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

A comparative evaluation of models to predict human intestinal metabolism from nonclinical data

Extensive gut metabolism is often associated with the risk of low and variable bioavailability. The prediction of the fraction of drug escaping gut wall metabolism as well as transporter-mediated secretion (F_g ) has been challenged by the lack of appropriate preclinical models. The purpose of this study is to compare the performance of models that are widely employed in the pharmaceutical industry today to estimate F_g and, based on the outcome, to provide recommendations for the prediction of human F_g during drug discovery and early drug development. The use of in vitro intrinsic clearance from human liver microsomes (HLM) in three mechanistic models - the ADAM, Q_gut and Competing Rates - was evaluated for drugs whose metabolism is dominated by CYP450s, assuming that the effect of transporters is negligible. The utility of rat as a model for human F_g was also explored. The ADAM, Q_gut and Competing Rates models had comparable prediction success (70%, 74%, 69%, respectively) and bias (AFE = 1.26, 0.74 and 0.81, respectively). However, the ADAM model showed better accuracy compared with the Q_gut and Competing Rates models (RMSE =0.20 vs 0.30 and 0.25, respectively). Rat is not a good model (prediction success =32%, RMSE =0.48 and AFE = 0.44) as it seems systematically to under-predict human F_g . Hence, we would recommend the use of rat to identify the need for F_g assessment, followed by the use of HLM in simple models to predict human F_g . © 2017 Merck KGaA. Biopharmaceutics & Drug Disposition Published by John Wiley & Sons, Ltd.

Regulatory and molecular aspects of mammalian amino acid transport

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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.

Intestinal and hepatic nitrogen balance in the rat after the administration of an oral protein load

The fate of a small oral dose of protein given to overnight-starved rats was studied. After 3 h, 62% of the protein amino acids had been absorbed. Most of the absorbed N went into the bloodstream through the portal in the form of amino acids, but urea and ammonia were also present. About one-quarter of all absorbed N was carried as lymph amino acids. The liver was able to take all portal free ammonia and a large proportion of portal amino acids, releasing urea. The hepatic N balance was negative, indicating active proteolysis and net loss of liver protein.

Intestinal handling of a glucose gavage by the rat

An oral gavage of either 3, 1 or 0.1 mmoles of 14C-labelled glucose was given to rats under standard feeding conditions or food deprived for 24 hr. The fate of the glucose label was determined at 10, 15, 30 and 60 min after gavage; at 60 min 40% of the glucose was absorbed in fed rats (60% in food deprived). The portal vein blood flows were determined and the levels of glucose, lactate, alanine and pyruvate, and their radioactivity, as well as that of CO2 were measured in both portal and arterial blood. The net computed glucose and 3-carbon carriers (lactate, alanine and pyruvate) actually released into the portal system by the intestine was lower than the amount of glucose taken up from the intestinal lumen in one hour. Oxidation to 14CO2 accounted for a 12-15% of the absorbed glucose. The size of the gavage deeply affected the proportion of glucose released into the portal blood (c. 50% with a 3 mmoles gavage and practically nil with a 0.1 mmoles gavage), but it affected much less the generation of lactate and other 3 C carriers. In fed rats, the net intestinal balance of non-radioactive glucose was negative, and that of lactate positive; when radioactive glucose was considered, the pattern was inverted. In starved rats, both glucose and lactate were released in large proportions by the intestine, but alanine efflux was lower. It can be concluded that the intestine consumes a considerable proportion of glucose in the fed state. Glucose handling by the intestine is compartmentalized in two functional circuits: glucose is taken up from the arterial blood and used for intestinal metabolism and lactate production, luminal glucose is absorbed mainly unaltered and transferred to the portal blood. Thus, the generation of lactate is mainly related to the availability of arterial glucose. In addition to the release of the ingested glucose as 3 C carriers or glucose, an extraportal pathway for glucose transfer into the bloodstream is postulated.

Role of the rat liver in the disposal of a glucose gavage

An oral gavage of either 3, 1, or 0.1 mmoles of glucose was given to rats under standard feeding conditions or food deprived for 24 hr. The blood flow of the portal and suprahepatic veins as well as the hepatic balances for glucose, lactate, alanine and pyruvate were estimated. In fed rats, after the administration of an oral 3 mmoles load, the liver actually released 310 mumoles of glucose and 90 of lactate, amounts that could be accounted for by the uptake of alanine (148 mumoles) and small loss of glycogen (275 mumoles of glycosyl residues). In starved rats, however, the liver took a very high proportion (c. 71%) of the glucose absorbed, both as glucose (780 mumoles), lactate and pyruvate (892 mumoles) or alanine (134 mumoles). The synthesis of glycogen was considerably limited, accounting for only 205 mumoles, and leaving practically one mmol of glucose equivalent energy available for liver function and the synthesis of other compounds. Practically all glycogen was synthesized directly from glucose, since the synthesis from 3 C carriers was less than a 5%. Smaller gavages (1 or 0.1 mmoles) resulted in a much lower liver uptake activity. The strikingly different activity of the liver with respect to the available glucose and 3 C fragments could not be explained alone by the circulating levels of these compounds, suggesting a very deep influence of the intestine in hepatic function. The liver plays a very passive role in fed animals, with a very small involvement in the disposal of a glucose load, whereas it takes on an important role when the overall availability of energy is diminished.

In the rat, intestinal lymph carries a significant amount of ingested glucose into the bloodstream

Intestinal lymph flow in rats receiving an oral load of 3 mmoles of glucose was determined by measuring the amount of tritiated gavage water entering via the portal route [computed differential arterio-venous tritium water concentrations multiplied by portal blood flow) and measuring the total amount of tritium transferred from the intestinal lumen to the rest of the rat in a given time [from the blood tritium specific activity and the water space (795 ml.kg-1) of the rat]. The approximate figure obtained (79.8 ml.kg-1.h) was used for the evaluation of the amount of glucose (from 14C-labelled glucose in the gavage) transfer via intestinal lymph from the label present in the lymph and its flow. Lymph glucose concentration (c. 11 mM) and specific radioactivity were higher than those of blood. The amount of glucose actually taken up from the intestinal lumen was determined measuring the remaining label in the alimentary canal. The portal contribution to label distribution was measured as for tritium water transfer. Most glucose label passed through in the form of glucose, with much smaller proportions as CO2 and even smaller as lactate and other labelled compounds. The results suggest that the contribution of lymph circulation to dietary glucose incorporation into the bloodstream may be significant, amounting to values higher than 10% of the glucose actually incorporated.

Amino acid uptake by liver of genetically obese Zucker rats

Alanine and glutamine uptake by the liver of 50-52-day-old genetically obese Zucker rats and their lean littermates has been studied. The net uptake in vivo of L-alanine is 2-fold higher in the obese animals. No significant change in L-glutamine net balance was found. We also studied the Na(+)-dependent uptake of L-alanine and L-glutamine into plasma-membrane vesicles isolated from either obese- or lean-rat livers. Vmax. values of both L-alanine and L-glutamine transport were 2-fold higher in those preparations from obese rats. No change in Km was observed. As suggested by inhibition studies, this seemed to be mediated by an enhancement of the activities of systems A, ASC and N. We conclude that the liver of the obese Zucker rat is extremely efficient in taking up neutral amino acids from the afferent blood, which results in an enhanced net uptake of L-alanine in vivo. The changes in transport activities at the plasma-membrane level might contribute to increase amino acid disposal by liver, probably for lipogenic purposes, as recently reported by Terrettaz & Jeanrenaud [Biochem. J. (1990) 270, 803-807].

Differences in L-alanine uptake by livers of Wistar and lean Zucker rats

The L-alanine uptake by livers of Wistar and lean Zucker rats has been studied. The hepatic uptake and fractional extraction rates of alanine were estimated in 50-55 day old rats. No significant differences in amino acid concentrations and blood flows in afferent and efferent liver vessels were seen in lean Zucker rats when compared with Wistar rats. However, the hepatic uptake (1.6 +/- 0.1 and 0.7 +/- 0.1 mumol/min/100 g bw, p less than 0.01) and the fractional extraction (26.8 +/- 2.1 and 15.2 +/- 3.1%, p less than 0.05) were much lower in Zucker than in Wistar rats. The hepatic active transport of L-alanine was determined in vitro using isolated plasma membrane vesicles. Vesicles isolated from livers of lean Zucker rats showed similar values of Km (2.5 +/- 0.7 vs. 2.0 +/- 0.5 mM for Wistar and Zucker respectively, N.S.), but lower values of Vmax when compared with Wistar rats (1.1 +/- 0.1 vs 0.6 +/- 0.005 nmol/mg prot 5 s, p less than 0.01, for Wistar and lean Zucker rats respectively). These results indicate that, the liver of lean Zucker rats concentrates alanine less efficiently than the liver of Wistar rats. This fact correlates well with a lower capacity of the Na(+)-dependent L-alanine transport in liver plasma membrane vesicles from lean Zucker rats.

Cellular energy metabolism and regulation

Consistent with the increased demand for nutrients imposed by lactation and growth, those tissues directly involved in the digestion, absorption, and processing of the required additional nutrients show response to these states. During lactation, the rumen, upper intestine, and liver increase in size, and more energy is spent on Na+,K+ transport and on protein turnover. The massive endocrine influences during lactation suggest that the metabolism of other tissues besides these and mammary tissue would be influenced, but evidence is rather sparse. Ion transport and protein metabolism in some muscles may indeed be increased. Although substrate cycles characteristically account for a substantially smaller portion of the energy expenditure in the intact animal than do ion transport and protein turnover, stage of lactation influences some of these cycles, particularly the triacylglycerol fatty acid cycle. The needs for additional quantitative in vivo measurements of metabolic conversions and for mechanistic model description of metabolic events in nonmammary tissues are discussed.

L-lactate uptake by rat liver. Effect of food deprivation and substrate availability

We have studied the role of substrate availability on net L-lactate uptake by liver of anaesthetized fed and 24 h-fasted rats. L-Lactate was infused through a mesenteric vein at infusion rates equivalent to 0, 0.125, 0.25 and 0.5 times the basal turnover rate (Rt). By these means we were able to increase L-lactate portal concentrations up to 5.5 mM, without significant changes in portal pH. In the basal state (0 Rt), a net L-lactate uptake by liver was found in 24 h-fasted animals. No net balance was observed in fed rats. Infusion of L-lactate in fed animals failed to induce a net hepatic uptake, except when L-lactate levels in portal vein were raised above 5 mM. In fasted animals, net L-lactate uptake by liver increased linearly (r = 0.99) as a function of L-lactate concentration in the portal vein, even beyond the saturation of its specific carrier. It is concluded that, first, the L-lactate carrier does not limit net L-lactate uptake, and second, that substrate availability is an important factor modulating net L-lactate uptake by liver.

Rat splanchnic net oxygen consumption, energy implications

1. The blood flow, PO2, pH and PCO2 have been estimated in portal and suprahepatic veins as well as in hepatic artery of fed and overnight starved rats given an oral glucose load. From these data the net intestinal, hepatic and splanchnic balances for oxygen and bicarbonate were calculated. The oxygen consumption of the intact animal has also been measured under comparable conditions. 2. The direct utilization of oxygen balances as energy equivalents when establishing the contribution of energy metabolism of liver and intestine to the overall energy expenses of the rat, has been found to be incorrect, since it incorporates the intrinsic error of interorgan proton transfer through bicarbonate. Liver and intestine produced high net bicarbonate balances in all situations tested, implying the elimination (by means of oxidative pathways, i.e. consuming additional oxygen) of high amounts of H+ generated with bicarbonate. The equivalence in energy output of the oxygen balances was then corrected for bicarbonate production to 11-54% lower values. 3. Intestine and liver consume a high proportion of available oxygen, about one-half in basal (fed or starved) conditions and about one-third after gavage, the intestine consumption being about 15% in all situations tested and the liver decreasing its oxygen consumption with gavage.

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.

The effects of tumour necrosis factor-alpha (cachectin) and tumour growth on hepatic amino acid utilization in the rat

The effects of acute administration of tumour necrosis factor-alpha (cachectin) (TNF-alpha) or of malignant tumour growth (Walker-256 carcinosarcoma) on hepatic availability and uptake of individual amino acids were compared. The results show that, in spite of lowering the hepatic availability of alanine, aspartate, serine, glycine and proline, the cytokine increased both the total amino acid hepatic uptake and the individual uptakes of alanine, glutamate, serine, threonine, proline, lysine and arginine, while decreasing those of leucine, isoleucine and phenylalanine. Tumour burden resulted in an increase in the hepatic availability of glutamine, threonine, glycine, lysine, leucine, isoleucine, valine and phenylalanine. Total liver amino acid uptake was unaffected, whereas the individual uptakes of alanine, threonine and proline were increased and those of glutamate, glutamine, serine and leucine were decreased. When effects of the cytokine are compared with those induced by tumour growth, there are similar increases in net utilization for alanine, proline and leucine, and a 3-fold difference in the increase observed for threonine. Unmatched effects are seen for glutamate, glutamine, aspartate, glycine, lysine, arginine, valine, phenylalanine and serine.

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 as a major nitrogen carrier to the liver in suckling rat pups

We measured the amino acid concentrations in the afferent and efferent vessels of the liver in anaesthetized fed adult rats and in fed suckling rat pups. A much higher content of glutamine in the portal vein and the aorta than in hepatic veins suggests that this amino acid is actively taken up by the liver of fed suckling rat pups, conversely to what is found in adult rats. In an attempt to characterize further the mechanism(s) contributing to this enhanced glutamine uptake, we monitored the time course of 1 mM-glutamine transport into plasma-membrane vesicles purified from the livers of either adult or suckling rats. The concentrative Na+-dependent uptake of glutamine was lower in those vesicles obtained from pups than in those obtained from adult rats. Glutaminase and glutamine synthetase activities in livers from both experimental groups were also measured. Glutaminase and glutamine synthetase activities in suckling rats were about 3-fold higher and 2-fold lower respectively than those in adult rats. It is concluded that glutamine is a main nitrogen carrier to the liver in fed suckling rats. A high availability of this amino acid and an enzyme imbalance between glutamine-synthesizing and -degrading activities may account for the net uptake found in vivo.

Hepatic uptake of amino acids in late-pregnant rats. Effect of food deprivation

Hepatic availability, uptake and fractional extraction of amino acids were estimated in anaesthetized 21-day-pregnant and age-matched virgin rats, either fed or after 24 h starvation. Amino acid availability was unaltered in fed pregnant rats as compared with fed virgin controls. However, the hepatic uptake of these compounds was higher in the former than in the latter. These adaptations were mediated by an increase in the hepatic capability to take up amino acids in late-pregnant rats, as reflected by the changes found for the fractional extraction rates. The decrease in amino acid availability found after starvation was more pronounced in pregnant than in virgin rats. Nevertheless, the hepatic uptake was similar in both groups. These results indicate that amino acids are not limiting for ureagenesis during late pregnancy, strongly suggesting that the mechanism(s) which modulate urea synthesis may be intracellular in origin.

Hepatic uptake of gluconeogenic substrates in late-pregnant and mid-lactating rats

Lactate uptake by liver is markedly increased in late-pregnant and mid-lactating rats without concomitant changes in its availability. Glycerol contribution to the liver 3-C unit uptake is only significant at term gestation (50% of lactate uptake) but almost negligible at mid-lactation (10% of lactate uptake). Pyruvate is only taken up by the liver of 15-day lactating rats. As a general trend, the livers of either pregnant or lactating rats are provided with an enhanced capacity to take up gluconeogenic substrates.