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J Appl Physiol 90: 606-614, 2001;
8750-7587/01 $5.00
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Vol. 90, Issue 2, 606-614, February 2001

Effects of 2 G on adiposity, leptin, lipoprotein lipase, and uncoupling protein-1 in lean and obese Zucker rats

L. Elisabeth Warren, Barbara A. Horwitz, Jock S. Hamilton, and Charles A. Fuller

Section of Neurobiology, Physiology, and Behavior, Division of Biological Sciences, University of California, Davis, California 95616-8519


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Male Zucker rats were exposed to 2 G for 8 wk to test the hypothesis that the leptin regulatory pathway contributes to recovery from effects of 2 G on feeding, growth, and nutrient partitioning. After initial hypophagia, body mass-independent food intake of the lean rats exposed to 2 G surpassed that of the lean rats maintained at 1 G, but food intake of the obese rats exposed to 2 G remained low. After 8 wk at 2 G, body mass and carcass fat were less in both genotypes. Leptin and percent fat were lower in lean rats exposed to 2 G vs. 1 G but did not differ in obese rats exposed to 2 G vs. 1 G. Although exposure to 2 G did not alter uncoupling protein-1 levels, it did elicit white fat pad-specific changes in lipoprotein lipase activity in obese but not lean rats. We conclude that 2 G affects both genotypes but that the lean Zucker rats recover their food intake and growth rate and retain "normal" lipoprotein lipase activity to a greater degree than do the obese rats, emphasizing the importance of a functional leptin regulatory pathway in this acclimation.

centrifugation; body mass; food intake; fat metabolism; gravity


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

THE PHYSIOLOGICAL CHANGES observed in organisms exposed to hyperdynamic fields via centrifugation are extensive (see Ref. 35 for review). Acute responses to chronic centrifugation include a period of hypophagia (19), impaired thermoregulation (15), altered glucose and fat metabolism (6-8), and a decrease in body mass (3). After acclimation, growth resumes, and organisms such as chickens (40), rabbits (19), and rats (28, 31) exhibit increased steady-state maintenance feed requirements. The distribution of body mass between fat and fat-free components is also altered by exposure to increased force environments; there is a specific reduction in body fat that is proportional to field strength. Burton and Smith (3) showed that chickens decreased body fat from 30% at 1 G to 3% at 3 G. Similar findings have been observed in mice (27), rats (28), and rabbits (19).

In a previous experiment on lean and genetically obese Zucker rats exposed to 2 G for 8 wk, our laboratory found that lean Zucker rats responded similarly to other species studied at 2 G (43, 44). There was an initial reduction in body mass, a resumption of growth at a reduced mass, an initial hypophagia followed by a body mass-independent food intake (MIFI) similar to that of the controls maintained at 1 G by the end of week 6, and a ~50% reduction of both absolute and percent carcass fat. In contrast, obese Zucker rats exposed to 2 G showed 1) an initial hypophagia with only partial recovery of MIFI by 8 wk at 2 G, 2) retardation of growth, and 3) a larger decrease in carcass fat mass but a smaller decrease in percent carcass fat than that measured in the lean rats. These differences in response suggest that, although obese Zucker rats respond to 2 G, their acclimation is blunted compared with that of their lean counterparts. Obesity in the Zucker fatty (fa/fa) rat is inherited as a single Mendelian recessive trait (47). The fa mutation has been identified as a point mutation in the extracellular domain of the leptin receptor (5). Thus obese Zucker rats possess defective signaling in the leptin pathway regulating food intake, and one result of the mutation is hyperphagia (46). Additionally, sympathetic neural activity is attenuated in the Zucker fatty rats, resulting in depressed metabolism (2, 20) and blunted regulatory thermogenesis (21, 22). Zucker rats, therefore, are useful animal models for evaluating the etiology of energy-balance disorders such as obesity.

In this study, we used Zucker rats to evaluate the effects of gravity on the regulation of energy intake, carcass fat, growth rate, nonshivering thermogenic (NST) capacity, and nutrient partitioning when the leptin signaling pathway was defective. Specifically, we tested the following two hypotheses: 1) that circulating leptin, adiposity, white adipose lipoprotein lipase (LPL) activity, and NST capacity, as indexed by brown adipose tissue (BAT) uncoupling protein-1 (UCP1) levels, would be lower in lean but not in obese Zucker rats at 2 G vs. 1 G; and 2) that the recovery from 2 G would be greater in the lean vs. obese rats. Our data indicate that at 2 G serum leptin levels and percent carcass fat were indeed lower in lean but not obese rats, that UCP1 levels did not differ from 1 G values in lean or in obese rats and that there were pad-specific changes in adipose LPL-specific activity in the obese but not the lean rats. These data are consistent with the importance of a functional leptin regulatory pathway for acclimation to the effects of 2 G.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Animals. Six groups of male Zucker rats (22-24 wk of age) were obtained from the Animal Models Core of the University of California, Davis, Clinical Nutrition Research Unit: lean rats maintained at 1 G (1GL; n = 7), obese rats exposed to 1 G (1GO; n = 8), lean rats exposed to 2 G (2GL; n = 6), obese rats exposed to 2 G (2GO; n = 11), lean rats maintained at 1 G and pair fed (PF) to their lean counterparts at 2 G (PFL; n = 9), and obese rats maintained at 1 G and pair fed to their obese counterparts at 2 G (PFO; n = 8). All animals were individually housed in stainless steel metabolism cages (10 × 7 × 7 in.) with ad libitum access to rat chow (Lab Diet) and water (except the PF groups). The PF rats, which began the study 1 wk after the ad libitum-fed rats, had access to water ad libitum and were provided with the same quantity of food that their 2-G-exposed counterparts consumed during the previous week. This quantity was divided into seven equal daily meals for the week. All animals were housed in a 12:12-h light-dark cycle and at 25 ± 2°C. The 2G groups were maintained on a centrifuge, whereas the 1G and PF groups were housed in an adjacent room under similar conditions. Data (body mass, food intake) were collected on all groups for 2 wk at 1 G. After this 1-G baseline period, centrifugation was initiated, and the 2G groups were constantly exposed to acceleration equivalent to twice Earth's normal gravitational force for 8 wk.

Centrifugation. The 2-G field was produced via centrifugation at the Chronic Acceleration Research Unit at the University of California, Davis. The cage-mounting system allowed for one degree of freedom so that the resultant acceleration vector was always perpendicular to the cage floor. The 3.5-m-diameter centrifuge was briefly stopped daily (~20 min) for animal husbandry; body mass and food and water intakes were measured once weekly.

Dissection. The 2G animals were killed by decapitation within the first 4 h after lights on and within 30 min of cessation of centrifugation. The 1G groups were killed at the same time. Trunk blood was collected for leptin analysis while the epididymal, retroperitoneal, and brown fat (interscapular and cervical) pads were removed from the carcass, weighed, and frozen. All tissues were frozen in liquid nitrogen and stored at -80°C until biochemical analysis could be completed.

Carcass composition. Carcass composition was determined by the gravimetric method described by Bell and Stern (1). Briefly, carcasses were eviscerated, freeze-dried for 7 days (or until 2 consecutive daily weighings differed by no more than 2%), and weighed to obtain the water content. The remainder was ether extracted for 7 days and acetone extracted for 5 days. Once the solvents had evaporated, the carcass was freeze-dried for 24 h and weighed to determine fat-free dry mass. The latter, when subtracted from dry mass, yielded fat content. Lean body mass was calculated as the difference between carcass mass and fat mass.

Serum leptin. Serum leptin was measured using a radioimmunoassay kit for rat leptin (Linco Research, St. Charles, MO) with a detection range of 0.5-50 ng/ml. All samples were run in duplicate.

LPL. Retroperitoneal and epididymal fat depots were assayed for total extractable LPL as described by Chan and Stern (4). Because the adipose tissue was frozen immediately after removal from the rat, we were unable to measure heparin-releasable LPL.

UCP1. UCP1 was measured in brown fat homogenates using immunoblotting techniques (16). Immunoblots were quantified via scanning densitometry, and homogenate protein was determined with the bicinchoninin acid assay (Pierce, Rockford, IL) with BSA as standard.

Statistics. Comparisons between the experimental and control groups were analyzed by ANOVA with Fisher's protected least significant difference post hoc test (StatView 5.0.1, SAS Institute). P values < 0.05 were considered significant. All data are expressed as means ± SE.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Body mass. During the 2-wk 1-G control period, the obese rats were consistently heavier (~100 g) than the lean rats with no differences within the lean or obese groups (Fig. 1). After the onset of 2 G, the 2GL and 2GO rats displayed a decrease in body mass, and after 2 wk at 2 G, they resumed growth. The PFL and PFO rats also resumed growth after their first week of pair feeding. Regression analysis of body masses during the last 5 wk of the study indicated that there were no differences in growth rates between the lean groups (Table 1). Whereas body masses of the PFL rats remained similar to those of 1GL rats after the first week of pair feeding to the 2GL rats, body masses of the 2GL rats remained lower than those of the 1GL controls. After 8 wk at 2 G, body mass of the lean rats averaged ~10% less than that of the 1GL and PFL groups (P < 0.01), which were similar to each other. Growth of the 2GO rats was significantly less than that of the 1GO rats (P < 0.05; Table 1). As with the lean rats, after 8 wk at 2 G, body mass of the 2GO group remained less than that of both the 1GO and PFO groups (P < 0.01). Although not significantly different from either the 1GO or 2GO groups, the growth pattern of the PFO group resembled that of the 2GO rats more than that of the 1GO rats. Contrary to the growth of the 2GL rats, growth of the 2GO rats appeared to be diverging from the 1-G control levels.


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  		Fig. 1.   Body mass. 1G lean and 1G obese, lean and obese rats exposed to 1 G, respectively; 2G lean and 2G obese, lean and obese rats maintained at 2 G, respectively; PF lean and PF obese, 1G lean and 1G obese rats pair fed to their counterparts at 2 G. Body mass decreased in lean and obese rats at the onset of 2 G. Body mass of the PF groups also decreased. Growth resumed to varying degrees in all groups, although mass remained low in the 2G lean, 2G obese, and PF obese rats. Values are means ± SE. Bar, period of centrifugation.


                              
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  		Table 1.   Carcass composition and growth rates

Food intake. During the 2-wk 1-G control period, absolute food intake (g) of the obese rats was greater (~40 g) than that of the lean rats with no differences within the lean or obese groups (Fig. 2). There were no differences in food intake between the PF and 2G rats. After the onset of 2 G, both 2GL and 2GO rats had significantly decreased (~40%) their food intake. By the second week at 2 G, recovery toward 1 G control levels had begun in both genotypes. The 2GL (and PFL rats) recovered to the 1GL control level by the end of the fourth week of centrifugation (Fig. 2A). After reversal of the initial drop in food intake, recovery of the 2GO and PFO rats slowed (Fig. 2B), with values still being significantly less than that of the 1GO controls at the end of the study.


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  		Fig. 2.   Food intake. A: food intake of the 2G lean rats decreased at the onset of 2 G, and consequently so did that of the PF rats. Food intake of the 2G and PF groups began to increase by the second week at 2 G, rising to the 1G lean control level by the end of the fourth week at 2 G. B: food intake of the 2G obese rats decreased at the onset of 2 G as did that of the PF rats. The 2G and PF obese rats' food intake did not recover to the 1G obese control level. Values are means ± SE. Bars, period of centrifugation.

Food intake was also expressed independent of body mass (grams food intake/grams body mass2/3; Fig. 3). During the first week of centrifugation at 2 G, the lean rats' MIFI decreased by ~40% (Fig. 3A). By the end of the second week at 2 G, MIFI had begun to recover toward that of the 1GL rats, which by the fourth week at 2 G it had surpassed. MIFI of the 2GL rats remained greater than that of the 1G controls for the remainder of the study. The MIFI of the PFL rats closely followed that of the 2GL group until the second week of centrifugation. Whereas MIFI of both the 2GL and PFL rats increased toward the 1GL level, that of the PFL rats diverged from the MIFI of the 2GL rats and remained lower than it for the remainder of the study. Thus, whereas MIFI of the 2GL rats surpassed that of the 1G controls, once MIFI of the PFL rats increased to the 1-G control level, it closely followed it. During the first week at 2 G, MIFI of the obese rats also decreased by ~40% (Fig. 3B), and by the end of the second week of centrifugation, recovery toward the 1GO control level had begun. However, MIFI of the 2GO group remained lower than that of the 1GO control group until the end of the experiment. The MIFI of the PFO rats closely followed that of the 2GO group. Differences in MIFI values for the 2G vs. PF groups of both genotypes reflect differences in body masses.


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  		Fig. 3.   Mass-independent food intake. A: mass-independent food intake of the 2G lean rats decreased at the onset of 2 G as did that of the PF rats. After 4 wk at 2 G, mass-independent food intake of the 2G group surpassed that of the 1G and PF rats. B: mass-independent food intake of the 2G (and PF) obese rats decreased at the onset of 2 G and remained lower than that of the 1G controls. Values are means ± SE. Bars, period of centrifugation.

Carcass composition. Exposure to 2 G resulted in loss of carcass fat in both genotypes, with the absolute amount of this loss being greater in the obese rats (Table 1). However, lean rats lost a greater percentage of their fat than did the obese rats (Fig. 4A). That is, after 8 wk at 2 G (Table 1), the lean Zucker rats had 50% less fat than did the 1G animals and 48% less than that of the PF animals (P < 0.001). Fat mass of the 2GO rats was 24% lower than that of the 1GO rats (P < 0.01) and 22% lower than that of the PFO rats (P < 0.01). When expressed relative to carcass weight (i.e., as percent carcass fat; Fig. 4A), 2GL rats had values that were 44% less than those of the 1GL rats (6.83 ± 0.49 vs. 12.20 ± 1.22%, respectively; P < 0.001) and 40% less than those of the PFL rats (11.34 ± 0.92%; P < 0.01). Percent carcass fat in the 2GO rats (32.32 ± 0.70%) was 12% less than that of the PFO rats (36.59 ± 1.00%; P < 0.05) but did not differ significantly from that of the 1GO rats (36.09 ± 2.79%).


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  		Fig. 4.   Carcass composition. A: percent carcass fat was lower in both the lean and obese rats at 2 G. B: percent lean carcass mass was higher in both the lean and obese rats at 2 G. C: percent carcass water was higher in the 2G lean rats compared with the 1G and PF rats. Values are means ± SE. Within a given genotype: a different from 1G rats, P < 0.05; b different from PF rats, P < 0.05.

Lean carcass mass (Table 1) was unchanged in the 2GL vs. 1GL rats but was 8% less than that of the PFL rats (P < 0.05). Lean carcass mass was 11% lower in the 2GO compared with 1GO controls (P < 0.05) but was not different from the PFO controls. Percent lean carcass mass (Fig. 4B) was 6% higher in the 2GL rats compared with the 1GL rats (93.17 ± 0.49 vs. 87.80 ± 1.21%, respectively; P < 0.001) and 5% higher compared with that in the PFL rats (88.66 ± 0.92%; P < 0.01). Percent lean carcass mass was 6% higher in the 2GO rats compared with the PFO rats (67.68 ± 0.70 vs. 63.41 ± 1.00%; P < 0.05) but did not differ in the 2GO vs. 1GO rats.

Water mass (Table 1) was unchanged in both the 2GL rats vs. 1GL rats and in the 2GO vs. 1GO and PFO rats. However, water mass in the 2GL group was 8% less than that in the PFL group (P < 0.05). Percent body water (Fig. 4C) in the 2GL rats was 6% higher than that of the 1GL rats (P < 0.01) and 5% higher than that of the PFL rats (P < 0.01). This variable was unchanged in the 2GO vs. 1GO and PFO rats.

Fat pads. Three fat pads were dissected from the carcass and weighed. In all cases, the fat pads from the 2GL weighed significantly less than those from PFL rats. Epididymal fat from the 2GL rats weighed 41% less than that from the 1GL rats (2.19 ± 0.13 vs. 3.71 ± 0.28 g, respectively; P < 0.01) and 38% less than that from the PFL group (3.55 ± 0.36 g; P < 0.01). There was no significant difference in epididymal fat mass between the 2GO and 1GO rats (9.89 ± 0.61 vs. 11.54 ± 0.82 g, respectively) or the PFO rats (11.64 ± 0.79 g). Retroperitoneal fat from the 2GL rats weighed 53% less than that from the 1GL controls (1.27 ± 0.12 vs. 2.70 ± 0.25 g, respectively; P < 0.01) and 55% less than that from the PFL group (2.81 ± 0.37 g; P < 0.01). There was no significant difference in retroperitoneal fat mass among the 2GO, 1GO, and the PFO rats (14.40 ± 1.04, 15.31 ± 2.01, and 18.47 ± 1.97 g, respectively). The cervical and interscapular brown fat pads (BAT) were dissected and weighed together. BAT removed from the 2GL rats (0.46 ± 0.06 g) weighed 34% less than BAT removed from the 1GL and the PFL rats (0.70 ± 0.08 and 0.70 ± 0.06 g, respectively; P < 0.05). Similarly, the BAT removed from the 2GO rats weighed 22% less than the BAT removed from the 1GO rats (1.65 ± 0.11 vs. 2.11 ± 0.16 g, respectively; P < 0.05). BAT mass of the 2GO rats was not different from that of the PFO rats (1.93 ± 0.09 g).

Leptin. Serum leptin (Fig. 5A) was 79% lower in the 2GL rats compared with that from 1GL rats (1.75 ± 0.15 vs. 8.44 ± 2.34 ng/ml, respectively; P < 0.05) and 76% lower than that of the PFL rats (7.37 ± 1.55 ng/ml; P < 0.05). In addition, leptin expressed as a function of carcass fat mass (Fig. 5B) was 54% lower in the 2GL rats vs. 1GL rats (0.08 ± 0.00 vs. 0.17 ± 0.04 ng · ml-1 · g-1, respectively; P < 0.05) and 51% lower than the PFL rats (0.16 ± 0.02 ng · ml-1 · g-1; P < 0.05). Leptin values (concentrations or relative to carcass fat) were not significantly altered in the 2GO vs. 1GO rats. However, leptin relative to carcass fat was 38% lower in the PFO rats than in the 2GO rats (0.16 ± 0.02 vs. 0.26 ± 0.04 ng · ml-1 · g-1; P < 0.05).


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  		Fig. 5.   Serum leptin. A: serum leptin of the 2G lean rats was lower than that of both the 1G and PF lean rats. B: serum leptin as a function of grams body fat was lower in the 2G lean rats than in either the 1G or PF lean rats. Serum leptin as a function of body fat was higher in the 2G obese than in the PF obese rats. Values are means ± SE. Within a given genotype: a different from 1G rats, P < 0.05; bdifferent from PF rats, P < 0.05.

LPL. Epididymal fat was examined for protein content and LPL activity (Fig. 6A). There were no differences in protein per gram of fat between the 2GL rats (5.65 ± 0.61 mg/g) and the 1GL or PFL rats (4.00 ± 0.47 and 6.41 ± 0.93 mg/g, respectively), although the value of the PFL rats was 60% higher than that of the 1GL rats (P < 0.05). There were no significant differences in LPL specific activity (µmol free fatty acids released per h per mg protein) between the 2GL, 1GL, or PFL groups (0.18 ± 0.03 vs. 0.22 ± 0.04 vs. 0.18 ± 0.03 µmol · h-1 · mg-1, respectively). In the obese rats, milligrams of protein per gram of fat did not differ in the 2G vs. 1G or PF groups (5.09 ± 0.84 vs. 3.90 ± 0.39 vs. 3.78 ± 0.23 mg/g, respectively). However, LPL specific activity was 38% lower in the 2GO vs. 1GO rats (0.12 ± 0.02 vs. 0.20 ± 0.03 µmol · h-1 · mg-1, respectively; P < 0.05). There was no difference in epididymal LPL specific activity between the 2GO and the PFO rats (0.17 ± 0.02 µmol · h-1 · mg-1).


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  		Fig. 6.   Lipoprotein lipase specific activity. FFA, free fatty acids. A: specific activity of epididymal fat lipoprotein lipase in the obese rats at 2 G was lower than that of those at 1 G. B: there were no differences in specific activity of retroperitoneal fat lipoprotein lipase in the lean or obese rats. Values are means ± SE. Within a given genotype: a different from 1G rats, P < 0.05.

The same variables were measured in the retroperitoneal fat with no differences being observed among the three groups of lean and three groups of obese rats with respect to LPL specific activity (Fig. 6B). However, protein per gram of fat was 75% higher in the 2GL vs. 1GL rats (5.86 ± 1.36 vs. 3.34 ± 0.61 mg/g; P < 0.05). In contrast, protein per gram of retroperitoneal fat was 18% lower in the 2GO rats (2.46 ± 0.13 mg/g; P < 0.05) and 25% lower in the PFO rats (2.27 ± 0.15 mg/g; P < 0.01) compared with the 1GO rats (3.01 ± 0.20 mg/g).

UCP1. Total BAT protein did not differ among the 2GL, 1GL, or PFL rats (23.39 ± 2.75, 28.52 ± 3.39, and 26.16 ± 3.11 mg, respectively). Similarly, there was no difference in total BAT protein between the 2GO, 1GO, and PFO rats (47.70 ± 4.56, 46.88 ± 2.56, and 66.63 ± 15.51 mg, respectively). Although BAT protein concentration (i.e., per gram of BAT) did not differ between the 2GL and 1GL rats (51.84 ± 4.55 vs. 42.30 ± 4.68 mg/g, respectively), it was 43% higher in the 2GL rats compared with that in the PFL rats (36.16 ± 2.52 mg/g; P < 0.01). There were no differences in BAT protein concentration between the 2GO, 1GO, or PFO rats (28.72 ± 1.40, 25.77 ± 4.73, and 33.83 ± 7.81 mg/g, respectively).

Total UCP1 protein in the lean and in the obese rats was unchanged by exposure to 2 G [lean values: 2.1 ± 0.4 mg (2G), 2.3 ± 0.6 mg (1G), 2.1 ± 0.2 mg (PF); obese values: 4.4 ± 0.7 mg (2G), 4.9 ± 2.1 mg (1G), 4.9 ± 1.6 mg (PF)]. Neither UCP1 concentration (µg UCP1/µg BAT protein; Fig. 7A) nor UCP1 content per gram of body mass2/3 (an index of NST capacity; Fig. 7B) was altered by exposure to 2 G in the lean or in the obese rats.


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  		Fig. 7.   Uncoupling protein-1 (UCP1). A: there was no effect of 2 G or pair feeding on UCP1 concentration. B: there was no effect of 2 G or pair feeding on brown fat thermogenic capacity as indicated by body mass-independent UCP1 levels. Values are means ± SE.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The results of this study support our hypothesis that 2-G exposure results in lower circulating leptin concentrations and percent carcass fat in lean but not in obese Zucker rats. However, they negate the hypothesis that a similar pattern occurs with adipose LPL or UCP1. Our data also indicate that recovery from the 2-G effects on food intake and growth rate is greater in the lean rats.

Perhaps the most striking result of this study was the dramatically lower serum leptin concentration of the lean rats after 8 wk of centrifugation at 2 G (Fig. 5A). This decrease is consistent with the marked loss of carcass fat in the centrifuged lean rats (~50% in terms of grams and ~40% in terms of percent; Fig. 4A). On the basis of the current view that decreased circulating leptin stimulates feeding behavior in part by modulating the expression and release of neuropeptides that either stimulate or inhibit feeding (9, 18, 23, 32, 33, 41, 45), we would expect that the 2GL rats would have greater energy intake than the controls. In fact, the MIFIs are elevated after 4 wk at 2 G (Fig. 2A). In contrast, the leptin-resistant obese Zucker rats did not exhibit a significant change in serum leptin with 2-G exposure, and their MIFI remained lower in the 2G rats compared with 1G controls.

In addition to lower serum leptin concentrations, the leptin concentration as a function of body fat was also less in the 2GL vs. 1GL or PFL rats (Fig. 5B). This could reflect reduced synthesis and/or secretion per gram of adipocytes and/or greater rates of leptin degradation in the circulation, possibilities that require further study. Although fat mass was significantly lower in the 2GO vs. 1GO and PFO controls, leptin concentrations were not (Fig. 5).

Our laboratory (43, 44), as well as others, has demonstrated that hypergravity induces a greater loss in fat than in protein or lean body mass. In this, as in our laboratory's previous study with Zucker rats (43), the absolute amount of fat lost was greater in the obese than in the lean rats although the percent change was less. These results indicate that the effect of 2 G on nutrient partitioning may differ in these two genotypes. One index of nutrient partitioning is LPL activity. This enzyme, which catalyzes the hydrolysis of lipoproteins and triglycerides into free fatty acids, is often considered a "gatekeeper" with respect to the transfer of fatty acids into tissue stores of triacylglycerol (17). Although both genotypes mobilized (rather than increased) body fat at 2 G, we saw diminished adipose LPL activity only in the epididymal fat depot of the 2GO rats. The fact that there were no differences in adipose LPL activity between 2G and 1G lean rats is consistent with the return of food intake in the former to 1-G levels. That is, by the end of the experiment, the 2G rats may have no longer been in a fat-mobilizing mode. It is also possible, however, that heparin-releasable LPL activity, generally thought to be a better index of endothelial LPL, may have shown 2-G effects in the lean rats. Unfortunately, we were unable to make such measurements.

Core body temperature decreases and thermoregulation is impaired during acute exposure to hypergravity (15). We found no significant effect of 2 G on the amount of UCP1 per milligram of protein or UCP1 expressed in terms of metabolic body mass (i.e., body mass to the <FR><NU>2</NU><DE>3</DE></FR> power; Fig. 7B) in the lean or obese rats, indicating that the nonshivering thermogenic capacity of the 2G rats was unchanged. The fact that there were no significant changes in the amount of BAT protein despite significant decreases in BAT mass in both the 2GL and 2GO rats indicates that this loss in mass was due primarily to reduced triacylglycerol stores. This could reflect increased lipolysis resulting from the need to mobilize fat stores at least during the periods of negative energy balance.

There are numerous reports indicating that metabolism is elevated during exposure to increased ambient force environments (24). Maintenance energy expenditure has been postulated to increase directly with the ambient force environment (3, 26, 34-40). To maintain energy balance, maintenance nutritional requirements in several species have also been shown to increase directly with the level of acceleration (19, 38). During exposure to hypergravity, there is evidence of increased glucose utilization (6, 10), increased carbohydrate utilization (24, 25), and increased fat utilization (11-13). In perhaps the most direct demonstration of the metabolic cost of gravity, oxygen consumption in small mammals of various sizes increased at 2 G at an approximately constant rate per gram of body weight, i.e., linearly with total body weight (14, 29, 30). In contrast to these studies, Wade et al. (42) saw no change in total daily energy expenditure using double-labeled water in rats during 14 days of hypergravity. It can be argued, however, that the rats in their study had yet to acclimate to the hypergravity environment. This would be consistent with our data, wherein food intake was still low relative to that of controls at day 14 of centrifugation.

Although we did not measure energy expenditure, our food intake and body mass data support the conclusion that the rats at 2 G, both lean and obese, had higher metabolic rates than did their PF counterparts. This conclusion is based on the finding that, although the 2G and PF rats consumed the same amount of food on an absolute basis, the latter weighed significantly more than did the 2G rats at the end of the 8-wk experiment. However, we cannot extrapolate this conclusion to the comparison between 2G and 1G ad libitum-fed rats because of the possibility that the PF rats reduced their metabolic rate below that of the 1G ad libitum-fed animals in the face of decreased food availability. That this was the case in the lean rats is indicated by the fact that the 1GL and PFL rats weighed the same amount at the end of the experiment, despite the lower food intake of the latter. The situation in the obese rats is not as clear.

In conclusion, exposure to altered gravity has been shown to influence body size, energy intake, growth rate, energy expenditure, and adiposity. This study demonstrates that the levels of circulating leptin, a key hormone involved in the regulation of adiposity, are altered by exposure to 2 G, in accordance with the loss of body fat. It also demonstrates that, during the early period of 2-G exposure, compensatory increases in food intake (which would be expected as body mass and fat decreased and leptin levels fell) did not occur. However, as the lean rats acclimated to the 2-G environment, such compensation was apparent as was recovery of their growth rate. The differences in response to 2 G between the lean and obese Zucker rats indicate that, although both genotypes are affected by 2 G, acclimation by the obese rats is blunted and quantitatively different from that of their lean counterparts. This is likely due to the fact that obese Zucker rats are leptin resistant and provides further support for the importance of a functional leptin regulatory pathway in the acclimation of the lean rats to 2 G. Because body fat storage underlies survival capacity, an understanding of fat regulation and its responsiveness to changes in the ambient force environment may be critical to humans involved in long-duration exposure to altered gravitational fields.


    ACKNOWLEDGEMENTS

The authors thank the University of California, Davis, Clinical Nutrition Research Unit for making available the Zucker rats; Sue Bennett for the body composition analysis; Sue Hansen for the LPL measurements; and Kimber Stanhope for analyzing the serum leptin concentrations.


    FOOTNOTES

This work was supported by National Aeronautics and Space Administration Grant NAG5-3949 and National Institute of Diabetes and Digestive and Kidney Diseases Grant DK-35747.

Address for reprint requests and other correspondence: C. A. Fuller, Sect. of Neurobiology, Physiology, and Behavior, Div. of Biological Sciences, Univ. of California, One Shields Ave., Davis, CA 95616-8519 (E-mail: cafuller{at}ucdavis.edu).

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. Section 1734 solely to indicate this fact.

Received 3 May 2000; accepted in final form 28 August 2000.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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