Effect of feed intake on ovine hindlimb protein metabolism based on thirteen amino acids and arterio–venous techniques

Abstract

It has been suggested that protein synthesis in peripheral tissues: (1) responds in a curvilinear manner to increasing feed intake over a wide range of feeding levels; and (2) has a greater sensitivity to intake than protein breakdown. The aim of the present experiment was to test these hypotheses across the ovine hindlimb. Six growing sheep (6 -8 months, 30 -35 kg), with catheters in the aorta (two), posterior vena cava and jugular vein, received each of four intakes of dried grass pellets (0 : 5, 1 : 0, 1 : 5 and 2 : 5 Â maintenance energy; M) for a minimum of 7 d. A U-13 C-labelled algal hydrolysate was infused intravenously for 10 h and from 3 -9 h para-aminohippuric acid was infused to measure plasma flow. Arterial and venous plasma were obtained over the last 4 h and the concentrations and enrichments of thirteen 13 C-labelled amino acids (AA) were determined by GC -MS. As intake increased, a positive linear response was found for plasma flow, arterial concentrations of the aromatic and branched-chain AA, total flow of all AA into the hindquarters and net mass balance across the hindquarters (except glycine and alanine). Based on two separate statistical analyses, the data for protein synthesis showed a significant linear effect with intake (except for phenylalanine, glycine and alanine). No significant curvilinear effect was found, which tends not to support hypothesis 1. Nonetheless, protein synthesis was not significantly different between 0 : 5, 1 : 0 and 1 : 5 Â M and thus the 2 : 5 Â M intake level was largely responsible for the linear relationship found. There was no significant response in protein breakdown to intake, which supports hypothesis 2.

Effect of intake on whole body plasma amino acid kinetics in sheep

“…Furthermore, for muscle-dominated effects on curvilinearity then the proportional contribution of hind-quarter metabolism to whole body AA flux might be expected to increase. In fact, the proportion remained fairly constant (or tended to decrease) even when the conditions changed from net catabolism to net anabolism [19]. Of course, such responses may be masked due to the changes in the ratio of enrichments for 'true' precursor: blood (or plasma) as intake varies, but this would have entailed the measurement of the aminoacyl tRNA which was out-of the scope of the current study of low enrichment multiple tracers.…”

“…Muscle has been postulated to exhibit a curvilinear response to intake [16] and this would then account, in part, for the pattern of whole body protein flux. More detailed companion analysis [19] of muscle responses in the current animals did not show any curvilinear effect over the intake range for either protein synthesis or breakdown. Furthermore, for muscle-dominated effects on curvilinearity then the proportional contribution of hind-quarter metabolism to whole body AA flux might be expected to increase.…”

“…Six Suffolk cross wether lambs (age 6-8 months, 30-35 kg live weight) were surgically prepared with a polyvinyl or silicone rubber catheter into the aorta as previously described [19]. This catheter was used for arterial blood sampling.…”

“…AA kinetics across the hind-quarters were calculated as described previously [19], but with the enrichment of the AAs in the artery (rather than the vena cava) selected as the precursor pools. This allowed more direct comparison with the whole body flux data.…”

“…The kinetics were based on the infusion of a multiple U-13 C-labelled AA mixture that permits the simultaneous evaluation and comparison of the responses of 15 AA, both essential and non-essential, to intake. The contribution of hindquarter metabolism, based on data reported previously [19], to whole body responses as altered intake was also assessed. Finally, the absolute and relative changes associated with selection of either plasma or whole blood AA fluxes was also investigated.…”