Measurement of protein turnover in the small intestine

of lambs. 2. Effects of feed intake
Authors: NEUTZE S. A. , GOODEN J. M. , ODDY V. H. 
Published: July 2002

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.