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Institut National de la Recherche Agronomique (INRA), Unité Mixte de Recherche sur le Veau et le Porc (UMRVP), 35590 Saint-Gilles, France
2To whom correspondence should be addressed. E-mail: louis.lefaucheur{at}rennes.inra.fr
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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-cardiac MyHC positive fibers in RH (P < 0.05). The LDH/CS ratio decreased dramatically (P < 0.001) after restriction, to a greater extent in LL than in RH. These changes denoted a more oxidative metabolism using fewer carbohydrates and more lipids in restricted pigs, as suggested by the increased activity of HAD (P < 0.001) and decreased respiratory quotient (P < 0.001).
KEY WORDS: • myosin heavy chain isoforms • metabolic enzymes • fiber type • undernutrition • swine
Skeletal muscle is essential for locomotion, postural maintenance, breathing and thermogenesis. In pigs, as in most mammals, ontogenesis of myofibers is a biphasic phenomenon that consists in the formation of a primary generation of muscle fibers between 35 and 55 d of gestation (dg)3, followed by a second generation between 55 and 90 dg (1
). The secondary fibers appear around each primary myotube, using them as a scaffold. Piglets can walk as soon as they are born (114 dg) and dramatic changes occur in skeletal muscle during the first postnatal wk, including a rapid rate of myofibrillar protein accretion (2
), changes in myosin heavy chain (MyHC) polymorphism, an increase in aerobic and glycolytic metabolisms and a hypertrophy of myofibers (3
–6
). At least eight isoforms of MyHC, each known to be encoded by a distinct gene, have been identified in mammalian striated muscle (7
). Thus, two "cardiac" genes, ß-cardiac-MyHC (or slow-twitch type I) and -cardiac-MyHC, are tandemly arranged on chromosome 14 in humans, whereas six other genes, embryonic, IIa, IIx (or IId), IIb, fetal and extraocular MyHC are located in this order in another cluster on chromosome 17. MyHC isoforms are temporally and spatially regulated and complex transitions between MyHC isoforms occur during the early postnatal period in pigs (4
–6
,8
). This includes a decrease in fetal MyHC, an increase in adult slow type I and fast type II MyHC and a transitory atypical expression of the -cardiac-MyHC. Only type I, IIa, IIx and IIb MyHC are still present in mature pig skeletal muscle (9
). At 100 kg body, they represent about 10, 7, 15 and 68% in longissimus muscle, and 68, 12, 20 and 0% in rhomboideus muscle.
It has been suggested that a defect in normal muscle development during the early postnatal or posthatch period can permanently alter later growth, as well as muscle contractile and metabolic maturation (10
–14
). In pigs, the natural high prolificity of the sow and its further improvement by selection and breeding techniques induce a high competition between newborn piglets for milk, as indicated by the high variation in body weight gain (15
). Therefore, risks of early undernutrition occur. Most studies on the influence of undernutrition on developing myofibers have been carried out after weaning in different species. They show that undernutrition decreases fiber size (16
) and affects more glycolytic fast-twitch type II than other types of myofibers (17
). In pigs, undernutrition between 3 and 7 wk of age has also been shown to induce an increase in the proportion of slow type I fibers in rhomboideus muscle, whereas longissimus muscle was not affected (18
). Whether immature muscle fibers exhibit the same differential response to undernutrition is not clear. Studies involving the effects of undernutrition during the early postnatal or posthatch period mostly used laboratory mice or chicks and generally report a delayed muscle maturation (13
,19
,20
). Very little information is available in larger mammals such as pigs. Therefore, the objective of the present study was to determine whether undernutrition during the first postnatal wk influenced the maturation of contractile and metabolic properties and the hypertrophy of myofibers according to their type in two different pig skeletal muscles.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The pigs were reared in compliance with national French regulations for human care and use of animals in research. Eighteen newborn crossbred Large White-Landrace X Piétrain piglets (Sus scrofa) from the national INRA herd were used. Parturition was induced with an intramuscular injection of a prostaglandin analog (Planate, Pitman-Moore, France) on d 113 of gestation to ensure farrowing on d 114. Immediately after birth, unsuckled newborn piglets were weighed and placed at thermoneutrality in individual small metal wire cages (40 x 40 x 50 cm) to minimize the effect of physical locomotory activity. The ambient temperature changed from 34 to 30°C from birth to slaughter at 7 d of age to take into account the decrease in the lower critical temperature with age (21
). Relative humidity was between 40 and 60%. Within the litter, three littermates of similar body weights (six replications) were randomly allocated to one of the three levels of nutrition (H = 300, M = 200 or L = 100 g/(kg body · d) of colostrum or milk). The H group was close to the ad libitum intake (22
), whereas the low nutritional level was chosen to allow a slight positive growth, thus avoiding pathologic changes associated with starvation. The colostrum and milk were pooled samples. Two pools of colostrum were collected from several sows during or soon after parturition (C0), and around 24 h after parturition (C24). To mimic changes occurring in the composition of colostrum, an intermediate colostrum (C12) was produced by mixing equal amounts of C0 and C24. A pool of milk was collected at 6 d of lactation. Colostrum and milk were stored at -20°C until used.
Two samples were taken from each pool of feed and analyzed in triplicate for dry matter, nitrogen (Kjeldahl), lactose, fat and gross energy as previously described (23
) (Table 1
). Piglets were bottle-fed sow colostrum C0 from birth to 8 h, colostrum C12 from 8 to 16 h, colostrum C24 from 16 to 24 h and then sow’s milk up to slaughter at 7 d of age. Piglets were fed every 2 h from 0700 to 2300 h, and only once during the night at 0300 h. To compensate for the long interval between meals, the size of the 0300 h meal was increased by 50%. Body weights (BW) of selected piglets ranged from 1360 to 1815 g at the beginning of the experiment, and feed intake was adjusted daily according to BW measured 90 min after the 0900 h meal. Feed intake was determined by weighing the bottle (±0.1 g) before and after each feeding. O2 consumption and CO2 production were measured by use of an open-circuit temperature-controlled respiration chamber, as previously described (24
), to determine the respiratory quotient RQ (RQ = CO2 produced/O2 consumed). Piglets were placed in the respiration chambers 100–110 min before the measurements, and became calm after 5–10 min. At the time of feeding, the respiration chambers were opened and piglets were bottle-fed while remaining in their cages. Measurements started 10 min later for 110 min. Two successive measurements of 110 min were performed on d 6 and d 7. The averaged RQ d6/d7 ratio was 1.0007, indicating the stability of energy metabolism. The mean of the two values was used in the analysis. At 90 min after the last meal, pigs were weighed, anesthetized using halothane and killed by exsanguination.
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At slaughter, blood was collected in heparinized tubes. After immediate centrifugation, plasma samples were collected and stored at -70°C. Total 3,5,3'-triiodothyronine (T3) was determined by radioimmunoassay by use of a commercially available standard kit (ICN Pharmaceuticals, Costa Mesa, CA).
Muscle sampling.
Two morphologically and functionally distinct skeletal muscles were selected: the longissimus lumborum (LL) muscle at the last rib level, a predominantly fast-twich glycolytic muscle involved in providing a propulsive voluntary rapid thrust to the hindlimb, and the tubular portion of rhomboideus (RH) muscle, a postural mixed slow- and fast-twitch oxido-glycolytic muscle of the neck involved in posture by supporting the head. Immediately after slaughter, muscle samples were taken, mounted on flat sticks, frozen in 2-methylbutane (isopentane), cooled by liquid nitrogen and stored at -70°C until further histological and biochemical analysis. The weight of the semitendinosus (ST) muscle, a large muscle of the hindlimb, was recorded as an indicator of muscle mass.
Histological analysis.
Because numerous myofibers contain several isoforms of MyHC during the first postnatal wk in pigs, the conventional histochemical acto-myosin-ATPase technique (25
) is inadequate to accurately type myofibers at this early stage (4
). Therefore, only immunocytochemical techniques using a panel of monoclonal antibodies (MAB) raised against different MyHC isoforms were used. The MAB were previously shown to be specific of the slow type I (NLC-MHCs; Novocastra, Newcastle, UK), fetal (F88 4C10; Biocytex, Marseille, France), -cardiac (F88 12F8; Biocytex), IIa (6B8, from Dr. D. Gerrard, Purdue University, West Lafayette, IN), IIb (BF-F3, from Dr. S. Schiaffino, Padova University, Padova, Italy) and IIa + IIx (S5–7D4; Biocytex) MyHC in pigs (4
,9
,26
). To our knowledge, antibodies raised against the other MyHC are not yet available in pigs. Transverse serial 10-µm cross sections were cut on a cryostat at -20°C (2800 frigocut N; Reichert-Jung, Heidelberg, Germany) and processed as previously described (4
). Antibodies NLC-MHCs, F88 4C10 and F88 12F8 were hybridoma supernatants diluted 1:20, 1:10 and 1:2, respectively, whereas 6B8 was produced from ascites and diluted 1:24,000. The specific binding was revealed by the avidin biotin peroxidase technique (Vectastain ABC kit; Vector Laboratories, Burlingame, CA). Fibers showing either intermediate or dark staining were counted as positive for the MyHC isoform of interest, and 
1000 fibers from five ramdom fields within each section were counted to determine the percentage of positive fibers through use of an image analysis system (Optimas; Media Cybernetics, Silver Spring, MD). This number was about twice that recommended by White et al. (27
) for the same muscles at 6 wk of age. The same observer analyzed all the sections to reduce the variability attributed to the operator. The mean cross-sectional area (CSA) of each fiber type was estimated from about 150 fibers of each type by use of a programmable planimeter (Hitachi, Japan).
Biochemical analysis.
Activities of citrate synthase (CS, EC 1.1.3.7), ß-hydroxy-acyl-coenzyme A-dehydrogenase (HAD, EC 1.1.1.35) and lactate dehydrogenase (LDH, EC 1.1.1.2.7) were used as markers of overall oxidative capacity (tricarboxylic cycle), lipid ß-oxidation and glycolytic potential, respectively. Enzyme activities were measured on LL and RH muscles as described previously (28
) and expressed as µmol of substrate degraded · min-1 · g-1 of protein. Total muscle protein determination was performed on muscle homogenate after digestion by 1 mol/L NaOH according to the method of Lowry, using BSA as a standard.
Statistical analysis.
The data were analyzed by ANOVA using the General Linear Model procedure of SAS (SAS Institute, Cary, NC). Because LL and RH muscles exhibited drastically different contractile and metabolic characteristics, the effects of diet were analyzed within each muscle type. The model included the effects of diet and litter, and the statistical significance of the diet effect was tested against the residual error of the model. When diet effect was significant, differences between means were analyzed using the Fisher’s LSD test and differences were considered significant at the 5, 1 or 0.1% level. The interactions between diet and muscle type were analyzed by ANOVA using a model including the effects of diet, muscle type, litter and the diet x muscle type interaction. The residual error of the model was used to test for the significance of the interaction. Data in the figures are presented as means ± SE.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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At the start of the investigation, birth weights did not differ among groups and were 1595 ± 100, 1523 ± 127 and 1540 ± 150 g in groups H, M and L, respectively. Piglets gained 1105 (+72%), 687 (+45%) and 185 g (+12%) between birth and slaughter at 7 d in the H, M and L groups, respectively (Fig. 1
). Total metabolizable energy (ME) intakes were 18.59 ± 2.35, 12.50 ± 1.26 and 5.68 ± 0.63 MJ in groups H, M and L, respectively. Body weight gain was linearly related to the total ME intake (R2 = 0.9996). Semitendinosus muscle weight (STW) decreased to the same extent as BW in group M (-16.5 vs. -16.3%), and to a greater extent than BW in group L (-44.0 vs. -35.7%, P < 0.05), suggesting that skeletal muscle was more affected than whole BW in the most restricted piglets (Fig. 1)
. Restriction significantly decreased the RQ (Fig. 1)
and there was a strong linear relationship (R2 = 0.9999) between total ME intake and RQ, suggesting a linear increase in the oxidation of lipids with the intensity of the restriction. Plasma total T3 concentration was decreased in restricted piglets, with a mean reduction of 10% (P = 0.06) in group M and 30% (P < 0.001) in group L (Fig. 1)
.
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As expected, there were marked differences (P < 0.001) in fiber distribution between the two muscles, with LL and RH containing 5.6 ± 1.2 and 57.5 ± 8.5% slow-twitch type I fibers, respectively (Fig. 2A and B
). There was no significant staining with the anti-IIb MyHC MAB BF-F3 (not shown). Pig skeletal muscles exhibited a highly organized pattern of fiber types consisting of islets of slow-twitch type I fibers surrounded by fast-twitch type II fibers. In LL muscle, primary fibers (i.e., the first generation of fibers appearing during myogenesis) could still be distinguished from secondary fibers on the basis of their location (center of islets), size (larger) and reactivity with antibodies [positive with anti-type I, and negative with anti-fetal MyHC MAB (Fig. 2
A and E, respectively)]. On the other hand primary fibers could not be easily identified in the RH muscle. However, one of the central type I fibers within each islet of type I fibers is a primary fiber, whereas all others were secondary fibers using it as a scaffold (1
). Therefore, this means that the secondary/primary ratio can be determined in the RH muscle as well. The secondary/primary ratio was significantly (P < 0.001) higher in LL than in RH groups (28.1 ± 4.6 vs. 20.1 ± 3.0, respectively) and was not influenced by food restriction.
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). -Cardiac MyHC positive fibers always coexpressed the slow MyHC isoform and were localized in the direct vicinity of primary myotubes (Fig. 2
C and D). Their percentage was lower in LL than in RH muscle (4.4 ± 1.1 vs. 15.7 ± 2.8) and smaller than that of slow MyHC positive fibers (5.6 ± 1.2 vs. 57.5 ± 8.5%, respectively). Food restriction increased (P < 0.05) the proportion of -cardiac MyHC positive fibers in RH muscle of the most restricted piglets (group L). The proportion of fetal MyHC positive fibers was higher in LL than in RH muscle (24.4 ± 11.2 vs. 13.2 ± 6.6%, P < 0.001) and was slightly increased in the LL muscle of group L piglets (+58%, P < 0.05). Interestingly, secondary fibers labeled with the fetal MyHC MAB exhibited a strikingly different spatial arrangement in LL and RH muscles (Fig. 2
E and F). Schematically, labeled secondary fibers were located in the center of each cluster in the direct vicinity of the primary fiber in LL muscle, and at the periphery of each cluster, far from the primary fiber, in the RH muscle. Despite weak staining, a clear-cut classification between positive and negative fibers could be made with the IIa MyHC MAB 6B8 (Fig. 2
G and H). The percentage of IIa MyHC positive fibers did not differ between LL and RH muscles at 7 d of age (51.3 ± 2.2 vs. 53.0 ± 5.9). Food-restricted piglets exhibited a higher proportion of IIa MyHC positive fibers only in LL muscle (diet x muscle type interaction, P = 0.11), with a stronger effect in group L than in group M.
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As already mentioned, primary fibers were distinguishable only in LL muscle and were pooled with the slow MyHC positive secondary fibers in RH muscle. Food restriction selectively decreased CSA of the fast-twitch type II fibers in LL muscle, whereas myofibers of RH were not affected (Figs. 4
, 5
). Moreover, a complementary analysis based on the reaction with the anti-IIa MyHC MAB 6B8 showed that within fast fibers of LL muscle, only negatively stained fibers were affected (P < 0.001) by undernutrition (205, 152 and 88 µm2 in groups H, M and L, respectively), located at the periphery of the clusters. All these fibers were strongly stained with the S5–7D4 MAB (IIa + IIx) and could be classified as IIx fibers at this stage.
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Muscle protein content (g/100 g wet muscle) was reduced in the most restricted piglets (Fig. 6
). Activity of LDH (glycolytic capacity) was higher in LL than in RH muscle in piglets from group H (Fig. 7
). Food restriction decreased LDH activity in both LL and RH muscles. However, there was a strong interaction (P < 0.001) between the fixed effects of the diet and muscle type because of a stronger effect of undernutrition on LDH activity in LL than in RH muscle, so that LDH activity did not differ between the muscles in the most restricted piglets. The CS activity (global oxidative capacity) was higher in RH than in LL muscle and was not affected by food restriction in either muscle. The HAD activity (lipid ß-oxidation capacity) was higher in RH than in LL muscle and was increased in both muscles of restricted piglets, to a greater extent in group L than in group M. The LDH/CS ratio decreased in both muscles as the intensity of food restriction increased, but to a greater extent in LL than in RH muscle (diet x muscle type interaction, P < 0.001). The LDH/CS ratio was positively and linearly related to the total ME intake in LL (R2 = 0.991) and RH (R2 = 0.990) muscles. The HAD/CS ratio, which can be interpreted as the relative importance of lipid ß-oxidation with respect to the global oxidative capacity, was higher in RH than in LL muscle. It increased to the same extent in both muscles as the intensity of food restriction increased.
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Muscle growth is more affected than body weight gain.
ST muscle maintained a normal weight relative to BW in group M, whereas it was more decreased than overall BW in group L, showing that the effect of undernutrition on relative growth of ST depends on the severity of the nutritional restriction. Using ST muscle weight as an indicator of muscle mass, our results suggest that other tissues more vital than muscle were more preserved during undernutrition, in accordance with other studies (29
,30
).
The total number of myofibers is not affected.
Previous studies carried out in pigs suggested that the total number of fibers (TNF) could increase during the first postnatal wk (4
,31
). Because all primary myotubes differentiate before 55 dg in pigs, the number of secondary fibers surrounding each primary myotube (secondary/primary ratio) can be used to follow changes in the hyperplasia of secondary fibers after 55 dg. This ratio was not influenced by the nutritional level in either muscle, suggesting that hyperplasia of secondary fibers was not modified. This confirms previous results in pigs (16
) and mice (32
) and is consistent with the general idea that the TNF is definitely established before birth in pigs (1
).
Cross-sectional area of future fast-twitch glycolytic fibers is selectively reduced.
A dramatic increase in the CSA of secondary fibers occurs during the first postnatal wk in pig LL and RH muscles (6
). A major effect of undernutrition was a reduction in this postnatal increase in myofiber CSA. However, this had a greater effect on LL than on RH muscle, where changes were not significant (P = 0.12), showing that the postural RH muscle was less sensitive than the phasic LL muscle to undernutrition. Moreover, the reduced hypertrophy in LL mostly affected a subpopulation of fast secondary fibers negatively stained with the anti-IIa MyHC 6B8 MAB, which were located at the periphery of each cluster. As suggested by the negative staining with BF-F3, they did not yet express the IIb MyHC at the protein level, in accordance with other results reporting the presence of the IIb MyHC mRNA only from 5 d onward in pig LL muscle using in situ hybridization (8
). The positive labeling of these fibers with the S5–7D4 MAB (IIa + IIx) shows that they were in fact IIx fibers at this stage, before maturing to large glycolytic fast-twitch IIb fibers in the adult (9
). Interestingly, all fast fibers of the RH muscle were positively stained with 6B8 and S5–7D4 MAB, showing that no pure IIx fibers were present at this stage. An obligatory pathway for MyHC transition in the rank order I 
IIa IIx IIb was previously reported (33
,34
) and the better resistance of fast fibers to undernutrition in RH than in LL muscles at 7 d is likely related to the presence of the IIa MyHC in all fast fibers in the RH muscle. Overall, despite the immaturity of the fibers during the first postnatal wk, our results showed a differential sensitivity of fiber types to undernutrition that is close to that reported in mature muscles in different species (i.e., a selective preservation of CSA of the most oxidative types I and IIa fibers). This contrasts with findings in pigs in which energy deficiency (-50%) between 3 and 7 wk of age resulted in a similar decrease in the CSA of all fiber types in LL, RH and soleus muscles (18
). These contrasting results are not easy to explain but may be related to the stage of development and/or the relative severity of undernutrition.
From a physiological standpoint, the selective preservation of the CSA of the type I and IIa MyHC positive fibers is likely advantageous to conserve energy because of the lower energy expenditure per unit tension developed in these fibers (35
). However, the decreased myofiber CSA and muscle protein concentration could alter muscle function by decreasing the strength of muscle contraction, thus leading to muscle weakness. This hypothesis is supported by previous studies showing that muscle strength is directly related to the myofibril content (36
). In this case, underfed piglets would be weaker and less competitive than their littermates to reach good teats, which will still increase the severity of undernutrition.
Muscle maturation is delayed.
A dramatic increase in protein concentration and glycolytic capacity, and a decrease in fetal MyHC isoform occur during the first postnatal wk in pig skeletal muscle (3
–6
). All of these characteristics can be used as markers of animal maturity. The percentage of fibers labeled by the anti-IIa MyHC MAB 6b8 can be added to these markers. Indeed, the percentage of positive fibers decreases postnatally, to reach 7% in LL and 12% in RH at 6 mo of age (9
). In accordance with previous results (6
), the proportion of fibers still containing the fetal MyHC isoform was higher in LL than in RH muscle, showing a lower rate of maturity of LL than of RH muscle. This is consistent with previous data reporting a lower postnatal allometric growth rate relative to total side muscle for RH muscle (0.84) than for the earlier maturing LL muscle (1.10) (37
). In LL, there was a lower protein concentration and LDH activity, and a higher percentage of fibers still containing the fetal or IIa MyHC isoforms in the most restricted piglets (group L), suggesting a delay in postnatal maturation of this muscle. The differences were significant only for LDH and the percentage of IIa MyHC positive fibers in group M, showing that the delay was influenced by the severity of the restriction. In RH, only the percentage of protein and the activity of LDH were reduced, showing that the delay in maturation was more pronounced in the later maturing LL than RH muscle. Present results are consistent with previous data reporting a delayed muscle maturation after undernutrition in rats and chicks (13
,19
,20
).
Undernutrition during the early postnatal period decreased plasma T3, in accordance with the abundant literature on this topic (38
). Because thyroid hormones have been shown to regulate the transition from fetal to adult fast MyHC isoforms during normal early postnatal development in different species (39
,40
), the delayed maturation in restricted piglets could be related, at least partly, to the reduced level of plasma T3. Alternatively, because both myofibrillar protein synthesis and degradation are downregulated as an adaptation to a restricted intake (41
,42
), it is also possible that maturational changes are delayed because of a reduction in the rate at which developmental protein isoforms are cleared and replaced by adult isoforms.
Expression of type I and -cardiac MyHC are upregulated.
The present study confirms the atypical expression of the -cardiac MyHC isoform during the early postnatal period in pig skeletal muscle (4
–6
). The -cardiac MyHC has been considered to be intermediate between type I and IIa MyHC in models of MyHC transition by use of electric stimulation in rabbits (43
). Undernutrition slightly (P < 0.10) increased the percentage of slow MyHC positive fibers in both LL and RH muscles, and of -cardiac MyHC positive fibers in RH (P < 0.05). An increase in the expression of type I MyHC in RH but not in LL was also previously reported at the mRNA and protein levels in piglets, at thermoneutrality, placed on a low energy intake between 3 and 6 or 7 wk of age (18
,27
). Mechanistically, it is thought that MyHC expression is regulated at the transcriptional level, and that contractile activity and thyroid status are two dominant regulators (34
,44
). In particular, hypothyroidism, increased neuromuscular activity and mechanical loading induce fast to slow transitions, whereas hyperthyroidism, reduced neuromuscular activity and mechanical unloading cause changes in the opposite direction. The effect of physical activity was reduced to a minimum in the present experiment by placing the piglets in small cages, at thermoneutrality. However, we cannot totally exclude an effect of physical activity through the effect of BW, which was lower in restricted piglets, thus decreasing the gravitational force. This should have decreased the proportion of slow-twitch type I fibers, which contradicts the present results. Therefore, other mechanisms must be involved such as the reduced plasma T3 level, a lower energy charge of the fibers (45
,46
) and/or the maturation of innervation (47
). The mechanisms involved in the atypical expression of -cardiac MyHC in pig skeletal muscle during the first postnatal wk are unknown. In the present experiment, its upregulation in the RH of undernourished piglets cannot be explained by the reduced plasma T3 levels because the -cardiac MyHC promoter is directly stimulated by T3 (48
). Other factors that are affected by nutrition are likely involved.
Glycolytic capacity is reduced.
During normal pig muscle development, the most dramatic change in metabolic enzyme activity during the early postnatal period is an increase in the activity of glycolytic enzymes in future fast-twitch glycolytic muscles, such as LDH in LL muscle (3
,6
). In the present experiment, the most striking effect of undernutrition on metabolic enzyme activities was a decrease in the glycolytic capacity (LDH) of LL, so that no difference between LL and RH muscles could be observed in the most restricted piglets. Whether the dramatic decrease in LDH activity in LL was primary to the selective failure to grow of future fast-twitch type II glycolytic fibers remains to be established. Otherwise, the overall oxidative (CS) capacity was not affected in either muscle, suggesting that the global oxidative capacity was preserved. Our results are consistent with most of the studies previously reported (49
–52
). However, they contrast with other findings using histochemistry in pigs in which energy deficiency (-67%) between 3 and 6 wk of age did not change the glycolytic metabolism in LL and decreased it in RH, whereas oxidative capacities were increased in both muscles (27
). The discrepancy could be attributable to the stage of development, the relative severity of the undernutrition or the techniques used.
ß-oxidation capacity is increased.
A shift toward a higher capacity of lipid ß-oxidation was observed in both muscles, as suggested by the increased HAD activity and HAD/CS ratio, and the decreased RQ. Thus, our results suggest that undernutrition during the early postnatal period in pigs increased the contribution of dietary fatty acids and decreased the glucose utilization to energy metabolism.
In conclusion, the present data show that metabolic properties were drastically altered by early postnatal undernutrition through a reduction of the glycolytic capacity and an increase in the potential for ß-oxidation of fatty acids. On the other hand, the effects on MyHC isoform expression were of relatively small magnitude. These changes were dependent on the muscle and reflected a delay in muscle maturation. Besides, undernutrition decreased muscle protein concentration and selectively reduced the CSA of the future fast-twitch glycolytic fibers in LL muscle. Further research is needed to establish the functional consequences of these early changes.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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3 Abbreviations used: BW, body weight; CS, citrate synthase; CSA, cross-sectional area; dg, day of gestation; HAD, ß-hydroxy-acyl-CoA-dehydrogenase; LDH, lactate dehydrogenase; LL, longissimus lumborum; MAB, monoclonal antibody; ME, metabolizable energy; MyHC, myosin heavy chain; RH, rhomboideus; RQ, respiratory quotient; ST, semitendinosus; STW, semitendinosus weight; T3, 3,5,3'-triiodothyronine; TNF, total number of fibers.
Manuscript received 30 July 2002. Initial review completed 17 September 2002. Revision accepted 15 October 2002.
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LITERATURE CITED |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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