This study used an experimental model, described in a companion
paper, to examine the effects of
feed intake on protein turnover in the small intestine of lambs. Ten
male castrate lambs (∼ 10 months
old) were offered, via continuous feeders, either 400 (n = 5)
or 1200 (n = 5) g/day lucerne chaff, and
mean experimental liveweights were 28 and 33 kg respectively. All lambs
were prepared with catheters
in the cranial mesenteric vein (CMV), femoral artery (FA), jugular vein
and abomasum, and a blood
flow probe around the CMV. Cr-EDTA (0·139 mg Cr/ml,
∼ 0·2 ml/min) was infused abomasally for
24 h and L-[2,6-3H]phenylalanine (Phe)
(420±9·35 μCi into the abomasum) and
L-[U-14C]phenylalanine
(49·6±3·59 μCi into the jugular vein) were
also infused during the last 8 h. Blood from the
CMV and FA was sampled during the isotope infusions. At the end of
infusions, lambs were killed and tissue (n = 4) and digesta
(n = 2) samples removed from the small intestine (SI) of each
animal. Transfers of labelled and unlabelled Phe were measured between
SI tissue, its lumen and blood, enabling both fractional and absolute
rates of protein synthesis and gain to be estimated.Total SI mass increased significantly with feed intake
(P < 0·05), although not on a liveweight
basis. Fractional rates of protein gain in the SI tended to increase
(P = 0·12) with feed intake; these
rates were −16·2 (±13·7) and 23·3
(±15·2) % per day in lambs offered 400 and 1200 g/day
respectively. Mean protein synthesis and fractional synthesis rates
(FSR), calculated from the mean
retention of 14C and 3H in SI tissue, were both
positively affected by feed intake (0·01 < P < 0·05).
The choice of free Phe pool for estimating precursor specific radioactivity
(SRA) for protein synthesis had a major effect on FSR. Assuming that tissue
free Phe SRA represented precursor SRA, mean
FSR were 81 (±15) and 145 (±24) % per day in lambs offered
400 and
1200 g/day respectively. Corresponding estimates for free Phe SRA in
the FA
and CMV were 28 (±2·9) and 42 (±3·5) % per
day on 400 g/day, and 61 (±2·9) and 94 (±6·0)
on
1200 g/day. The correct value for protein
synthesis was therefore in doubt, although indirect evidence suggested
that blood SRA (either FA or CMV) may be closest to true precursor SRA.
This
evidence included (i) comparison with flooding
dose estimates of FSR, (ii) comparison of 3H[ratio ]14C
Phe SRA in free Phe pools with this ratio in SI
protein, and (iii) the proportion of SI energy use associated with
protein synthesis.Using the experimental model, the proportion of small intestinal protein
synthesis exported was estimated as 0·13–0·27
(depending on the choice of precursor) and was unaffected by feed intake.
The
contribution of the small intestine to whole body protein synthesis
tended to be higher in lambs offered 1200 g/day (0·21) than
in those
offered 400 g/day (0·13). The data obtained in this study
suggested a role for the small intestine in modulating amino acid supply
with changes in feed intake. At high intake (1200 g/day), the small
intestine
increases in mass and CMV uptake of amino acids
is less than absorption from the lumen, while at low intake (400 g/day),
this organ loses mass and CMV uptake of amino acids exceeds that absorbed.
The
implications of these findings are discussed.
The measurement of protein turnover by density labelling