Abstract/Summary

The contribution of the total splanchnic tissue (TSP; portal-drained viscera (PDV) plus liver) to whole-body protein metabolism was estimated in relation to intake (0×6, 1×0 and 1×6 maintenance requirements), in six multicatheterized growing beef steers used in a double 3x3 Latin square design. At the end of each 21 d experimental period, [1-13C]leucine was infused into a jugular vein (1×05 mmol/h for 5 h, preceded by a priming dose of 1×05 mmol). Arterial, portal and hepatic blood samples were collected hourly during the infusion. The increment in TSP leucine irreversible loss rate (ILR) observed with increasing intake reached significance (P,0×10) only for PDV, while whole-body ILR increased markedly (P,0×001) with intake. The relative contribution of TSP to whole-body leucine ILR averaged 44% (25% from PDV and 19% from the liver). Although these proportions were not affected by intake, on an incremental basis more than 70% of the increase of whole-body leucine ILR between the 0×6 and 1×0 X maintenance originated from the changes in TSP ILR, while the corresponding value was below 13% between 1×0 and 1×6 ´ maintenance. Total whole-body leucine oxidation and fractional oxidation increased (P,0×05) with intake. Protein retention increased with intake (P,0×01), as a result of a greater increase in protein synthesis compared with protein degradation. Protein breakdown had a major impact on protein turnover as 65% of the protein synthesized was degraded when intake varied from 1×0 to 1×6 x maintenance. Net leucine portal absorption increased (P,0×001) with intake and represented 1, 16 and 23% of whole body leucine ILR, for 0×6, 1×0 and 1×6 ´ maintenance, respectively. Although leucine oxidation was not a major component of whole body ILR (9×3–19×9 %), it represented 69% of the net available leucine (portal absorption) even at 1×6 ´ maintenance. The lower relative contribution of the TSP to whole-body leucine ILR at higher intake indicates the proportional increase in the metabolic activity of peripheral tissues as the animals moved into positive protein balance.