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*
Institut National de la Recherche Agronomique, Station de Recherches Porcines, 35590 Saint Gilles
Institut National de la Recherche Agronomique, Station de Recherches Cunicoles, 31326 Castanet Tolosan Cedex, France
2To whom correspondence should be addressed.
 |
ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: • food restriction • myofiber • intramuscular fat • lipogenesis • rabbits
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Restricted feeding alters the size of the myofibers (Harrison et al. 1996
, Solomon et al. 1988
in pigs,
Yambayamba and Price 1991
in beef) and favors the
oxidative metabolism pathway, as evidenced by the higher percentage of
oxidative fibers in muscles of feed-restricted compared to
well-nourished animals (Seideman and Crouse 1986
in
beef, Solomon and Lynch 1988
in lambs, Solomon et al. 1988
in pigs). But conflicting data are often reported
depending on species, muscle considered (Brandstetter et al. 1998
), age of the animal (Harrison et al. 1996
) and whether the effect of feed restriction was studied at
similar age or at similar body weight (Candek-Potokar et al. 1999
).
Restricting feed allowance may also result in a reduction of muscle
lipid concentration, as evidenced in the longissimus muscle of pigs
slaughtered at the same weight as their well-nourished counterparts
(Candek-Potokar et al. 1998
, Wood and Warris 1990
). However, the mechanisms whereby restricted feed intake
impairs intramuscular fat level are not well understood. In
subcutaneous and internal adipose tissues (e.g., Ingle et al. 1973
in sheep, Mills et al. 1989
in steers,
Mersmann et al. 1981
, Steele and Frobish 1976
, in pigs), feed restriction and deprivation depress the
rate of de novo lipogenesis, as well as the activities of numerous
enzymes involved in the synthesis of fatty acids
(acetyl-CoA-carboxylase and fatty acid synthase) or responsible for
generating NADPH for the support of lipogenesis [malic enzyme and
glucose 6 phosphate dehydrogenase
(G6PDH)3
]. However, it is not known whether moderate feed restriction also
affects lipogenic rate and enzyme activities in muscles of locomotion.
To our knowledge, there is only one study that demonstrates that severe
feed deprivation has no effect on the acetyl-CoA-carboxylase activity
in a glycolytic hindlimb muscle of rats (Winder et al. 1995
). As a whole, little information is available on the
regulation of the different lipogenic enzymes in the muscle.
Muscles which differ in their metabolic properties are not homogeneous
in final lipid content. Variation between muscle types in intramuscular
fat is primarily related to differences in the accumulation of
adipocytes along fiber fasciculi; however, the precise relationships
between metabolic type and intramuscular fat remain rather
controversial (Candek-Potokar et al. 1999
,
Gondret et al. 1998
). Moreover, the relationship between
lipogenic enzyme activities and muscle carbohydrate metabolism is
mainly unknown.
Therefore, the present study was undertaken to examine the effects of a
30% food-intake restriction on morphological, metabolic and
biochemical characteristics of three rabbit muscles. Moderate but long
restriction of feed intake during fattening was chosen because such
feeding system is sometimes carried out in rabbit meat industry
(Ouhayoun et al. 1986
). We measured de novo lipogenic
enzyme activities in muscle homogenates in order to study the cellular
mechanism for putative differences in lipid content between restricted
and well-nourished rabbits. In addition, the experiment
investigates some aspects of intramuscular lipid deposition in relation
to muscle energy metabolism.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Rabbits were reared and slaughtered in compliance with national
regulations for humane care and use of animals in research (certificate
of authorization to experiment of living animals n°02376 delivered by
the French Department of Agriculture to F. Lebas). Male White New
Zealand rabbits (NZW) from the strain INRA 1077 were purchased from the
Animal Breeding Unit (Institut National de la Recherche Agronomique,
Castanet Tolosan, France). From weaning (4 wk) to 11 wk of age
(beginning of the period of accelerated fat deposition), rabbits were
reared collectively (n = 6 per cage), and had free
access to water and feed. At 11 wk of age, 60 rabbits of average body
weight (2377 ± 21 g) were allotted to one of two dietary
groups. They were given free access to feed (control, n
= 30) or received 70% of the control feed intake (restricted,
n = 30). During the subsequent experimental period, the
rabbits were housed in individual cages. They had free access to water
and were fed the same pelleted diet based on alfalfa, wheat, beet pulp,
soybean and sunflower meals. The diet contained 14.7% cellulose,
16.5% crude protein, 2.8% fat and 10.3 MJ/kg digestible energy. Feed
was distributed three times a wk (2/7, 2/7 and 3/7 of the weekly
allowance; Ouhayoun et al. 1986
). The amount of feed
allocated to restricted rabbits at each distribution was calculated on
the basis of data of the weekly spontaneous feed consumption obtained
in a preliminary study carried out on 20 NZW rabbits reared under the
same conditions. Feed refusals did not occur in the restricted group,
whereas they were removed and measured just before each distribution in
the control group. Live body weight was recorded weekly during the
experiment.
At 15 wk of age (2905 ± 31 g body weight), 15 rabbits were
selected from the control group, so that their weight was
representative of the mean body weight of their counterparts. They were
then feed-deprived overnight, and slaughtered the following morning
by electric stunning and exsanguination. Restricted rabbits were
maintained on the above regimen until they reached the average weight
of 2905 g, at which time 15 rabbits were selected on the basis of
their weight, and slaughtered under the same conditions. Carcasses were
prepared as described by Blasco et al. (1993)
, by
removing the skin, feet, paws, genital organs, urinary bladder and
digestive tracts. Carcass, liver, and perirenal fat were weighed, and
dressing out percentage (carcass weight/body weight, a commonly used
indicator of rabbit carcass quality) was determined. Immediately after
slaughter, samples from the left and right longissimus lumborum (LL,
white dorsal) at the level of the 3–7th lumbar vertebra, biceps
femoris (BF, white thigh), and semimembranosus proprius (SMP, red
thigh) muscles were excised from the carcass. These three muscles were
chosen because of their importance for rabbit meat industry and their
differences in contractile and metabolic characteristics. Muscles were
carefully trimmed from external fat and epimysium. The left sample was
immediately frozen in liquid nitrogen for biochemical measurements. The
right sample was restrained on flat sticks and frozen in isopentane
(cooled by liquid nitrogen) for subsequent histological examinations.
All muscle samples were stored at -80°C until analyses.
Determination of muscle fiber characteristics: morphometrics, contractile and oxidative activities.
From the right part of each muscle sample, serial cross-sections
(10 µm) were cut on a cryostat (2800 Frigocut Reichert-Jung,
Francheville, France). All sections were taken at a comparable relative
position in each muscle. One section was stained with azorubin, and
mean fiber cross-sectional area (µm2) was measured
with a projection microscope (Visopan Reichert, Wien, Austria) and a
programmable planimeter (Hitachi Siko, Tokyo, Japan), using 200 fibers
counted over three fields per each section. The second section was
processed for the actomyosin ATPase activity (Guth and Samaha 1969
), and myofibers were classified on the basis on their
contractile properties as type I (slow-twitch), type IIA or type IIB
+ IIX (fast-twitch). The third section was stained for succinate
dehydrogenase (SDH, an enzyme specific of the aerobic oxidative
pathway) activity in order to evaluate the oxidative capacity of the
myofibers (Nachlas et al. 1957
). Myofibers have been
defined as either oxidative (SDH activity = high) or nonoxidative
(SDH activity = low or none). Proportions of the different fiber
types were evaluated on 900–1300 fibers counted over three fields for
each muscle section.
Determination of muscle lipid traits: biochemical content and lipogenesis.
Total lipid contents were extracted according to Folch et al (1957)
, using a 10-g sample of the left part in each LL and BF
muscles, and a 1-g sample in each SMP muscle. Total lipid contents were
expressed as g/100 g of fresh tissue. The activities of enzymes
considered as rate-controlling steps in the process of lipogenesis
(acetyl-CoA carboxylase ACC; EC 6.4.1.2), or related to NADPH
production for fatty acid synthesis (G6PDH, EC 1.1.1.49; and malic
enzyme, EC 1.1.1.40) were measured on muscle homogenates. Briefly, a
weighed quantity of tissue (1.5 g for BF and LL, and 0.5 g for
SMP) was homogenized in 2.5 mL of saccharose (0.25 mol/L) ice-cold
buffer. After centrifugation at 30,000 x g at 4°C
for 40 min, the resulting supernatant fraction was collected and used
for enzyme assays. The activity of muscle ACC was determined by the
H14CO3--fixation method
(Chang et al. 1967
). The activities of malic enzyme and
G6PDH were assayed as described by Hsu and Lardy (1969)
and Ficht et al. (1959)
, respectively, using
spectrophotometric absorbance at 340 nm. Lipogenic enzyme activities
were expressed as unit/min/g of fresh tissue, and were also normalized
using cytosol protein content (Lowry et al. 1951
).
Statistical analyses.
Growth performances and slaughtering data were compared for the effect
of feeding level, using the one-way ANOVA (General Linear Model procedure of SAS, 1990
). Muscle histological and biochemical data were
analyzed by multifactor ANOVA, including the main effects of feeding
level (F), muscle (M) and their interaction (F x M). Where
applicable, multiple comparison of means was performed, using the
LSMEANS statement of the GLM procedure. Data are presented as the means
of each feeding group within each muscle, and pooled SEM
together with the significance levels of the main effects and
interactions. A further analysis was performed using Student’s t test
to compare restricted to control data within each muscle.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Growth performance and carcass data of the 30 experimental rabbits are
given in Table 1.
The overall level of restriction achieved was 29.8% in the restricted
group compared to control rabbits. From 11 wk to slaughter age, growth
rate was 53% lower (P < 0.001) and the gain/feed
ratio was 36% lower (P < 0.001) in
feed-restricted rabbits than in controls. At the same slaughter
weight, restricted rabbits were 3 wk older than controls. Carcass
weight, dressing out percentage, as well as amount of perirenal fat,
were significantly reduced (P < 0.001) in restricted
rabbits compared with controls. In contrast, liver weight was not
significantly different (P > 0.05) between the two
groups.
|
No difference was observed for mean cross-sectional area of fibers
between restricted and control rabbits compared at the same weight at
slaughter. Mean comparisons revealed that, irrespective of the feeding
status, mean fiber size was higher in SMP than in LL, and it was the
lowest in BF (Table 2
)None of the muscles studied showed evidence for an effect of feed
restriction on contractile fiber-type composition (Table 2)
. As
expected, there were marked differences in fiber-type composition
between the three muscles with SMP being a pure slow-twitch type I
muscle, whereas LL and BF were mixed fast-twitch muscles. LL had
the lowest proportion of type I and the highest proportion of type IIB
+ IIX fibers, whereas BF displayed the highest proportion of
type-IIA fibers. A muscle-specific response to restriction was
observed (P < 0.01 for F x M) for the percentage
of oxidative fibers which was not significantly affected by feed
restriction in SMP and BF muscles whereas it was lowered (P
< 0.001) in LL muscle (Table 2)
.
|
Restricted vs. control feeding resulted in a significantly lower total
lipid content in all three muscles (Fig. 1
) A significant effect of interaction (F x M) was observed
(P < 0.001), in which the relative reduction induced
by feed restriction was larger in SMP (-39%) than in BF (-31%),
whereas it was the smallest in LL (-25%). Irrespective of feeding
group, total lipid content was higher in SMP than in either of the
other two muscles (P < 0.0001). It was slightly lower
(P < 0.01) in LL than in BF in control rabbits,
whereas LL and BF muscles of restricted rabbits had lipid contents that
did not differ (P = 0.3).
|
).Whatever the muscles, there was no significant difference between
feeding groups for acetyl-CoA carboxylase (ACC) activity. Malic
enzyme and glucose-6 phosphate dehydrogenase (G6PDH) were slightly
lower (P < 0.05) in feed-restricted rabbits than
in controls. The relative reductions in malic enzyme and G6PDH
activities were greater (P < 0.05) in SMP than in BF
or LL muscles.
|
.
Indeed, malic enzyme level was higher in SMP than in the other two
muscles (P < 0.05) in control rabbits, whereas it was
not significantly different from that in BF and LL muscles of
restricted rabbits. The reason for this discrepancy was mainly due to
differences in the cytosol protein content, which was much higher in LL
and BF than in SMP (data not shown). No clear-cut differences in
enzyme activities were found for between the two fast-twitch
glycolytic BF and LL muscles (Table 3)
. Moreover, muscle type markedly
affected (P < 0.001) the ratio of malic enzyme to
G6PDH. Indeed, whatever the feeding group, the malic enzyme level was
about 9- and 15-fold higher than that of G6PDH in BF and LL,
respectively, whereas the ratio of malic enzyme to G6PDH was only about
1.5 in SMP muscle. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, Ouhayoun et al. 1986
), we
have investigated for the first time the effects of moderate feed
restriction during fattening on morphological, biochemical and
metabolic characteristics of three different skeletal muscles.
Furthermore, the parallel study of muscle fiber-type composition,
lipid concentration and lipogenic enzyme activities provided new
insight in the regulation of lipid deposition in skeletal muscles, in
rabbits as in other species. Effects of feed restriction on muscle characteristics.
In the present work, rabbits were subjected to a 30% restriction of
feed intake during fattening period, which resulted in a 53% decrease
in growth rate. When slaughtered at the same body weight (2.9 kg), but
at different ages (18 vs. 15 wk), restricted and control rabbits
displayed similar mean myofiber cross-sectional areas. This
suggests that, in rabbits, enlargement of muscle fibers is correlated
with body weight rather than age. In the three muscles, the lack of
differences in contractile fiber-type composition between the two
groups is consistent with the fact that the muscle contractile
differentiation is mostly completed at 2 mos of age in rabbits
(Gondret et al. 1996
). The present study has also shown
that the percentage of oxidative fibers was not altered in SMP and BF
muscles of restricted compared with control rabbits. In other species,
feed restriction has either no effect on muscle metabolism
(Brandstetter et al. 1998
in cattle,
Candek-Potokar et al. 1999
in pigs, Maxwell et al. 1992
in rats), or increases the percentage of oxidative
fibers (Harrison et al. 1996
in pigs, Seideman and Crouse 1986
in cattle, Solomon and Lynch 1988
in lambs). In contrast to these latter results, we
found a markedly lower proportion of oxidative fibers in the LL muscles
of feed-restricted rabbits compared to control rabbits. However,
the observed variation in the metabolic activity of LL muscles probably
reflected differences in age rather than in nutritional status. Indeed,
in rabbits, it has been reported that age-related changes toward a
less oxidative pathway continue at least until 12 wk of age in the LL
muscle, whereas metabolic myofiber differentiation is already finished
before 9 wk of age in the other muscles (Dalle-Zotte and Ouhayoun 1995
).
Previous studies in rabbits (Gondret et al. 1997
,
Gondret et al. 1998
) have shown that the
muscle fat content markedly increases from 15 wk onward. In contrast,
data from the current investigation demonstrate that, in the three
muscles studied, intramuscular fat contents were much lower in the 18
wk-old restricted rabbits than in the 15 wk-old controls. The
lower muscle fat content of feed-restricted rabbits, slaughtered at
the same weight as their well-nourished counterparts, is consistent
with results in pigs (Candek-Potokar et al. 1998
,
Wood and Warris 1990
). The precise mechanisms by which
accumulation of intramuscular lipids is impaired by food restriction
are not completely understood. Marbling may be due to de novo
lipogenesis in the intramuscular adipose tissue (Chakrabarty and Romans 1972
, Gondret et al. 1997
in rabbits,
Lee and Kauffman 1974
in pigs, May et al. 1995
in cattle) and/or occurs as a result of an uptake of fatty
acids via lipoprotein lipase activity (Haugebak et al.
1974
in sheep). Complete feed deprivation depresses the rate of
de novo lipogenesis in extramuscular adipose tissues of various species
(e.g., Ingle et al. 1973
), however data are lacking on
the variations of lipogenic enzyme activities in skeletal muscles of
animals adapted to feed intake below ad libitum. The current results
show that both slow- and fast-twitch muscles of restricted rabbits
exhibited lower activity levels of G6PDH and malic enzyme than those
found in controls. The hexose monophosphate pathway and the malic
enzyme activity generate NADPH for fatty acid synthesis in extra- and
intra-muscular adipose tissues (e.g., Ingle et al. 1973
, Lee and Kauffman 1974
). Furthermore, we
previously reported (Gondret et al. 1997
) that changes
with age in lipid and triglyceride contents of rabbit longissimus
muscle are closely associated with the age-related patterns of
malic enzyme and G6PDH activities. Therefore, it is reasonable to
speculate that when carbohydrate availability is reduced in the
food-restricted rabbits, the enzymatic machinery of the
intramuscular adipocyte has adapted in such a way as to reduce the
capacity for NADPH generation for fatty acid synthesis, thereby
facilitating the use of C-substrates toward maintenance processes
and skeletal muscle fiber size preservation. However, the measured
activities of the NADPH-producing enzymes reflected metabolic
capacities and not in vivo actual rates. Furthermore, a precise
temporal association between variations in these enzyme activities and
changes in the muscle lipid concentration is lacking. Therefore,
further investigations, combining the use of C14-labeled
substrates, determination of tissue NADPH concentrations and isolation
of intramuscular adipocytes, are needed to draw such a conclusion.
The finding that the activity of ACC in rabbit muscles was not affected
by dietary manipulations (long-term feed restriction) is in accordance
with the results of Winder et al. (1995)
in a glycolytic
muscle of starved/refed rats. These observations suggest that there is
no clear association between muscle ACC activity and intramuscular
total lipid content. ACC has been postulated as the rate-limiting
enzyme in the process of de novo fatty acid synthesis in liver and
extramuscular adipose tissue. However, in nonlipogenic tissues such as
cardiac and skeletal muscles, a 280 kDa isoform of ACC, distinct from
the 265 kDa species found in the main lipogenic sites, is predominantly
expressed (Bianchi et al. 1990
, Trumble et al. 1995
). This isoenzyme of ACC very likely plays an important
role in governing the rate of fatty acid oxidation during muscle
contraction (Trumble et al. 1995
, Winder et al. 1995
), rather than controlling de novo lipogenesis.
As a whole, the results reported herein demonstrate that feed restriction during fattening affects intramuscular lipid deposition without any modification of fiber characteristics.
Muscle-dependent variations in lipid deposition.
This study had the secondary objective of documenting aspects of
intramuscular lipogenesis in relation to muscle energy metabolism.
Significant interactions between feeding level and muscle metabolism
were observed, in which the effects of feed restriction on lipid
concentrations and enzyme activities were more acute in the pure
oxidative SMP than in the two glycolytic BF and LL muscles. The great
differences in triglyceride content among SMP, BF and LL muscles in
normal-fed rabbits (Gondret et al. 1998
) have very
likely accounted for the different degrees of response to
undernutrition of these muscles.
Muscle lipid content as well as enzyme activities (expressed on a
soluble protein basis) were higher in SMP than in BF or LL muscles. The
highest ACC activity found in SMP is in agreement with the highest
ability of the pure oxidative muscles to oxidize fatty acids during
muscle contraction (Goodpaster and Kelley 1998
for
review). The higher activities of G6PDH and malic enzyme found in SMP
compared to BF and LL may be related to the greater capacity of the
former muscle to accumulate lipids in the interfascicular adipocytes
(Gondret et al. 1998
). The much lower activity of G6PDH
compared to that of malic enzyme observed in the two glycolytic muscles
confirms previous studies on rabbit longissimus muscle (Gondret et al. 1997
, Gondret et al. 1998
).
But surprisingly, the ratio of malic enzyme to G6PDH activities was
markedly (P < 0.0001) lower in SMP (about 1.5) than in
the other two muscles (about 9 and 15 in BF and LL, respectively). The
reason for such a lower activity ratio in the oxidative muscle than in
the two glycolytic muscles is unclear. Studies on isolated
intramuscular porcine adipocytes show that less than half of G6PDH
activity is present in muscle adipocytes, whereas more than 80% of
malic enzyme activity measured on homogenates could be ascribed to the
intramuscular adipose tissue (Mourot and Kouba 1998
).
Furthermore, porcine myofibers have been shown to react positively for
G6PDH (Allen et al. 1967
), the pattern of G6PDH stain
being almost identical to that of SDH positive fibers, indicative of
the oxidative capacity of the fibers. These findings suggest that some
portion of the G6PDH activity might be restricted to an oxidative
pathway of carbohydrate metabolism within the myofibers, as suggested
by Lee and Kauffman (1974)
in pigs. The fact that a same
rank order of SMP > BF > LL was observed for the ratio of
G6PDH to malic enzyme activities and for the proportion of oxidative
fibers supports this hypothesis. However, other authors have detected
very little G6PDH activity in the myofibers of the rats (Glock and McLean 1954
), mice, pigs and rabbits (Ogata and Mori 1964
). Therefore, further investigations are needed to
characterize the role of G6PDH in the energy metabolism of individual
muscle fibers in the rabbits, and to better understand the pattern of
intramuscular lipid deposition in oxidative compared to glycolytic
muscles.
|
FOOTNOTES |
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3 Abbreviations used: ACC, acetyl-CoA-carboxylase; BF, biceps femoris; G6PDH, glucose-6P dehydrogenase; LL, longissimus lumborum; NZW, New Zealand White; SDH, succinate dehydrogenase; SMP, semimembranosus proprius.
Manuscript received April 29, 1999. Initial review completed July 26, 1999. Revision accepted October 18, 1999.
|
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) and feed-restricted (
) rabbits slaughtered at
the same weight. Values are means ± SEM, n = 15.
Within each defined muscle, restricted values are significantly
different from control data at **P < 0.01 or
***P < 0.001 (Student t test). Data were also compared
using a two-way ANOVA with the main effects of feeding group (F),
muscle type (M) and their interaction (F x M). Significant
P-values were found for feeding (P = 0.0001), muscle (P = 0.0001) and their interaction
(P = 0.0001).