Anionic amino acid transporter expression in late gestation rodent yolk sac

The yolk sac plays an important role in fetal nutrition. Transport of amino acids by the rodent visceral yolk sac has been shown previously. We have demonstrated the presence of several amino acid transport proteins capable of the Na(+)-dependent transport of anionic amino acids within late gestation mouse visceral yolk sac and uterine epithelium. We speculate that these proteins may be involved in the efflux of glutamate from the fetal to the maternal circulations.

Placental anionic and cationic amino acid transporter expression in growth hormone overexpressing and null IGF-II or null IGF-I receptor mice

The role of growth hormone (GH), insulin-like growth factor (IGF)-II and the IGF-I receptor (IGF-Ir) in the regulation of the in vivo expression of Na(+)-coupled anionic [System X-AG; GLAST1 (EAAT1), GLT1 (EAAT2), EAAC1 (EAAT3), EAAT4; where the human homologues of amino acid transport proteins first cloned in the rat are given in parentheses] and Na(+)-independent cationic (System y(+);CAT1) amino acid transport proteins was evaluated by comparing transporter expression in day 17 placentae of mice that overexpressed bovine GH (GH+) or that carried null gene mutations for IGF-II or IGF-Ir. Northern analysis revealed no apparent difference in the mRNA content of GLAST1 (EAAT1), EAAC1 (EAAT3), or EAAT4, in homogenates of GH+ placentae, but levels of GLT1 (EAAT2) and CAT1 mRNA were increased. Immunoblot analysis revealed that whole-placental steady-state GLAST1 (EAAT1), EAAC1 (EAAT3), and EAAT4 protein levels were not affected by GH+, whereas GLT1 (EAAT2) levels were increased. Immunohistochemical analysis showed that the cell-specific expression of the anionic and CAT1 transporters was not affected by overexpression of GH. Similar analyses of null IGF-II placentae demonstrated increases in GLAST1 (EAAT1), EAAT4 and CAT1 mRNAs. Parallel immunoblot analysis demonstrated decreased expression of GLT1 (EAAT2), GLAST1 (EAAT1) and EAAC1 (EAAT3) protein, but an increased expression of EAAT4. In null IGF-II and IGF-Ir placentae, however, GLT1 (EAAT2) and EAAC1 (EAAT3) protein content was decreased in junctional zone cells, whereas CAT1 content was increased in junctional and labyrinth zone cells. These data indicate that an excess level of GH stimulates GLT1 (EAAT2) expression and that a normal level of IGF-II is required for typical expression of GLT1 (EAAT2), GLAST1 (EAAT1) and EAAC1 (EAAT3), but that IGF-II downregulates the expression of EAAT4 and CAT1.

The serendipity of faculty practice: strategies for success

The contemporary social issues impacting healthcare coupled with the increasing demands for academic units to generate income have contributed to the emergence of faculty practice as an integral component of the nurse educator's role. As a result, faculty are encouraged increasingly to assume entrepreneurial joint appointments with the service industry. For nurse educators who engage in faculty practice, serendipity occurs when they immerse themselves in situations and emerge from the experience making unexpected discoveries. The author shares practical recommendations and strategies resulting from a successful 9-month faculty practice.

Nurse case managers' opinions of their role

Delphi and focus group methods were used to identify nurse case managers' (NCMs') opinions regarding their professional role. This study convened 15 NCMs from a regional medical center in the southeastern United States. Findings present the highest-rated items influencing the NCM role, as well as a definition and model depicting the role's essential elements. In study questionnaires, participants were asked to identify and subsequently rate items in seven categories. The overall highest-rated item was the personal attribute of critical thinking and prioritizing. The second highest-rated response was the critical function of coordinating a multidisciplinary plan of care. Interdisciplinary support and participative decision making were consistently ranked as important organizational factors. In the category of impact on health care outcomes, identifying patient needs and services was highly rated. Participants suggested that NCM assignments should be based on clinical knowledge and expertise, with a focus on prevention. Practice implications, based on the findings, are emphasized.

Activity and protein localization of multiple glutamate transporters in gestation day 14 vs. day 20 rat placenta

Concentrative absorption of glutamate by the developing placenta is critical for proper fetal development. The expression of GLAST1, GLT1, EAAC1, and EAAT4, known to be capable of D-aspartate-inhibitable and Na(+)-coupled glutamate transport (system X-AG), was evaluated in day 14 vs. day 20 rat chorioallantoic placenta. Steady-state mRNA levels were greater at day 20 for all transporters. Immunohistochemistry determined that the expression of GLAST1, GLT1, and EAAC1 was greater throughout the day 20 placenta and was asymmetric with respect to cellular localization. EAAT4 protein was not detected. System X-AG activity was responsible for most of the Na(+)-dependent glutamate uptake and was greater in day 20 than in day 14 apical and basal membrane subdomains of the labyrinth syncytiotrophoblast. Greater quantities of EAAC1 and GLAST1 protein were identified on day 20, and quantities were greater in basal than in apical membranes. GLT1 expression, unchanged in apical membranes, was decreased in basal membranes. These data correlate transporter mRNA and protein content with transport activity and demonstrate an increasing capacity for glutamate absorption by the developing placenta.

An expression system for mammalian amino acid transporters using a stably maintained episomal vector

Despite its versatility and effectiveness in numerous studies, the vaccinia/HeLa cell expression model may not be optimal for the study of all transport proteins. To evaluate an alternative expression model for amino acid transport Systems ASC and X-AG, the mRNA content and transport activity encoded by human hippocampal ASCT1 cDNA and rat hippocampal EAAC1 cDNA, respectively, were measured in pDR2-cDNA-transfected human embryonic kidney 293 cells made competent by stable transfection with the Epstein-Barr neutral antigen-1 (EBNA-1) cDNA (293c18 cells) to evaluate the EBNA-1/293c18 expression system. The results show that (i) the EBNA-1/293c18 expression system results in a larger increase over background of Systems ASCT1 (6.4x) and EAAC1 (39x) transport activity than does the vaccinia/HeLa expression system (2.6x and 22x, respectively); (ii) transfection and hygromycin B selection for the pDR2 vector do not affect the endogenous transport velocities of Systems ASC, X-AG, or A; and (iii) the endogenous transport velocities of Systems ASC and X-AG in 293c18 cells were not affected by the expression of exogenous EAAC1 or ASCT1. We conclude that the EBNA-1/293c18 cell expression model represents a useful transient expression regimen to characterize mammalian amino acid transport proteins, especially for transporters that may exhibit relatively low activity in transient expression systems lacking a selection mechanism.

Demonstration of system y+L activity on the basal plasma membrane surface of rat placenta and developmentally regulated expression of 4F2HC mRNA

Na(+)-independent cationic amino acid transport in the rat placenta occurs by leucine-sensitive and leucine-insensitive pathways. The ontogeny of these transport mechanisms within the rat placenta has been described recently. To assign the leucine-inhibitable portion of uptake definitively the uptake of [3H]arginine was studied in the presence of both BCH (to inhibit system Bo,+) and varied concentrations of leucine. Uptake of arginine into basal-enriched membrane vesicles derived from rat placenta was, in the presence of sodium, inhibited by micromolar concentrations of leucine, consistent with assignment of this activity to system y+L. In contrast, the majority of arginine uptake into apical-enriched membrane vesicles was leucine insensitive. Messenger RNA derived from rat placenta at days 14, 16, 18 and 20 of gestation was hybridized with full-length rat cDNA probes against NBAT and 4F2HC (thought to encode proteins associated with system bo,+ and y+L activities, respectively). No NBAT mRNA was detected, whereas 4F2HC mRNA was present at all gestational stages, increasing 12-fold over the last third of gestation. It is concluded that system y+L is present in the basal plasma membrane of the rat placenta syncytium and is subject to developmental regulation by a mechanism that alters the steady content of 4F2HC mRNA.

Transplantation in children. A longitudinal assessment of mothers' stress, coping, and perceptions of family functioning

This study examined the parenting stress, coping resources, and family functioning among 27 mothers of children undergoing bone marrow, liver, kidney, and heart transplantation. The mothers completed a comprehensive battery of psychological instruments at the pretransplant stage and at 1- and 6-month posttransplant stages. Increased parenting stress, financial strain, caregiver burden, and family stress were reported following transplantation and persisted for several months. The mothers reported using coping strategies characterized by attempts to maintain family integration and to understand the child's medical situation. Development and evaluation of intervention programs to enhance parents' ability to cope with stress and maintain family stability are warranted.

Glutamine transport in human and rat placenta

Glutamine plays an important role in fetal nutrition. This study explored the transport of [3H]glutamine into apical and basal predominant membrane vesicles derived from rat and human placenta. Na+-dependent glutamine transport was present in both apical and basal predominant vesicles derived from 20- and, to a lesser degree, 14-day gestation rat placenta. Amino-acid transport systems A, ASC-like, B(o,+) (in apical membrane vesicles) and, perhaps, y+L were involved in Na+-dependent glutamine transport. Na+-dependent glutamine uptake into human placental microvillus and basolateral membrane vesicles also occurred via several distinct transport activities. Glutamine transport via system N was not detected in either rat or human placental preparations. Na+-dependent glutamine transport in the rat was more pronounced in basal as compared to apical membrane vesicles. Conversely, in the human preparations, activity was significantly higher in microvillus as compared to basolateral membrane vesicles. It is concluded that Na+-dependent glutamine transport occurs through a variety of transport agencies in both the rat and human placenta. Transport varies with ontogeny and between species.

Perceptions of parenting stress and family relations by fathers of children evaluated for organ transplantation

18 fathers of children evaluated for solid organ or bone marrow transplantation completed measures of parenting stress and family functioning. Comparisons with normative data indicated that these fathers reported less parenting stress, less family conflict, more concern about family finances, and more limitations in family activities. These data highlight the need for family-based assessments in pediatric transplantation.

Ontogeny of amino acid transport system A in rat placenta

Amino acid transport System A has previously been demonstrated in apical membranes derived from rat placenta, as well as in apical and basal membranes derived from human placenta. We have studied Na(+)-dependent alpha-(methylamino)isobutyric acid (MeAIB) transport in apical and basal predominant membrane fractions prepared from 14 and 20 day gestation rat placenta. Marker enzyme recoveries did not differ significantly between age groups. Markers for intracellular organelles were also found to be comparable. Na(+)-dependent MeAIB transport was not sensitive to freezing and could be found in all membrane components tested. Kinetic parameters were studied--Km = 852 +/- 215 microM, Vmax = 718 +/- 126 pmol/5 sec/mg protein--20 day apical; Km = 748 +/- 269 microM, Vmax = 610 +/- 176 pmol/5 sec/mg protein--20 day basal-predominant; Km 614 +/- 261 microM, Vmax = 123 +/- 45 pmol/5 sec/mg protein-14 day apical. Kinetic parameters could not be determined in the 14 day gestation basal-predominant fraction because of the small amount of uptake present. We conclude that System A like activity is found in both apical and basal predominant membrane fractions derived from rat placenta, and that this activity increases over the last one third of gestation.

Mothers of children evaluated for transplantation: stress, coping resources, and perceptions of family functioning

The stress, coping resources, and family functioning of 36 mothers of children awaiting transplantation were evaluated. Using standardized assessment techniques, we found that, overall, 20% of mothers reported clinically elevated stress responses and that parenting stress was higher for mothers of children evaluated for solid organ transplantation (vs. bone marrow transplantation). Coping strategies characterized by maintenance of personal and family stability were strongly associated with lower levels of parenting stress. Findings also showed significant disruption in family planning and activities, as well as a strong association between lower socioeconomic status and higher parenting stress. There is a need for the longitudinal assessment of parental and family functioning throughout the transplantation process as well as for interventions designed to reduce parenting distress.

Effect of low-protein diet-induced intrauterine growth retardation on rat placental amino acid transport

Given the central role of the placenta in nutrient transport to the fetus, one might propose that maternal nutrition would have a regulatory effect on this nutrient delivery. We have examined the effect of a low-protein adequate-calorie diet on specific amino acid transport processes by the rat placenta. Maternal weight, fetal weight, and placental weight were all significantly reduced in dams fed a low-protein (5% casein), isocaloric diet when compared with dams pair-fed a control (20% casein) diet. Even though maternal serum amino acid levels were maintained in the low-protein animals, fetomaternal serum amino acid ratios were significantly reduced, suggesting a reduction in nutrient transfer to the fetus. Apical and basal membrane vesicles were isolated from the placental trophoblast and were used to examine the amino acid transport capacity of both maternal-facing and fetal-facing membranes, respectively. Na+-dependent neutral amino acid transport mediated by system A was decreased in both membrane preparations, while transport mediated by system ASC was unaffected. The Na+-dependent anionic amino acid uptake by system X(-)AG (EAAC1) was reduced on the basal membrane, while the Na+-independent component was similar between the low-protein and control diet-fed dams. Cationic amino acid uptake was also reduced on both membrane surfaces. A decreased steady-state mRNA content for EAAC1 and CAT1 (system y+) suggests that reduced synthesis of the transporter proteins is responsible for the decrease in transport activity. Taken together, these data support the hypothesis that maternal protein malnutrition affects nutrient delivery to the fetus by downregulation of specific amino acid transport proteins.

Gastroesophageal reflux in the preterm infant

Gastroesophageal reflux is a problem familiar to most pediatricians. The focus of this article is to provide a synopsis of the current state of knowledge regarding gastroesophageal reflux in the premature population, as well as to provide the practitioner with a rational basis upon which to diagnose and treat gastroesophageal reflux in this population.

Reconstitution and characterization of ATP-dependent bile acid transport in human and rat placenta

Bile acid (BA) transport across the human microvillus maternal-facing trophoblast plasma membrane (mTPM) has been recently reported to be stimulated by the presence of ATP [Marin, Bravo, El-Mir and Serrano (1993) J. Hepatol. 18, S41]. Reconstitution of BA transport activity in proteoliposomes from human mTPM is reported in this paper. Typical characteristics of BA transport in native mTPM vesicles, including a requirement for ATP hydrolysis and inhibition by other BA species, were preserved in proteoliposome preparations. BA transport into 20- and 14-day-gestation rat mTPM vesicles was also stimulated by the presence of ATP as noted in human mTPM and in the rat liver canalicular membrane. Besides this functional similarity, these ATP-dependent carriers may share structural characteristics, as demonstrated by studies using an antibody (100 Ab) raised against the 100 kDa BA carrier of the canalicular membrane from rat liver which recognized proteins in both human and rat brush-border trophoblast membranes.

Moderating Effects of Family Functioning on the Social Adjustment of Children With Liver Disease

In this study, we examined the moderating effects of family functioning on parent- and child-reported indices of social functioning in 30 children with liver disease. Consistent with previous research, children with liver disease exhibited significantly lower levels of social functioning compared to normative data. Hierarchical multiple-regression analyses were conducted to examine the relative influence of cognitive functioning, disease severity, and family functioning (i.e., family cohesion and adaptability, parenting stress, and parenting esteem) on children's social functioning. Family functioning accounted for an additional 23% of the variance in parent-reported social functioning and only 4% of the variance in child-reported social competence after controlling for cognitive functioning and disease severity. These preliminary results underscore the importance of considering family factors in designing interventions to enhance the social functioning of children with liver disease.

Effect of chronic cocaine administration on amino acid uptake in rat placental membrane vesicles

This study evaluated the effects of chronic exposure to cocaine during pregnancy on amino acid uptake in placental membrane vesicles. Pregnant rats received 62 mg/kg of cocaine hydrochloride by intraperitoneal (IP) injection as a divided daily dose on gestation days 8-19 inclusive. Fetal body weights were significantly decreased by 19% in the cocaine group, while placental weights were unchanged. Placental apical membrane vesicles were prepared from control and cocaine-treated animals, and marker enzyme enrichments for alkaline phosphatase and [3H]-dihydroalprenolol binding did not differ between cocaine and control groups. Rates of uptake (10 sec) of selected radiolabeled amino acids were measured utilizing a rapid filtration technique. Na(+)-dependent apical membrane [3H]-glutamine transport (50 microM) was reduced by 95% (p < 0.05) in cocaine-treated compared to control placentas. Uptake of 50 microM [3H]-methyl aminoisobutyric acid (MeAIB) into apical membranes was also decreased by 43% (p < 0.05) in cocaine membranes. Na(+)-independent [3H]-arginine transport (10 microM), however, did not differ between control or cocaine-treated groups. In summary, chronic cocaine administration selectively inhibited the transport of glutamine and MeAIB into apical membrane vesicles, but had minimal effect on arginine transport. We postulate that this diminution in uptake may contribute to the fetal growth retardation noted in our model.

Ontogeny of cationic amino acid transport systems in rat placenta

Gestational regulation of the placental transfer of amino acids from maternal to fetal circulations is essential for the proper development of the fetus. The cationic amino acid transport systems of the microvillous (maternal facing) and basal (fetal facing) membranes of the rat placental syncytiotrophoblast were examined. Inhibition analysis documented the presence of three kinetically distinct cationic amino acid transport mechanisms: a single Na(+)-dependent mechanism in the microvillous membrane, which increased in activity from 14 to 20 days gestation but was absent from the basal membrane throughout the entire gestational period (system Bo,+), and two Na(+)-independent transport systems in both membrane domains, one that is completely inhibited by leucine, which increased in activity in both the microvillous and basal membrane domains, and the other that is leucine insensitive, which remained fairly constant in the basal membrane and increased throughout gestation in the microvillous membrane (system y1+). Northern analysis with the system y1+ cDNA revealed a specific band of approximately 7.4-7.9 kb, which increased with increasing gestational age.

Ontogeny and plasma-membrane domain localization of amino acid transport system L in rat liver

Na(+)-independent hepatic transport of branched-chain amino acids occurs via at least two distinct transport processes. System L1, characterized by micromolar Km values, predominates in hepatoma and fetal hepatocytes, whereas System L2, distinguished by Km values in the millimolar range and sensitivity to inhibition by N-ethylmaleimide (NEM), predominates in adult hepatocytes. To determine the plasma-membrane domain localization and ontogeny of System L activity in the rat, we prepared membrane vesicles from the livers of suckling (10 days old) and adult rats enriched for either basolateral (BLMV) or canalicular (CMV) domains. The initial rate of [3H]leucine uptake into BLMV and CMV derived from adult liver was significantly inhibited by the addition of 5 mM NEM; transport into BLMV and CMV derived from 10-day-old rat liver was not affected. Michaelis-Menten kinetic parameters estimated in BLMV derived from adult liver were consistent with System L2 (Km = 2.16 +/- 0.62 mM, Vmax. = 781 +/- 109 pmol/5 s per mg of protein), as were those estimated in adult CMV (Km = 0.83 +/- 0.21 mM, Vmax. = 385 +/- 38 pmol/5 s per mg of protein). Conversely, kinetic parameters estimated in BLMV derived from livers of suckling rats were consistent with System L1 (Km = 0.041 +/- 0.024 mM, Vmax. = 8.8 +/- 1.5 pmol/5 s per mg of protein), as were those from CMV of suckling rats (Km = 0.023 +/- 0.09 mM, Vmax. = 28.1 +/- 2.1 pmol/5 s per mg of protein). We conclude that NEM-inhibitable Na(+)-independent leucine transport activity consistent with System L2 is present in both BLMV and CMV derived from adult rat liver, whereas System L1 predominates in 10-day-old rat liver tissue.

Diet affects hepatocyte membrane composition, fluidity, and taurocholate transport in suckling rats

We postulated that age-related changes in hepatocyte basolateral membrane lipid composition might contribute to the diminished Na(+)-dependent taurocholate transport noted in suckling animals. Basolateral membrane vesicles (BLMVs) were prepared from suckling rats (day 11) whose dams were fed diets predominant in lard (LBLMV), corn oil (COBLMV), or fish oil (FOBLMV). Fatty acid compositions of milk and BLMV differed significantly among the groups. Membrane cholesterol was higher in FOBLMV compared with the COBLMV and LBLMV groups; lipid phosphorus and the relative distribution of phospholipid classes were similar. Fluorescence anisotropy (1,6-diphenyl-1,3,5-hexatriene) was higher in FOBLMV (0.230) than in LBLMV (0.222) or COBLMV (0.217). Excited state lifetimes were similar in all groups. Na(+)-dependent taurocholate transport was increased at 5 and 20 s in LBLMV and COBLMV compared with FOBLMV. In vitro alteration of membrane cholesterol-fluidity did not alter taurocholate transport. In conclusion, although affected by alterations in diet, simple changes in membrane fluidity-cholesterol content do not affect Na(+)-dependent taurocholate transport.

Dietary nucleotides affect hepatic growth and composition in the weanling mouse

The effect of dietary nucleotides upon hepatic growth and composition was examined in weanling mice. For 5 weeks, mice were fed either Purina Rat Chow, a nucleotide-free diet (NT-), a nucleotide-free diet supplemented with a mixture of five nucleotides (0.21% w/w), (NT+) or a nucleotide-free diet supplemented with adenosine 5'-monophosphate (0.0425% w/w) (NTA). Hepatic cholesterol and lipid phosphorous were significantly higher, whereas liver weight (expressed as a percentage of body weight), and glycogen were lower in animals fed NT- vs all other groups. NTA-fed animals presented a greater contrast to the NT- group than did animals fed the mixture of nucleotides. Liver fatty acid composition and distribution of phospholipid subclasses were not affected by dietary nucleotide supplementation. Dietary nucleotide supplementation in weanling mice affects hepatic growth and composition; adenosine 5'-monophosphate may play a unique role in these effects.

Recent advances in mammalian amino acid transport

During the last four decades, mammalian amino acid transport systems have been described at the cellular level through general properties such as ion-dependence, kinetics, substrate specificity, regulation of activity, and numerous other characteristics. These studies have allowed the definition of multiple transport systems for neutral, anionic, and cationic amino acids. Each system is distinct but exhibits overlapping substrate specificity. Direct measurement of transport has permitted a wealth of information to be accumulated regarding the regulation of overall activity, but the underlying molecular mechanisms have not been investigated because of a lack of the appropriate tools. Recent research designed to obtain these tools has proven fruitful, and the field of amino acid transport clearly is entering a new era. In the immediate future, transporter properties such as hormonal regulation, adaptive control, ion-dependence, and trans-effects will be studied at the molecular level by assaying mRNA or protein content and by analyzing results obtained with altered protein structures following site-directed mutagenesis. Identification of specific proteins associated with activities already well described will provide answers to heretofore untestable questions. For example, is Na(+)-independent transport mediated by the same proteins that mediate Na(+)-dependent uptake except that their function in this mode does not require sodium binding? What is the protein composition of amino acid transporters? As discussed above, emerging evidence suggests that transporter proteins have different molecular structure, 12 versus 1 transmembrane domains, or that they exist as heterodimers or heterotetramers. Identification of certain transporter proteins and cloning of the respective genes also will provide valuable information about a number of inheritable diseases that are thought to be caused by defects in transporter synthesis or function. The opportunity to ask these questions will certainly generate renewed interest in the field of amino acid transport and lead to exciting advances in our knowledge.

Postnatal expression of the canalicular bile acid transport system of rat liver

Canalicular plasma membrane (CPM) vesicles prepared by a Ca2+ precipitation method from developing (7 and 14 days old) and adult rat liver were used to directly examine the postnatal ontogenesis of taurocholate (TC) transport. The initial rate of 50 microM TC uptake by vesicles derived from 14-day-old and adult but not 7-day-old animals was markedly inhibited by the anion transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). DIDS-sensitive TC uptake was 21.6 +/- 5.6 (SE) at 14 days compared with 58.1 +/- 8.1 pmol.mg protein-1.5 s-1 in adults (P less than or equal to 0.01). Kinetic studies were performed by preloading these predominantly "right-side out" vesicles with TC (25-800 microM) and measuring the initial rate (5 s) of efflux into bile salt-free medium. Computer analysis of the DIDS-sensitive portion of efflux revealed saturable kinetics with a similar Vmax (2.72 +/- 0.36 vs. 1.97 +/- 0.17 nmol.mg protein-1.min-1; P = NS) but a threefold higher Km (0.35 +/- 0.09 vs. 0.11 +/- 0.02 mM; P less than or equal to 0.05) in 14 day vs. adult CPM vesicles. In contrast, efflux from 7 day CPM vesicles increased linearly with increasing concentrations of TC and was not inhibited by DIDS. Immunoblots of canalicular membranes, probed with an antibody against the 100-kDa bile acid transport protein, showed that the amount of immunoreactive carrier protein in the membranes of 14-day-old and adult rats was similar but was only 37% of the adult level at 7 days of age.(ABSTRACT TRUNCATED AT 250 WORDS)

Taurocholate transport by basolateral plasma membrane vesicles isolated from human liver

Transport of taurocholate into the hepatocyte against unfavorable chemical and electrical gradients occurs via a sodium-dependent, carrier-mediated transport system. Although this cotransporter has been characterized in the rodent, it has not been demonstrated in man. Therefore, we utilized human liver, obtained via multiorgan donation but not used for transplantation, to prepare basolateral (sinusoidal) liver plasma membrane vesicles by a Percoll gradient method. Na+,K+-ATPase, a marker enzyme for the basolateral domain, was enriched 28.9-fold in the final membrane fraction compared with homogenate, whereas the bile canalicular membrane enzymes Mg++-ATPase and alkaline phosphatase were enriched only 3.4- and 6.4-fold, respectively. Marker enzyme activities for endoplasmic reticulum, lysosomes and mitochondria were not enriched compared with homogenate. Integrity of the membrane vesicles was confirmed by the demonstration of Na+-dependent concentrative uptake of the amino acid L-alanine (estimated intravesicular volume of 0.59 microliter per mg protein). An inwardly directed 100 mM Na+ gradient stimulated the initial rate of 2.5 microM taurocholate uptake and energized a transient 2-fold accumulation of the bile acid above equilibrium ("overshoot"). In contrast, uptake was slower and no overshoot occurred with a K+ gradient. A negative intravesicular potential, created by altering accompanying anions or by valinomycin-induced K+ diffusion potentials, did not enhance taurocholate uptake, suggesting an electroneutral cotransport mechanism. Chloride as the accompanying anion stimulated the initial rate of uptake compared with anions of lesser or greater lipid permeability. Na+-dependent taurocholate (4 microM) uptake was significantly inhibited by 250 microM cholate, taurocholate, glycocholate, taurochenodeoxycholate and bromsulfophthalein.(ABSTRACT TRUNCATED AT 250 WORDS)

Determinants of bile formation during development: ontogeny of hepatic bile acid metabolism and transport

The studies cited in this brief review stress that the development of hepatic transport processes is extraordinarily complex. Important changes in hepatic morphology and synthetic capacity are required before maturation of membrane carriers for bile acids. Transport systems at both poles of the hepatocyte develop independently. An increase in bile acid synthesis at several stages during the development appears to be an ontogenic event that is programmed to occur in concert with functional maturation of the enterohepatic circulation. Expression of specific membrane transporters for bile acids can be observed in fetal liver and postnatal ileum during periods of expansion of the bile acid pool. It is likely that specific defects, such as congenitally absent or defective bile acid transport proteins, will eventually be discovered in rare patients with undefined cholestatic syndromes. The absence of active ileal bile acid transport has recently been demonstrated in several children with congenital bile acid malabsorption. Whether bile acids can actually induce or regulate production of their own carriers during development has not been determined, but an increase in bile acid pool through feeding of exogenous bile acid has been shown to stimulate an increase in plasma membrane carriers for bile acids in adult rat liver. Thus, a number of factors, including available driving forces for transport, bile acid pool size and composition, effectiveness of intracellular compartmentation and transfer, and the function of membrane carriers, can all contribute to low rates of bile flow and bile acid secretion, depending on the stage of development.