Utilization of L-alanine and L-glutamine by lactating mammary gland of the rat. A role for L-alanine as a lipogenic precursor

Plasma amino acid profiles at various reproductive stages in female rats

We measured the plasma levels of amino acids at various reproductive stages in female rats, including the estrous cycle, pregnancy and lactation, and compared the resulting amino acid profiles using two- or three-dimensional figures. These figures revealed that the amino acid profiles of pregnant and lactating dams differed considerably from those during the estrous cycle or in male rats. The plasma levels of individual amino acids were almost the same between proestrus, estrus, metestrus and diestrus, and their profiles did not differ significantly. However, the amino acid profiles changed during pregnancy and lactation in dams. The plasma Ser level decreased significantly in mid and late pregnancy, whereas Tyr, Gly and His decreased significantly in the late and end stages of pregnancy, and Trp and Lys significantly decreased and increased at the end of pregnancy, respectively. Much larger changes in amino acid profiles were observed during lactation, when the levels of many amino acids increased significantly, and none showed a significant decrease. Plasma Pro, Ser and Gly levels increased continuously from day 1 until day 15 of lactation, whereas Asn and Met increased significantly from days 1 and 5 respectively until the end of lactation. These results suggest that the profiles of plasma amino acids show characteristic changes according to reproductive stage and that it may be necessary to consider such differences when performing amino acid-based diagnosis.

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.

Alanine uptake by liver of mid-lactating rats

L-Alanine transport in liver plasma membrane vesicle preparations from fed virgin and 15-day-lactating rats was studied. Lactation was found to induce a decrease of the maximal rate (Vmax) of a high-capacity-low-affinity component of the Na(+)-dependent L-alanine uptake. However, a high-affinity-low-capacity agency was significantly induced in lactating-rat livers. L-Alanine uptake was differentially inhibited by other amino acids in those preparations from lactating rats, and showed different sensitivity to Li+ as a cosubstrate instead of Na+ and to inhibition by sulfhydryl modifying reagents (N-ethylmaleimide [NEM] and p-chloromercuribenzosulfonate [PCMBS]). All of these observations taken together suggest that system A is upregulated in lactating-rat livers, thus resulting in a different contribution of both agencies A and ASC to the total Na(+)-dependent alanine transport into liver plasma membrane vesicles. This was demonstrated using the analogue alpha-methyl-aminoisobutyric acid (MeAIB), a specific system A substrate. L-Alanine uptake rates, as calculated from plasma membrane enzyme marker recoveries, were also enhanced in the physiologic range of alanine concentrations in blood. Our results prove that the physiologic adaptation to lactation involves modulation of system A activity in the liver.

Amino acid metabolism and protein synthesis in lactating rats fed on a liquid diet

1. Amino acid metabolism was studied in control virgin rats, lactating rats and virgin rats protein-pair-fed with the lactating rats (high-protein virgin rats). 2. Urinary excretion of nitrogen and urea was higher in lactating than in control virgin rats, and in high-protein virgin rats it was higher than in lactating rats. 3. The activities of urea-cycle enzymes (units/g) were higher in high-protein virgin than in lactating rats, except for arginase. In lactating rats the activities of carbamoyl-phosphate synthase, ornithine carbamoyltransferase and argininosuccinate synthase were lower than in control virgin rats. When the liver size is considered, the activities in lactating rats were similar to those in high-protein virgin rats, except for arginase. 4. N-Acetylglutamate content was higher in high-protein virgin rats than in the other two groups. 5. The rate of urea synthesis from precursors by isolated hepatocytes was higher in high-protein virgin rats than in the other two groups. 6. The flooding-dose method (L-[4-3H]phenylalanine) for measuring protein synthesis was used. The absolute synthesis rates of mammary gland, liver and small-intestinal mucosa were higher in lactating rats than in the other two groups, and in high-protein virgin rats than in control virgin rats 7. These results show that the increased needs for amino acids during lactation are met by hyperphagia and by a nitrogen-sparing mechanism.

Reciprocal changes in amino acid metabolism in mammary gland and liver of the lactating rat on starvation and refeeding as indicated by the tissue accumulation of alpha-amino[1-14C]isobutyrate

Measurements of the tissue accumulation in vivo and in vitro by hepatocytes and mammary-gland acini of alpha-amino[1-14C]isobutyrate ([1-14C]AIB) were compared in virgin and lactating rats. The results indicate the existence of a reciprocal relationship between mammary gland and liver for AIB accumulation that is dependent on the lactational and the nutritional state of the rat. This suggests that amino acids are preferentially directed to the mammary gland during active lactation.

Comparative effects of tumour necrosis factor-alpha (cachectin), interleukin-1-beta and tumour growth on amino acid metabolism in the rat in vivo. Absorption and tissue uptake of alpha-amino[1-14C]isobutyrate

The effects of acute administration of either tumour necrosis factor-alpha (cachectin) (TNF) or interleukin-1-beta (IL-1), or of tumour growth (Walker-256 carcinosarcoma), on blood amino acid concentrations and tissue alpha-amino[1-14C]isobutyrate (AIB) uptake in virgin and lactating rats were compared. Both monokines decreased the blood concentrations of those amino acids (serine, glycine, alanine and proline) transported via the A system. Tumour growth decreased the blood concentrations of serine, proline and histidine, whereas the concentrations of glutamine and leucine were increased. IL-1 decreased the intestinal absorption of AIB in all groups studied; TNF or tumour growth had no effect. Tissue AIB uptake was increased (1.5-2.5-fold) in liver, whereas it was decreased in heart and skeletal muscle of the three treatment groups (except skeletal muscle of the IL-1-treated rats). Lactating rats had lower hepatic uptake of AIB compared with livers of virgin rats. IL-1 increased the hepatic uptake of AIB in lactating rats, but not to the values seen in virgin rats treated with IL-1; there was no effect of the cytokine on muscle or mammary-gland uptake. In adrenalectomized rats, the stimulatory effect of IL-1 on hepatic AIB uptake was diminished, whereas that of TNF still persisted. IL-1 caused a marked decrease of AIB uptake in muscle and heart of adrenalectomized rats, which was accompanied by an increase in the blood concentrations of branched-chain amino acids. These effects did not occur with TNF. It is concluded that the effects of the cytokines on tissue amino acid metabolism may depend on a differential endocrine response involving glucagon and/or glucocorticoids.

Glutamine and ketone-body metabolism in the small intestine of starved peak-lactating rats

1. The effect of starvation on the metabolism of gut glutamine and ketone-bodies of peak lactating, non-lactating and virgin rats was investigated. 2. The arterial blood ketone-body concentration was increased by approximately 7-, 6- and 13-fold in 48 h-starved virgin, non-lactating and lactating rats, respectively. 3. The arterial blood glutamine concentration was decreased by approximately 32% in 48 h-starved lactating rats (p less than 0.001). 4. The maximal activity of phosphate-dependent glutaminase was increased or decreased in the small intestine of fed or 48 h-starved peak-lactating rats, respectively. 5. Portal drained viscera blood flow increased by approximately 25% in peak-lactating rats. 6. Arteriovenous difference measurements for ketone-bodies across the gut of 48 h-starved rats showed an increase in net uptake of ketone-bodies by approximately 10-, 17- and 29-fold in virgin, non-lactating and lactating rats, respectively. 7. Glutamine was extracted by the gut of peak-lactating rats at a rate of 487 nmol/100 g of body wt. which was greater by approximately 33% (p less than 0.001) than that of virgin or non-lactating animals. In peak lactating rats, 48 h-starvation resulted in marked decreases in the rates of glutamine removal from the circulation (p less than 0.001) which was accompanied by decreased rates of release of glutamate, alanine and ammonia.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic uptake of amino acids at mid-lactation in the rat

Hepatic availability and uptake of amino acids were measured in fed virgin and 15-day-lactating rats. Lactation did not induce any change in total amino acid availability (expressed per 100 g body wt.). Virgin rats showed a nil hepatic balance, and lactation induced a high net uptake. The high drainage of amino acids by mammary gland does not affect hepatic availability.

Decreased urea synthesis in cafeteria-diet-induced obesity in the rat

Feeding rats with a cafeteria diet resulted in increases in total body weight and in epididymal-adipose-tissue weight. Those rats excreted significantly less N than did controls. The amount of N ingested by cafeteria-diet-fed rats was kept equal to that of controls. This decrease in N excretion is explained by a decrease in urinary excretion of urea. This may be due to the following facts. The rate of synthesis of urea from precursors by isolated hepatocytes from cafeteria-diet-fed rats was lower than in controls. In cafeteria-diet-fed rats the activities of all the enzymes of the urea cycle are decreased. The major percentage decreases are those of carbamoylphosphate synthetase (EC 6.3.4.16) and of argininosuccinate synthetase (EC 6.3.4.5), the enzymes probably involved in the regulation of the overall rate of the cycle. When rats are switched to normal chow diet, the enzyme activities return to normal values. The uptake of amino acids by liver of cafeteria-diet-fed rats is lower than in controls. These results contrast with those obtained previously by using other models of obesity in rat (i.e. genetic or hypothalamic), in which N excretion was increased.

Amino acid transport systems in bovine mammary tissue

Nutrient provision to the lactating mammary gland involves three factors: blood nutrient concentration, blood flow, and cellular uptake. This paper reviews uptake of amino acids by bovine mammary tissue relative to interorgan blood flows, red blood cell contribution, arteriovenous differences, specific mammary amino acid transport systems, and glutathione and gamma-glutamyl transpeptidase. Recent studies with ruminant amino acid blood fluxes and the role of the red blood cell in providing nitrogenous substrates to tissues have brought to light new considerations of nutrient availability to mammary tissue. Previous studies measured arteriovenous differences to quantitate net amino acid uptake. These studies are considered relative to seven specific and separate amino acid transport systems, some of which have been identified in bovine mammary tissue. Uptake of sulfur amino acids by mammary tissue has been of interest because it appeared that insufficient quantities were provided during lactation. Glutathione, a tripeptide, may be a principle source of cysteine to mammary tissue via mechanisms involving gamma-glutamyl transpeptidase, glutathione, and red blood cells. The paper considers these mechanisms in relationship to amino acid transport systems.

Effect of prolactin and glucocorticoids on P-enolpyruvate carboxykinase activity in liver and mammary gland from diabetic and lactating rats

The administration of 2 bromo-alpha-ergocryptine, to reduce serum prolactin decreased the activity of cytosolic P-enolpyruvate carboxykinase (GTP) (EC 4.1.1.32) about 50% in both liver and mammary gland of lactating animals. Adrenalectomy had similar effects to those of bromo-alpha-ergocryptine. In contrast, there was a 50% increase in enzyme activity in the mammary gland of diabetic, lactating rats and a 10-fold increase in liver as compared with normal rats. P-enolpyruvate carboxykinase activity in mammary gland as liver is coordinately regulated by prolactin, glucocorticoids and insulin.

Lactose and fatty acid synthesis in lactating-rat mammary gland. Effects of starvation, re-feeding, and administration of insulin, adrenaline, streptozotocin and 2-bromo-alpha-ergocryptine

Lactose synthesis and fatty acid synthesis in intact lactating-rat mammary gland were measured simultaneously by incorporation of [U-14C]glucose and of both [U-14C]glucose and 3H2O respectively. Both processes were almost abolished by overnight starvation. Self-re-feeding caused recovery of lipogenesis to 100% of normal by 2 h and to 170% by 5 h. Lactose synthesis recovered to 80% of normal by 5 h. Food intubated to starved rats caused partial recovery in 3 h, standard diet favouring lactose synthesis and sugars favouring lipogenesis. Casein and starch were ineffective. Olive oil intubated to fed rats suppressed lipogenesis greatly and lactose synthesis slightly. Paraffin oil or water partly mimicked these effects. Adrenaline (subcutaneous) decreased lipogenesis from glucose, whereas insulin (subcutaneous) caused hypoglycaemia associated with loss of lactose synthesis but unchanged fatty acid synthesis. Streptozotocin and 2-bromo-alpha-ergocryptine (CB-154) impaired lipogenesis but not lactose synthesis. The results are interpreted in terms of competition for intracellular glucose by biosynthetic pathways for lactose and fat, and the possible implications for variations in milk composition are discussed.

Changes induced in rat plasma composition by lactation

Changes in rat plasma composition in post-partum period were studied from day 1 to post weaning recovery situation (day 40). Urea levels showed a significant increase at the peak of lactation, and glycerol levels showed a decrease towards late lactation. Changes in the plasma aminogram were more marked during the maximal intensity of lactation (days 10-20) and in postlactation. Low trypthophan levels were sustained throughout the lactation period. The combined amino acids showed significant increases on day 10 of lactation, and increases in non-essential and total amino acids in post-lactation (40 days). These results point towards increased amino-acid utilization during lactation.

Effect of premature weaning on amino acid uptake by the mammary gland of lactating rats

1. Arteriovenous differences of amino acids across the lactating mammary gland were measured in normal rats and weaned for 4, 5 and 24h. 2. Uptake of amino acids by mammary glands of rats weaned for 5h or more was significantly lower than that of controls. This was not reversed by injection of prolactin. 3. By using 'unilaterally weaned' rats we showed that milk accumulation plays an important role in amino acid uptake by mammary gland. 4. gamma-Glutamyltransferase activity was significantly lower in 'weaned' glands than in 'normal' glands. This provides further support for the hypothesis of the function of the gamma-glutamyl cycle in the mammary gland in vivo.