Changes In Amino Acid Transport Research Articles

Reduction of In Vivo Placental Amino Acid Transport Precedes the Development of Intrauterine Growth Restriction in the Non-Human Primate.

Intrauterine growth restriction (IUGR) is associated with reduced placental amino acid transport (AAT). However, it remains to be established if changes in AAT contribute to restricted fetal growth. We hypothesized that reduced in vivo placental AAT precedes the development of IUGR in baboons with maternal nutrient restriction (MNR). Baboons were fed either a control (ad libitum) or MNR diet (70% of control diet) from gestational day (GD) 30. At GD 140, in vivo transplacental AA transport was measured by infusing nine (13)C- or (2)H-labeled essential amino acids (EAAs) as a bolus into the maternal circulation at cesarean section. A fetal vein-to-maternal artery mole percent excess ratio for each EAA was measured. Microvillous plasma membrane (MVM) system A and system L transport activity were determined. Fetal and placental weights were not significantly different between MNR and control. In vivo, the fetal vein-to-maternal artery mole percent excess ratio was significantly decreased for tryptophan in MNR. MVM system A and system L activity was markedly reduced in MNR. Reduction of in vivo placental amino acid transport precedes fetal growth restriction in the non-human primate, suggesting that reduced placental amino acid transfer may contribute to IUGR.

Viral model of maternal immune activation alters placental AMPK and mTORC1 signaling in rats

IntroductionMaternal immune activation (MIA) is associated with neurodevelopmental disorders in offspring. We previously demonstrated that poly(I:C)-mediated MIA alters placental and fetal brain amino acid transporter expression in rats, which could potentially play a role in altered neurodevelopment; however, the mechanism(s) underlying these changes in amino acid transporter expression remain unknown.The objective of the current study was to investigate the mechanism(s) underlying poly(I:C)-mediated changes in the expression of the amino acid transporters in the placenta. MethodsPregnant rats received poly(I:C) on gestational day 14 and placentas were collected 6 h later. Mass spectrometry-based proteomics of placentas was performed followed by pathway enrichment analysis. Activation of mTORC1 and its upstream regulator, AMPK, was investigated using immunoblotting. Finally, the role of mTORC1 and AMPK in regulating the expression and localization of the amino acid transporters EAAT2 and ASCT1 was investigated in the human choriocarcinoma cell line JAR. ResultsThe impact of poly(I:C) on the placental proteome was highly sexually dimorphic. While proteomics-based pathway enrichment analysis indicated enrichment of mTOR signaling in male placentas only, further investigation revealed inhibition of mTORC1 in both male and female placentas in addition to activation of AMPK. In vitro, activation of AMPK and inhibition of mTORC1 decreased membrane localization of EAAT2 and ASCT1. DiscussionPoly(I:C)-mediated MIA activates AMPK and inhibits mTORC1 in rat placenta, both of which decrease expression and membrane localization of EAAT2 and ASCT1 in vitro. Thus, AMPK/mTORC1 signaling could be a novel treatment target for alleviating MIA-mediated changes in placental amino acid transport.

Maternal Immune Activation Alters Placental and Fetal Brain Amino Acid Transport in Rats

ProblemMaternal immune activation (MIA) during pregnancy is an environmental risk factor for neurodevelopmental disorders such as autism and schizophrenia. However, the exact mechanism through which this occurs is unknown. Due to the important roles that amino acids play in neurodevelopment, altered transport of amino acids across the placenta and throughout the developing fetal brain could play a role in neurodevelopmental disorders. Our objectives were to determine whether virally mediated MIA alters amino acid transporter expression in the placenta and fetal brain, and to explore the underlying mechanism(s) associated with these changes.MethodsPregnant rats received poly(I:C) on gestational day 14 and placental and fetal tissues were collected 6, 24, and 48 hours later. Amino acid transporter expression in the placenta and fetal brain was measured using qPCR and immunoblotting. Free amino acid concentrations in the fetal brain were quantified using HPLC. To explore a possible mechanism, the expression and phosphorylation of components of the placental mTOR and AMPK signaling pathways were investigated using immunoblotting.ResultsPoly(I:C) induced the mRNA expression of multiple amino acid transporters in the placenta and fetal brain over all three timepoints. Conversely, poly(I:C) significantly decreased protein expression of ASCT1 and EAAT2 in the placenta, and SNAT5, EAAT1, and GLYT1 in the fetal brain. Changes in transporter expression were accompanied by functional changes in the concentrations of numerous amino acids in the fetal brain. We observed activation of AMPK and inhibition of mTOR signaling in the placentas of poly(I:C)‐treated dams.ConclusionsMIA induced by poly(I:C) significantly alters placental and fetal brain amino acid transporter expression as well as amino acid concentrations in the fetal brain. Due to the importance of amino acids in neurodevelopment, changes in amino acid transporters could contribute to the development of a neurodevelopmental disorder. AMPK and mTOR have been previously shown to regulate amino acid transporter expression. Since poly(I:C) altered the activation of placental and AMPK and mTOR, these could provide novel treatment targets to alleviate MIA‐induced changes in amino acid transport and investigate the implications of these changes in neurodevelopment.Support or Funding InformationThis work was supported by an operating grant from the Canadian Institutes of Health Research (CIHR) awarded to Dr. Piquette‐Miller [MOP 13346]. Eliza McColl was a recipient of a CIHR Canadian Graduate Scholarship (Master’s) and an Ontario Graduate Scholarship, and is currently funded by a CIHR Banting and Best Canadian Graduate Scholarship (Doctoral) [GSD‐164238].

Amino Acid Uptake Measured by [18F]AFETP Increases in Response to Arginine Starvation in ASS1-Deficient Sarcomas.

Rational: In a subset of cancers, arginine auxotrophy occurs due to the loss of expression of argininosuccinate synthetase 1 (ASS1). This loss of ASS1 expression makes cancers sensitive to arginine starvation that is induced by PEGylated arginine deiminase (ADI-PEG20). Although ADI-PEG20 treatment is effective, it does have important limitations. Arginine starvation is only beneficial in patients with cancers that are ASS1-deficient. Also, these tumors may metabolically reprogram to express ASS1, transforming them from an auxotrophic phenotype to a prototrophic phenotype and thus rendering ADI-PEG20 ineffective. Due to these limitations of ADI-PEG20 treatment and the potential for developing resistance, non-invasive tools to monitor sensitivity to arginine starvation are needed.Methods: Within this study, we assess the utility of a novel positron emission tomography (PET) tracer to determine sarcomas reliant on extracellular arginine for survival by measuring changes in amino acid transport in arginine auxotrophic sarcoma cells treated with ADI-PEG20. The uptake of the 18F-labeled histidine analogue, (S)-2-amino-3-[1-(2-[18F]fluoroethyl)-1H-[1,2,3]triazol-4-yl]propanoic acid (AFETP), was assessed in vitro and in vivo using human-derived sarcoma cell lines. In addition, we examined the expression and localization of cationic amino acid transporters in response to arginine starvation with ADI-PEG20.Results: In vitro studies revealed that in response to ADI-PEG20 treatment, arginine auxotrophs increase the uptake of L-[3H]arginine and [18F]AFETP due to an increase in the expression and localization to the plasma membrane of the cationic amino acid transporter CAT-1. Furthermore, in vivo PET imaging studies in mice with arginine-dependent osteosarcoma xenografts showed increased [18F]AFETP uptake in tumors 4 days after ADI-PEG20 treatment compared to baseline.Conclusion: CAT-1 transporters localizes to the plasma membrane as a result of arginine starvation with ADI-PEG20 in ASS1-deficient tumor cells and provides a mechanism for using cationic amino acid transport substrates such as [18F]AFETP for identifying tumors susceptible to ADI-PEG20 treatment though non-invasive PET imaging techniques. These findings indicate that [18F]AFETP-PET may be suitable for the early detection of tumor response to arginine depletion due to ADI-PEG20 treatment.

Decreased placental amino acid transport and intrauterine growth restriction: which is the chicken and which is the egg?

Decreased placental amino acid transport and intrauterine growth restriction: which is the chicken and which is the egg?

Proliferation-dependent changes in amino acid transport and glucose metabolism in glioma cell lines

Proliferation-dependent changes in amino acid transport and glucose metabolism in glioma cell lines

Waterlogging affects nitrogen transport in the xylem of soybean

Transfer of nodulated and non-nodulated plants grown in vermiculite to hydroponic culture without soil was used to study waterlogging and nitrogen transport in the xylem of soybean. Non-aeration, aeration or aeration with nitrogen gas were used to obtain different levels of oxygen in the culture solutions. Ureides, the principal form of nitrogen transport in nodulated plants, were considerably reduced in waterlogged plants or after transfer to water-culture, especially when not aerated or aerated with nitrogen gas. Aeration of the water-culture following a period of non-aeration allowed some recovery of ureides, as did the return of plants to drained vermiculite. Although smaller changes in the total amino acid fraction were observed for the different treatments, marked changes occurred in the composition depending on the treatment imposed. A high proportion of asparagine and low glutamine characterised non-nodulated plants grown on nitrate, or nodulated plants subsequently fed nitrate. A higher level of glutamine and lower level of asparagine characterised nodulated plants dependent on nitrogen fixation. High levels of aspartic acid characterised plants transferred to water-culture with aeration, especially in N-deficient solution, while alanine and serine were very prominent in non-aerated or hypoxic water-culture. These changes also occurred in non-nodulated plants and plants kept in vermiculite in a flooded condition. Some of the changes in transport were accompanied by similar changes in the free amino acid fraction of the roots. It is suggested that an alteration in asparagine metabolism may underlie the changes in amino acid transport in the xylem associated with waterlogging.

Influence of epidermal growth factor (EGF) on renal transport of PAH and amino acids in amino acid loaded rats

In anaesthetized adult female rats, the influence of epidermal growth factor (EGF) on renal transport of p-amino-hippurate (PAH), electrolytes, and amino acids was investigated. After loading with PAH (200 mg/100 g b.wt. iv.), PAH excretion in EGF treated rats (8 microg/100 g b.wt. subcutaneously for 8 days, twice daily 8 a.m. and 4 p.m.) was increased by about 20 %. Continuous infusions of glutamine, arginine (both 50 mg/100 g b.wt. per hour), or alanine (90 mg/ 100 g b.wt. per hour) were followed by an increase in the fractional excretion (FE) of the administered amino acids as well as of the other endogenous amino acids. Under load conditions (alanine, arginine or glutamine), EGF pretreatment was followed by a stimulation of renal amino acid reabsorption. These changes in amino acid transport were connected with a significant reduction of GFR after EGF pre-treatment (0.96+/-0.10 vs. 0.62+/-0.07 ml/min x 100 g b.wt.), with a distinct increase in sodium excretion (2.98+/-0.55 vs. 4.97+/-0.71 microval/100 g b.wt. x 20 min) and with a retarded normal kidney weight gain (874+/-18 vs. 775+/-32 mg/100 g b.wt.). A simultaneous PAH load reduced amino acid reabsorption as a sign of overloading of renal tubular transport capacity, but in EGF pretreated animals the amino acid excretion was only slightly increased under these conditions.

Epidermal growth factor (EGF) increases the renal amino acid transport capacity in amino acid loaded rats.

In anaesthetized adult female rats, the influence of epidermal growth factor (EGF) on renal amino acid handling was investigated in glutamine, arginine (both 50 mg/100 g b.wt. per hour), or alanine (90 mg/100 g b.wt. per hour) loaded animals. Continuous infusions of the three amino acids were followed by an increase in the fractional excretion (FE) of the administered amino acids as well as of the other endogenous amino acids. Under load conditions (alanine, arginine or glutamine), EGF pretreatment (8 micrograms/100 g b.wt. subcutaneously for 8 days, twice daily 8 a.m. and 4 p.m.) was followed by a stimulation of renal amino acid reabsorption. The increase in the fractional excretion of the administered amino acids was significantly lower than in non-EGF-treated rats. These changes in amino acid transport were connected with a significant reduction of GFR after EGF pretreatment (0.96 +/- 0.10 vs. 0.62 +/- 0.07 ml/min x 100 g b.wt.) and a distinct increase in sodium excretion (2.98 +/- 0.55 vs. 4.97 +/- 0.71 muval/100 g b.wt. x 20 min). After loading with p-aminohippurate (PAH; 200 mg/100 g b.wt.), PAH excretion in EGF rats was increased by about 20%, whereas urinary protein excretion was lower in EGF pretreated rats (control: 0.45 +/- 0.04 vs. EGF: 0.18 +/- 0.03 mg/100 g b.wt. x 20 min). The PAH load reduced amino acid reabsorption as a sign of overloading of renal tubular transport capacity, but in EGF pretreated animals the amino acid excretion was only slightly increased under these conditions. Furthermore, EGF pretreatment depressed normal kidney weight gain significantly (874 +/- 18 vs. 775 +/- 32 mg/100 g b.wt.). EGF can improve the renal tubular transport capacity, but, compared to well-known stimulators of renal transport like dexamethasone or triiodothyronine, its effect is only of a moderate degree.

Signaling elements involved in amino acid transport responses to altered muscle cell volume.

Skeletal muscle glutamine uptake via the transport system Nm is subject to rapid (t(1/2) = approximately 1 min) regulation after changes in cell volume by mechanisms that remain to be elucidated. Wortmannin (phosphatidylinositol 3-kinase inhibitor) but not rapamycin (inhibitor of p70S6 kinase activation) prevents both hypo-osmotic swelling-induced stimulation and hyperosmotic shrinkage-induced inhibition of Na+-dependent glutamine uptake in primary culture of rat skeletal muscle. G-protein inhibitors (cholera, pertussis toxins) also abolished responses of glutamine transport to cell volume changes whereas these responses were sustained in the presence of G-protein activators (MAS 7, lysophosphatidic acid). Swelling-induced activation of glutamine transport does not seem to involve release of autocrine factors because "conditioned" medium from swollen cells has no effect on previously unstimulated cells. System A amino acid transport exhibits responses to cell volume change that are opposite to those of system Nm, but these are also blocked by wortmannin. Active phosphatidylinositol 3-kinase appears to be required to enable muscle cells to exhibit rapid, volume-induced changes in amino acid transport when suitably stimulated.

Involvement of system A in proline transport in the intestinal crypt-like cell line IEC-17

Although the morphologic and enzymatic acquisitions associated with enterocytic differentiation have been extensively investigated, little is known about the changes in amino acid transport during this process. The transport of proline was examined in the rat epithelial cell line IEC-17, which presents the characteristics of an undifferentiated crypt cell. Proline uptake involves both Na +-independent non-saturable diffusion and Na +-dependent components. The Na +-dependent transport of proline is sensitive to the pH and is inhibited by hydroxyproline, glycine, alanine, and MeAIB. Non-α amino acids, such as β-alanine and γ-aminobutyric acid, do not compete with proline, whereas leucine is a weak inhibitor. Therefore, proline transport appears to occur through the system A, whereas the intestinal specific systems (B and IMINO) are not involved in this process. These observations suggest that these two systems are either absent or expressed at a very low level in this undifferentiated epithelial cell line.

Application of alpha-aminoisobutyric acid, L-methionine, thymidine and 2-fluoro-2-deoxy-D-glucose to monitor effects of chemotherapy in a human colon carcinoma cell line.

Up to 4 h after treatment of human SW 707 colon carcinoma cells with the antineoplastic drug 4-amino-N-(2'-aminophenyl)-benzamide (GOE 1734, dinaline), the effects of tumour cell metabolism and proliferation were examined in vitro. Four tracers which can be labelled with isotopes suitable for positron emission tomography (PET) were used for this purpose: alpha-aminoisobutyric acid (AIB) and methionine to study changes in amino acid transport and protein synthesis, thymidine to observe changes in tumour proliferation and 2-fluoro-2-deoxy-D-glucose (FDG) to estimate glucose metabolism. Dinaline showed an inhibition of the sodium-dependent and -independent uptake of AIB. The methionine uptake was found to increase shortly after therapy. Thymidine incorporation into DNA was impaired and the FDG uptake showed a maximally 2.2-fold enhancement. Inhibition of AIB uptake suggests changes in amino acid transport, whereas increased uptake of methionine and FDG points to an enhancement of protein synthesis and glycolysis caused by repair mechanisms. The cytostatic and antiproliferative effect of dinaline, observed in cell growth curves, could be demonstrated by the impaired thymidine incorporation into DNA. This study demonstrates that in vitro screening with radiotracers suitable for PET can help to clarify effects of new antineoplastic substances on tumour cell metabolism. These data may be applied to choose the appropriate time schedule for monitoring therapeutic effects on tumour tissue.

Brush border membrane and amino acid transport

Fundamental differences in midgut structure, physiology, brush border proteins, and transporters among Leptinotarsa decemlineata, lepidopteran caterpillars, other insect taxa, and vertebrates are reviewed. The effects of dietary protein concentration on Manduca sexta midgut amino acid transport and brush border membrane proteins are reported. M. sexta fed diet with reduced protein had elevated levels of leucine aminopeptidase in the brush border membrane. No changes in amino acid transport or alkaline phosphatase activity due to dietary differences were detected. Changes in brush border proteins could affect the toxicity and pathogenicity of microbial agents. © 1996 Wiley-Liss, Inc.

Substrate-dependent regulation of intracellular amino acid concentrations in cultured bovine aortic endothelial cells

Amino acid deprivation induces adaptive changes in amino acid transport and the intracellular amino acid pool in cultured cells. In this study intracellular amino acid levels were determined in cultured bovine aortic endothelial cells (EC) deprived of L-arginine or total amino acids for 1, 3, 6 and 24 h. Amino acid concentrations were analyzed by reverse phase HPLC after precolumn derivatisation. Under normal culture conditions levels of L-arginine L-citrulline, total essential and non-essential amino acids were 840 +/- 90 microM, 150 +/- 40 microM, 11.4 +/- 0.9 mM and 53.3 +/- 3.4 mM (n = 9), respectively. In EC deprived of L-arginine or all amino acids for 24 h L-arginine and L-citrulline levels were 200 microM and 50 microM, and 670 microM and 100 microM Deprivation of L-arginine or total amino acids induced rapid (1 h) decreases (30 - 50%) in the levels of other cationic (lysine, ornithine) and essential branched-chain (valine, isoleucine, leucine) and aromatic (phenylalanine, tryptophan) amino acids. L-glutamine was reduced markedly in EC deprived of total amino acids for 1 h - 6 h but actually increased 3-fold in EC deprived of L-arginine for 6 h or 24 h. Arginine deprivation resulted in a rapid decrease in the total intracellular amino acid pool, however concentrations were restored after 24 h. Increased amino acid transport and/or reduced protein synthesis may account for the restoration of amino acid levels in EC deprived of L-arginine. The sustained reduction in the free amino acid pool of EC deprived of all amino acids may reflect utilization of intracellular amino acids for protein synthesis.

Dimorphism-associated changes in amino acid transport of Candida albicans

Amino acid uptake was followed during pH-regulated dimorphism of Candida albicans. It was observed that transport activities of various amino acids differed with the morphological phenotype. The uptake rates of l-alanine, l-phenylalanine and of l-lysine were lower and those of l-methionine were higher in elongated hypha (germ tube), while the rates of glycine, l-glutamic acid and l-proline were similar in bud and hyphal phenotypes. Minimum threshold of amino acids transport activity is required at the time of phenotypic commitment in a diverging population of Candida albicans.

Dexamethasone selectively increases sodium-dependent alanine transport across neonatal piglet intestine.

1. Lysine and alanine uptake by pig enterocytes has been measured in piglet mid intestine both during normal development and 3 days after injection of dexamethasone and epidermal growth factor (EGF) into 3-day-old animals. 2. Alanine uptake measured in the presence of sodium increased markedly during the first 4 weeks of post-natal life. Similar effects on alanine uptake could be produced through injection of dexamethasone, but not EGF, into 3-day-old piglets. Alanine uptake measured in the absence of sodium and lysine uptake measured in the presence of sodium remained unchanged during development and unaffected by injection of dexamethasone or EGF. 3. Enterocytes capable of transporting alanine in the presence of sodium were found, by quantitative autoradiography, to cover the top 400 micron of the villus in 6-day-old and 3-4-week-old control pigs. Alanine concentrations in villus tip enterocytes in 3-4-week-old pigs were four times those found in 6-day-old animals. Qualitative examination of selected villi, however, showed alanine uptake taking place over a considerably greater area of villus surface in 6-day-old compared with 3-4-week-old animals. 4. Injection of dexamethasone and EGF into 3-day-old piglets caused an increase in crypt depth without apparent change in crypt cell proliferation. The rate at which enterocytes migrated out of the crypt and the length of individual villi also remained unchanged by dexamethasone or EGF injection. 5. Dexamethasone produces its effect on alanine uptake by acting on older enterocytes present on the upper part of the villus. These enterocytes can be shown, by calculations based on enterocyte migration rate, to have already been present on the villus at the time the pig was born. 6. The above findings are discussed in relation to the ability of villus as well as crypt enterocytes to change their programme of differentiation in response to external stimuli. The particular ability of dexamethasone to induce system A type carrier function is further discussed in relation to normal changes found to occur during neonatal development. It is finally suggested, as a working hypothesis, that endogenous glucagon might act as the final mediator of both developmentally controlled and dexamethasone-induced changes in amino acid transport.

Characterization of amino acid transport during erythroid cell differentiation.

We have studied the changes in amino acid transport in fetal erythroid cells isolated from rat fetal liver at different gestation days. Our results show that System A transport as measured by the Na+-dependent uptake of 2-(methylamino)isobutyric acid (MeAIB) was conspicuous at day 13 but virtually disappeared between days 16 and 18. In contrast, the activity of System ASC measured by the Na+-dependent uptake of MeAIB-insensitive threonine uptake increased after day 14 and was optimal between days 16 and 18. This transport system regressed in activity with further maturation, but remained conspicuously saturable in the matured red blood cell. Interestingly, the newly discovered Na+-independent System asc (Vadgama, J. V., and Christensen, H.N. (1985) J. Biol. Chem. 260, 2912-2921), selective for the uptake of test substrates threonine, serine, and alanine, was present in these erythroid cells. Its activity increased during gestation days 16-18. System L transport was present simultaneously with the Na+-independent System asc. As we had previously demonstrated for the pigeon red blood cell, these two transport systems are kinetically independent as confirmed with inhibition studies and the special selectivity of System L to trans stimulation. Tryptophan uptake could be attributed predominantly to System L, as also observed for the nucleated pigeon red blood cells and certain other cells. Arginine showed its familiar Na+-independent mode of uptake as a cation throughout the interval of study. An exceptional Na+-dependent component of arginine uptake emerged after day 14, peaked at day 18, and then disappeared on further maturation of the erythroid cell.

Parallel changes in amino acid transport and protein kinase C localization in LLC-PK1 cells treated with TPA or diradylglycerols.

Protein kinase C is considered to be a major target for tumor promoting phorbol esters such as 12-0-tetradecanoylphorbol-13-acetate (TPA). We have analyzed the correlation between A-system amino acid transport and the distribution of protein kinase C (PKC) between a membrane-rich fraction (100,000 g pellet) and cytosol (supernatant) from homogenized LLC-PK1 cells, a pig kidney epithelial cell line grown in culture. During log growth 1 day after seeding the cells onto culture plates, PKC activity is high in the membrane fraction and low in the cytosol. As the cells become confluent the PKC distribution shifts to a cytosolic pool. Concomitantly, A-system amino acid transport, as measured by methylaminoisobutyric acid [14C]MeAIB uptake, decreases. TPA (0.01-1.0 microM) induces a shift of PKC activity from the cytosol back to the membrane-rich fraction in post-confluent cells with a concomitant 2-3 fold stimulation of MeAIB uptake. The same responses can be achieved by treating cells with certain diradylglycerols, either diacylglycerols such as 1-oleyl-2-acetyl-sn-glycerol (OAG) or alkylacylglycerols such as 1-hexadecenyl-2-oleyl-sn-glycerol. Both responses to TPA are blocked by cytochalasin B, but cycloheximide inhibits the transport response without affecting PKC redistribution. It is suggested that the redistribution may be a necessary but not sufficient concomitant to the transport activation.

Intestinal Absorption of Glucose and Amino Acids in Chickens Administered Monensin

Effects of the ionophoric anticoccidial agent monensin on glucose and amino acid absorption were investigated in young broiler chicks. Chicks housed in cages with wire floors were fed a practical diet containing 0 or 121 ppm monensin for 2 to 3 weeks prior to measurement of nutrient absorption. Chicks were anesthetized with halothane, and the intestine was accessed through an incision in the abdominal wall. A 10-cm segment of the intestine immediately anterior to Meckel’s diverticulum was rinsed free of digesta and used for measurement of absorption in situ. One milliliter of saline solution containing 2 mM of glucose or a mixture of amino acids was injected into the lumen of the ligated segment, and recovery of glucose and amino acids in the lumen was determined after 4- and 5-min intervals, respectively. In one glucose study, the solution contained 0, 1, 10, or 80 μg/ml of monensin. In one amino acid study, the solution contained 0, 5, or 80 μg/ml of monensin; in subsequent studies, monensin was not included in the solution.Results show that neither dietary monensin nor monensin in the saline solution affected glucose absorption. Dietary monensin tended to depress lysine absorption but increased the absorption of tryptophan and arginine, the latter amino acid being markedly affected by monensin. Dietary monensin alone had no effect on absorption of cystine, isoleucine, methionine, or threonine. Inclusion of monensin in the saline solution depressed methionine absorption but only when birds also had been fed the diet containing monensin. Monensin in the saline solution depressed isoleucine absorption irrespective of dietary treatment. We conclude that monensin may cause subtle changes in amino acid transport in the gut or other tissues and suggest that this may be a factor in the depression of food intake and growth rate that occurs in uninfected chicks receiving dietary monensin.

Effects of Ultrasound on Placental Function

Reports indicate that exposure to ultrasound can produce adverse effects on blood Row and uterine tissue under appropriate con- ditions. Thus it is important to directly investigate the effects of ultra- sound on placental function. To accomplish this, a perfusion technique (previously reported), based on isolation of the fetal circulation of the guinea pig placenta in siru was used. Tritiated water and %cY-ami- noisobutyric acid were introduced into the maternal circulation, their isobutyric concentrations determined in the umbilical perfusing fluid, and clearances calculated for periods before, during, and after expo- sure to pulsed and continuous ultrasound. The preparation thus al- lowed us to examine the effects of ultrasound on blood flow to the pla- centa and transplacental movements of a neutral amino acid. No effects on either measurement were observed at intensities of 1 W/cm2 contin- uous or 0.02 W/cmZ pulsed modes (both SATA). Ten-minute exposures appeared to have an effects threshold between 0.06 W/cm2 and 0.09 W1 cm2 in the pulsed mode and between 6.3 Wicm' and 10.5 W/cm2 in the continuous mode (both SATA). Noconsistent changes in amino acid transport occurred independently of changes in maternal blood flow to the placenta. I. INTRODUCT~ON