Effects of Individual Essential Amino Acids on Growth Rates of Young Rats Fed a Low-Protein Diet.

To investigate the effects of individual essential amino acids (EAA) on growth and the underlying mechanisms, EAA individually supplemented a low-protein (LP) diet fed to young rats in the present study. Treatments were an LP diet that contained 6% crude protein (CP), a high-protein (HP) diet that contained 18% CP, and 10 LP diets supplemented with individual EAA to achieve an EAA supply equal to that of the HP diet. The CP concentration of the LP diet was ascertained from the results of the first experiment, which examined the effects of dietary CP concentrations on growth rates, with CP ranging from 2% to 26%. Weight gain was increased with the supplementation of His, Ile, Lys, Thr, or Trp as compared to the LP diet (p < 0.05). Feed intake was greater for the His-, Lys-, and Thr-supplemented treatments as compared to the LP group (p < 0.05). Protein utilization efficiency was lower for the HP group than other groups (p < 0.01). The supplementation of Leu, Lys, and Val led to reduced protein utilization efficiency (p < 0.05), but the supplementation of Thr and Trp led to greater efficiency than the LP group (p < 0.05). Compared to the LP group, plasma urea concentrations were elevated with individual EAA supplementation, with the exception of the Thr addition. The added EAA resulted in increased concentrations of the corresponding EAA in plasma, except for Arg and Phe supplementation. The supplementation of Arg, His, Leu, Lys, and Met individually stimulated mTORC1 pathway activity (p < 0.05), and all EAA resulted in the decreased expression of ATF4 (p < 0.05). In summary, the supplementation of His, Ile, Lys, Thr, or Trp to an LP diet improved the growth performance of young rats. Responses to His and Lys additions were related to the activated mTORC1 pathway and feed intake increases. The improved growth performance resulting from the addition of a single EAA is not solely attributed to the increased plasma availability of EAA. Rather, it may be the consequence of a confluence of factors encompassing signaling pathways, the availability of amino acids, and other associated elements. The additivity of these factors results in independent responses to several EAA with no order of limitation, as is universally encoded in growth models for all production animal species.

Supplementing Ruminally Protected Lysine, Methionine, or Combination Improved Milk Production in Transition Dairy Cows.

The objectives of this study were to evaluate the effects of dietary supplementation of ruminally protected lysine (RPL), or methionine (RPM), and their combination (RPML) on the production efficiency of transition cows. A total of 120 pre-partum multiparous Holstein cows were assigned to four treatments based on previous lactation milk production, days (d) of pregnancy, lactation, and body condition score (BCS). Cows were fed a basal diet [pre-calving: 1.53 Mcal/kg dry matter (DM) and post-calving: 1.70 Mcal/kg DM] with or without supplemental ruminally protected amino acids (RPAA). Treatments were the basal diets without supplemental amino acids (CONTROL, n = 30), with supplemental methionine (RPM, pre-calving at 0.16% of DM and post-calving at 0.12% of DM, n = 30), with supplemental lysine (RPL, pre-calving at 0.33% of DM and post-calving at 0.24% DM, n = 30), and the combination (RPML, pre-calving at 0.16% RPM + 0.33% RPL of DM and post-calving at 0.12% RPM + 0.24 % RPL DM, n = 30). The dietary content of lysine was balanced to be within 6.157.2% metabolizable protein (MP)-lysine and that of methionine was balanced within 2.1-2.35% MP-methionine. Dry matter intake (DMI) was measured daily. Milk samples were taken on d 7, 14, and 21 days relative to calving (DRC), and milk yields were measured daily. Blood samples were taken on d -21, -14, -7 before expected calving and d 0, 7, 14, and 21 DRC. Data were analyzed using SAS software. There were significant Trt × time interactions (P < 0.01) for DMI pre- and post-calving period. The CON cows had lower DMI than RPM, RPL, and RPML, both pre-calving (P < 0.01) and post-calving periods (P < 0.01). Energy-corrected milk (P < 0.01), milk fat (P < 0.01), protein (P = 0.02), and lactose (P < 0.01) percentage levels were greater for RPM, RPL, and RPML cows compared to CON. Supplementing RPAA assisted in maintaining BCS post-calving than CON (P < 0.01). Blood concentrations of ß-hydroxybutyrate decreased with RPM or RPL or the combination pre-calving (P < 0.01) and tended to decrease post-calving (P = 0.10). These results demonstrated that feeding RPL and RPM improved DMI and milk production efficiency, maintained BCS, and reduced ß-hydroxybutyrate concentrations of transition cows.

Isoleucine and leucine independently regulate mTOR signaling and protein synthesis in MAC-T cells and bovine mammary tissue slices.

Understanding the regulatory effects of individual amino acids (AA) on milk protein synthesis rates is important for improving protein and AA requirement models for lactation. The objective of this study was to examine the effects of individual essential AA (EAA) on cellular signaling and fractional protein synthesis rates (FSR) in bovine mammary cells. Omission of L-arginine, L-isoleucine, L-leucine, or all EAA reduced (P < 0.05) mammalian target of rapamycin (mTOR; Ser2448) and ribosomal protein S6 (rpS6; Ser235/236) phosphorylation in MAC-T cells. Phosphorylation of mTOR and rpS6 kinase 1 (S6K1; Thr389) decreased (P < 0.05) in the absence of L-isoleucine, L-leucine, or all EAA in lactogenic mammary tissue slices. Omission of L-tryptophan also reduced S6K1 phosphorylation (P = 0.01). Supplementation of L-leucine to media depleted of EAA increased mTOR and rpS6 and decreased eukaryotic elongation factor 2 (Thr56) phosphorylation (P < 0.05) in MAC-T cells. Supplementation of L-isoleucine increased mTOR, S6K1, and rpS6 phosphorylation (P < 0.05). No single EAA considerably affected eukaryotic initiation factor 2-α (eIF2α; Ser51) phosphorylation, but phosphorylation was reduced in response to provision of all EAA (P < 0.04). FSR declined when L-isoleucine (P = 0.01), L-leucine (P = 0.01), L-methionine (P = 0.02), or L-threonine (P = 0.07) was depleted in media and was positively correlated (R = 0.64, P < 0.01) with phosphorylation of mTOR and negatively correlated (R = -0.42, P = 0.01) with phosphorylation of eIF2α. Such regulation of protein synthesis will result in variable efficiency of transfer of absorbed EAA to milk protein and is incompatible with the assumption that a single nutrient limits protein synthesis that is encoded in current diet formulation strategies.

Amino acid supplementation does not alter whole-body phenylalanine kinetics in Arabian geldings.

Stable isotope infusion methods have not been extensively used in horses to study protein metabolism. The objectives were to develop infusion and sampling methodologies for [1-(13)C] phenylalanine and apply these methods to determine whether the addition of supplemental amino acids to a control diet affected whole-body phenylalanine kinetics in mature horses. Arabian geldings were studied using a 6-h primed (9 µmol/kg), constant (6 µmol · kg(-1) · h(-1)) i.v. infusion of L-[1-(13)C] phenylalanine, with blood and breath sampled every 30 min, to measure whole-body phenylalanine kinetics in response to receiving the control diet (n = 12) or the control diet supplemented with equimolar amounts of glutamate (+Glu; 55 mg · kg(-1) · d(-1); n = 5), leucine (+Leu; 49 mg · kg(-1) · d(-1); n = 5), lysine (+Lys; 55 mg · kg(-1) · d(-1); n = 5), or phenylalanine (+Phe; 62 mg · kg(-1) · d(-1); n = 6). The plasma concentrations of the supplemented amino acid in horses receiving the +Leu, +Lys, and +Phe diets were 58, 53, and 36% greater, respectively, than for the control treatment (P < 0.05). Isotopic plateau was attained in blood [1-(13)C] phenylalanine and breath (13)CO(2) enrichments by 60 and 270 min, respectively. Phenylalanine flux (+20%) and oxidation (+110%) were greater (P < 0.05) in horses receiving the +Phe treatment than in those fed the control diet. There was no effect of treatment diet on nonoxidative phenylalanine disposal or phenylalanine release from protein breakdown. The developed methods are a valuable way to study protein metabolism and assess dietary amino acid adequacy in horses and will provide a useful tool for studying amino acid requirements in the future.