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Chronic improvement of amino acid nutrition stimulates initiation of global mRNA translation in tissues of sheep without affecting protein elongation

M. T. Connors^*, D. P. Poppi^* and J. P. Cant

^* Schools of Animal Studies and Veterinary Science, University of Queensland, St Lucia 4072, Brisbane, Australia Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, N1G 2W1, Canada

jcant{at}uoguelph.ca

Abstract

Initiation of mRNA translation and elongation of the polypeptide chain are two regulated processes responsible for the short-term postprandial acceleration of protein synthesis in animal tissues. It is known that a chronic increase in the absorptive supply of amino acids stimulates protein synthesis in ruminant animals but effects on translation initiation and elongation are unknown. To determine whether initiation or elongation phases of global mRNA translation are affected by chronic elevation of amino acid supply, 24 ewe lambs of 25.9 ± 2.5 kg BW were randomly allocated to 4 treatment groups of 6 lambs each. All lambs received a basal diet of barley and hay at 1.2 times maintenance ME intake. Treatments were an i.v. saline infusion as a control, i.v. infusion of 6 essential amino acids (EAA; Arg, Lys, His, Thr, Met, Cys) for 10 d, i.v. infusion of the same EAA excluding Met and Cys (EAA-SAA) for 10 d, and an oral drench of fishmeal twice daily for 17 d. Fishmeal supplementation supplied an extra 719 mg N•kg^-0.75•d^-1 and N retention was increased 519 mg•kg^-0.75•d^-1 over the control. The EAA treatment supplied an extra 343 mg N•kg^-0.75•d^-1 directly into the blood and N balance was increased by 268 mg•kg^-0.75•d^-1. Deletion of Met plus Cys from EAA had no effect on N balance. The results indicate that Met plus Cys did not limit body protein gain on either the basal diet alone or the basal diet plus six amino acids. Protein fractional synthesis rates in liver, duodenum, skin, rumen, semimembranosus, and LM were measured by a flooding dose procedure using L-[ring-2,6-³H]-Phe. Ribosome transit times were estimated from the ratio of nascent to total protein-bound radioactivities. Fishmeal and EAA treatments had no effect on RNA, DNA, or protein contents of tissues but FSR, translational efficiency, and concentrations of active ribosomes were consistently elevated. Ribosome transit time was not affected by long-term amino acid supply. We conclude that the chronic stimulation of protein synthesis by long-term i.v. infusion of EAA or supplementation with an undegradable protein source is brought about by an improvement in the rate of initiation of mRNA translation with no change in the rate of polypeptide chain elongation.

Key Words: ribosome transit time • protein synthesis • sheep • amino acids • fishmeal