This study was undertaken to examine the hypothesis that increasing the arterial flow of amino acids to the hindlimb would lead to increased hindlimb protein synthesis and muscle growth. The hypothesis was examined in two different ways on crossbred lambs. Firstly, the nitrogen balance response to a mix of six essential amino acids or casein infused into the abomasum was measured in lambs on a ryegrass hay diet (130gCP/kgDM, 0.644 DMD). Secondly, hindlimb muscle growth was compared after the infusion of the same mix of six essential amino acids into the external iliac artery of one hindlimb, while the other hindlimb was infused with a saline control in lambs on a similar ryegrass hay diet (167gCP/kgDM, 0.715 DMD). This contra-lateral infiision method enabled the arterial concentration of amino acids passing into the infused hindlimb to be increased to about 70% above that in the control leg.
The results showed that abomasal
infusion of limiting amino acids and protein increased nitrogen balance (by 111-530%), but that iliac artery infusion of amino acids did not significantly increase hindlimb muscle growth.
Fat deposition and clean wool production were altered to a greater extent than muscle growth by the hindlimb infusion of amino acids, though usually not significantly. In experiment 1, large rapidly growing wethers on a lucerne pellet diet showed a 4% increase in subcutaneous fat and a 10% increase in wool production on infused compared to control legs, though neither were significant (p>0.05). In experiment 2, smaller slow growing ewes on a ryegrass hay diet showed a decrease of 8-10 % (p>0.05) in subcutaneous and inter-muscular fat, an increase of 4-17% (significant for some animals) in intra-muscular fat and an increase (significant for some animals) in clean wool production of 8-37%.
It was concluded that simply increasing the
concentration of limiting amino acids arriving at a tissue bed such as muscle was not sufficient to elicit a response of muscle growth.