Responses in tissue protein synthesis to sub- and supra-maintenance intake in young growing sheep: comparison of large-dose and continuous-infusion techniques.

In ten lambs (average live weight 33 kg), five offered 300 g/d (approximately 0.6 x maintenance; L) and five 900 g/d (1.8 x maintenance; H), tissue protein synthesis was measured by three procedures simultaneously. The techniques involved continuous infusion of [U-14C]phenylalanine and [1-13C]leucine over 7-8 h followed by a terminal large dose of [15N]phenylalanine during the last 30 or 60 min. Rates of protein synthesis were then calculated based on the free amino acid or oxo-acid isotopic activity in either arterial, iliac venous blood or tissue homogenate for the continuous-infusion studies, or on plasma or tissue homogenate for the large-dose procedure. For muscle (> 99%), and to a lesser extent skin (85-93%), effective flood conditions were achieved with the [15N]phenylalanine but were either not established or maintained for liver and tissues of the gastrointestinal tract (< 50%). The large dose of phenylalanine also caused changes in the concentration and isotopic activity of blood leucine and 4-methyl-2-oxo-pentanoate. Based on the assumption that the large-dose procedure yields the closest value for the true rate of protein synthesis (L 1.97%/d, H 2.85%/d) then, for muscle, only values based on the homogenate as precursor gave comparable results for both leucine (L 1.83%/d, H 3.01%/d) and phenylalanine (L 1.67%/d, H 2.71%/d) continuous infusion. The values based on the arterial or venous amino or oxo-acid were significantly less, more so at the lower intake. In contrast, for skin, a tissue dominated by export protein synthesis, values from the large-dose procedure (L 6.37%/d, H 10.98%/d) were similar to those derived with arterial or venous metabolites as precursor (L 5.23 and 6.93%/d, H 9.98 and 11.71%/d for leucine), but much less than those based on homogenate data. Based on the large-dose technique, protein synthesis increased with intake in muscle (P < 0.001), skin (P = 0.009) and liver (26.7 v. 30.5%/d; P = 0.029). The contributions of muscle and skin to total protein synthesis were approximately equal. The incremental efficiency of conversion for muscle of synthesized protein into deposition appeared to be similar to values reported for rodents.