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ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
* Institut National de la Recherche Agronomique (INRA), Unité Mixte de Recherche (UMR) 1079 Systèmes d’Elevage Nutrition Animale et Humaine (SENAH), 35590 Saint Gilles, France;
and
Institute for Animal Science and Technology, Polytechnic University of Valencia, PO Box 22012, 46022 Valencia, Spain; and
INRA, Université de Toulouse, UMR 1289, Tissus Animaux, Nutrition, Digestion, Ecosystème et Métabolisme (TANDEM), Chemin de Borde-Rouge, Auzeville, BP 52627, 31326 Castanet-Tolosan Cedex, France
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Abstract |
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0.05) in EXE rabbits compared with SEDN rabbits. This experiment demonstrates that rabbit muscles turn to a more oxidative metabolic pattern in response to jump exercise. The quality of attachment of cooked meat to bone is also improved in active rabbits.
Key Words: fiber type composition • intramuscular lipid • jump exercise • meat quality • oxidative enzyme • tendon
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Rabbits reared in large pens show active behavior, spending more time in social activity and movement (runs and jumps) than rabbits held in conventional cages (Dal Bosco et al., 2002; Postollec et al., 2008). Presumably as a consequence of physical exercise, pen-reared rabbits show greater development of the hind parts (Dal Bosco et al., 2002; Combes et al., 2003). Inconsistent effects of alternative rearing systems have been found on rabbit meat quality (Combes and Lebas, 2003), resulting from a combination of the structural, biochemical, and functional properties of muscles. To propose rearing systems compatible with good meat quality, it is important to determine responses to physical activity per se.
Electrical nerve stimulation experiments have been undertaken to examine how rabbit myofiber properties adapt to forced contractile activity (e.g., Pette, 1992). However, the time course of changes and the degree of change induced on fiber properties by chronic repetitive jumps might be different. Intramuscular lipids are energy substrates during long periods of exercise (Romijn et al., 1993), and muscle lipid content at the time of slaughter is an important trait of rabbit meat quality (Gondret et al., 1998a). The possible modifications of muscle lipid content in response to jump exercise in rabbits remain to be investigated. An improvement in physical activity may also modify the biomechanical properties of tendons that transmit the force generated from muscles to bone (Buchanan and Marsh, 2002). In small species such as poultry and rabbits, the quality of attachment of meat to bone is of particular interest because whole retail cuts are eaten.
This study aimed to describe histological, biochemical, and biomechanical responses in muscles and connective tendons, and their relation to meat quality traits, in rabbits performing repetitive jumps. Sedentary rabbits were used for comparison.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Experimental Animals
At 35 d of age, 152 male weaned crossbred Hyplus PS 39 x White New Zealand 1077 rabbits originating from 39 litters (approximately 4 rabbits per litter) were allocated to 2 groups: a jump exercise-trained group (EXE) and a sedentary (SEDN) control group. Sedentary rabbits (n = 62) were kept individually in cages with a restricted area (0.25 m length, 0.42 m width, 0.80 m height), limiting their possibility of movement. Exercised animals (n = 90) were collectively reared (n = 10 rabbits per cage) in 9 large cages (2.13 m length, 0.62 m width, 0.80 m height). These large cages were equipped with 2 gates dividing the cage into 3 equal spaces (see Ducomps et al., 2003, for a detailed scheme). The gates were equipped with an obstacle adjustable in height according to the size and age of the animal, as follows: 0.25 m between 35 and 49 d of age, and 0.35 m from 50 d of age to slaughter at approximately 10 wk of age. Rabbits had to jump over these obstacles to have access to feed and water, which were located at each side of the cage. All rabbits were reared at INRA TANDEM (Toulouse, France) in 2 successive series at 15-d intervals. Rabbits in the first series (n = 50 EXE; n = 40 SEDN) were used for measurements of body composition, meat quality, and tendon biomechanical traits; rabbits in the second series (n = 40 EXE; n = 22 SEDN) were used for muscle histological and biochemical studies.
During the entire study period, all rabbits had free access to water and to a standard pelleted diet based on wheat, alfalfa, and sunflower meal; the diet contained 16% protein, 3% fat, 60% carbohydrates, and 17% cellulose, and had 2,303 kcal/kg of DE (as-fed basis). The rabbits were housed in a climate-controlled room at an ambient temperature of 20 ± 2°C (mean ± SD) under a regimen of 12 h of light per day (lights on at 0700 h). At 66 d of age, 3 rabbits per giant cage were individually marked with colors on their backs. The number of jumps performed by these rabbits was counted by continuous video monitoring during a 24-h period. Highly sensitive cameras allowed data acquisition during light and dark periods. All rabbits were individually weighed every week from 35 to 70 d to calculate ADG (g/d). Feed consumption was measured weekly in each SEDN and EXE cage to estimate ADFI (g/d). Mortality rate was unexpectedly high (23%) in the SEDN group, especially in the second series (data not shown); it was approximately 10% in the EXE group. Mortality affected rabbits during the first 2 wk of the experiment in both groups. Two rabbits in each group were also removed because of diarrhea. The total number of rabbits under final consideration for growth performance was 46 for the SEDN group and 79 for the EXE group.
Body Composition and Meat Quality Indicators
In the first series, 30 SEDN and 30 EXE rabbits were chosen such that their BW at 63 d were representative of the mean BW of their groups. At 70 d of age, those rabbits were removed from their cages, weighed (2,479 ± 43 g and 2,367 ± 52 g in the SEDN and EXE groups, respectively), and transported to the slaughterhouse (INRA Toulouse, 5-min transport duration). They were immediately preanesthetized by electronarcosis and slaughtered, without prior fasting, by exsanguination. The hot carcass was prepared by removing the lungs, genital organs, viscera, urinary bladder, and skin. Because of aberrant HCW, 3 animals in the SEDN group were discarded. After 24 h of chilling, the cold carcass was weighed and divided according to the norms of the World Rabbit Scientific Association (Blasco and Ouhayoun, 1996). Dressing out [(chilled carcass weight/preslaughter BW) x 100], and proportions [(BW/chilled carcass weight) x 100] of the sum of perirenal and interscapular fat depots, and of the fore (from the atlas vertebra to the seventh thoracic vertebra), intermediate (from the seventh thoracic vertebra to the sixth lumbar vertebra), and hind (from the sixth lumbar vertebra) parts were calculated. Legs were then separated and weighed. Meat color was assessed on the surface over the biceps femoris (BF) in the hindleg, and on a freshly exposed cut surface of LM at the level of the seventh lumbar vertebra. A Minolta CR-300 chromameter (Minolta, Osaka, Japan) was set to the L* (lightness), a* (redness), b* (yellowness) scale. Values corresponded to the average of 3 measurements at each point. Ultimate pH was then obtained at the surface over the BF and in the LM adjacent to the seventh lumbar vertebra level by using a combined glass penetrating electrode (Ingold, Mettler Toledo, Greifensee, Switzerland) and portable pH meter (WTW 340i, WTW, Weilheim, Germany). The legs and a portion of the LM separated from the intermediate part of the carcass were then vacuum packed and frozen at –20°C. Thereafter, the left hind legs were thawed for 24 h at 4°C, and cooked at 80°C for 2 h and 30 min in a wet oven (EX435, HMI-Thirode, Mitry Mory, France) after the tendons had been removed. The muscle-to-bone ratio was then calculated as the ratio of deboned cooked meat to bone weight x 100, and was used as an indicator of the lean-to-bone ratio in rabbit carcasses (Blasco and Ouhayoun, 1996). Water-holding capacity was estimated by centrifuging raw LM portions for 10 min at 1,500 x g at 4°C and determining the residual water by drying the sample at 103°C overnight according to the method of Castellini et al. (1998).
Measurements of Biomechanical Properties of Tendons
After thawing overnight at 4°C, the Achilles tendon (tendo calcaneus) and the patellar ligament (ligamentum patellae) linked to the tibia were precisely dissected from the left hind leg in 27 SEDN and 30 EXE rabbits. These were placed into sealed plastic bags and cooked at 85°C for 10 min. Passive tensile tests were performed with a universal testing machine (MTS, Eden Prairie, MN) as described recently in chicken (Moussa et al., 2007). Briefly, the extremities of the tendons were first frozen in liquid nitrogen to ensure a good adhesion. One extremity was then inserted into a jaw fixed on the crosshead of the universal testing machine, and was able to move vertically. The second extremity (i.e., the bone part of the samples) was fixed on the base of the machine. The speed of the crosshead was set to 25 mm/min. The length and force values applied to the tendon were recorded until the tendon ruptured. From the force displacement curve, the variables recorded or calculated were the maximal load (in N), the stiffness value (slope of the elastic part, in N/mm), and the longitudinal strain corresponding to the ratio between elongation of the tendon at the maximum load and initial length (in mm), measured with a caliper before beginning the test.
Skeletal Muscle Sampling
In the second series, SEDN and EXE rabbits were chosen (n = 10 in each group) so that their BW at 63 d were representative of the mean BW of their groups. At 73 d of age, those rabbits were then removed from their cages, weighed (2,508 ± 59 g and 2,441 ± 95 g, respectively), and slaughtered as described above. One rabbit in the SEDN group was discarded after slaughter because of sanitary considerations. Three skeletal muscles were removed from the hindlegs and immediately weighed. The semimembranosus proprius (SMP) muscle offers the advantage of being one of the rare slow-twitch muscles displaying a pure type I oxidative fiber phenotype; it also displays a high intramuscular lipid content (Gondret et al., 1998b). However, it was impossible to do all the analyses on this muscle because of its small size. The muscles semimembranosus accessorius (SMA) and BF have a quantitative importance in rabbit leg meat. They show a mixed-fiber type distribution, with a predominantly glycolytic metabolism. Muscle samples were carefully trimmed of the external fat located on the surface. They were then prepared following the longitudinal fiber axis, attached to flat sticks, frozen in isopentane cooled by liquid nitrogen, and finally stored at –70°C until histological analysis. Other muscle samples were cut in small pieces, frozen in liquid nitrogen, and stored at –70°C until used for enzymatic assays and biochemical measurements.
Measurements of Histochemical Characteristics
Ten-micrometer-thick transverse serial cross-sections of SMP, SMA, and BF muscles were cut with a cryostat (2800 Frigocut, Reichert-Jung, Francheville, France). Serial sections from each muscle were stained for myofibrillar actomyosin ATPase activity by using preincubation pH values of 4.25, 4.35, 4.45, and 4.6 to identify type I, IIA, and IIB fibers (Brooke and Kaiser, 1970). It was not possible to obtain a repeatable distinction between the IIB and IIX subtypes identified in rabbits by Hämäläinen and Pette (1995); therefore, these fibers were classified as IIB+X. An additional cross-section was stained for succinate dehydrogenase (SDH) activity to estimate the individual muscle fiber oxidative capacity (Nachlas et al., 1957). A minimum of 400 fibers in 3 randomly selected fields from each muscle were then counted to determine the relative percentages of contractile types and SDH-positive fibers from images digitized by using a microscope (Leitz, Wetzlar, Germany) equipped with a charge-coupled device camera (CV-M90, JAI Corporation, Yokohama, Japan). Three other serial cross-sections (40-µm interval each) were fixed for 10 min in glutaraldehyde [Sigma, Saint-Quentin Fallavier, France; 2.5% (vol/vol) in phosphate buffer]. They were then stained for 4 min in a solution of Oil red O (Sigma), an oil-soluble dye that stains neutral lipids with an orange-red tint. The Oil red O working solution was prepared from a stock solution (500 mg of crystallized Oil red O dissolved in 100 mL of isopropanol), which was further diluted in distilled water (48 mL of Oil red O stock + 32 mL of water) and filtered through Whatman No. 42 paper. Slides were then counterstained for 20 s in an aqueous solution of kristall-violet (VWR International, Fontenay-sous-Bois, France), as described previously (Gondret et al., 1998b). Glycerol gelatin (Sigma) was used as an organic mounting medium. Surfaces of perimysial adipocytes filled with triglycerides were carefully reproduced on transparent plastic sheets by using a projection microscope (Visopan, Reichert-Jung, Vienna, Austria); sheets were then digitized (Gel Doc 2000, Bio-Rad, Hercules, CA). Individual areas were measured by using a macroprogram developed on an image analysis system (Optimas 6.5, Media Cybernetics, Silver Spring, MD) before the mean diameter of adipocytes in each sample could be calculated, assuming that adipocytes are spherical cells.
Determination of Catabolic Enzyme Activities
Marker enzymes for primary energy-yielding pathways of metabolism were assayed on SMP, SMA, and BF muscle portions. Each sample (approximately 200 mg) was homogenized in 50 vol (wt/vol) of ice-cold 0.1 M phosphate buffer (pH 7.5) containing 2 mM EDTA by using a motor-driven homogenizer. Homogenates were then sonicated. After centrifugation at 1,700 x g for 15 min at 4°C, the supernatant fractions were collected and used for further analyses. Citrate synthase (CS) was selected as a marker enzyme of the tricarboxylic acid cycle, reflecting mitochondrial density, to estimate whole-muscle oxidative capacity. Enzyme activity was assayed according to the method of Srere (1969), with oxaloacetic acid (Sigma) as the substrate initiating the reaction. The activity of 3-hydroxyacyl-CoA dehydrogenase (HAD), reflecting fatty acid β-oxidation, was determined according to the method described by Bass et al. (1969). Lactate dehydrogenase (LDH), as a marker of the glycolytic pathway, was assayed by the method of Bergmeyer and Bernt (1974). These assays were either colorimetric (for CS) or NADH linked (for HAD and LDH) in nature, and were conducted at 30°C in an automatic spectrophotometric analyzer (Cobas Mira, Roche, Basel, Switzerland). Protein content was determined by using the bicinchoninic acid (BCA)-containing protein assay (Stich, 1990) provided by Pierce (Rockford, IL), and enzyme activities were expressed as nanomoles of NADH (LDH, HAD) or of mercaptide ion (CS) released per minute and per milligram of protein.
Measurements of Lipogenic Enzyme Activities
Activities of fatty acid synthase (FAS), controlling a key step of fatty acid synthesis, and of glucose-6-phosphate dehydrogenase (G6PDH) and malic enzyme, providing reduced NAD phosphate for lipogenesis, were assessed spectrophotometrically according to the methods described by Bazin and Ferre (2001). Portions of muscles (approximately 500 mg in SMP to 1 g in SMA and BF, respectively) were homogenized with a motor-driven homogenizer in 2 mL of ice-cold 0.25 M sucrose solution containing 1 mM dithiothreitol and 1 mM EDTA. The mixture was centrifuged at 100,000 x g at 4°C for 1 h, and cytosolic supernatants were collected for enzyme assays. In all assays, enzymes were maximally activated by the substrate provided in excess and the cofactors, so the activity represents the maximum potential of the enzymes under optimal conditions. Cytosolic protein content was determined by using the BCA Protein Assay Kit as described above. Enzyme activities were expressed in nanomoles of NADH phosphate oxidized (FAS) or produced (G6PDH, malic enzyme) per minute and per milligram of cytosolic protein.
Determination of Total Lipid Content
Total lipid content was measured from approximately 3 g of wet SMA or BF muscle in each rabbit. Because of the lack of enough biological material remaining after other analyses, SMP samples were pooled from 4 rabbits within each treatment group (n = 2 pools per treatment). Total lipids were extracted by using a 17-fold dilution of tissue in a 2:1 chloroform:methanol mixture (vol/vol), and the homogenate was filtered according to the method outlined by Folch et al. (1957). Two washing procedures were performed with adequate salt solutions. Drying was performed by vacuum distillation of solvents. Total lipid content was expressed as grams per 100 g of wet tissue.
Statistical Analyses
All data are presented as arithmetic means, and were analyzed using SAS (SAS Institute, Cary, NC). Differences in individual muscle weights at slaughter were analyzed by the GLM procedure, including treatment (i.e., EXE or SEDN) as the main effect and end BW as a covariable. Other data were subjected to 1-way ANOVA with the fixed effect of treatment. Results are presented as arithmetic means with residual SD. The residual SD is the root mean square of the residual error and applies to the whole model, not an individual estimate within the model.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Videotapes revealed approximately 98 jumps per EXE rabbit during the 24-h period; approximately 42% of these jumps were performed during the night. During the growth period, ADFI tended (P = 0.07) to be less in EXE rabbits compared with SEDN animals (Table 1); however, significant reductions (P < 0.01) in daily food consumption occurred during the last week of the experiment (data not shown). Average daily gain from weaning to slaughter tended (P = 0.08) to be less in EXE rabbits than in SEDN animals (Table 1); weekly ADG was especially reduced (P < 0.01) in EXE rabbits during the last week of the experiment (data not shown). Consequently, end BW tended (P < 0.1) to be less in EXE rabbits than in SEDN animals. There was a trend (P = 0.07) for a greater slaughter yield in the former compared with the latter group. Carcass adiposity did not differ between the 2 treatments (Table 1). The hind part proportion was greater (P = 0.02) in EXE rabbits compared with SEDN animals, and the same trend (P = 0.09) was observed for the fore part percentage. The muscle-to-bone ratio in the hindleg was less (P < 0.01) in EXE rabbits compared with SEDN animals. Meat color assessed on the BF surface was changed (P 0.023) toward greater red (a*) and yellow (b*) values in EXE rabbits compared with SEDN animals, but LM color did not differ between the 2 treatments. Ultimate pH, which was reached 24 h postmortem in BF muscle, was similar in the 2 groups, whereas it was 0.02 point less (P = 0.03) in the LM of EXE rabbits compared with SEDN animals. Water-holding capacity in LM meat was similar in both treatment groups.
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Biomechanical data recorded for the cooked Achilles tendon and the patellar ligament are reported in Table 2. Maximum load did not vary in Achilles tendons of the 2 groups, whereas it was greater (P = 0.01) in patellar ligaments of EXE rabbits compared with SEDN animals. Both the Achilles tendon and the patellar ligament showed greater stiffness (+25%, P = 0.05, and +34%, P = 0.01, respectively) in EXE than in SEDN rabbits. Longitudinal strain in the Achilles tendon was 30% greater (P < 0.05) in EXE than in SEDN rabbits, whereas the same trait in the patellar ligament did not vary between the 2 groups.
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Results for muscle masses corrected for BW indicated no changes in SMP (1.53 g ± 0.05 g) and BF (17.33 ± 0.55 g) at the end of the experiment (data not shown). In contrast, the weight of SMA was slightly less (P = 0.02) in EXE rabbits (15.95 ± 0.67 g) compared with SEDN animals (17.33 ± 0.78 g). As expected, the SMP muscle consisted of only slow-twitch type I fibers in both groups. In EXE rabbits, the BF muscle was composed of fewer type IIB+X fibers (P < 0.001) and greater numbers of type I (P = 0.05) and type IIA fibers (P < 0.01) than the BF muscle of SEDN animals (Table 3). The relative proportion of SDH-positive fibers in BF muscle was also 19% greater (P = 0.05) in EXE rabbits compared with SEDN animals. There was no effect of jump exercise on the fiber type proportions in SMA muscle.
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The greatest activities of the 2 mitochondrial enzymes, CS and HAD, were found in SMP. Conversely, SMA displayed the greatest LDH activity for anaerobic glycolytic metabolism. A 38-d jump-training program clearly resulted in elevated activities of HAD in the 3 muscles (Table 4). Except in SMA, CS activity was also greater (P = 0.03) in the muscles of EXE rabbits than in those of SEDN animals. The magnitude of increase was generally less for SMP (+12 to 14% for CS and HAD, respectively) than for SMA (+17 to 24% for CS and HAD, respectively) and for BF (+25 to 27% for CS and HAD, respectively). Conversely, LDH activity was decreased in SMA (P = 0.05) from EXE rabbits compared with SEDN control animals. The activity of LDH was unchanged in response to jump training in the SMP and BF muscles.
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There was a trend (P = 0.06) for a slightly reduced diameter of adipocytes in the SMP of EXE rabbits compared with that of SEDN animals (Table 5). Specific activities of the lipogenic enzymes FAS and G6PDH also tended (P < 0.07) to be depressed in the SMP of EXE rabbits compared with those of SEDN animals. Conversely, intramuscular lipid content, mean diameter of adipocytes interspersed between fiber fasciculi, and lipogenic enzyme activities measured in the fast-twitch glycolytic SMA and BF did not vary between EXE and SEDN rabbits.
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DISCUSSION |
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; Postollec et al., 2008), and treatment differences on growth performance were minimal. The trends observed for a reduced growth rate and a reduced ADFI in EXE rabbits compared with SEDN animals are similar to housing effects on the same traits reported for rabbits in pens compared with standard-sized collective cages (Maertens and Van Herck, 2000; Maertens and Van Oeckel, 2001) or bi-cellular cages (Dal Bosco et al., 2002). Presumably, a larger area allowance better satisfied the spatial and social needs of rabbits, which spent less time inactive and were then generally less interested in feed (Dal Bosco et al., 2002). The heights of obstacles relative to BW during the last week of the experiment might also have depressed the motivation of EXE rabbits for feed intake during this last period.
Presumably as a consequence of improved physical activity, the proportion of hind parts was greater in the carcasses of EXE rabbits compared with SEDN animals. This modification in body conformation has also been observed for rabbits reared in pens that allow spontaneous physical activity, compared with rabbits held in standard-sized collective cages (e.g., Dal Bosco et al., 2002; Combes and Lebas, 2003). In addition, EXE rabbits in the current experiment displayed a decreased meat-to-bone ratio compared with SEDN rabbits. It is likely that the jump activity pattern imposed a pronounced physical demand on the skeletons of EXE rabbits, resulting in greater bone weights, similar to the results of experiments on rabbits reared in pens or in double cages equipped with a platform, compared with animals held in small collective cages (Martrenchar et al., 2001; Jehl et al., 2003). Similarly, jump exercise resulted in promoting bone mass on the lower limbs in rats (e.g., Umemura et al., 1995). Additionally, the change in muscle-to-bone ratio in EXE rabbits compared with SEDN animals might have been caused by a reduction in growth rate observed in the former group. Indeed, it is generally assumed (Ouhayoun, 1998) that reduced ADG enhances the relative growth of early-maturing tissues (bones) at the expense of late-maturing ones (muscle, fat). Considering the reduced ADG coupled with the reduced ADFI and improved physical activity of EXE rabbits, one might have expected decreased carcass fatness in these animals compared with SEDN rabbits. We did not find such an adaptation, however. Similarly, Maertens and Van Oeckel (2001) did not report any differences in carcass fatness for rabbits housed in pens or in standard-sized collective cages, whereas other studies generally reported decreased fat pad percentages in the carcasses of rabbits reared in large pens compared with caged animals (Dal Bosco et al., 2002; Combes and Lebas, 2003).
Skeletal muscle is a heterogeneous tissue composed of muscle fibers of different contractile and metabolic types, together with interspersed fat cells and blood vessels. Because feed rationing during postweaning growth did not influence oxidative energy metabolism in rabbit muscles (Dalle-Zotte et al., 2005), the increase in oxidative enzyme activities observed in the current study in the muscles of EXE rabbits compared with SEDN animals likely occurred as an adaptation to physical activity. It is difficult to state whether jumping represents strength, endurance, or mixed exercise in rabbit species with a natural aptitude to perform repetitive jumps. However, the increase in maximal activities of oxidative enzymes elicited in the 3 muscles under study was rather similar to that reported to occur in response to treadmill endurance training in other species (e.g., McAllister et al., 1997; Siu et al., 2004; Kim et al., 2005). This shift toward a more oxidative metabolism was likely not accompanied by a reduction in lactate production, except for SMA muscle, in which LDH activity was slightly depressed in EXE rabbits compared with SEDN animals. These observations are also in accordance with other studies on LDH activity in muscles from endurance-trained pigs (McAllister et al., 1997) and rats (Kovanen and Suominen, 1987). Despite similar modifications in energy metabolism, differences in contractile adaptations were observed according to muscle type in the current study. Indeed, the fast-twitch BF of EXE rabbits showed greater percentages of type I and type IIA fibers and a correlative decrease in the type IIB+X fiber proportion compared with SEDN rabbits, whereas jump exercise training did not affect the proportions of myofiber types in the SMP and SMA muscles. Ducomps et al. (2004) have also reported greater percentages of type I or type IIA fibers at the expense of fast-twitch IIB+X in other fast-twitch muscles of rabbits that were jump exercised for 59 d. Taking the 2 studies together, modifications of the rabbit contractile apparatus induced by jump exercise are generally not in agreement with those observed in jump-exercised rats, which showed decreased percentages of type IIA fibers to the benefit of type IIB fibers in a fast-twitch muscle (Dooley et al., 1990) or of type I fibers to the benefit of type II fibers in a red slow-twitch muscle (Pousson et al., 1991).
In the muscles of humans and rodents, intramyocellular triacylglycerols adjacent to mitochondria represent a large energy source during exercise (Romijn et al., 1993) and the postexercise period (Kiens and Richter, 1998). However, the extent to which triglycerides stored in the adipocytes clustered along fiber fasciculi could also be used during exercise, and the existence of differences in their use between low-fat, fast-twitch and high-fat, slow-twitch muscles have been poorly investigated to date. To evaluate these possibilities, we performed Oil red O staining of interspersed adipocytes in the skeletal muscles of SEDN and EXE rabbits, and measured the specific activities of lipogenic enzymes that might have contributed to lipid filling. There was no change in total lipid content or mean diameter of adipocytes in response to jump exercise in SMA and BF. Only small decreases were apparent in adipocyte diameter and lipogenic enzyme activities in the SMP of EXE rabbits compared with SEDN animals. The lack of effect of physical activity on lipid content in rabbit fast-twitch SMA and BF agrees with other reports in skeletal muscles of pigs performing spontaneous or treadmill-forced walking activities (Petersen et al., 1998). In rabbits trained for jump exercise over a long period, Ducomps et al. (2004) showed a decrease in the surface area occupied by adipocytes around a vessel in muscle cross-sections. Taken together, it is likely that the fatty acids used for energy production in muscles of moderately jump-exercised rabbits under the current protocol originated from intramyocellular triglyceride droplets rather than from lipids stored in intramuscular adipocytes.
The current study revealed only small modifications in rabbit meat quality indicators. The greater redness index in the BF of EXE rabbits compared with SEDN animals was likely due to its improved oxidative energy metabolism and a greater proportion of SDH-positive myofibers rich in mitochondria and myoglobin in EXE rabbits relative to SEDN animals. However, this did not lead to a difference in ultimate pH of BF meat. Conversely, the pH value of LM was slightly less in EXE rabbits compared with SEDN animals, in accordance with other data obtained from the LM of rabbits reared in wire-netted pens compared with caged animals (Dal Bosco et al., 2002). Poor cohesion between meat and bone after cooking might also be an important factor in consumer decisions, because whole anatomical parts (legs) of rabbit species are generally eaten. To understand the quality of the attachment of muscle to bone, biochemical properties of cooked tendons were assessed in both groups. Stiffness was greater in both the cooked Achilles tendon and patellar ligament of EXE rabbits than in those of SEDN animals. Tendons have previously been shown to undergo remodeling in response to endurance training, including greater stiffness for at least 1 of the tendons studied (Buchanan and Marsh, 2002 for a review). In particular, Viidik (1967, 1969) reported an increase in the stiffness of the Achilles and tibialis posterior raw tendons in rabbits after 40 wk of training on a running machine. The physiological significance of increased stiffness in tendons in response to physical exercise remains elusive, but it might represent a mechanism to resist damage caused by mechanical fatigue (Buchanan and Marsh, 2002). Reports dealing with the effects of exercise on cooked tendons are very limited. Moussa et al. (2008) recently reported that the stiffness of cooked gastrocnemius tendon did not differ between active chickens and control birds. However, possible effects of exercise on biomechanical properties of the tendon were confused with breed and feeding effects in that study, which compared standard and Label Rouge broilers. Taken together, our observations of greater longitudinal strain in cooked Achilles tendon and of maximal force required to achieve rupture in the cooked patellar ligament of EXE rabbits compared with SEDN animals might suggest an improved attachment of cooked meat to bone in EXE rabbits.
In conclusion, the physical exercise defined in this experiment for pen-housed rabbits, compared with sedentary control animals, slightly affected growth performance and muscle oxidative energy metabolism. It also changed some meat quality attributes, with an improvement in the quality of the attachment of cooked meat to bones.
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Footnotes |
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2 Corresponding author: Florence.Gondret{at}rennes.inra.fr
Received for publication July 10, 2008. Accepted for publication September 29, 2008.
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