Effect of plasma insulin and branched-chain amino acids on skeletal muscle protein synthesis in fasted lambs

Abstract

The increase in fractional rate of protein synthesis (Ks) in the skeletal muscle of growing rats during the transition from fasted to fed state has been explained by the synergistic action of a rise in plasma insulin and branched-chain amino acids (BCAA). Since growing lambs also exhibit an increase inKswith level of feed intake, the objective of the present study was to determine if this synergistic relationship between insulin and BCAA also occurs in ruminant animals. Six 30 kg fasted (72 h) lambs (8 months of age) received each of four treatments, which were based on continuous infusion into the jugular vein for 6 h of: (1) saline (155 mmol NaCl/l); (2) a mixture of BCAA (0·778 μmol leucine, 0·640 μmol isoleucine and 0·693 μmol valine/min·kg); (3) 18·7 μmol glucose/min·kg (to induce endogenous insulin secretion); (4) co-infusion of BCAA and glucose. Within each period all animals received the same isotope of phenylalanine (Phe) as follows: (1) l-[1-13C]Phe; (2) l-phenyl-[ring2H5]-alanine; (3) l-[15N]Phe; (4) l-[ring 2,6-3H]Phe. Blood was sampled serially during infusions to measure plasma concentrations of insulin, glucose and amino acids, and plasma free Phe isotopic activity; biopsies were taken 6 h after the beginning of infusions to determineKsinm. longissimus dorsiandvastusmuscle. Compared with control (saline-infused) lambs,Kswas increased by an average of 40 % at the end of glucose infusion, but this effect was not statistically significant in either of the muscles sampled. BCAA infusion, alone or in combination with glucose, also had no significant effect onKscompared with control sheep.Kswas approximately 60 % greater forvastusmuscle than form. longissimus dorsi(P>0·01), regardless of treatment. It is concluded that there are signals other than insulin and BCAA that are responsible for the feed-induced increase inKsin muscle of growing ruminant animals.