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Growth Hormone Blunts Protein Oxidation and Promotes Protein Turnover to a Similar Extent in Abdominally Obese and Normal-Weight Women

Department of General Internal Medicine (M.M.B., M.F., A.E.M., H.P.), Department of Endocrinology (J.A.R.), and Centre for Human Drug Research (J.B., M.L.D.K., A.F.C.), Leiden University Medical Centre, 2300 RC Leiden, The Netherlands; and Center for Liver, Digestive and Metabolic Diseases, Laboratory of Paediatrics (F.S.), University Hospital Groningen, 9713 GZ Groningen, The Netherlands

Address all correspondence and requests for reprints to: M. M. Buijs, M.D., Department of General Internal Medicine, Leiden University Medical Centre, C1-R39, P.O. Box 9600, 2300 RC Leiden, The Netherlands. E-mail: m.m.buijs{at}lumc.nl.

Abstract

Abdominally obese individuals have reduced 24-h plasma GH concentrations. Their normal plasma IGF-I levels may reflect GH hypersensitivity. Alternatively, obesity-associated hyposomatotropism may cause less biological effect in target tissues. We therefore determined whole-body responsiveness to the anabolic effects of GH in abdominally obese (OB) and normal weight (NW) premenopausal women. A 1-h iv infusion of GH or placebo was randomly administered to six NW (body mass index, 21.1 ± 1.9 kg/m²) and six OB (body mass index, 35.5 ± 1.5 kg/m²) women in a cross-over design. Endogenous insulin, glucagon and GH secretion was suppressed by infusion of somatostatin. Whole-body protein turnover was measured using a 10-h infusion of [¹³C]-leucine. GH administration induced a similar plasma GH peak in NW and OB women (49.8 ± 10.4 vs. 45.1 ± 5.6 mU/liter). GH, compared with placebo infusion, increased nonoxidative leucine disposal, P < 0.0001) and endogenous leucine appearance (R_a, P = 0.0004) but decreased leucine oxidation (P = 0.0051). All changes were similar in both groups. Accordingly, whole-body GH responsiveness, defined as the maximum response of nonoxidative leucine disposal, leucine R_a, and oxidation per unit of GH, was not different in OB and NW women (0.25 ± 0.18 vs. 0.19 ± 0.17 µmol/kg·h, 0.21 ± 0.23 vs. 0.13 ± 0.17 µmol/kg·h, and -0.10 ± 0.08 vs. -0.08 ± 0.05 µmol/kg·h, respectively). These results indicated that whole-body tissue responsiveness to the net anabolic effect of GH is similar in OB and NW women. Hence, we inferred that hyposomatotropism may promote amino acid oxidation and blunt protein turnover in abdominal obesity. However, hyposomatotropism cannot account for all anomalous features of protein metabolism in abdominally obese humans.

NUMEROUS STUDIES HAVE shown the importance of GH in the regulation of protein metabolism. A single infusion of GH reduces amino acid oxidation (1) and stimulates muscle protein synthesis (2, 3), whereas more prolonged administration reduces urinary urea excretion (4) and increases whole-body protein synthesis (5, 6). Stimulation of endogenous GH secretion by fasting or administration of GH during other catabolic conditions diminishes protein breakdown (7, 8). These protein-conserving characteristics of GH promote accrual of lean body mass (LBM) in time. Hence, untreated patients with GH deficiency as a sequel of pituitary disease have reduced amounts of LBM (9) that is regained by GH substitution therapy (10, 11).

Abdominally obese humans also have profoundly reduced 24-h plasma GH concentrations (12, 13). However, the biological significance of hyposomatotropism in abdominal obesity is unknown. In view of the anabolic properties of GH, diminished GH availability in abdominal obesity may impair protein accrual. Alternatively, it has been proposed that obesity is a GH-hypersensitive state because plasma IGF-I and IGF-binding protein 3 (IGFBP-3) levels, both measures of hepatic GH bioactivity, are normal in obese individuals (14). Up-regulation of GH receptor number and/or sensitivity has been suggested to explain this phenomenon.

In the present study, we determined whole-body responsiveness to the protein-conserving action of GH in abdominally obese (OB) and normal-weight (NW) premenopausal women. To this end, we conducted a placebo-controlled study in which we randomly infused recombinant human GH (rhGH), to mimic physiological plasma GH pulse concentration, or placebo in abdominally OB and NW women. Somatostatin was infused to inhibit endogenous GH, insulin, and glucagon secretion. Whole-body protein kinetics was determined using a primed, continuous infusion of [¹³C]-leucine. Differences in response to GH, corrected for the response to placebo, were compared between the two groups and used to estimate tissue responsiveness to the anabolic actions of GH. The control study is used to specifically judge the effects of GH because many other endocrine cues that differ between OB and NW individuals (e.g. insulin, cortisol) will also affect protein metabolism.

Subjects and Methods

Subjects

Six NW (body mass index <25 kg/m²) and six abdominally OB (body mass index >29 kg/m² and waist circumference >88 cm) premenopausal women participated in the study (Table 1). The subjects were healthy, nonsmoking, and not taking any medication (including oral contraception). They all had plasma cholesterol <6.5 mmol/liter, fasting triglycerides <4.0 mmol/liter, and hemoglobin A₁C <6.7%. The subjects were weight stable for at least 3 months and did not exercise for more than 3 h/wk. During the 3 d preceding the study, the participants consumed a weight-maintaining diet containing at least 250 g carbohydrate. The study protocol was approved by the Medical Ethics Committee of Leiden University Medical Centre. Subjects were recruited using the Centre for Human Drug Research volunteer pool and advertisements in the local media. Written informed consent was obtained from all participants after explaining the nature of procedures.

The study comprised two occasions and was performed in a randomized, placebo-controlled, and double-blind fashion. An interval of at least 8 wk separated both occasions. All studies were performed in the early follicular phase of each woman’s menstrual cycle to eliminate the confounding variable of changing serum estrogen concentrations. The subjects were admitted to the Clinical Research Unit of the Centre for Human Drug Research the evening before the study. They were allowed to drink only water until the end of experiments.

The following morning, a cannula to infuse stable isotopes and hormones was inserted into an antecubital vein. Another catheter was placed into a dorsal hand vein of the contralateral arm, and this hand was kept in a thermoregulated (60 C) box to sample arterialized venous blood (15).

At 0830 h, a 0.2 mg/kg priming dose of NaH¹³CO₃ was immediately followed by a primed (1.0 mg/kg), continuous (1.0 mg/kg·h) iv infusion of [¹³C]-leucine (99% atom percent excess, Cambridge Isotope Laboratories, Andover, MA). This infusion was continued for 600 min, using a calibrated 6000 + pump (Sigma, St. Louis, MO). At 1100 h, a continuous infusion of somatostatin (Ferring Pharmaceuticals Ltd. BV, Hoofddorp, The Netherlands) was started at a rate of 300 µg/1.73 m²·h (max. 350 µg/h) for 450 min, using a calibrated syringe pump 3200 (Graseby, Watford, UK). At 1130 h, rhGH (12 mU/kg·h, Pharmacia Corp., Peapack, NJ) or placebo was randomly administered for 60 min using a calibrated syringe pump (Graseby). NaCl 0.9% was used for the control study. The start of the GH or placebo infusion was set at t = 0 h.

Before isotope infusion (t = -190 min), blood and breath samples for background isotope enrichment of, respectively, [¹³C]-leucine and [¹³C]-ketoisocaproate acid (KICA), and [¹³C]CO₂ were collected. At t = -46 min, three arterialized blood samples were obtained at 7-min intervals to determine basal protein kinetics. Breath samples were taken at two of these time points as well. Blood and breath samples to estimate the anabolic response to GH vs. placebo injection were drawn every 60 min from t = 120 to 420 min. Plasma GH levels were determined every 15 min from t = -46 to 150 min and henceforth every 30 min. Plasma IGF-I levels were determined at t = -32, -5, 120, and 420 min and IGFBP-3 concentrations only at t = -32 min. Blood samples for insulin and glucose concentration were obtained at t = -46 and -32 min and henceforth every 30 min. Glucagon and free fatty acid (FFA) levels were determined at t = -46 and -32 min and thereafter hourly.

Indirect calorimetry using a ventilated hood (Oxycon ß; Jaeger Toennies, Breda, The Netherlands) was performed for 20 min at t = -60, 100, 160, 220, 280, 340, and 400 min to estimate total CO₂ production. The initial 3 min of calorimetry were used for acclimatization and calculations were based on the mean value of 17 1-min measurements.

On a separate day, total body fat mass and total LBM were assessed using dual-energy x-ray absorptiometry (QDR 4500; Hologic, Inc., Waltham, MA) (16). The scanner had a coefficient of variation for body fat mass of 2.1% and LBM of 1.0%.

Assays

Blood for [¹³C]-leucine and [¹³C]-KICA enrichment and plasma GH, IGF-I, IGFBP-3, and leucine concentration was collected on heparin. Blood for glucagon and FFA level was collected on EDTA, and insulin and glucose was determined in serum. Heparin and EDTA samples were kept on ice and all were centrifuged within 30 min of sampling (2000 x g at 4 C for 10 min). Samples were stored at -40 C and transported on dry ice before assay. Breath sample collection was performed in triplicate by breathing through a straw into a 30-ml gas collection tube (Exetainers, Van Loenen Instrumenten, Zaandam, The Netherlands).

Plasma GH, IGF-I, IGFBP-3, insulin, and glucose were measured at the laboratory for clinical chemistry at Leiden University Medical Centre as described before (17). Additionally, plasma glucagon concentration was determined by RIA (Daiichi, Tokyo, Japan). The detection limit was 15 ng/liter, with an interassay coefficient of variation between 5.1% and 7.2%. Plasma FFA level was measured using an enzymatic colorimetric assay kit (Roche Molecular Biochemicals, Mannheim, Germany).

Plasma leucine concentration and [¹³C]-leucine, [¹³C]-KICA and [¹³C]CO₂ enrichment were determined at the laboratory of pediatrics of University Hospital Groningen. Plasma leucine level was measured by an amino acid analyser (Biochrom 20, Phamacia Biotech, Cambridge, UK). Measurement of [¹³C]CO₂ enrichment was performed directly in breath with a Finnigan TracerMat continuous-flow isotope ratio mass spectrometer (Finnigan MAT, San Jose, CA). For plasma [¹³C]-leucine enrichment, amino acids were isolated from 500 µl plasma by cation exchange chromatography and derivatized to the N(O,S)-methoxycarbonylmethyl derivative (18) and as described before (19). Analysis of plasma [¹³C]-leucine enrichment was carried out on an SSQ 7000 gas chromatograph/quadruple-mass spectrometer (GC/MS) (Finnigan MAT) by use of methane-positive ion chemical ionization and applying an OV-1701 capillary column (AT 1701, 20 m x 0.18 mm, film thickness 0.40 µm, Alltech Associates Inc., Deerfield, IL). The MS was operated in the selected ion monitoring mode at fragments m/z 204 and 205 of the [MH]⁺ and [MH+1]⁺ ions of the N(O,S)-methoxycarbonylmethyl derivative of unlabeled leucine and [¹³C]-leucine, respectively.

To determine [¹³C]-KICA enrichment, 500 µl plasma was deproteinized with sulfosalicylic acid and the supernatant derivatized to the quinoxalinol-O-t-butyldimethylsilyl derivative (20). [¹³C]-KICA enrichment was measured by use of electron impact ionization GC/MS in the selected ion monitoring mode recording the fragments m/z 259 en 260 of the quinoxalinol-O-t-butyldimethylsilyl derivative. The same GC/MS and GC column were used as described for the [¹³C]-leucine enrichment measurement.

Calculations

Plasma hormone and substrate concentrations. The maximum GH concentration in response to GH injection was analyzed to determine whether comparable GH peaks in both groups were achieved. In addition, GH areas under the curve (AUCs) (using the linear trapezoidal rule and 0 mU/liter as baseline) were calculated from t = -2 to 420 min. In each group, GH AUC on the control day was subtracted from the GH AUC on the GH day to estimate the difference in GH exposure between both occasions ( GH AUC). These GH AUCs were compared between the two study groups.

Mean prevalues for insulin, glucagon, IGF-I, glucose, and FFA concentrations were calculated from t = -46 to -32 min. Insulin and glucagon secretion was suppressed by somatostatin infusion that started at t = -30 min. Therefore, average hormone concentration was calculated from t = -2 to 420 min. Differences in response of plasma IGF-I, glucose, and FFA levels to GH vs. placebo were determined by subtracting the response to placebo from that of GH administration (-score) for each time point.

Whole-body protein kinetics and tissue responsiveness. The only source of endogenous leucine appearance (R_a) during fasting is protein breakdown. Hence, endogenous leucine R_a reflects the rate of leucine release from whole-body proteolysis. Total leucine R_a represents the sum of endogenous leucine R_a and exogenous administration of leucine. For leucine, there are two pathways of disposal: oxidation and nonoxidative leucine disposal (NOLD). NOLD reflects the uptake of leucine for protein synthesis and is equal to the difference between total leucine R_a and leucine oxidation at steady state. During non-steady state, NOLD is equal to the difference between the leucine disappearance and oxidation rate. Protein turnover estimates are expressed per kilogram LBM, unless specified otherwise.

Basal endogenous leucine R_a was calculated from steady-state plasma [¹³C]-leucine abundance that was achieved between t = -46 and -32 min (before somatostatin and GH infusion). Basal leucine oxidation was calculated from [¹³C]CO₂ abundance in the expired air, CO₂ production rate, and plasma [¹³C]-KICA (21), which best represents the immediate intracellular precursor for irreversible decarboxylation of leucine (22). The correction factor for the incomplete recovery of [¹³C]CO₂ was assumed to be 0.8 in all subjects.

Because metabolic non-steady state existed between t = 120 and 420 min, Steele’s equations for non-steady state conditions adjusted for stable isotopes (23, 24) were used as previously described (25, 26) to calculate the response of leucine R_a and leucine disappearance to GH vs. placebo injection. The effective volume of distribution of leucine was assumed to be the total amount of body fluid as measured by dual-energy x-ray absorptiometry. To compensate for nonuniform mixing, the correction factor p of the non-steady state equations was assumed to be 0.25 (26). Because oxidation calculations do not depend on the existence of a steady state (26), leucine oxidation rates between t = 120 and 420 min were calculated as described above.

Differences in response of NOLD, leucine oxidation, and endogenous leucine R_a to GH vs. placebo injection were calculated by subtracting the placebo response from the GH response (-score) for each time point. Maximum -scores were determined in NW and OB women. Whole-body GH responsiveness, defined as the maximum change of each parameter per unit of GH, was calculated by dividing the maximum -score by the GH AUC.

Statistics

Basal values, maximum -scores and responsiveness estimates were compared between the two groups using ANOVA with group (NW vs. OB) as factor. Comparisons of -scores between NW and OB subjects, with data obtained at subsequent time points, were made by an ANOVA for repeated measurements with time and group as factors, the interaction time per group, and prevalues as covariate, using SAS software for Windows V8.1 (SAS Institute, Inc., Cary, NC). Because -scores are used, significance on the factor time indicates a difference between GH and placebo treatment. A significant group effect indicates a different level of response of NW and OB subjects. A significant interaction indicates a different reaction to GH treatment for the obese, compared with the normal weights. No post hoc tests were performed. Data are presented as mean ± SD. The 95% confidence intervals of the difference between means are given if indicated. A P value of less than 0.05 was considered statistically significant.

Results

Hormone concentrations

Plasma GH peak concentration in response to GH infusion was similar in NW and OB subjects (49.8 ± 10.4 vs. 45.1 ± 5.6 mU/liter, Fig. 1). Because somatostatin suppressed GH secretion completely in both NW and OB subjects, GH AUCs were also similar in both groups (NW: 52.4 ± 11.1 mU/liter·h, OB: 48.0 ± 6.5 mU/liter·h). Basal insulin concentration was significantly lower in NW, compared with OB, women (6.1 ± 2.5 vs. 13.5 ± 4.4 mU/liter, P = 0.005), whereas basal glucagon level was similar in both groups (NW: 120 ± 41 ng/liter, OB: 136 ± 35 ng/liter, P > 0.05). In all studies, somatostatin infusion decreased insulin level to values below the assay detection limit. Plasma glucagon concentration was slightly decreased to a similar extent in both groups (Table 2).

View larger version (18K): [in this window] [in a new window]   Figure 1. Plasma GH ± SD concentrations in NW (squares) and OB (circles) subjects. and •, Profiles after GH infusion; and , profiles after placebo administration.

OB subjects had significantly higher fasting endogenous leucine R_a, leucine oxidation, and NOLD, compared with NW subjects (Fig. 2). However, when expressed per kilogram body weight (BW), endogenous leucine R_a, and leucine oxidation were similar in OB and NW women (60 ± 3 vs. 61 ± 6 µmol/kg·h and 14 ± 2 vs.14 ± 1 µmol/kg·h, respectively), whereas NOLD tended to be lower in OB, compared with NW, subjects (50 ± 3 vs. 55 ± 6 µmol/kg·h, P = 0.078).

View larger version (37K): [in this window] [in a new window]   Figure 2. Mean ± SD overnight fasting endogenous leucine Ra, leucine oxidation, and NOLD in µmol/kg LBM per hour in NW () and OB () women. *, P < 0.05 compared with NW women.

View larger version (50K): [in this window] [in a new window]   Figure 3. Mean ± SD nonoxidative leucine disposal (A) and leucine oxidation (B) per kilogram lean body mass after placebo () and after GH infusion () in NW and OB subjects. Somatostatin was infused during both conditions from t = -30 to 420 min, and GH or placebo from t = 0 to 60 min. GH, as opposed to placebo, significantly increased nonoxidative leucine disposal (P < 0.0001) and decreased leucine oxidation (P = 0.0051). These effects were not different between both groups (time/group effect, P > 0.05).

Additional effects

Basal IGF-I and IGFBP-3 levels in NW subjects were similar to those in OB subjects (IGF-I: 15.8 ± 6.4 vs. 12.3 ± 5.8 nmol/liter; IGFBP-3: 101.8 ± 28.8 vs. 108.8 ± 17.2 nmol/liter). Although GH, compared with placebo, did not affect IGF-I levels in general, -scores slightly increased in NW and decreased in OB women (time effect: P > 0.05, time/group effect: P = 0.016).

Basal glucose level was equal in NW and OB subjects (4.7 ± 0.3 vs. 4.8 ± 0.2 mmol/liter). After an initial decrease, somatostatin infusion increased plasma glucose concentration in both NW and OB women. The increase in glucose was significant and similar in both groups in response to GH, compared with placebo, administration (time effect, P < 0.0001; time/group effect, P > 0.05). Overnight fasting FFA concentration was similar in NW and OB women (451 ± 174 vs. 429 ± 149 µmol/liter). Plasma FFA levels rapidly increased during somatostatin infusion. GH administration induced a significant further rise of FFA levels that was similar in both groups (time effect, P = 0.0033; time/group effect, P > 0.05, Fig. 4).

View larger version (25K): [in this window] [in a new window]   Figure 4. Plasma FFA ± SD concentration in NW (squares) and OB (circles) subjects. and •, Profiles after GH infusion; and , profiles after placebo administration. GH significantly enhanced the somatostatin-induced rise of FFA levels that was similar in both groups (time effect, P = 0.0033; time/group effect, P > 0.05).

To unravel the biological significance of reduced GH availability in abdominal obesity, we investigated in vivo whole-body tissue responsiveness to the anabolic action of GH. In particular, we measured leucine kinetics in response to GH and placebo administration in abdominally OB and NW women. The results clearly demonstrated the acute protein-conserving effect of GH. In both NW and abdominally OB subjects, leucine oxidation decreased, whereas nonoxidative leucine disposal (a measure of protein synthesis) increased in response to GH, compared with placebo infusion. The maximum change of these parameters per unit of GH was similar in both groups of subjects. Therefore, our data suggest that abdominally OB and NW women are equally responsive to the anabolic action of GH and argue against the concept that abdominally OB humans are more sensitive to GH action, at least with respect to protein metabolism.

Although many human tissues, including the liver and skeletal muscle, express GH receptors (27), it is not clear whether GH directly affects protein metabolism. Administration of GH invariably leads to promotion of lipolysis, stimulation of IGF-I production, and hyperinsulinemia. All of these secondary hormonal and metabolic changes promote protein accretion (28, 29, 30, 31). Because we completely suppressed insulin secretion by concomitant somatostatin administration, insulin can be excluded as a mediator of GH action in the present study. Although plasma IGF-I levels did not change in our subjects during the observation period, GH administration promotes local production of IGF-I in addition to its effects on systemic concentrations (32, 33). Because IGF-I appears to be a critical mediator of GH anabolic action (34), it is conceivable that locally produced hormone was involved in the effects of GH that we observed. Finally, GH administration increased lipolysis in the present study because it significantly enhanced the rise of plasma FFA levels, compared with placebo conditions (Fig 4). Therefore, the protein-sparing effect of GH that we observed may have been induced by GH directly, locally produced IGF-I, and/or an increased availability of FFAs.

In addition to the anabolic effect of GH, we observed a significant increase in leucine R_a (as a measure of proteolysis) in response to GH in both groups. This is in apparent contrast to the well-established idea that GH affects protein anabolism by selectively increasing protein synthesis without altering protein breakdown (3, 10, 35). Our experimental condition, in which GH was infused and plasma insulin levels were suppressed, may explain this observation. Insulin primarily affects protein metabolism by inhibition of protein breakdown (36). In the absence of a concomitant somatostatin infusion, GH induces hyperinsulinemia, which may prevent any GH-induced increase in proteolysis. Only one previous study examined the acute effect of GH on leucine kinetics during somatostatin infusion (1). However, in that study insulin was replaced by continuous infusion, and as a result its plasma levels increased 2-fold above basal, which may have counteracted the effect of GH per se on protein breakdown. Thus, the present data suggest that GH promotes whole-body protein turnover with its anabolic action prevailing in this context. Specifically, it stimulates whole-body protein synthesis with a concomitant increase in proteolysis and reduces protein oxidation, resulting in a net anabolic effect. In physiological conditions, insulin may be instrumental in counteracting the intrinsic proteolytic impact of a plasma GH pulse. Our inference of GH being a hormone that promotes protein turnover is in keeping with reports of reduced proteolysis in GH-deficient patients (37, 38).

It has been suggested that obesity is a GH hypersensitive state because plasma IGF-I and IGFBP-3 levels, both measures of hepatic GH bioactivity, are normal in OB individuals despite GH hyposecretion (14). Indeed, plasma IGF-I and IGFBP-3 concentrations were similar in our abdominally OB and NW women. In apparent contrast, our data on leucine kinetics suggest that whole-body tissue responsiveness to GH action with respect to protein metabolism is similar in abdominally OB and NW women. To reconcile these observations, it is conceivable that GH signal transduction is governed differentially in various GH sensitive tissues. Alternatively, hepatic IGF-I and IGFBP-3 production may be controlled by factors other than GH alone. For example, insulin stimulates hepatic IGF-I production (39), and therefore, obesity-associated hyperinsulinemia, instead of a GH hypersensitive state, might explain normal plasma IGF-I values in abdominal obesity.

Hyposomatotropism in the face of normal tissue responsiveness to GH action presumably translates diminished biological effect of GH to target tissues in abdominally OB women. By analogy of observations in GH-deficient adults (37, 38), hyposomatotropism thus potentially reduces proteolysis and protein synthesis and promotes protein oxidation in abdominal obesity. Indeed, leucine oxidation in basal conditions was increased in our abdominally OB women, compared with controls. However, in contrast, both protein degradation and protein synthesis were enhanced in OB women. These observations agree with other studies showing enhanced postabsorptive leucine turnover rates with increasing adiposity (40, 41, 42). Thus, although hyposomatotropism may explain the fact that protein oxidation is increased in OB humans, it cannot account for simultaneous elevations of protein breakdown and synthesis that are consistently observed in OB individuals. However, alternative mechanisms may overrule the impact of hyposomatotropism in this context. Firstly, hyperinsulinemia and insulin resistance may play a role. Insulin is the primary antiproteolytic hormone and obesity is accompanied by defects in insulin-mediated leucine metabolism (43, 44). Also, visceral obesity is associated with higher free concentrations of anabolic hormones, such as testosterone (45, 46, 47), which may enhance protein turnover rates in OB, compared with NW, humans (48). Thus, other endocrine features of obesity are likely to be also involved in the pathogenesis of anomalous protein metabolism in abdominally obese individuals.

It is important to recognize that our study design did not include multiple doses of GH. Thus, we cannot rule out the possibility that GH affects protein metabolism differentially in OB and NW humans at distinct plasma levels. Also, our measure of GH action cannot be interpreted as sensitivity (hence, the term tissue responsiveness).

Although indices of protein metabolism per kilogram LBM differed between OB and NW women, these measures did not differ if expressed per kilogram BW. Adipose tissue is a quantitatively important site of proteolysis (49, 50). Therefore, fat mass should somehow be accounted for in the description of protein kinetics. However, weighing fat and lean mass equally in this context will certainly overestimate the role of fat mass with regard to protein metabolism because only 5–10% of systemic leucine release is derived from adipose tissue (50). Thus, it may be important to develop an algorithm to adequately weigh the contributions of lean and adipose tissue to the various measures of protein metabolism for future studies in OB humans.

In conclusion, the present study indicates that GH acutely inhibits whole-body protein oxidation and simultaneously stimulates whole-body protein synthesis and proteolysis, resulting in a net anabolic effect. Moreover, tissue responsiveness to this anabolic effect of GH is similar in abdominally OB and NW women. Hyposomatotropism in the face of normal whole-body tissue responsiveness to GH action will translate less biological effect of GH to target tissues in abdominally OB women. Hence, we infer that hyposomatotropism enhances protein oxidation and blunts protein turnover in abdominal obesity. However, hyposomatotropism alone cannot account for all anomalous features of protein metabolism in OB humans.

Acknowledgments

The excellent technical assistance of Eric Gribnau and Hermi Kingma is greatly appreciated. We thank Pharmacia Corp. (Peapack, NJ) for its generous gift of rhGH and Ferring Pharmaceuticals Ltd. BV (Hoofddorp, The Netherlands) for its gift of somatostatin for this study.

Footnotes

Abbreviations: AUC, Area under the curve; BW, body weight; FFA, free fatty acid; GC/MS, gas chromatograph/quadruple-mass spectrometer; IGFBP, IGF-binding protein; KICA, ketoisocaproate acid; LBM, lean body mass; NOLD, nonoxidative leucine disposal; NW, normal weight; OB, obese; R_a, endogenous leucine appearance; rhGH, recombinant human GH.

Received June 28, 2002.

Accepted September 4, 2002.

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