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*
Zoological Laboratory, University of Groningen, Haren, the Netherlands;
Department of Anatomy and Physiology, University of Dundee, Dundee, United Kingdom and
Centre for Isotope Research, University of Groningen, Groningen, the Netherlands
2To whom correspondence should be addressed at RIKILT, P.O. Box 230, 6708 PD Wageningen, the Netherlands. E-mail: p.e.boon{at}rikilt.wag-ur.nl
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: • stable isotopes • protein synthesis • feeding • starvation • birds
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, Schanbacher et al. 1981
). This effect is
produced by the influence of day length on daily energy intake and
energy expenditure (Boon et al. 1997
, Charles et al. 1992
, Schanbacher et al. 1981
). Much of the
growth observed in young animals is due to increases in body protein,
with skeletal muscle showing the greatest percentage change in body
composition as animals grow. Growth is achieved when protein synthesis
rates are higher than protein breakdown rates (Waterlow 1984
). It therefore seems feasible that day length, via time
available for food intake, activity and energy expenditure, will affect
protein balance and thus net body growth through changes in protein
synthesis and/or breakdown rates.
Changes in timing of food intake have been shown to induce daily
variations in protein turnover. In animals experiencing a balanced
diurnal periodicity, daytime feeding results in protein gain, and
nighttime starvation leads to protein loss (Clugston et al. 1982
, Millward et al. 1988
and 1991
,
Murphy et al. 1990
, Pocknee et al. 1978
).
For net growth of tissues to occur, as in growing and lactating
animals, the deposition of protein should exceed protein losses
(Garlick et al. 1994
). Exposure to short day
lengths
(SDL),3
as, for example, occurs seasonally, results in long nocturnal periods
of food deprivation and large amounts of food consumed during short
light periods (Boon et al. 2000
). During these short
feeding periods, large amounts of protein amino acids become available
to the body. These dietary amino acids have to cater for protein
synthesis to cover immediate protein needs (repair, growth and
maintenance) and/or as a protein store for use in subsequent periods of
food deprivation. Normally, when the supply of amino acids exceeds the
protein synthesis capacity of the animal, the excess will be oxidized
(Gibson et al. 1996
, Millward et al. 1988
and 1991
, Waterlow 1984
). During starvation, e.g.,
overnight, protein synthesis rates drop, and during prolonged periods
of starvation, rates may even fall below breakdown levels. Starvation
over a period of 2–4 d reduces protein synthesis rates considerably in
many different tissues and whole body (Cherel et al. 1991
, McNurlan et al. 1979
and 1980
,
Muramatsu et al. 1987
). Even short periods of starvation
(8–18 h) have been shown to reduce protein synthesis rates in rats and
mice (Davis et al. 1993
, Garlick et al. 1983
, Pocknee et al. 1978
, Yoshizawa et al. 1997
). Thus, long nocturnal periods of food deprivation may
be a limiting factor for growth.
In most studies, starvation and feeding were imposed to investigate acute effects on protein synthesis rates. There is little information concerning the changes in tissue protein turnover rates during periods of starvation and feeding that return daily over a longer time span, as will be experienced by animals exposed to long dark periods and short light periods, such as during the late winter and early spring months. These animals are still able to grow, but to what extent is protein turnover affected? What we aimed to investigate was whether the rates of tissue protein synthesis in animals exposed to such conditions adapted to day length. Are animals that are habituated to short daily feeding periods, and consequently long nocturnal periods of starvation, able to maintain growth at the same rates during the overnight starvation by compensatory stimulation of protein synthesis when food becomes available? We also aimed to investigate whether there were differences between any feeding effects on the rates of protein synthesis in fast turnover (i.e., liver) and slower turnover (i.e., skeletal muscle) tissues.
We investigated the effect of day length on the response of protein
synthesis rates to starvation and feeding in a rapidly growing strain
of Japanese quail (Coturnix c. japonica) subjected to either
a long day length (18 h light/6 h dark) (LDL) or SDL (6 h light/18 h
dark) with free access to food during the light period. We measured the
fractional synthesis rate (KS) of
tissue mixed protein after an overnight period of starvation and after
2-h food access at dawn. KS was
measured of two different types of muscle tissue, skeletal and cardiac,
and of liver. Skeletal muscle and liver were chosen because both
tissues contribute considerably to whole-body protein synthesis. In
all organisms where it has been measured, hepatic
KS is high, whereas muscle, with a
much lower KS, contributes by its
large size of the protein pool (McNurlan et al. 1980
,
Waterlow 1984
). Cardiac muscle was investigated because
of its insensitivity to short-term variation in food availability,
as far as function and size are concerned. Japanese quail were used, a
species with the highest growth rate of the family Phasianidae
(Ricklefs 1973
) and therefore likely to be responsive to
variations in day length and food availability. In the present study,
all measurements were conducted on 2- to 3-wk-old birds, the period of
highest rate of body weight gain (10%/d; Boon et al. 2000
).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Japanese quail (Coturnix c. Japonica) neonates, of a
strain selected for maximum body mass at the age of 5 wk, were obtained
from a commercial quail farm (N.V. Nouwen, Lommel, Belgium). Birds were
kept in wooden cages [67 x 39 x 44 cm3 (l
x b x h)] with sawdust bedding at conditions of continuous
light and free access to quail starter food and water, as previously
shown to ensure maximum possible body mass gain, until 6 d old. A
40-W heating lamp was placed in each cage 20 cm above the bedding to
provide a temperature gradient sufficient to allow for selection of the
preferred temperature by the chicks. A maximum of 10 chicks were housed
in each box. At 6 d of age, the chicks were assigned to either LDL
[lights on at 9 h Mid-European Time (MET)] or SDL (lights on
at 9 h MET), with free access to food during the light period. At
this time, each box housed a maximum of four chicks per box. The 40-W
heating lamp was gradually removed to allow the ambient temperature to
decrease to room temperature (
21°C). Throughout the experimental
period, a pelleted diet (Institute for Animal Science and Health,
ID-DLO, Lelystad, the Netherlands; Table 1
) containing 27.7 g/100 g crude protein and 17 kJ/wet g (gross energy by
bomb calorimetry) was fed. Water was always freely available.
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Experiment 1: Time course of enrichment in plasma, liver and muscle.
To establish a protocol for measuring tissue mixed protein fractional
synthesis rates by a flooding dose technique (McNurlan et al. 1979
), we examined the time course of
L-[13C-]Leu enrichment in the free amino acid
in plasma and intracellular pools and measured the incorporation of
L-[13C-]Leu into mixed tissue protein in
pectoral muscle and liver after the administration of a flooding dose
of L-[1-13C]Leu. Quail (14 d old, 85 ± 15 g), subjected to an LDL, received a single intramuscular (i.m.)
injection in the left pectoral muscle at dawn after an 18-h period of
starvation of 1 mL/100 g body mass containing 200 µmol/L leucine, of
which 20% was labeled (99 atom %
L-[1-13C]Leu; Masstrace Inc., Woburn, MA).
After 5 (n = 7), 15 (n = 4) and
30 (n = 4) min, taken as the time after the whole
solution was injected, blood was collected from the right wing vein
into heparinized tubes, which were then centrifuged (1500 x g, 15 min at 4°C) to separate plasma from whole blood.
Plasma was stored at -70°C until analysis. Birds were then killed
and quickly immersed in liquid nitrogen to cool the body. Whole liver
and a sample of the right pectoral muscle were immediately removed and
washed in ice-cold saline (9 g sodium chloride/L) to minimize blood
contamination. Excess saline was blotted before rapid tissue freezing
in liquid nitrogen, weighed (to 0.0001 g) and stored at -70°C until
analysis. The tissue harvesting procedure took <2 min, and all tissues
were removed in the same order each time. A group of three birds was
used for baseline measurements of protein bound and free leucine
natural abundance in blood and tissue.
Experiment 2: The effect of starvation and day length on KS in fed and starved animals.
Having established that flooding was achieved during 15–30 min in all
tissues, we used this time period to measure
KS of pectoral muscle, liver and heart in
14-d-old SDL and LDL quail starved overnight or after refeeding for
2 h at dawn. The rate of protein turnover is related to body mass
and age (Goldspink et al. 1984
, Millward et al. 1981
, Waterlow 1984
). We therefore repeated the
experiment in 21-d-old SDL chicks. The body mass of SDL chicks at that
age was closer to that of 14-d-old LDL chicks (Table 2
). We also measured KS in 14-d-old LDL
chicks starved for 18 h that were refed for 2 h at dawn,
similar to the daily starvation duration of SDL chicks. To measure
KS under all conditions, we injected
chicks as described in expt. 1 and killed them either 15 or 30 min
after injection. Whole liver, a sample of the right pectoral muscle and
whole heart were taken and processed as described earlier.
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A 100- to 200-mg sample of tissue was homogenized in liquid nitrogen using a mortar and pestle. The tissue was mixed with 3 mL of ice-cold 0.2 mol perchloric acid/L, and after centrifugation (1500 x g), the supernatant, containing the tissue free amino acids, was removed and neutralized with potassium hydroxide. The amino acids from this solution were purified by ion-exchange chromatography (Dowex H+ form), dried, and derivatized with 50 µL of pyridine and 50 µL of methylsilyl-tert-butylsilyltrifluoroacetamide at 80°C for 1 h. Labeling of the tissue free leucine 13C enrichment, as tert-butyl dimethylsilyl (t-BDMS) derivative, was then measured by gas chromatography–mass spectrometry (MD 800; Thermoquest, Hemel Hempstead, U.K.) operated in selected ion monitoring mode (m/z 302 and 303).
To determine the bound leucine enrichment, the pellet obtained from the
treatment of the tissue with perchloric acid underwent acid and alkali
treatment to remove RNA and DNA. The protein was then subjected to
hydrolysis in 3 mL of 6 mol/L hydrochloric acid at 110°C for 15–18
h. Resulting free amino acids were purified by ion-exchange
chromatography, as above, before being vacuum dried. The leucine from
this hydrolysis was collected by preparative gas chromatography of the
t-BDMS derivatives (Smith et al. 1988
).
Enrichment of the tissue protein leucine was determined by isotope
ratio mass spectrometry (Europa Scientific Instruments 2020, Crewe,U.K.) after liberation of the carboxyl carbon as
13CO2 by the ninhydrin reaction (Smith et al. 1988
).
Calculations and statistical analysis.
Mixed protein KS (%/h) was calculated as
 |
where EB is the enrichment of
the tissue protein-bound leucine,
EF is the enrichment of the
tissue-free leucine and t is the time of labeling in h
(Garlick et al. 1980
, McNurlan et al. 1979
). The mean enrichments of the free and bound pool over the
different time periods
(x1–x2
min; Table 3
) after administration of the flooding dose were used. We calculated for
each individual quail the difference in enrichment of the free and
bound leucine pools over the indicated time periods. Mean values for
the free amino acid enrichment over the time course of the experimental
period were obtained from the group values.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The enrichment of the free pools
(EF; atom % excess) increased rapidly
after injection (Fig. 1
). In plasma and liver, the peak enrichment was reached at 5 min and
remained constant over the next 10 min. In muscle, the peak was
achieved after 15 min. In the protein bound pool of muscle and liver,
we observed a linear increase in leucine enrichment
(EB) over the whole measurement, when
the free leucine pool was flooded. The relative enrichment of the
tissue free pools to that of plasma is shown in Table 4
. It is evident from these results that flooding conditions were reached
after 5 min for liver. Flooding conditions for muscle were reached only
after 15 min and were maintained for the period chosen for protein
synthesis measurements (see later; Table 4
). Calculated mixed protein
KS for the different time intervals
are presented in Table 3
. We decided to take the 15- to 30-min interval
for the experimental protocol because flooding was achieved and
maintained in all tissues and intracellular and plasma free leucine
pools were labeled to the same extent. Incorporation of label into
protein was linear during this time interval.
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Body mass.
Body mass varied significantly with day length: SDL chicks weighed 47%
less than LDL chicks (P < 0.001; Table 2
). SDL chicks
of 21 d, regardless of feeding state (118 ± 17.6 g),
weighed significantly more (18%) than 14-d-old LDL chicks
(P < 0.001). After correction for body mass, pectoral
muscle weighed significantly more and heart less in LDL chicks compared
with SDL chicks (P < 0.05). A comparison of these LDL
chicks with 21-d-old SDL chicks demonstrated that LDL chicks had a
significantly lower muscle weight (P < 0.05) and
higher liver weight (P < 0.001), corrected for body
mass (Table 2)
. After correction for body mass, feeding significantly
influenced liver weight in only 21-d-old SDL chicks (P
< 0.001).
Mixed protein KS.
In LDL chicks, there was no significant effect of refeeding on
KS of any of the tissues
(Table 5
) after 6 h of starvation. Starvation for 18 h, however,
caused a significant decrease in KS of
pectoral muscle and liver compared with a 6-h starvation period
(P < 0.01 and P < 0.05, respectively;
Table 5
). Refeeding after an 18-h period of starvation resulted in a
marked increase in the rate of synthesis by approximately twofold in
both pectoral muscle (P < 0.01) and liver
(P < 0.05; Table 5
). In SDL chicks, regardless of age,
there was a significant increase in KS
in both pectoral muscle (P < 0.05) and liver
(P < 0.01) after re-feeding. Again, this increase
in protein synthesis rate was in the order of twofold to fourfold
(Table 5)
. The magnitude of change in
KS in 18-h starved individuals was the
same for the LDL and SDL groups. Interestingly, 14-d-old SDL chicks
showed the greatest magnitude of change in protein synthesis rate in
pectoral muscle and liver due to refeeding (Table 5)
. There was no
significant alteration in heart KS on
refeeding in either of the SDL groups.
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To test for an effect of day length on tissue
KS, we compared for each tissue
14-d-old LDL chicks with 21-d-old SDL chicks, because of more
comparable body mass (Table 2)
. For this, we entered day length,
feeding state and day length x feeding state interaction in
one-way ANOVA. Pectoral muscle and heart
KS were significantly higher in the
LDL group for both the starved and fed state (P < 0.01). Liver KS was not significantly
affected by feeding state or day length.
To test for an effect of day length on pectoral muscle and
liver KS corrected for duration of
food deprivation, we compared 14-d-old LDL chicks starved for 18 h
with 21-d-old SDL quail (Table 2)
. To this end, day length, feeding
state and day length x feeding state interaction were analyzed
using a one-way ANOVA. Pectoral muscle (P < 0.005)
and liver KS (P < 0.01) varied only significantly with feeding state:
KS of both tissues was greater in the
fed compared with the starved state.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). Day length reduction would be expected to have a
profound influence on body protein turnover (and consequently growth).
To measure the likely effects of day length on protein synthesis, we
first demonstrated that it is possible to use the flooding dose method
to measure tissue protein synthesis in rapidly growing birds. The
flooding dose method is a widely adopted technique to measure whole
body and tissue protein synthesis rates in small animals
(Garlick et al. 1994
). It is a convenient method for
measuring acute changes in tissue protein synthesis as can occur, for
example, after feeding (Davis et al. 1993
,
Garlick et al. 1983
and 1994
). Flooding dose experiments
in rats (Garlick et al. 1980
, McNurlan et al. 1979
, Obled et al. 1989
), dogs (Jahoor et al. 1992
) and birds (Murphy et al. 1995
) show a
linear decline in free pool enrichment
(EF) with time shortly after injection
of the label. This was not apparent in our study, most likely due to a
difference in the route of label administration. In rat studies, the
label is usually injected intravenously, leading to a rapid rise of
plasma and tissue free pool enrichment (to a maximum level within 2
min), that drops curvilinearly later. In a study with the
White-crowned sparrow (Zonotrichia leucophrys gambelii),
a large dose of 3H-phenylalanine was injected in
the pectoral muscle, similar to our study (Murphy et al. 1995
). In these birds, free pool enrichment of muscle and liver
reached their highest levels within 5 min after injection. The pectoral
muscle in quail contains mainly white fast-twitch glycolytic fibers
that are not highly vascularized (Hohtola et al. 1998
).
Because of this poor vascularization, the movement of the injected
label from the left pectoral muscle into the bloodstream and then into
the muscle free amino acid pools may be attenuated. This may explain
the slow rise with time in free pool enrichment of pectoral muscle in
our study compared with experimental protocols using intravenous or
better vascularized routes. In plasma and liver, we observed the
highest EF levels within 5 min. After
15 min, EF decreased linearly in all
compartments (Fig. 1)
. We calculated protein
KS using measured and estimated
enrichments 15 and 30 min after injection, because we demonstrated
clearly that at these time points, flooding conditions were reached in
muscle and liver (Table 4)
. Mixed protein
KS calculated over this time period
were comparable to those reported for other bird species (Maeda et al. 1984
and 1986
, Murphy et al. 1995
).
Using this method, we showed that exposure to SDL (and
consequently long overnight periods of food deprivation) resulted in
a) a pronounced decrease in tissue mixed protein synthesis
rates during the overnight starvation and b) a steep rise in
measured synthesis rates during feeding (Table 5)
. Shorter periods of
starvation, at least as long as 6 h, appeared to have no
significant effect on muscle or liver protein synthesis rates. Exposure
to an SDL resulted in a pronounced rise in tissue mixed protein
synthesis rates during food availability. This increase was absent in
birds conditioned to an LDL and hence longer periods of food
availability. Being habituated to an SDL did not appear to induce any
compensatory higher tissue protein synthesis rates at night and/or
increase the deposition of dietary amino acids during feeding. It has
been suggested that molting birds, which face high energy demands, may
undergo pronounced daily cycling of protein, involving net protein
degradation during the night when exposed to long overnight periods of
starvation (
8 h) and net synthesis by day (Murphy et al. 1990
). The young birds investigated here were rapidly growing
animals, so it is likely that energy demands in these birds will be
high. SDL chicks are therefore likely to show a discontinuous 24-h
growth curve; net protein gain during the day alternates with net
protein loss during the relatively long night. Overall, birds were
clearly in a net positive protein balance as they grew, albeit more
slowly than the LDL group, over the experimental period. Quail chicks
starved overnight for 6 h had prefeeding tissue
KS values comparable to fed rates,
suggesting that these birds are able to maintain protein synthesis
rates throughout the 24-h period and that the processes involved in
regulating protein synthesis are not sensitive to feeding in this
condition. A daily starvation period of 6 h also ensured chicks of
relatively high tissue KS values
throughout the 24-h period (Table 5)
. These findings suggest that LDL
chicks are able to maintain high rates of protein synthesis, which
would contribute to their rapid growth rate. The mechanism for this is
not clear at present, but it may involve maintained delivery of food
from crop storage over the nightly period.
Because growth in protein is the difference between protein
synthesis and protein breakdown (Waterlow 1984
), an
increase in KS will result only in
increased tissue protein growth when breakdown levels are increased
less or even reduced during feeding. However, from evidence gained from
other experiments, it is possible to predict that protein breakdown
rates have likely fallen either during or immediately after feeding. In
mice it has been shown that refeeding for 1 h after an 18-h period
of starvation resulted in a decrease in muscle protein breakdown and an
increase in muscle protein synthesis (Yoshizawa et al. 1997
). The inhibition of protein breakdown rates due to feeding
has also been shown in human studies (Gibson et al. 1996
, Pacy et al. 1994
). It is therefore likely
that in our experiment, the increase in
KS, and any decrease in protein
breakdown due to feeding, has resulted in an increase in tissue protein
growth. Unfortunately, there is no simple method for measuring protein
breakdown rates in vivo. However, we can estimate the likely changes in
the groups of animals based on their net growth rates. If we assume
that net growth over this period is approximately that of protein
growth and that the fraction of the tissue weight that is protein has
not changed significantly during the experimental period, we can
estimate the average rate of protein breakdown in LDL chicks. Muscle
growth over this period was 18%/d, and protein synthesis rate was
38%/d. Therefore protein breakdown would average 20%/d. Clearly in
this tissue, synthesis, at nearly twice the rate of protein breakdown,
would appear to be the most important energy demanding process for
growth. In liver, the rate of tissue growth was only 5%/d, and the
protein synthesis rate was 145%/d, meaning that protein breakdown rate
would be 140%/d, a much smaller difference in the rates of the
processes regulating tissue protein mass. The protein synthesis rates
we measured in tissues from this LDL group were not stimulated by
feeding, unlike after the longer period of starvation in both SDL
groups. It would appear that during the 6 h of starvation, they
are receiving a sufficient supply of amino acids from food still being
processed in their gut or from other storage forms for protein
synthesis to be maintained during the whole day and during the 6-h
starvation period. Alternatively, there may be attenuation of the
translational process or its signaling pathway, and the process of
protein synthesis is not stimulated, in this group, by the postprandial
surge in amino acids and hormones that normally takes place. We have
shown earlier that feeding during the first 2 h of food access is
largely increased in SDL chicks and only minimally in LDL chicks:
11 g versus 4 g, respectively (Boon et al. 2000
). This may explain the steep rise in muscle and liver
KS at refeeding in SDL chicks. The
fact that no stimulation of protein synthesis occurred in LDL chicks
after 2 h food access may also be due to a low food intake level
during this period.
LDL chicks starved for 18 h had comparable tissue protein
synthesis rates at dawn as birds habituated to repeated 18-h periods of
starvation (Table 5)
. We had thus no support for our first hypothesis
that birds habituated to 6-h feeding periods develop adaptive
mechanisms by which they can maintain high protein synthesis rates
during daily periods of prolonged starvation. We cannot, however,
exclude that the drop in KS during the
overnight starvation was delayed in SDL chicks compared with 18-h
starved LDL chicks, resulting in less protein loss during the night. We
have shown that growing quail habituated to a long daily overnight
period of starvation were able to offset (partially) the effects of
starvation by exploiting their crop as a temporary place for food
storage and slow delivery for digestion and absorption (Boon et al. 2000
). In this way, food is present in the gastrointestinal
tract during a large part of the night, as was evident from a high RQ.
LDL chicks starved for 18 h may not have developed this strategy
of using their crop as a food store, or their crop may be less
accommodating to large amounts of storage. There may be a role of
adaptations in crop volume, emptying regulation (neural or endocrine)
or hypertrophy that requires further investigation. The loss of protein
during the night may, therefore, be less in SDL chicks than LDL chicks
starved for 18 h. LDL chicks that were refed for 2 h after an
18-h period of starvation showed a comparable stimulation of muscle and
liver KS as 21-d-old SDL chicks during
feeding (Table 5)
. We hypothesized that SDL chicks, conditioned to
repeated long daily overnight periods of starvation, would show a
higher increase in tissue KS during
feeding than LDL chicks. Because feeding is restricted to only 6 h
of the 24-h period, a large amount of amino acids become available to
the body in a relatively short period. For efficient growth to occur,
these dietary amino acids must be rapidly and efficiently deposited as
body protein with minimal oxidative losses. Adopting this strategy,
chicks could enlarge their amino acid store for periods during which no
feeding can occur. Muscle KS in
particular may be responsive. Muscle is known to contain the largest
reserve of mobilizable protein in the body (Waterlow 1984
) and has been suggested to perform a storage function in
meeting amino acid demands in the postabsorptive state (Murphy et al. 1995
). However, we were unable to find evidence for this
hypothesis (Table 5)
. Results indicate that protein synthesis rates are
highly flexible and respond quickly to ad hoc changes in food supply.
Feeding had a significant effect on liver and pectoral
muscle KS after an 18-h period of
starvation. This has been demonstrated in other studies after prolonged
overnight starvation (Cherel et al. 1991
, Davis et al. 1993
, Garlick et al. 1983
, Pocknee et al. 1978
, Rennie et al. 1982
,
Yoshizawa et al. 1995
and 1997
). Feeding, however, did
not influence the protein synthesis rate of cardiac muscle. Heart is a
rhythmically active organ that supplies tissues with nutrients and
perfuses organs that remove waste products from the body. It is vital
for life and growth, and its function cannot be compromised due to the
short-term absence of dietary nutrients. Other studies have shown
that in rats, cardiac muscle is sensitive to hormones and amino acids
after prolonged periods of starvation (Preedy et al. 1984
and 1995
). This relative insensitivity of heart muscle in quail to
periods of starvation and feeding could be due to the time periods used
in this study.
Whole body absolute protein turnover rates in mature animals of
different species are positively related to metabolic mass
(g0.75; Goldspink et al. 1984
,
Millward et al. 1981
, Reeds et al. 1980
,
Waterlow 1984
, Waterlow et al. 1978
).
Protein turnover rates per unit mass on the other hand are negatively
related to metabolic mass (g-0.25). In our
study, there was a significant difference in body mass between 14-d-old
LDL and 21-d-old SDL chicks (96 g versus 118 g). However, based on
the relationship between protein turnover rate per unit mass and
metabolic mass, we calculated that the difference in body mass between
the two groups could only account for 5% of the change in mass
corrected protein turnover rates. This is a marginal effect considering
the scale of differences observed after feeding and the variation
within the groups. It is therefore unlikely that this difference in
body mass has affected the comparison between tissue
KS values. This relationship also
explains the higher KS values in the
14-d-old SDL group that had a significantly lower body mass than
equal-aged LDL chicks (Tables 2
and 5)
.
In conclusion, the results showed that day length affects protein synthesis rates by determining the daily duration of food access in diurnal animals. In this way, day length will determine the level of overall growth juvenile animals are able to achieve when exposed to different day lengths.
|
ACKNOWLEDGMENTS |
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|
FOOTNOTES |
|---|
3 Abbreviations used:
EA, enrichment of the tissue-free leucine; EB, enrichment of the tissue protein bound leucine; KS, percent of protein mass synthesized/h; LDL, long day length (18 h light/6 h dark); MET, Mid-European Time; SDL, short day length (6 h light/18 h dark); t, time of labeling (in h).
Manuscript received July 12, 2000. Initial review completed August 7, 2000. Revision accepted November 10, 2000.
|
REFERENCES |
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|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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