The market for amino acids: understanding supply and demand of substrate for more efficient milk protein synthesis

Feed restriction of lactating cows triggers acute downregulation of mammary mammalian target of rapamycin signaling and chronic reduction of mammary epithelial mass

While there is generally no consensus about how nutrients determine milk synthesis in the mammary gland, it is likely that the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) plays a role as a key integrator of nutritional and mitogenic signals that can influence a multitude of catabolic and anabolic pathways. The objectives of this study were to evaluate acute changes (<24 h) in translational signaling, in addition to chronic changes (14 d) in mammary gland structure and composition, in response to a severe feed restriction. Fourteen lactating Holstein dairy cows were assigned to either ad libitum feeding (n = 7) or a restricted feeding program (n = 7). Feed-restricted cows had feed removed after the evening milking on d 0. Mammary biopsies and blood samples were collected 16 h after feed removal, after which cows in the restricted group were fed 60% of their previously observed ad libitum intake for the remainder of the study. On d 14, animals were slaughtered and their mammary glands dissected. In response to feed removal, an acute increase in plasma nonesterified fatty acid concentration was observed, concurrent to a decrease in milk yield. In mammary tissue, we observed downregulation of the mTORC1-S6K1 signaling cascade, in addition to reductions in mRNA expression of markers of protein synthesis, endoplasmic reticulum biogenesis, and cell turnover (i.e., transcripts associated with apoptosis or cell proliferation). During the 14 d of restricted feeding, animals underwent homeorhetic adaptation to 40% lower nutrient intake, achieving a new setpoint of 14% reduced milk yield with 18% and 29% smaller mammary secretory tissue DM and CP masses, respectively. On d 14, no treatment differences were observed in markers of protein synthesis or mammary cell turnover evaluated using gene transcripts and immunohistochemical staining. These findings implicate mTORC1-S6K1 in the early phase of the adaptation of the mammary gland's capacity for milk synthesis in response to changes in nutrient supply. Additionally, changes in rates of mammary cell turnover may be transient in nature, returning to basal levels following brief alterations that have sustained effects.

Effects of isoleucine, lysine, valine, and a group of non-essential amino acids on mammary amino acid metabolism in lactating dairy cows

Intracellular amino acids (AA) regulate milk protein synthesis within the mammary glands by modifying mammary plasma flow (MPF) and AA transporter activity. Amino acid transporters catalyze translocation using Na+-gradient, substrate gradient (uniporters), and exchange mechanisms; further, they exhibit specificity for individual AA or groups of AA with similar side-chain properties within each transport system. Non-essential AA are actively transported through Na+-dependent transporters and, thus, are often utilized as intracellular currencies for EAA transport through exchange transporters. Therefore, it was hypothesized that individual EAA supplementation would compete with other EAA for shared transporters, and supplementation with Ala, Gln, and Gly would stimulate EAA transport through exchange transporters. Ten primiparous lactating dairy cows were divided into 2 groups based on milk production and were randomly assigned to treatment sequences within 2 balanced 5 × 5 Latin Squares by group. Period length was 14 d. Treatments were 9-d jugular infusions of 1) saline; 2) 34.5 g Val/d; 3) 32.7 g Ala/d: 40 g Gln/d: 26.7 g Gly/d (AQG); 4) 43 g Lys/d; or 5) 33.5 g Ile/d. All cows were fed a common base diet formulated to contain 15.0% CP. Ile, Lys, or AQG infusions did not affect milk protein or milk production; however, Val infusion decreased both. The effects of Val infusion on milk protein production appeared to be partially driven by decreased DMI. The decline in milk protein percentage indicated that milk lactose production was also affected. Additionally, Val infusion increased MPF efficiency (MPF/Milk; L/L) by approximately 44%. Val infusion tended to decrease or decreased mammary net uptakes of Lys, Leu, Met, and total AA. Ile infusion tended to increase its mammary net uptakes but did not affect any other AA. Lys and AQG infusions did not affect any mammary net uptakes. Val infusion tended to decrease Phe and total NEAA mammary clearance rates. AQG infusion stimulated Tyr clearance rates and tended to decline System N mammary clearance rates. Mammary uptake to milk protein output ratios (U:O) of BCAA did not differ from 1 for Val-infused cows, which indicated that little intramammary catabolism was occurring. Additionally, the average NEAA U:O in response to all treatments except Val was 0.70, but Val-infused cows had NEAA U:O that averaged 0.09 indicating increased synthesis within the glands. The effects of Val on mammary net clearance rates of multiple EAA support the incorporation of AA limitations in ration optimizers to prevent AA imbalances. It is possible that over-supplementation of EAA other than Val may also decrease DMI and mammary activity. Identifying efficiency apexes for each of the EAA will allow more precise diet formulation and supplementation, leading to improved production efficiency.

Metabolomics and amino acid profiling of plasma reveals the metabolite profiles associated with nitrogen utilisation efficiency in primiparous dairy cows

Nitrogen (N) utilisation efficiency (NUE, milk N yield [g/d]/N intake [g/d]) is an important performance indicator in dairy farming. Determining the NUE-associated blood metabolite profile will contribute to the optimisation of nutritional strategies to further improve NUE among dairy cows. Here, 20 primiparous lactating cows with days in milk ranging from 95 to 115 days were selected from a total of 1 221 cows. Each cow's N intake and milk N yield were measured for 7 days. Subsequently, blood samples were collected before morning feeding. Based on analysis and calculations, cows were retrospectively classified into two groups based on their NUE values, namely, a low NUE group (LNUE, NUE = 24.8 ± 1.6%, n = 10, mean ± SD) and a high NUE group (HNUE, NUE = 35.2 ± 1.7%, n = 10, mean ± SD). Plasma samples were selected from six cows in each group for metabolomics and amino acid profiling. Among the 41 differential metabolites (DMs) identified in the metabolomic analysis, sucrose, MG(0:0/22:1(13Z)/0:0), 2-amino-6-hydroxyhexanoic acid, and L-glutamine exhibited significant correlations with NUE, milk yield, and BW (P < 0.05). Moreover, the five differential amino acids and amino acid metabolites (DAAs) identified in the amino acid profiling and 5 of the 6 differential amino acids and amino acid conjugates identified by plasma metabolomics were found to be less abundant in the HNUE group (P < 0.05). Specifically, there was a 39.4% decrease in L-arginine content and a 29.2% decrease in L-glutamine content (P < 0.05). Pathway analysis indicated that the DMs and DAAs were mainly involved in arginine biosynthesis, glutathione metabolism, arginine and proline metabolism, and tryptophan metabolism (pathway impact > 0.1). These results provided new insights into the new blood metabolite profile associated with NUE in dairy cows. These new insights can provide foundational information for the formulation of new strategies to further enhance NUE in dairy cows.

Intravenous infusion of 5-hydroxytryptophan to mid-lactation Holstein cows transiently affects milk production and circulating amino acid concentrations

In dairy cows, the lactating mammary glands synthesize serotonin, which acts in an autocrine-paracrine manner in the glands and is secreted into the periphery. Serotonin signaling during lactation modulates nutrient metabolism in peripheral tissues such as adipose and liver. We hypothesized that the elevation of circulating serotonin during lactation would increase nutrient partitioning to the mammary glands, thereby promoting milk production. Our objective was to elevate circulating serotonin via intravenous infusion of the serotonin precursor 5-hydroxytryptophan (5-HTP) to determine its effects on mammary supply and extraction efficiency of AA, and milk components production. Twenty-two multiparous mid-lactation Holstein cows were intravenously infused with 5-HTP (1 mg/kg body weight) or saline, in a crossover design with two 21-d periods. Treatments were infused via jugular catheters for 1 h/d, on d 1 to 3, 8 to 10, and 15 to 17 of each period, to maintain consistent elevation of peripheral serotonin throughout the period. Milk and blood samples were collected in the last 96 h of each period. Whole-blood serotonin concentration was elevated above saline control for 96 h after the last 5-HTP infusion. Dry matter intake was decreased for cows receiving 5-HTP, and on average they lost body weight over the 21-d period, in contrast to saline cows who gained body weight. Milk production and milk protein yield were lower in cows receiving 5-HTP during the 3 infusion days, but both recovered to saline cow yields in the days after. Although milk fat yield exhibited a day-by-treatment interaction, no significant difference occurred on any given day. Milk urea nitrogen concentration was lower in 5-HTP cows on the days following the end of infusions, but not different from saline cows on infusion days. Meanwhile, plasma urea nitrogen was not affected by 5-HTP infusion. Circulating concentrations of AA were overall transiently decreased by 5-HTP, with concentrations mostly returning to baseline within 7 h after the end of 5-HTP infusion. Mammary extraction efficiency of AA was unaffected by 5-HTP infusion. Overall, both lactation performance and circulating AA were transiently reduced in cows infused with 5-HTP, despite sustained elevation of circulating serotonin concentration.

Relationship between rate of glucose or propionate infusion and milk protein yield and concentration in dairy cows: A meta-regression

Although postruminal glucose infusion into dairy cows has increased milk protein yield in some past experiments, the same trend has not been observed in others. A meta-regression of 64 sets of observations from 29 previously published glucose and propionate infusion studies in dairy cattle, treating study and experiment (study) as random effects, was performed to establish the general effects of glucose equivalent (GlcE) infusion rate on milk true protein (MTP) yield and content, if any, and to identify independent, fixed-effect variables that accounted for the changes in MTP yield and content that were observed. Candidate explanatory variables included rate and site of infusion, diet composition and intake, body weight and lactation stage of the cows, and the change in nutrient intake between GlcE and control treatments. Across all studies, according to a model containing only the random effects of study and experiment, GlcE infusion at an average of 954 g/d increased MTP yield by 26 g/d, on average, whereas mean MTP content was not affected. Backward stepwise elimination of potential explanatory variables from a full mixed model produced a final, reduced model for MTP yield that retained a positive, second-order quadratic effect of infusion rate of GlcE and a positive, linear effect of the change in crude protein intake (CPI) between GlcE treatment and control. This change in CPI due to GlcE infusion ranged from -0.546 to 0.173 kg/d in the dataset. The model fit indicated that when CPI was allowed to drop during GlcE infusion, the effect of GlcE on MTP yield was smaller than when CPI was maintained or increased, in a manifestation of the classic protein:energy interaction. The final reduced model for MTP content contained the same explanatory variables as for MTP yield, plus a negative effect of intravenous compared with gastrointestinal infusion. Overall, the meta-analysis revealed that both MTP yield, and content were positively related to GlcE infusion rate and to the change in CPI between glucose treatment and control.

Effects of L-arginine and arginine-arginine dipeptide on amino acids uptake and αS1-casein synthesis in bovine mammary epithelial cells

Arginine (Arg), as an important functional amino acids (AA), is essential for milk protein synthesis in lactating ruminants. Arg shares transporters with cationic and neutral AA in mammary epithelial cells. Therefore, competitive inhibition might exist among these AA in uptake by mammary epithelial cells. In this study, cultured bovine mammary epithelial cells (BMEC) were used as the model to investigate whether the availability of L-Arg (0.7, 1.4, 2.8, 5.6, and 11.2 mM) affects the uptake of other AA and if this related to αS1-casein synthesis, and whether Arginine-Arginine (Arg-Arg) substituting part of free L-Arg can alleviate competitive inhibition among Arg and other AA, so as to promote αS1-casein synthesis. Our results showed that 2.8 mM L-Arg generated the greatest positive effects on αS1-casein synthesis and the activation of mammalian target of rapamycin (mTOR) signaling pathway (P < 0.01). With L-Arg supply increasing from 0.7 to 11.2 mM, the net-uptake of other AA (except Glu and Ala) decreased linearly and quadratically (Plinear < 0.01; Pquadratic < 0.01). Compared with 2.8 mM, the net-uptake of essential amino acids (EAA) and total amino acids (TAA) were lower at 11.2 mM L-Arg group, while greater at 1.4 mM L-Arg group (P < 0.01). Arg-Arg dipeptide replacing 10% free L-Arg increased αS1-casein synthesis (P < 0.05), net-uptake of EAA and TAA, as well as phosphorylation level of mTOR and p70 ribosomal protein S6 kinase (P70S6K) and mRNA expression of oligopeptide transporter 2 (PepT2; P < 0.01). These observations suggested that the increased αS1-casein synthesis by 10% Arg-Arg dipeptide might be related to the increase of AA availability and the activation of mTOR signaling pathway in BMEC.

Effects of methionine, leucine, and insulin on circulating concentrations and mammary extraction of energy substrates and amino acids in lactating dairy cows

The aim of this experiment was to test whether insulin potentiates the effects of two abomasally infused amino acids (AA), leucine and methionine (LM), on mammary extraction efficiency of energetic and nitrogenous nutrients. Six lactating Holstein cows (155 ± 9 DIM) were ruminally-cannulated and had the right carotid artery subcutaneously transposed. Cows were fed a 20% metabolizable protein-restricted diet and abomasally infused with water (8 L/d) or AA (Met 26 g/d, Leu 70 g/d) for 8 h/d, for 7 days. On the last day of each period, cows were intravenously infused with saline (0.9% NaCl, 110 mL/h) or subjected to 8 h hyperinsulinemic clamp (IC) alongside abomasal infusions. For IC, insulin was infused at 1 µg/kg/h. Normoglycemia was maintained by varying glucose (50% w/v in water) infusion rate based on coccygeal vein glucose concentration. Carotid arterial and subcutaneous abdominal (mammary) vein blood samples were collected at 0, 1, 2, 4, and 6 h from the start of infusions. Milk weights and samples for baseline measurements of production were taken on day 5 PM, day 6 AM and PM, and day 7 AM of the experimental period. A final milk weight and sample was taken immediately after abomasal and intravenous infusions on day 7 PM for assessing the interaction between insulin and the infused AA. The experiment had an incompletely replicated Latin square design with a 2 × 2 factorial arrangement of treatments (abomasal and intravenous infusion). Baseline milk production when cows were only receiving abomasal infusions was largely unaffected by LM, but milk protein yield tended to be decreased. On day 7, LM tended to positively increase milk fat and de novo fatty acid content, and IC tended to decrease milk protein content. Both milk urea nitrogen and plasma urea nitrogen were decreased by IC. Circulating AA concentrations in plasma were decreased by both LM and IC, but mammary extraction efficiency was affected by neither. Infusion of LM had no effect on any energy metabolite analyzed. Circulating non-esterified fatty acid concentration was decreased by IC, with no effect on mammary extraction efficiency. Mammary extraction efficiency of both acetate and β-hydroxybutyrate were decreased by IC. Overall, while both circulating concentrations of energy metabolites and amino acids were decreased in response to treatments, this was not due to improved mammary extraction efficiency.

Energy: Protein Ratio in Ruminants: Insights from the Intragastric Infusion Technique

Simple Summary

One key question that has confounded nutritional scientists for years is how the ruminant responds metabolically with respect to energy and nitrogen utilisation when no exogenous energy is consumed. Fasting metabolism studies using the intragastric infusion technique (IIT) showed this to be a glucose-deficient state characterised by elevated nitrogen excretion and heat production. However, modern feeding systems continue to adopt fasting as the basis for measuring utilisation efficiency of nutritionally balanced diets, giving rise to the false concept of greater feed utilisation below than above energy maintenance. Another IIT finding was that given the animal’s genetic capacity for protein accretion and provided a rumen undegradable protein is fed, ruminants do not catabolise amino acids as an energy source but instead retain these to attain substantial gains in tissue protein deposition, fuelled by endogenous energy reserves. This suggests that endogenous fat reserves could be used to retain body protein when feed supplies are scarce or of poor nutritive value and questions the need to use high-energy diets in the finishing pre-slaughter period. Moreover, body protein and body fat deposition were also shown to be negatively correlated, contradicting current feeding systems which assume that nitrogen retention is always negative in an underfeeding situation.

Abstract

Studies on energy:protein ratio in ruminants are constrained by rumen fermentation since it governs nutrient metabolism and the ratio of energy:protein yielding nutrients available for absorption. By circumventing rumen fermentation, the total intragastric infusion technique (IIT) allowed objective quantification of maintenance energy and protein requirements, volatile fatty acid utilisation efficiency, efficiency of energy utilisation for maintenance (K_m) and growth (K_f) and the origin of N retention responses to independent variation of energy and protein intake. This review outlines the key IIT findings and whether they are reflected in current feeding systems with implications for different production systems worldwide. Maintenance energy requirements are similar to those derived from comparative slaughter but maintenance N requirements are significantly lower. No differences in utilisation efficiency exist between acetic, propionic and butyric acids. At low energy intakes, endogenous energy reserves are utilised to retain amino acids and fuel substantial tissue protein gains. The use of fasting metabolism to measure the utilisation of nutritionally balanced diets is questioned since it is a glucose-deficient state. Inter-species differences in glucose metabolism appear to exist, suggesting that glucose requirements may be higher in cattle than sheep. The difficulty in predicting nutrient requirements, particularly protein, with any one technique is highlighted.