Evidence for an interaction between cationic and neutral amino acids at the blood-facing aspect of the lactating rat mammary epithelium

Amino acid transportation, sensing and signal transduction in the mammary gland: key molecular signalling pathways in the regulation of milk synthesis

The mammary gland, a unique exocrine organ, is responsible for milk synthesis in mammals. Neonatal growth and health are predominantly determined by quality and quantity of milk production. Amino acids are crucial maternal nutrients that are the building blocks for milk protein and are potential energy sources for neonates. Recent advances made regarding the mammary gland further demonstrate that some functional amino acids also regulate milk protein and fat synthesis through distinct intracellular and extracellular pathways. In the present study, we discuss recent advances in the role of amino acids (especially branched-chain amino acids, methionine, arginine and lysine) in the regulation of milk synthesis. The present review also addresses the crucial questions of how amino acids are transported, sensed and transduced in the mammary gland.

Reduced protein diet with near ideal amino acid profile improves energy efficiency and mitigate heat production associated with lactation in sows

Background

The study objective was to test the hypothesis that 1) lowering dietary crude protein (CP) increases dietary energetic efficiency and reduces metabolic heat associated with lactation, and 2) excessive dietary leucine (Leu) supplementation in a low CP diet decreases dietary energetic efficiency and increases metabolic heat associated with lactation.

Methods

Fifty-four lactating multiparous Yorkshire sows were allotted to 1 of 3 isocaloric diets (10.80 MJ/kg net energy): 1) control (CON; 18.75% CP), 2) reduced CP with a near ideal or optimal AA profile (OPT; 13.75% CP) and 3) diet OPT with excessive Leu (OPTLEU; 14.25% CP). Sow body weight and backfat were recorded on day 1 and 21 of lactation and piglets were weighed on day 1, 4, 8, 14, 18, and 21 of lactation. Energy balance was measured on sows during early (day 4 to 8) and peak (day 14 to18) lactation, and milk was sampled on day 8 and 18.

Results

Over 21-day lactation, sows fed OPT lost body weight and body lipid (P < 0.05). In peak lactation, sows fed OPT had higher milk energy output (P < 0.05) than CON. Sows fed OPTLEU tended (P = 0.07) to have less milk energy output than OPT and did not differ from CON. Maternal energy retention was lower (P < 0.05) in OPT and OPTLEU compared to CON sows, and did not differ between OPTLEU and OPT sows. Sows fed OPT had higher (P < 0.05) apparent energy efficiency for milk production compared to CON. Heat production associated with lactation was lower (P < 0.05) or tended to be lower (P = 0.082), respectively, in OPT and OPTLEU compared to CON sows.

Conclusion

The OPT diet, in peak lactation, improved dietary energy utilization for lactation due to less urinary energy and metabolic heat loss, and triggered dietary energy deposition into milk at the expense of maternal lipid mobilization. Leucine supplementation above requirement may reduce dietary energy utilization for lactation by decreasing the energy partitioning towards milk, partially explaining the effectiveness of OPT diet over CON diets.

Feeding a reduced protein diet with a near ideal amino acid profile improves amino acid efficiency and nitrogen utilization for milk production in sows1,2

Fifty-four lactating multiparous Yorkshire sows were used to test the hypothesis that feeding a reduced protein diet with a near ideal AA (NIAA) profile increases the biological utilization efficiency of N and essential AA (EAA) for milk production in part as a result of reduced dietary Leu concentration. Sows were fed 1 of 3 isocaloric diets containing the following concentration of CP (% as-fed, analyzed): 18.74 (Control: CON), 13.78 (Optimal: OPT), and 14.25 (Optimal + Leu: OPTLEU). The OPT and OPTLEU diets contained the same concentration of crystalline AA (CAA) to meet requirements of the limiting AA. Crystalline Leu was added to OPTLEU to contain the same SID Leu concentration as that of CON. Sows were weighed on days 1 and 21 of lactation and piglets on days 1, 4, 8, 14, 18, and 21 of lactation. Nitrogen retention was measured for 48 or 72 h between days 4 and 8 (early) and days 14 and 18 (peak) of lactation. Sow BW change and ADFI did not differ between diets. Litter growth rate (LGR) during early lactation did not differ between diets. At peak lactation, LGR was higher in sows fed OPT compared with CON (P < 0.05) and lower in sows fed OPTLEU compared with OPT (P < 0.05). In early and peak lactation, total N retention, and milk N output efficiency were greater in OPT (P < 0.01) and OPTLEU (P < 0.05) than CON. Compared with CON, overall biological efficiency of N, Arg, His, Ile, Leu, Phe, and Trp were greater (P < 0.05), whereas those of Lys, Met, Thr, and Val did not differ in sows fed OPT and OPTLEU, except for Leu which did not differ between OPTLEU and CON. Compared with OPT, only Leu and Met efficiency were lower (P < 0.01) and tended to be lower (P = 0.10), respectively, in sows fed OPTLEU. Reducing CP with a NIAA profile to attain the minimum Leu requirement maintained overall lactation performance, improved utilization efficiency of N, Arg, His, Ile, Leu, Phe + Tyr, and Trp for milk production, and maximized efficiency of Ile, Leu, Lys, Met + Cys, Phe + Tyr, Thr, Trp, and Val. Addition of Leu did not reduce N and EAA utilization efficiency. This study provides revised and novel maximum biological efficiency value (MBEV) for Ile (65.4), Leu (75.1), Lys (63.2), Met + Cys (78.2), Phe + Tyr (69.5), Thr (71.0), Trp (70.1), and Val (57.0). These MBEV can be used to more accurately predict the requirement for those AA during lactation.

Impact of improving dietary amino acid balance for lactating sows on efficiency of dietary amino acid utilization and transcript abundance of genes encoding lysine transporters in mammary tissue

Lactating multiparous Yorkshire sows ( = 64) were used in 2 experiments to test the hypothesis that reducing dietary CP intake and improving AA balance through crystalline AA (CAA) supplementation improves apparent dietary AA utilization efficiency for milk production and increases transcript abundance of genes encoding Lys transporter proteins in mammary tissue. In Exp. 1, 40 sows were assigned to 1 of 4 diets: 1) high CP (HCP; 16.0% CP, as-fed basis; analyzed concentration), 2) medium-high CP (MHCP; 15.7% CP), 3) medium-low CP (MLCP; 14.3% CP), and 4) low CP (LCP; 13.2% CP). The HCP diet was formulated using soybean meal and corn as the only Lys sources. The reduced-CP diets contained CAA to meet estimated requirements for essential AA that became progressively limiting with reduction in CP concentration, that is, Lys, Ile, Met + Cys, Thr, Trp, and Val. Dietary standardized ileal digestible (SID) Lys concentration was 80% of the estimated requirement. In Exp. 2, 24 sows were assigned to the HCP or LCP diets. In Exp. 1, blood samples were postprandially collected 15 h on d 3, 7, 14, and 18 of lactation and utilization efficiency of dietary AA for milk production was calculated during early (d 3 to 7) and peak (d 14 to 18) lactation. Efficiency values were estimated from daily SID AA intakes and milk AA yield, with corrections for maternal AA requirement for maintenance and AA contribution from body protein losses. In Exp. 2, mammary tissue was biopsied on d 4 and 14 of lactation to determine the mRNA abundance of genes encoding Lys transporter proteins. In peak lactation, Lys, Thr, Trp, and Val utilization efficiency increased with decreasing dietary CP (linear for Trp and Val, < 0.05; in sows fed the MHCP diet vs. sows fed the HCP diet for Lys and Thr, < 0.05). Total essential and nonessential 15-h postprandial serum AA concentrations increased with decreasing dietary CP (linear, = 0.09 and < 0.05, respectively), suggesting increased maternal body protein mobilization. Transcript abundance of several genes involved in Lys transport in mammary tissue did not differ between sows fed the LCP and HCP diets. Feeding lactating sows low-CP diets supplemented with CAA increases the efficiency of utilizing dietary Lys, Thr, Trp, and Val for milk protein production but is unrelated to abundance in mRNA of genes encoding Lys transport proteins in the mammary gland. Dietary Lys utilization for milk protein production in lactating sows appears to be optimized when crystalline Lys is included at a minimum of 0.10% in a diet containing 15.70% CP.

The functional and molecular entities underlying amino acid and peptide transport by the mammary gland under different physiological and pathological conditions

This review describes the properties and regulation of the membrane transport proteins which supply the mammary gland with aminonitrogen to support metabolism under different physiological conditions (i.e. pregnancy, lactation and involution). Early studies focussed on characterising amino acid and peptide transport pathways with respect to substrate specificity, kinetics and hormonal regulation to allow a broad picture of the systems within the gland to be established. Recent investigations have concentrated on identifying the individual transporters at the molecular level (i.e. mRNA and protein). Many of the latter studies have identified the molecular correlates of the transport systems uncovered in the earlier functional investigations but in turn have also highlighted the need for more amino acid transport studies to be performed. The transporters function as either cotransporters and exchangers (or both) and act in a coordinated and regulated fashion to support the metabolic needs of the gland. However, it is apparent that a physiological role for a number of the transport proteins has yet to be elucidated. This article highlights the many gaps in our knowledge regarding the precise cellular location of a number of amino acid transporters within the gland. We also describe the role of amino acid transport in mammary cell volume regulation. Finally, the important role that individual mammary transport proteins may have in the growth and proliferation of mammary tumours is discussed.

Effect of amino acids supply in reduced crude protein diets on performance, efficiency of mammary uptake, and transporter gene expression in lactating sows

To test the hypothesis that reduction in dietary CP concentration coupled with crystalline AA inclusion increases the efficiency of AA use for milk production, mammary AA arteriovenous concentration differences (A-V), AA transport efficiency (A-V/A × 100), and transcript abundance of AA transporters and milk protein genes were determined in lactating sows fed 1 of 3 diets containing 9.5% (Deficient), 13.5% (Ideal), or 17.5% (Standard) CP, with a similar profile of indispensable and dispensable AA. On d 7 and 18, arterial and mammary venous blood and mammary tissue were sampled postfeeding. Transcript abundance of AA transporters b(0,+)AT (SLC7A9), y(+)LAT2 (SLC7A6), ATB(0,+) (SLC6A14), CAT-1 (SLC7A1), and CAT-2b (SLC7A2) and milk protein β-casein (CSN2) and LALBA (α-lactalbumin) were determined using reverse transcription quantitative PCR. Piglet ADG increased curvilinearly (linear and quadratic, P < 0.03) with increasing percent CP from Deficient to Standard. On d 7, Lys and Arg A-V and transport efficiency increased quadratically (P < 0.05) with increasing percent CP. On d 18, Lys A-V tended to increase (linear, P = 0.08) with increasing percent CP. Increasing CP increased Ile and Val A-V on d 7 (linear, P = 0.05 and P = 0.08, respectively) and Leu and Val on d 18 (linear, P = 0.07 and P = 0.04, respectively). On d 7, plasma concentrations of branched chain AA (BCAA):Lys decreased quadratically (P < 0.05). Expression of genes SLC7A9, SLC7A6, SLC6A14, SLC7A1, SLC7A2, CSN2, and LALBA was unaffected by diet. In conclusion, decreasing the dietary CP from 17.5% to 13.5% with inclusion of crystalline AA did not affect piglet ADG, AA transporter, or milk protein gene expression but increased mammary transport efficiency and A-V of Lys and Arg on d 7 of lactation. This increase was associated with a decrease in plasma concentration of BCAA:Lys, suggesting a competitive mechanism between cationic and BCAA for transport of AA across mammary cells.

Changes in messenger RNA abundance of amino acid transporters in rat mammary gland during pregnancy, lactation, and weaning

During lactation, the mammary gland increases the needs for nutrients to fulfill the milk production requirements. Among these nutrients, amino acids play an important role for the synthesis of milk proteins. Amino acids are supplied to the mammary gland through amino acid transporters, although some are synthesized in situ. The purpose of this study was to establish the pattern of changes in messenger RNA abundance of the amino acid transporters ASC, system L, EAAC1, GLAST, CAT-1, and Tau in the mammary gland of the rat during different stages of pregnancy and lactation. Rats were fed during pregnancy and lactation a 20% casein diet. Food intake increased significantly during the lactation period. Amino acid transporter ASC expression increased during the first days of pregnancy about 2-fold, and it was increased in a lesser extent again during the peak of lactation. The expression of system L (LAT-1) and CAT-1 transporters was increased only during the lactation period. On the other hand, the expression of the transporters for anionic amino acids EAAC1 and GLAST was low during both stages. Finally, taurine transporter expression decreased during pregnancy; and it was significantly lower during lactation. These results showed that amino acid transporters were not expressed similarly in the mammary gland during pregnancy and lactation, indicating that the expression of these transporters did not respond only to the metabolic needs of the gland but depended on the dietary protein supply and possibly the specific hormonal changes that occur during pregnancy and lactation.

A compartmental capillary, convolution integration model to investigate nutrient transport and metabolism in vivo from paired indicator/nutrient dilution curves

Thirty-three paired indicator/nutrient dilution curves across the mammary glands of four cows were obtained after rapid injection of para-aminohippuric acid (PAH) plus glucose into the external iliac artery. For the measurement of extracellular volume and kinetics of nutrient uptake from indicator dilution curves, several models of solute dispersion and disappearance have been proposed. The Crone-Renkin models of exchange in a single capillary assume negligible washout of solutes from the extracellular space and do not describe entire dilution curves. The Goresky models include a distribution of capillary transit times to generate whole system outflow profiles but require two indicators to parametize extracellular behavior. A compartmental capillary, convolution integration model is proposed that uses one indicator to account for the extracellular behavior of the nutrient after a paired indicator/nutrient injection. With the use of an iterative approach to least squares, unique solutions for nonexchanging vessel transit time t(mu) and its variance sigma were obtained from all 33 PAH curves. The average of heterogeneous vascular transit times was approximated as 2sigma = 8.5 s. The remainder of indicator dispersion was considered to be due to washout from a well-mixed compartment representing extracellular space that had an estimated volume of 5.5 liters or 24% of mammary gland weight. More than 99% of the variation in the time course of venous PAH concentration after rapid injection into the arterial supply of the mammary glands was explained in an unbiased manner by partitioning the organ into a heterogeneous nonexchanging vessel subsystem and a well-mixed compartmental capillary subsystem.

Dietary protein concentration affects plasma arteriovenous difference of amino acids across the porcine mammary gland1

The objective of this study was to determine whether the porcine mammary gland responds to increasing dietary CP concentration through changes in AA arteriovenous difference (a-v). Sixteen Landrace x Yorkshire lactating sows were provided ad libitum access to one of four isocaloric diets varying in CP concentration (7.8, 13.0, 18.2, and 23.5 %; as-fed basis). Litters were adjusted to 11 pigs within 48 h of birth. Sows were fitted with catheters in the carotid artery and main mammary vein on d 4. On d 10, 14, 18, and 22 of lactation, arterial and venous blood samples were obtained every 30 min over 6 h. Milk yield was estimated on d 11 and 21 using the D2O dilution technique. Final litter sizes on d 21 were 10.3, 11, 9.5, and 11 piglets for sows fed the 7.8, 13.0, 18.2, and 23.5% CP diets, respectively. Piglet ADG tended (P = 0.088) to increase with increasing dietary CP concentration and were 186, 221, 220, and 202 g for sows fed the 7.8, 13.0, 18.2, and 23.5% CP diet, respectively. Daily total milk yield on d 21 (kg milk/d) tended (P = 0.099) to increase, and average milk yield per nursed piglet (kg of milk-pig(-1)d(-1)) increased (P < 0.05) with increasing CP concentration and were, on a per-piglet basis, 0.95, 1.19, 1.14 and 1.13 kg of milk/d for the 7.8, 13.0, 18.2, and 23.5% CP diets, respectively. As dietary CP increased from 7.8 to 23.5%, isoleucine and leucine a-v increased linearly only (linear, P < 0.01); all other AA a-v increased, reached a maximum in sows fed 18.2% CP, and decreased thereafter in sows fed 23.5% CP (quadratic, from P = 0.10 to P < 0.05). Amino acid uptake by the entire udder and by each gland increased (linear, P < 0.05) with increasing dietary CP. Arteriovenous differences response to increasing day of lactation varied among AA, from no change for histidine, isoleucine, lysine, methionine, tryptophan, and valine, to a linear trend increase for arginine (P = 0.055), leucine (P = 0.064), phenylalanine (P = 0.101), and threonine (P = 0.057). In summary, for the majority of AA, a-v increased with increasing dietary CP concentration from 7.8 to 18.2%, but decreased when CP concentration exceeded 18.2%. In contrast, mammary AA uptake, piglet ADG and milk yield per pig increased linearly with increasing dietary CP, suggesting a coordinated regulation between AA delivery and transport to meet the demand for milk yield.

Transport of milk constituents by the mammary gland

This review deals with the cellular mechanisms that transport milk constituents or the precursors of milk constituents into, out of, and across the mammary secretory cell. The various milk constituents are secreted by different intracellular routes, and these are outlined, including the paracellular pathway between interstitial fluid and milk that is present in some physiological states and in some species throughout lactation. Also considered are the in vivo and in vitro methods used to study mammary transport and secretory mechanisms. The main part of the review addresses the mechanisms responsible for uptake across the basolateral cell membrane and, in some cases, for transport into the Golgi apparatus and for movement across the apical membrane of sodium, potassium, chloride, water, phosphate, calcium, citrate, iodide, choline, carnitine, glucose, amino acids and peptides, and fatty acids. Recent work on the control of these processes, by volume-sensitive mechanisms for example, is emphasized. The review points out where future work is needed to gain an overall view of milk secretion, for example, in marsupials where milk composition changes markedly during development of the young, and particularly on the intracellular coordination of the transport processes that result in the production of milk of relatively constant composition at a particular stage of lactation in both placental and marsupial mammals.

The regulation of Na(+)-dependent anionic amino acid transport by the rat mammary gland

The regulation of anionic amino acid transport, using radiolabelled D-aspartate as a tracer, by rat mammary tissue explants has been examined. Na(+)-dependent D-aspartate uptake by mammary tissue increased between late pregnancy and early lactation and again at peak lactation but thereafter declined during late lactation. In contrast, the Na(+)-independent component of D-aspartate uptake by mammary explants did not change significantly with the physiological state of the donor animals. Premature weaning of rats during peak lactation markedly decreased Na(+)-dependent D-aspartate uptake by mammary tissue. In addition, premature weaning also reduced the effect of reversing the trans-membrane Na(+)-gradient on the fractional loss of D-aspartate from mammary tissue explants. Unilateral weaning of rats during peak lactation revealed that milk accumulation per se reduced the Na(+)-dependent moiety of D-aspartate uptake by mammary tissue suggesting that the transport of anionic amino acids is regulated to match supply with demand. Treating lactating rats with bromocryptine reduced D-aspartate uptake by mammary tissue explants suggesting that the transport of anionic amino acids by the rat mammary gland is regulated by prolactin.

Mammary gland membrane transport systems

The secretion of milk depends on the activity of a large number of membrane transport systems located on the apical and basolateral membranes of mammary secretory cells. It follows that a thorough knowledge of individual mammary tissue membrane transport systems is required if we are to fully understand the process of milk secretion. The distribution of the transporters between the apical and basolateral poles of the mammary epithelium must be asymmetrical given that the mammary gland is capable of vectorial transport. This is particularly evident in the case of glucose and amino acid transport systems: the transport mechanisms for these compounds are predominantly situated in the blood-facing aspect of the secretory cells. In addition. it is apparent that there is a polarized distribution of transport systems (carriers and channels) which accept sodium, potassium, chloride, phosphate, and calcium as substrates.

Volume-activated taurine efflux from the in situ perfused lactating rat mammary gland

The effect of cell swelling on the efflux of amino acids from the in situ perfused lactating rat mammary gland has been examined. Cell swelling, induced by a hyposmotic shock, increased the fractional release of [3H]taurine. In contrast, a hyposmotic shock did not stimulate the efflux of D-[3H]aspartate, suggesting that the effect of a hyposmotic challenge on taurine release cannot be attributed to cell lysis. Volume-activated taurine efflux was reversible, dependent upon the extent of the osmotic challenge and inactivated with a prolonged hyposmotic shock. The release of taurine was also reversibly increased following isosmotic cell swelling (using urea). The results confirm the presence of a volume-sensitive taurine efflux transport system in lactating rat mammary tissue and suggest that the volume-activated amino acid efflux pathway is located at the blood-facing aspect of the mammary epithelium.