The effects of breed and level of nutrition on whole-body and muscle protein metabolism in pure-bred Aberdeen Angus and Charolais beef steers

Eighteen pure-bred steers (live weight 350 kg) from each of two breeds, Abe rdeen Angus (AA) and Charolais (CH), were split into three equal groups (si x animals each) and offered three planes of nutrition during a 20-week peri od. The same ration formulation was offered to all animals with amounts adj usted at 3-week intervals to give predicted average weight gains of either 1 . 0 kg/d (M/M group) or 1 . 4 kg/d (WH group). The remaining group (M/H) were offered the same amount of ration as the M/M group until 10 weeks befo re slaughter when the ration was increased to H. Data on animal performance , carcass characteristics and fibre-type composition in skeletal muscle are presented elsewhere (Maltin et al. 2000; Sinclair et al. 2000). On three o ccasions (17, 10 and 2 weeks before slaughter) the animals were transferred to metabolism stalls for 1 week, during which total urine collection for q uantification of N-tau-methylhistidine (N-tau-MeH) elimination was performe d for 4d. On the last day, animals were infused for 11 h with [H-2(5)]pheny lalanine with frequent blood sampling (to allow determination of whole-body phenylalanine flux) followed by biopsies from, m. longissimus lumborum and m. vastus lateralis to determine the fractional synthesis rate of mixed mu scle protein. For both breeds, the absolute amount of N-tau-MeH eliminated increased with animal age or weight (P < 0 . 001) and was significantly gre ater for CH steers, at all intake comparisons, than for AA (P < 0 . 001). E stimates of fractional muscle breakdown rate (FBR; calculated from N-tau-Me H elimination and based on skeletal muscle as a fixed fraction of live weig ht) showed an age (or weight) decline for M/M and H/H groups of both breeds (P < 0 . 001). FBR was greater for the H/H group (P = 0 . 044). The M/H gr oup also showed a lower FBR for the first two measurement periods (both at M intake) but increased when intake was raised to H. When allowance was mad e for differences in lean content (calculated from fat scores and eye muscl e area in carcasses at the end of period 3), then were significant differen ces in muscle FBR with intake (P = 0 . 012) but not between breed. Whole-bo dy protein flux (WBPF: g/d) based on plasma phenylalanine kinetics increase d with age or weight (P < 0 . 001) and was similar between breeds. The WBPF was lower for M/M compared with H/H (P < 0 . 001) based on either total or per kg live weight(0 . 75). Muscle protein fractional synthesis rate (FSR) declined with age for both breeds and tended to be higher at H/H compared with M intakes (intake x period effects, P < 0 . 05). Changing intake from M to H caused a significant increase (P < 0 . 001) in FSR. The FSR values f or AA were significantly greater than for CH at comparable ages (P = 0 . 04 4). Although FSR and FBR responded to nutrition, these changes in protein m etabolism were not reflected in differences in meat eating quality (Sinclai r et al. 2000).