Hindlimb protein turnover and muscle protein synthesis in lambs: a comparison of techniques

A combination of arterio-venous difference, kinetic isotope transfer and blood flow rate techniques were used to measure tyrosine metabolism across hindlimb tissues of nine growing lambs (average live weight 36.5 kg) fed on a range of dry matter intakes. Muscle protein synthesis was measured using a continuous infusion technique and compared with simultaneous estimates of hindlimb protein turnover calculated from the values for tyrosine metabolism. When the specific radioactivity (SRA) of tyrosine in the arterial plasma free pool was assumed to be the same as the SRA of tyrosine in the direct precursor pool of protein synthesis, hindlimb protein synthesis (ksav; 3.66 (SEM 0.50) %/d) was significantly (P < 0.001) higher (68%) than muscle protein synthesis (ksp; 2.18 (SEM 0.31) %/d) but was similar to the value for muscle protein synthesis calculated using the homogenate free tyrosine SRA (ksh; 3.35 (SEM 0.42) %/d). Hindlimb and muscle protein synthesis (y) were both significantly related to dry matter intake (chi) (ksav, r2 0.667, P = 0.007; ksh, r2 0.968, P < 0.001) and there was no significant difference between the slopes (P = 0.532) and intercepts (P = 0.945) of the two regression lines. The results demonstrate that hindlimb protein turnover cannot be quantitatively compared with muscle protein synthesis, probably due to high protein metabolic activity in non-muscular tissues within the hindlimb, although similar responses in protein synthetic rate to the level of feed intake were observed between hindlimb and muscle tissues.