 292-298, 1998[ISI][Medline].

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