Kinetic impairment of nitrogen and muscle glutamine metabolisms in old glucocorticoid-treated rats

Regine Minet-Quinard1,2, Christophe Moinard1, Françoise Villie1, Stephane Walrand1, Marie-Paule Vasson1, Jean Chopineau2, and Luc Cynober1

1 Department of Biochemistry, Molecular Biology, and Nutrition and 2 Clinical Pharmacy, Human Nutrition Research Center, Pharmacy School, 63 000 Clermont-Ferrand, France


    ABSTRACT
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Abstract
Introduction
MATERIALS AND METHODS
RESULTS
DISCUSSION
References

Aged rats are more sensitive to injury, possibly through an impairment of nitrogen and glutamine (Gln) metabolisms mediated by glucocorticoids. We studied the metabolic kinetic response of adult and old rats during glucocorticoid treatment. The male Sprague-Dawley rats were 24 or 3 mo old. Both adult and old rats were divided into 7 groups. Groups labeled G3, G5, and G7 received, by intraperitoneal injection, 1.50 mg/kg of dexamethasone (Dex) for 3, 5, and 7 days, respectively. Groups labeled G3PF, G5PF, and G7PF were pair fed to the G3, G5, or G7 groups and were injected with an isovolumic solution of NaCl. One control group comprised healthy rats fed ad libitum. The response to aggression induced specifically by Dex (i.e., allowing for variations in pair-fed controls) appeared later in the aged rats (decrease in nitrogen balance from day 1 in adults but only from day 4 in old rats). The adult rats rapidly adapted to Dex treatment, whereas the catabolic state worsened until the end of treatment in the old rats. Gln homeostasis was not maintained in the aged rats; despite an early increase in muscular Gln synthetase activity, the Gln pool was depleted. These results suggest a kinetic impairment of both nitrogen and muscle Gln metabolisms in response to Dex with aging.

aging; dexamethasone; duration of treatment


    INTRODUCTION
Top
Abstract
Introduction
MATERIALS AND METHODS
RESULTS
DISCUSSION
References

ELDERLY PEOPLE have a reduced capacity to recover from a variety of stresses. Jeevanandam et al. (12) reported that, after injury, elderly patients recover their preinjury nutritional status more slowly and incompletely. Dardevet et al. (3) showed that, after dexamethasone (Dex) withdrawal, adult rats rapidly restored their muscle protein mass within 3 days, whereas, in old animals, this recovery was delayed to 7 days. In the same model of glucocorticoid-treated old rats, Savary et al. (31) described an impairment of muscle protein synthesis during recovery. However, although this process of recovery has been largely investigated, little is known about the metabolic response of elderly subjects during stress. This point deserves attention because morbidity among elderly subjects is increased during catabolic situations.

Glutamine (Gln) is the most abundant free amino acid (AA) in the human body and has several important metabolic roles, e.g., it is a fuel for rapidly dividing cells (33). It is the main carrier of alpha -amino nitrogen between tissues of the body and is therefore important in interorgan traffic and acid-base homeostasis (35). Skeletal muscle is a major site for Gln synthesis in the body via glutamine synthetase (GS) and serves as a Gln store (29). During stress, the muscle Gln pool is markedly decreased (2). The physiological relevance of this impairment may be great, since Gln may promote protein synthesis (13) and inhibit protein catabolism in muscle (32). These alterations of muscle Gln metabolism are probably heightened during aging, as elderly subjects already have decreased muscle protein stores (22). In addition, hypoglutaminemia is correlated with high mortality (25). However, although impairment of muscle Gln is largely described in injured adults, to the best of our knowledge only one study has been performed during aging (20).

Glucocorticoids such as Dex are important in three ways. First, they are a major mediator of catabolic response. They are known to induce an augmentation of net protein catabolism (negative nitrogen balance) and more specifically an increase in muscle myofibrillar protein catabolism (11, 16). This muscle Dex-mediated catabolic effect is integrated in the systemic response to stress that supplies AAs for gluconeogenesis and for the synthesis of inflammatory proteins by the liver. Consequently, these steroids are often used in models of stress. Second, glucocorticoids are implicated in the alterations of the muscle Gln pool by stimulating both GS activity (19) and muscle Gln efflux (17). Third, these hormones are specifically important during aging, since basal plasma concentration of corticosterone, the physiological glucocorticoid in rodents, may be higher in old rats than in adults (30). In addition, there are several other relevant lesions (an aging-related adenomatous hyperplasia and a cortical metaplasia of the periadrenal tissues). These lesions are generally regarded as "silent" because the elevation of hormones is not sufficient to cause overt Cushing's syndrome. Studies in humans support these observations (34).

To understand the greater sensitivity of elderly persons to catabolic conditions, we attempted to describe kinetic impairment of nitrogen metabolism during prolonged stress in elderly rats by means of a longitudinal study in adult and old glucocorticoid-treated rats. We extended our study to muscle Gln metabolism because of its relevance to protein metabolism. Three types of muscle have been studied that are recognized to have different sensitivities to Dex: extensor digitorum longus (EDL, a type II fiber-rich muscle), gastrocnemius (a mixed muscle composed of type I and type II fibers), and soleus (a type I fiber-rich muscle). Muscle atrophy in glucocorticoid-treated adult rats is generally observed for both EDL and gastrocnemius (7, 15) through a decreased concentration of RNA and a decreased protein synthesis (28). Conversely, Dex treatment has little effect on the mass of soleus, and protein synthesis in this muscle is not affected. The noteworthy resistance of soleus to treatment could be due to its antigravity function (31) or to the qualitative difference in its glucocorticoid receptors despite their greater number (4, 16).


    MATERIALS AND METHODS
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Abstract
Introduction
MATERIALS AND METHODS
RESULTS
DISCUSSION
References

Animals and experimental study. One hundred twenty-three male Sprague-Dawley rats, supplied by Iffa Credo (L'Arbresle, France), were used. They were 24 (n = 63) or 3 mo (n = 60) old. After their arrival in our animal facilities, the rats were maintained on a standard diet (17% protein, 3% fat, 59% carbohydrate, and 21% water, fibers, vitamins, and minerals) and water ad libitum. They were kept in a controlled environment (constant temperature 24°C and a 12:12-h light-dark cycle). After 10 days of acclimatization in standard cages and 5 days in individual metabolic cages, the rats were divided into 8 groups of 12 as follows: groups labeled G3, G5, G7, and G9 received, by intraperitoneal injection, 1.50 mg/kg of Dex for 3, 5, 7, and 9 days, respectively; groups labeled G3PF, G5PF, G7PF, and G9PF were pair fed with the G3, G5, G7, and G9 groups and were injected with an isovolumic solution of NaCl. There was also one healthy group of 27 rats (Al group). Each group comprised the same number of old (n = 6) and adult (n = 6) rats except the Al group, which comprised 15 old and 12 adult rats. The G3, G5, G7, and G9 groups were given Dex, as this synthetic steroid is more often used than corticosterone because of its greater affinity for glucocorticoid receptors (4). The dose of 1.50 mg/kg was chosen with reference to the literature and is known to induce a severe catabolic state in adults (19). Because treatment by glucocorticoids induces anorexia (2), the study in parallel of pair-fed rats is mandatory. Thus the G3PF, G5PF, G7PF, and G9PF control groups were pair fed with the G3, G5, G7, and G9 groups, respectively. They were managed as Dex-treated animals but were injected with an isovolumic solution of 0.9% NaCl instead of Dex. The Al group received no treatment and was fed ad libitum for the duration of the study. For statistical analysis reasons (see Statistical analysis) healthy rats were randomly assigned to the following two subgroups: G0 (n = 15) and G0PF (n = 12).

Body weight, urine volume, and food intake were recorded daily from day 0 to day 9. Rats were starved the evening before death. They were anesthetized with ether and killed by beheading 30 h after the last Dex or NaCl injections. Animal care and experimentation complied with the rules of our institution, and one of us (Cynober) is authorized by the French Ministry of Agriculture and Forestry to use animal models of stress.

Urinary parameters. Urine was collected in a container on a preservative (AMUKIN; Gifrer Barbezat, Decines, France). Nitrogen was quantified by chemoluminescence using an Antek 7000 apparatus (Antek, Houston, TX; see Ref. 8), and nitrogen balance was calculated as the difference between nitrogen intake and nitrogen urinary output. Limit of sensitivity and intra- and interassay coefficients of variation were 20 mg/l and 1 and 5%, respectively.

Muscle and plasma parameters. Muscles of the hindlimbs (soleus, EDL, gastrocnemius) were rapidly removed, weighed, and frozen in liquid nitrogen. Right muscles were used for determination of free AA concentrations. They were ground and deproteinized with 10% trichloroacetic acid-EDTA (10 mM). The supernatant was stored at -80°C until analysis of AAs by ion exchange chromatography with an AA autoanalyzer (model 6300; Beckman, Palo Alto, CA). For Gln, the limit of sensitivity and intra- and interassay coefficients of variation were 5 µmol/l and 0.9 and 4.2%, respectively. The results of our participation in the European Quality Control Scheme (ERNDIM) indicate the accuracy of our AA determinations, in particular for Gln. Left muscles were used to measure GS activities. Muscles were homogenized in 10 vol of 50 mM imidazole, and GS was measured as described by Minet et al. (21). The limit of sensitivity and intra- and interassay coefficients of variation were 2.7 nmol/min glutamyl hydroxamate formed and 1.5 and 2.6%, respectively.

Blood was withdrawn in heparinized tubes to measure concentrations of free AAs. Blood was centrifuged (4°C, 4,500 g, 10 min), and the plasma was deproteinized with sulfosalicylic acid (50 mg/ml). The supernatant was stored at -80°C for no longer than 15 days before the plasma AA analysis was performed as described for muscles.

Statistical analysis. Data are presented as means ± SE. For parameters measured daily in live animals, an analysis of variance (ANOVA) on repeated measurements and with one factor (effect of treatment) was performed in either adult or old rats for the G9 group, studied up to day 7 (most elderly rats were dead on day 8), with estimation of missing values. For parameters measured after death, a two-way ANOVA was performed in either adult or old rats with treatment and duration of treatment as main factors. The effects of factors (treatment or duration of treatment) were analyzed by the Newman-Keüls test (see RESULTS and Figs. 1-6 for additional information). Because the two-way ANOVA requires two groups of rats at each time (1 treated group and 1 control group), healthy ad libitum-fed rats (i.e., Al group) were randomly assigned to two subgroups: G0 and a so-called G0PF (n = 15, including 3 rats who died during the experiment, and n = 12, respectively). Therefore G3, G5, and G7 groups were compared with G0, and G3PF, G5PF, and G7PF groups were compared with G0PF. There was no difference for any parameter between G0 and G0PF. The PCSM software (Deltasoft, Grenoble, France) was used. Values of P < 0.05 were considered significant.


    RESULTS
Top
Abstract
Introduction
MATERIALS AND METHODS
RESULTS
DISCUSSION
References

Characteristics of animals. All of the adult rats survived the glucocorticoid treatment. In aged rats, the mortality rate was 14% (1 rat in G7, 5 rats in G9, and 3 rats in G0). As most old rats in the G9 group were dead before the 9th day of treatment, i.e., at day 8 (5/6), the results obtained at this time point were ignored for analysis of parameters measured at the time of killing but were considered for parameters measured daily (i.e., food intake, body weight, and nitrogen balance).

Dex induced anorexia from day 2 in adult and old rats. Food intake improved after day 3 in adult rats but worsened in aged animals (Fig. 1).


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Fig. 1.   Food intake of adult and old dexamethasone (Dex)-treated rats. Adult (filled bars) and old (open bars) experimental rats received 1.50 mg/kg of Dex for 7 days by ip injection. Adult and old control animals were pair fed during the same period (data not shown). Values are means ± SE for n = 6 rats. D0-D7, days 0-7, respectively. Analysis of variance on repeated measurements was performed in either adult or old rats to discriminate among effects of dexamethasone (D), duration of treatment (T), and their interaction (D · T). A significant effect of T (P < 0.0001) was observed for both ages considered. Comparison of means was carried out with the Newman-Keuls test: ** P < 0.01 vs. day 0.

Treatment by glucocorticoid and pair feeding induced a decrease in whole body weight from day 1. Body weight loss induced specifically by Dex (P < 0.05 between Dex-treated and pair-fed animals) occurred from day 2 in adult rats (Fig. 2A) and from day 1 in old animals (Fig. 2B).


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Fig. 2.   Variations of total body weight of adult (A) and old (B) rats. Adult (filled bars) and old (open bars) experimental rats received 1.50 mg/kg of Dex for 7 days by ip injection. Adult (horizontal-striped bars) and old (vertical-striped bars) control animals were pair fed during the same period. Values are means ± SE for n = 6 rats. Analysis of variance on repeated measurements was performed in either adult or old rats to discriminate among effects of dexamethasone (D), duration of treatment (T), and their interaction (D · T). A significant effect of D (P < 0.03), T (P < 0.0001), and D · T (P < 0.0001) was observed in each population. Comparison of means was carried out with the Newman-Keuls test: ** P < 0.01 vs. day 0, dagger dagger P < 0.01 vs. pair-fed control.

Nitrogen balance. Nitrogen balance decreased from day 1 in adult and old treated rats (Fig. 3). This decrease was due to the specific effect of Dex from day 1 in adult rats but only from day 4 in old rats (P < 0.01 between Dex-treated and pair-fed rats, data not shown). Nitrogen balance remained negative until the end of the experimentation in aged animals. In contrast, nitrogen balance increased at day 6, becoming positive at day 7, in adult rats.


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Fig. 3.   Nitrogen balance of adult and old Dex-treated rats. Adult (filled bars) and old (open bars) experimental rats received 1.50 mg/kg of Dex for 7 days by ip injection. Adult and old control animals were pair fed during the same period (data not shown). Values are means ± SE for n = 6 rats. Analysis of variance on repeated measurements was performed in either adult or old rats to discriminate among effects of dexamethasone (D), duration of treatment (T), and their interaction (D · T). A significant effect of D (P < 0.02) and T (P < 0.0001) was observed for both ages considered. A significant effect of D · T (P < 0.0001) was observed for adult rats. Comparison of means was carried out with the Newman-Keuls test: ** P < 0.01 vs. day 0, dagger dagger P < 0.01 vs. pair-fed control.

Muscle mass. An atrophy of EDL and gastrocnemius, due to the specific catabolic effect of Dex, was observed, respectively, from day 5 and day 3 in adult treated rats (Table 1). No similar change was noticed in muscles from aged rats. Soleus mass was not modified during experimentation, whatever the age of the animals.

                              
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Table 1.   Effect of Dex treatment on muscle weights in young adult and old rats

Plasma Gln concentrations. Plasma Gln concentrations were decreased from day 5 only in old rats because of the specific catabolic effect of Dex [for 5 days of treatment: experimental, 580 ± 38 µmol/l vs. control pair fed, 959 ± 28 µmol/l, P < 0.01; experimental, 763 ± 112 µmol/l vs. control pair fed, 924 ± 151 µmol/l (nonsignificant) in old and adult rats, respectively].

Muscle Gln metabolism. In adult rats, Gln concentrations decreased in both EDL (Fig. 4A) and gastrocnemius (Fig. 5A) from 3 days of treatment by glucocorticoids. In old animals, muscle Gln depletion occurred later: from day 5 and day 7, respectively, in EDL (Fig. 4A) and gastrocnemius (Fig. 5A).


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Fig. 4.   Glutamine concentrations (A) and glutamine synthetase activities (B) in extensor digitorum longus (EDL) according to glucocorticoid impregnation time. Glutamine concentrations and glutamine synthetase activities were measured in EDL of adult and old experimental rats of G3, G5, and G7 groups, i.e., treated for 3 (D3), 5 (D5), or 7 (D7) days by Dex (filled bars). Glutamine concentrations and glutamine synthetase activities were measured in EDL of adult and old control rats (open bars) treated for 3 (D3), 5 (D5), or 7 (D7) days by ip injection of NaCl solution and pair fed, respectively, with experimental groups treated during 3, 5, or 7 days by Dex. Glutamine concentrations and glutamine synthetase activities measured at day 0 in 2 groups of healthy rats (stippled bars) can be considered as basal values of rats. For each time point and each group, n = 6 rats. Two-way analysis of variance was performed in either adult or old rats to discriminate among effects of dexamethasone (D), duration of treatment (T), and their interaction (D · T). Significant effects of D, T, and D · T were observed for glutamine concentration (D: P < 0.0001, T: P < 0.0001, and D · T: P < 0.0001 in adult rats; D: P < 0.0001, T: P = 0.004, and D · T: P < 0.0001 in old rats) and glutamine synthetase (D: P = 0.001, T: P < 0.001, and D · T: P < 0.001 in adult rats; D: P < 0.0001, T: P < 0.0001, and D · T: P = 0.004 in old rats). Comparison of means was carried out with the Newman-Keuls test: ** P < 0.01 vs. day 0, dagger dagger P < 0.01 vs. pair-fed control.


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Fig. 5.   Glutamine concentrations (A) and glutamine synthetase activities (B) in gastrocnemius according to glucocorticoid impregnation time. Glutamine concentrations and glutamine synthetase activities were measured in gastrocnemius of adult and old experimental rats of G3, G5, and G7 groups (filled bars). Glutamine concentrations and glutamine synthetase activities were measured in gastrocnemius of adult and old control rats (open bars) treated for 3 (D3), 5 (D5), or 7 (D7) days by ip injection of NaCl solution and pair fed, respectively, to experimental groups treated for 3, 5, or 7 days by Dex. Glutamine concentrations and glutamine synthetase activities measured at day 0 in two groups of healthy rats (stippled bars) can be considered as basal values of rats. For each time point and each group, n = 6 rats. Two-way analysis of variance was performed in either adult or old rats to discriminate among effects of dexamethasone (D), duration of treatment (T), and their interaction (D · T). Significant effects were observed for glutamine concentration (D: P < 0.0001, T: P < 0.0001, D · T: P < 0.0001 in adult rats; D: P < 0.0001 in old rats) and glutamine synthetase (D: P < 0.0001, T: P < 0.0001, and D · T: P < 0.002 in adult rats; D: P < 0.0001, T: P < 0.0001, and D · T: P < 0.0001 in old rats). Comparison of means was carried out with the Newman-Keuls test: * P < 0.05 vs. day 0, ** P < 0.01 vs. day 0, dagger  P < 0.05 vs. pair-fed control, dagger dagger P < 0.01 vs. pair-fed control.

GS activity increased from day 5 and day 3 in EDL (Fig. 4B) and gastrocnemius (Fig. 5B), respectively, in adult rats. In old animals, this rise in GS activity occurred earlier: from day 3 in both EDL (Fig. 4B) and gastrocnemius (Fig. 5B).

Whatever the age of the rats, these muscle alterations of both the Gln pool and GS activity were due to the specific catabolic effect of Dex (P < 0.01 between treated and pair-fed rats; Figs. 4 and 5). In soleus, Dex induced no modification of Gln concentrations but an increase in GS activities at day 7 in both adult and old rats (data not shown).

Relationship between muscle Gln concentrations and muscle weights. A correlation was established between Gln concentration in EDL and its weight (y = 45.96x - 3.39; r2 = 0.48, P < 0.0001 in adults). Similar results were found for gastrocnemius of adult rats (y = 2.73x - 1.67; r2 =0.68, P < 0.0001; Fig. 6A). In contrast, in old rats, weight of muscles and Gln concentrations were not correlated (Fig. 6B).


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Fig. 6.   Correlation between gastrocnemius weight and muscle glutamine concentrations in adult (A) and old (B) rats.


    DISCUSSION
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Abstract
Introduction
MATERIALS AND METHODS
RESULTS
DISCUSSION
References

Surprisingly, none of the three types of muscles were atrophied in the old rats at any time during the glucocorticoid treatment. The hypothesis of an involution of type II fibers into type I fibers with aging (10) could explain this result. However, although such an involution has been shown in a mixed muscle, such as plantaris (10), none has been found in EDL (5).

The body weight decrease observed from day 1 in treated and pair-fed rats despite a nonsignificant decrease in food intake may result from loss of water, as indicated by the increase in their diuresis (data not shown), and from the stress of injection in experimental and control pair-fed rats, respectively.

Like weight loss, nitrogen balance decrease observed in old and adult treated rats was also more marked than in their pair-fed controls. However, this decrease in nitrogen balance was only transient in adults, unlike in old treated rats for which nitrogen balance worsened with the duration of treatment. This different kinetic response according to the age of rats may be due to the severity of anorexia, since decrease in food intake was longer in old rats than in adults. Taken as a whole, these data suggest that the metabolic response to treatment by glucocorticoids occurs in two phases: an early catabolic phase in adult rats, which is blunted in old rats, and a later phase where the response worsens only in aged rats. It is noteworthy that the first phase in old rats lags 1 or 2 days behind that of adults. These two phases have already been described in adult rats by Kayali et al. (14) and Odedra et al. (23). These authors demonstrated an early phase lasting at least 4 days, during which proteolysis increased (14) while the overall rate of protein synthesis in muscles fell gradually (23), followed by a later phase during which proteolysis decreased (14) while protein synthesis was maintained at the lower rate (23). In old rats, a decrease in protein synthesis has been described (3) after 6 days of treatment by glucocorticoids. All of these data emphasize the importance of the duration of treatment for the metabolic response. However, other works (6, 26) have focused only on the first and last days of treatment. Consequently, they failed to detect variations in a number of parameters under study. Thus one study (26) concluded wrongly that Dex was not responsible for anorexia, since food intake had been measured only at the beginning and end of treatment, i.e., when it had reverted to its basal value.

Like whole body nitrogen metabolism, Gln metabolism is not altered in the same way and for the same duration in adult and old rats.

In agreement with Parry-Billings et al. (26), the Gln plasma pool was not depleted in adult rats during the treatment, whereas it was lowered from day 5 in old rats. Consequently, either aged rats have greater requirements than adult rats and endogenous production does not supply them, or endogenous production is decreased and is not sufficient to supply the organism. The decrease in endogenous production can result either from a decrease in Gln muscle efflux or a decrease in de novo Gln synthesis by GS. This last hypothesis can be discarded since, in agreement with data from Meynial-Denis et al. (20), muscle GS activities were increased whatever the age of the rats. Consequently, Gln plasma depletion in old treated rats may result from a decrease in its muscle efflux, as previously shown by Parry-Billings et al. (27) in soleus from healthy aged rats. Irrespective of this mechanism, hypoglutaminemia seems important per se since Parry-Billings et al. (25) correlated mortality with hypoglutaminemia in burned patients. Interestingly, the mortality rate was high only in the old rat group, which suffered from marked hypoglutaminemia.

The sensitivity of muscle Gln to the duration of treatment by Dex is also different according to the age of animals. Dex induced an increase in GS activities in EDL and gastrocnemius of treated rats. As previously described by Meynial-Denis et al. (20), this augmentation was the same whatever the age of the animals but occurred earlier in old rats than in adult rats. Consequently, Gln depletion occurred later in aged animals than in adult animals and, at day 3, the muscle Gln pool was higher in the older rodents. In previous unpublished work, we showed that Dex plasma concentration kinetics were the same in adult and old rats after intraperitoneal injection of 1.50 mg/kg of this glucocorticoid. Consequently, Dex plasma concentrations cannot account for this different time response of GS activities according to the age of animals. We can speculate that GS activities in old rats are more sensitive to Dex than in adult rats. Therefore, in a recent study, we showed that intraperitoneal injection of 0.75 mg/kg of Dex for 5 days was sufficient to maximally enhance GS activity in EDL from old rats, whereas a dose of 2.50 mg/kg of Dex was required in adult rats. Induction of GS activity appears to occur at the transcriptional level, since there is an overall increase in the level of mRNA encoding the enzyme in both adult (1, 6, 19) and old rats (20). In soleus, an increase in GS activity occurred from day 7, in agreement with results of Max et al. (19) and Meynial-Denis et al. (20).

We have established a correlation between EDL or gastrocnemius weights and EDL or gastrocnemius Gln contents, respectively, only in adult rats. These results challenge the existence of an association between Gln and muscular nutritional status, as suggested by Mac Lennan (18). We can hypothesize that Gln muscle concentration and muscle weight may be the covariables of a third factor, itself the regulator of muscular nutritional status. With aging, this hypothetical regulator would rely neither on Gln concentration nor muscle weight. It has been suggested (9, 13) that insulin, thyroid hormones, or cell hydration could be the third factor. Finally, Olde Damink et al. (24) demonstrated that changes in Gln turnover and/or Gln supplementation were not related to whole body protein turnover by measuring plasma and muscle Gln concentration and whole body protein turnover in rats treated by methionine sulfoximine, an inhibitor of GS activity. Clearly, further work is needed to characterize both the exact nature of the association between muscle Gln content and muscular nutritional status and the factors that influence the concentrations of Gln in muscle, especially during aging.

In conclusion, this is the first study on the metabolic kinetic response of aged rats during treatment by glucocorticoids. This longitudinal study shows that the metabolic response of old rats to Dex is divided into two phases: an early phase where the response to Dex is blunted, followed by a later phase characterized by a worsening of nutritional status. Gln homeostasis was not maintained in the aged rats; despite an early increase in muscular GS activity, the Gln pool was depleted. Finally, it seems unlikely that nitrogen and muscle Gln metabolisms are causally related.


    ACKNOWLEDGEMENTS

We are indebted to P. Davot in our department and to P. Rousset (B. Beaufrère in the Laboratory of Human Nutrition) for expert technical assistance and to Dr. Meynial-Denis (INRA) for enthusiastic discussions throughout the study and during the drafting of the manuscript. We are also grateful to B. Normand (Statistics Department, Medicine School) for advice on statistical analysis.


    FOOTNOTES

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.

Address for correspondence and reprint requests: R. Minet-Quinard, Laboratoire de Biochimie, Biologie Moléculaire et Nutrition, Faculté de Pharmacie, 28 place Henri-Dunant BP 38, 63001 Clermont-Ferrand cedex 1, France (E-mail: M.-P.Vasson{at}u-clermont1.fr).

Received 30 March 1998; accepted in final form 11 November 1998.


    REFERENCES
Top
Abstract
Introduction
MATERIALS AND METHODS
RESULTS
DISCUSSION
References

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Am J Physiol Endocrinol Metab 276(3):E558-E564
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