Postnatal changes in extracellular concentrations of free amino acids are associated with declining fractional protein synthesis rates in skeletal muscles of fed pigs

Abstract

This study investigated postnatal changes of skeletal muscle protein fractional synthesis rates (FSR) and their correlations with serum concentrations of hormones (insulin, growth hormone and cortisol), IGF-I and plasma concentrations of free amino acids (AA) in fed pigs. Thirty-six purebred Yorkshire gilts were used for blood and muscle sampling at 1, 4, 6, 12, 20 and 28 (1 wk postweaning) d of age. The pigs received an ip injection of a flooding dose of Phe containing L-[ring-2H5]Phe (40 molar%) in saline. Serum samples before the Phe injection and plasma, loin and ham samples at 30 min post-injection were obtained for the determinations of tracer Phe enrichments by GC-MS, free AA by HPLC, and hormones and IGF-I by RIA. Tracer Phe enrichments in the plasma and the skeletal muscle free AA pools were flooded between 25 and 27 molar%. Loin and ham FSR decreased linearly (P<0.05) by 4 folds from d 1 to 28 of age. Postnatal changes of ham FSR were positively correlated (P<0.05, r=0.33) with serum insulin concentrations. Postnatal changes of ham and loin muscle FSR were positively correlated (P<0.05) with plasma concentrations of Asn, citrulline, Gln, His, Leu, ornithine, Phe, Ser, taurine, Tyr and Val (r=0.34–0.72) and negatively correlated (P<0.05) with Gly (r= −0.42) or Ile (r=−0.33). The results suggest that the postnatal decreasing skeletal muscle FSR in fed pigs is well associated with extracellular free AA concentrations as well as serum insulin levels.