Alanine uptake by liver at midpregnancy in rats

Na+-dependent nucleoside transport in liver: two different isoforms from the same gene family are expressed in liver cells

Hepatocytes show a Na+-dependent nucleoside transport activity that is kinetically heterogeneous and consistent with the expression of at least two independent concentrative Na+-coupled nucleoside transport systems (Mercader et al. Biochem. J. 317, 835-842, 1996). So far, only a single nucleoside carrier-related cDNA (SPNT) has been isolated from liver cells (Che et al. J. Biol. Chem. 270, 13596-13599, 1995). This cDNA presumably encodes a plasma membrane protein responsible for Na+-dependent purine nucleoside transport activity. Thus, the liver must express, at least, a second nucleoside transporter which should be pyrimidine-preferring. Homology cloning using RT-PCR revealed that a second isoform is indeed present in liver. This second isoform turned out to be identical to the 'epithelial-specific isoform' called cNT1, which shows in fact high specificity for pyrimidine nucleosides. Although cNT1 mRNA is present at lower amounts than SPNT mRNA, the amounts of cNT1 protein, when measured using isoform-specific polyclonal antibodies, were even higher than the SPNT protein levels. Moreover, partially purified basolateral plasma membrane vesicles from liver were enriched in the SPNT but not in the cNT1 protein, which suggests that the subcellular localization of these carrier proteins is different. SPNT and cNT1 protein amounts in crude membrane extracts from 6 h-regenerating rat livers are higher than in the preparations from sham-operated controls (3.5- and 2-fold, respectively). These results suggest that liver parenchymal cells express at least two different isoforms of concentrative nucleoside carriers, the cNT1 and SPNT proteins, which show differential regulation and subcellular localization.

Glucose turnover and gluconeogenesis in human pregnancy

The rate of appearance (Ra) of glucose in plasma and the contribution of gluconeogenesis were quantified in normal pregnant women early ( approximately 10 wk) and late ( approximately 34 wk) in gestation. Their data were compared with those of normal nonpregnant women. Glucose Ra was measured using the [U-13C]glucose tracer dilution method. Gluconeogenesis was quantified by the appearance of 2H on carbon 5 and 6 of glucose after deuterium labeling of body water pool. Weight-specific glucose Ra was unchanged during pregnancy (nonpregnant, 1.89+/-0.24; first trimester, 2.05+/-0.21; and third trimester 2.17+/-0.28 mg/kg.min, mean+/-SD), while total glucose Ra was significantly increased (early, 133.5+/-7.2; late, 162.6+/-16.4 mg/min; P = 0.005). The fractional contribution of gluconeogenesis via pyruvate measured by 2H enrichment on C-6 of glucose (45-61%), and of total gluconeogenesis quantified from 2H enrichment on C-5 of glucose (i.e. , including glycerol [68-85%]) was not significantly different between pregnant and nonpregnant women. Inasmuch as total glucose Ra was significantly increased, total gluconeogenesis was also increased in pregnancy (early pregnancy, 94.7+/-15.9 mg/min; late pregnancy, 122.7+/-9.3 mg/min; P = 0.003). These data demonstrate the ability of the mother to adapt to the increasing fetal demands for glucose with advancing gestation. The mechanism for this unique quantitative adjustment to the fetal demands remains undefined.

Inhibition of biliary bicarbonate secretion in ethinyl estradiol-induced cholestasis is not associated with impaired activity of the Cl-/HCO-3 exchanger in the rat

BACKGROUND/AIMS

Bicarbonate is a major component of bile salt independent bile flow, which is impaired in ethinyl estradiol (EE)-cholestasis. To examine this subject in EE-cholestasis, we studied: 1) basal and glucagon-stimulated biliary bicarbonate secretion both in vivo and in the isolated perfused rat liver (IPRL); 2) H+/HCO-3 transport processes in isolated rat hepatocyte couplets.

METHODS

Rats received EE (5 mg.kg b.w.-1) for 5 days. Intracellular pH (pHi) was measured (BCECF-AM) using a single-cell microfluorimetric setup.

RESULTS

Bile flow was markedly (p < 0.01) decreased in EE-treated rats. Bicarbonate concentration in bile was decreased (p < 0.01) and bicarbonate secretion was 2.5-fold lower in EE-treated animals than in controls, both in bile-fistula rats [19.5 +/- 5.1 (n = 23) vs 54.2 +/- 5.7 (n = 20) nmol.min-1g liver-1; p < 0.01] and in the IPRL [11 +/- 2 (n = 8) vs 24 +/- 3 (n = 8) nmol.min-1.g liver-1; p < 0.01]. In control IPRL, a bile/perfusate gradient for bicarbonate is maintained, while it is lost in EE-treated IPRL because of the lower bicarbonate concentration in bile. Glucagon stimulated bile flow and bicarbonate secretion to a similar extent in EE-treated and control IPRL (+25% vs +23%). Resting pHi of EE-treated hepatocyte couplets was higher in comparison with controls in KRB [7.25 +/- 0.07 (n = 35) vs 7.20 +/- 0.05 (n = 33); p < 0.02] but similar in Hepes [7.08 +/- 0.07 (n = 24) vs 7.05 +/- 0.06 (n = 26)]. Basal activity of the Cl-/HCO-3 exchanger was similar in EE-treated and control hepatocyte couplets [H+ flux = 2.87 +/- 1.12 (n = 18) vs 3.01 +/- 1.23 mM/min (n = 15)] and was stimulated to a similar extent by glucagon. Na+/HCO3-symport activity was increased in EE-treated hepatocyte couplets (p < 0.05) while the Na+/H+ exchanger was unchanged.

CONCLUSIONS

Bicarbonate biliary secretion is markedly impaired during EE-cholestasis in association with a marked decrease of bile salt independent bile flow. However, the Cl-/HCO-3 exchanger and its hormonal regulation are normal, indicating that the lower bicarbonate excretion in EE-cholestasis is not due to a compromised activity of this anion exchanger. Since the bile/perfusate gradient for bicarbonate is dissipated in EE-treated IPRL, the impaired bicarbonate excretion could be caused by a reflux of biliary bicarbonate via leaky tight junctions.

L-alanine uptake by rat liver parenchymal and haematopoietic cells during the perinatal period

Alanine disposal by liver parenchymal and haematopoietic cells from 21-day fetuses, newborns and adult rats was studied. Preparations selectively enriched in either haematopoietic cells or hepatocytes were obtained by direct perfusion of fetal- and neonatal-rat livers. L-Alanine transport into liver parenchymal cells was best fitted to two Na(+)-dependent saturable systems. The high-affinity system showed a much higher activity (Vmax.) in hepatocytes from fetuses and newborns than in those from adult rats (2.4, 4.3 and 0.3 nmol/8 min per 10(6) cells for fetuses, newborns and adults respectively). Vmax. for the low-affinity component was slightly lower during the perinatal period than in the adult (about 30 nmol/8 min per 10(6) cells for hepatocytes from fetuses and newborns, versus 48 nmol/8 min per 10(6) cells for adult rat parenchymal cells). Haematopoietic cells from fetal-rat livers showed significant Na(+)-dependent L-alanine uptake which was completely abolished after birth. These results show that the transport systems involved in L-alanine uptake by liver parenchymal cells are fully developed before birth. This probably contributes to fulfilling the high requirement for neutral amino acids for protein synthesis during development. Haematopoietic cells may play an important role in liver amino acid metabolism during fetal life.

Ethinylestradiol treatment induces multiple canalicular membrane transport alterations in rat liver

We investigated the effects of 17 alpha-ethinylestradiol treatment of rats on various transport functions in isolated basolateral and canalicular liver plasma membrane vesicles. Both membrane subfractions were purified to a similar degree from control and cholestatic livers. Although moderate membrane lipid alterations were predominantly observed in basolateral vesicles, no change in basolateral Na+/K(+)-ATPase activity was found. Furthermore, while Na(+)-dependent taurocholate uptake was decreased by approximately 40% in basolateral vesicles, the maximal velocity of ATP-dependent taurocholate transport was decreased by 63% in canalicular membranes. In contrast, only minimal changes or no changes at all were observed for electrogenic taurocholate transport in "cholestatic" canalicular membranes and total microsomes, respectively. However, canalicular vesicles from cholestatic livers also exhibited marked reductions in ATP-dependent transport of S-(2,4-dinitrophenyl)glutathione and in Na(+)-dependent uptake of adenosine, while in the same vesicles HCO3-/SO4- exchange and Na+/glycine cotransport activities were markedly stimulated. These data show that in addition to the previously demonstrated sinusoidal transport abnormalities ethinylestradiol-induced cholestasis is also associated with multiple canalicular membrane transport alterations in rat liver. Hence, functional transport alterations at both polar surface domains might ultimately be responsible for the inhibitory effects of estrogens on the organic anion excretory capacity and on bile formation in rat liver.

Uridine transport in basolateral plasma membrane vesicles from rat liver

The characteristics of uridine transport were studied in basolateral plasma membrane vesicles isolated from rat liver. Uridine was not metabolized under transport measurements conditions and was taken up into an osmotically active space with no significant binding of uridine to the membrane vesicles. Uridine uptake was sodium dependent, showing no significant stimulation by other monovalent cations. Kinetic analysis of the sodium-dependent component showed a single system with Michaelis-Menten kinetics. Parameter values were KM 8.9 microM and Vmax 0.57 pmol/mg prot/sec. Uridine transport proved to be electrogenic, since, firstly, the Hill plot of the kinetic data suggested a 1 uridine: 1 Na+ stoichiometry, secondly, valinomycin enhanced basal uridine uptake rats and, thirdly, the permeant nature of the Na+ counterions determined uridine, transport rates (SCN- greater than NO3- greater than Cl- greater than SO4(2-)). Other purines and pyrimidines cis-inhibited and trans-stimulated uridine uptake.

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].

Sensitivity of system A and ASC transport activities to thiol-group-modifying reagents in rat liver plasma-membrane vesicles. Evidence for a direct binding of N-ethylmaleimide and iodoacetamide on A and ASC carriers

1. In the present study we have examined the sensitivity of A and ASC amino-acid-carrier activities in rat liver plasma-membrane vesicles to the thiol-group modifying reagents N-ethylmaleimide (NEM) and iodoacetamide (IA). To this end, the different Na(+)-dependent entities involved in alanine transport were assessed. 2. NEM inactivated Na(+)-dependent alanine transport as a result of the inhibition of both system A and ASC transport activities. The functional sensitivity of system A to NEM was greater than that of system ASC. 3. The presence of L-alanine (10 mM) during the exposure of vesicles to NEM afforded partial protection to system A, but not to the ASC, carrier. This effect was specific, since the presence of L-phenylalanine (10 mM) did not cause any protection. 4. Na+ did not protect A or ASC carriers against NEM inactivation; however, the presence of Na+ (100 mM-NaCl) and L-alanine (10 mM) during the exposure of the vesicles to NEM protected against inactivation of system A and ASC transport activities. The extent of protection was greater in the case of the system ASC transport activity than in the case of the A carrier. 5. IA also diminished Na(+)-dependent alanine transport by inhibition of A and ASC transport activities. Sodium and L-alanine afforded protection to both A and ASC transport activities from the inhibitory action of IA. The extent of protection induced by substrates was similar for both carriers. 6. It is concluded that there is one, or several, free thiol groups in A and ASC carriers, the integrity of which is essential for transport activity. Sensitivity to thiol-group-specific reagents and the pattern of protection with substrates against inactivation is different in A and ASC carriers. That suggests the existence of topological dissimilarities regarding the thiol-group containing site(s) in A and ASC amino acid carriers.

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.

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 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.