Poly(A)+ RNA from the mucosa of rat jejunum induces novel Na(+)-dependent and Na(+)-independent leucine transport activities in in oocytes of Xenopus laevis

Na+-Coupled Nutrient Cotransport Induced Luminal Negative Potential and Claudin-15 Play an Important Role in Paracellular Na+ Recycling in Mouse Small Intestine

Many nutrients are absorbed via Na⁺ cotransport systems, and therefore it is predicted that nutrient absorption mechanisms require a large amount of luminal Na⁺. It is thought that Na⁺ diffuses back into the lumen via paracellular pathways to support Na⁺ cotransport absorption. However, direct experimental evidence in support of this mechanism has not been shown. To elucidate this, we took advantage of claudin-15 deficient (cldn15^-/-) mice, which have been shown to have decreased paracellular Na⁺ permeability. We measured glucose-induced currents (ΔI_sc) under open- and short-circuit conditions and simultaneously measured changes in unidirectional ²²Na⁺ fluxes (ΔJ^Na) in Ussing chambers. Under short-circuit conditions, application of glucose resulted in an increase in ΔI_sc and unidirectional mucosal to serosal ²²Na⁺ (∆J^Na_MS) flux in both wild-type and cldn15^-/- mice. However, under open-circuit conditions, ΔI_sc was observed but ∆J^Na_MS was strongly inhibited in wild-type but not in cldn15^-/- mice. In addition, in the duodenum of mice treated with cholera toxin, paracellular Na⁺ conductance was decreased and glucose-induced ∆J^Na_MS increment was observed under open-circuit conditions. We concluded that the Na⁺ which is absorbed by Na⁺-dependent glucose cotransport is recycled back into the lumen via paracellular Na⁺ conductance through claudin-15, which is driven by Na⁺ cotransport induced luminal negativity.

Renal handling of amino acids in 5/6-nephrectomized rats: stimulation of renal amino acid reabsorption after treatment with triiodothyronine or dexamethasone under amino acid load

In anaesthetized adult female rats, the renal amino acid handling was measured six days after 5/6 nephrectomy (5/6NX). The distinct rise in blood urea nitrogen as well as the significant reduction in urine flow and GFR indicate an impairment of kidney function. In principle, in 5/6NX rats amino acid plasma concentrations were comparable to those of control animals with two intact kidneys, whereas the fractional excretions (FEAA) of most endogenous amino acids measured were significantly enhanced. After bolus injection of leucine or taurine (each 20 mg/100 g b.wt.) or glutamine (90 mg/ 100 g b.wt.), dissolved in 2 ml normal saline per 100 g b.wt., the FEAA of both the amino acids administered and the endogenous amino acids increased as a sign of overloaded amino acid reabsorption capacity. This effect was more pronounced in 5/6NX rats than in controls. As early as one hour after amino acid load, plasma concentrations and FEAA returned to baseline values of 5/6NX rats. A pretreatment with triiodothyronine (20 micrograms/100 g b.wt.) or dexamethasone (60 micrograms/100 g b.wt.), both given intraperitoneally once daily for 3 days, stimulated the renal amino acid transport capacity in 5/6NX rats: the increase in FEAA after amino acid load was significantly lower compared to non-pretreated animals. This stimulation could be shown for the bolus amino acids and the endogenous amino acids and was more distinct in 5/6NX rats than in controls with two intact kidneys.

Changes of renal taurine transport after treatment with triiodothyronine or dexamethasone in amino acid loaded rats

In adult female anaesthetized rats, the influence of triiodothyronine or dexamethasone on renal amino acid (AA) handling was investigated in taurine (45 mg/100 g b.wt.) loaded animals. Bolus injections of taurine were followed by temporary increase in fractional excretion (FE(AA)) of taurine as well of the endogenous amino acids which were not administered. Under taurine load conditions, triiodothyronine treatment (20 microg/100 g b.wt. for 3 days, i.p. once daily) was followed by a slight stimulation of the renal taurine reabsorption: the increase in FE(taurine) after taurine load was lower than in untreated rats. Dexamethasone (60 microg/100 g b.wt. for 3 days, i.p. once daily) was without significant effect on FE(taurine) in taurine loaded rats. In non taurine loaded rats there was no hormone influence at all. Similarities and differences between the effects of bolus injections of taurine, glutamine, and leucine on the FE(AA) of these three amino acids were compared in detail to further clarify the reason for the increased amino acid reabsorption capacity after pretreatment with triiodothyronine or dexamethasone.

The hepato-renal syndrome: renal amino acid transport in bile duct ligated rats (DL)--influence of treatment with triiodothyronine or dexamethasone on renal amino acid handling in amino acid loaded rats

The influence of triiodothyronine or dexamethasone on renal amino acid handling was investigated in anaesthetized, bile duct-ligated (DL) adult female rats. 3 days after DL, glomerular filtration rate (GFR) was unchanged whereas urine flow was decreased. Plasma concentrations of 5 out of 16 amino acids were significantly enhanced after DL. On the other hand, the fractional excretion (FE) of 11 out of 16 amino acids was significantly reduced as a sign of improved reabsorption capacity. Bolus injections of leucine (20 mg/100 g b.wt.), glutamine (45 mg/100 g b.wt.), or taurine (45 mg/100 g b.wt.) were followed by a temporary increase in the FE of the administered amino acids as well of the endogenous amino acids which were not administered. This phenomenon was more pronounced in DL than in control rats. Under load conditions, dexamethasone (60 microg/100 g b.wt.) or triiodothyronine (20 microg/100 g b.wt.) treatment for 3 days, i.p. once daily, was followed by a stimulation of renal amino acid reabsorption in DL rats. The increase in fractional amino acid excretion after amino acid load was significantly lower than in untreated rats. This effect was also more pronounced in DL rats.

Cloning and functional expression of a cDNA from rat jejunal epithelium encoding a protein (4F2hc) with system y+L amino acid transport activity

Two different protein families, designated CAT (cationic amino acid transporter) and BAT (broad-specificity amino acid transporter) mediate the plasma membrane transport of cationic amino acids in animal cells. CAT transporters have 12-14 transmembrane domains and are selective for cationic amino acids. BAT proteins, in contrast, have one to four transmembrane domains and induce the transport of both cationic and zwitterionic amino acids when expressed in Xenopus oocytes. Mutations in the human BAT gene cause type I cystinuria, a disease affecting the ability of intestinal and renal brush border membranes to transport cationic amino acids and cystine. We have used functional expression cloning in oocytes to isolate a BAT-related cDNA from rat jejunal epithelium. The cDNA encodes the rat 4F2 heavy chain (4F2hc) cell-surface antigen, a 527-residue (60 kDa) protein that is 26% identical in amino acid sequence with rat renal BAT (also known as NBAT/D2). Expression of rat jejunal 4F2hc in oocytes induced the lysine-inhibitable Na+-dependent influx of leucine and the leucine-inhibitable Na+-independent influx of lysine. Lysine efflux was stimulated by extracellular (Na+ plus leucine). These characteristics identify the expressed amino acid transport activity as system y+L, a transporter that has been implicated in basal membrane transport of cationic amino acids in intestine, kidney and placenta.

Expression of the surface antigen 4F2hc affects system-L-like neutral-amino-acid-transport activity in mammalian cells

Mammalian cells possess a variety of amino acid-transport systems with overlapping substrate specificity. System L is one of the major amino acid-transport systems of non-epithelial cells. By expression cloning we have recently demonstrated that the surface antigen 4F2hc (CD98) is a necessary component for expression of system-L-like amino acid-transport activity in C6-BU-1 rat glioma cells [Bröer, Bröer and Hamprecht (1995) Biochem. J. 312, 863-870]. 4F2hc mRNA was detected in CHO cells, COS cells, activated lymphocytes isolated from mouse spleen and primary cultures of astrocytes. In all these cell types, Na+-independent isoleucine transport was mediated by system L. No contribution of system y+L to isoleucine or arginine transport was detected in C6-BU-1 cells. In lymphocytes, both system-L-like amino acid-transport activity and 4F2hc mRNA levels increased after treatment with phorbol ester plus ionomycin. Antisense oligonucleotides caused modest inhibition of Na+-independent isoleucine transport in C6-BU-1 cells and primary cultures of astroglial cells, whereas arginine transport was unaffected. Overexpression of 4F2hc cDNA in CHO cells resulted in an increase in Na+-independent isoleucine transport.

The 4F2hc surface antigen is necessary for expression of system L-like neutral amino acid-transport activity in C6-BU-1 rat glioma cells: evidence from expression studies in Xenopus laevis oocytes

Mammalian cells possess a variety of amino acid-transport systems with overlapping substrate specificity. System L is one of the major amino acid-transport systems in all non-epithelial cells. Its molecular structure is not known. To clone the neutral amino acid-transporter system L, we followed an expression cloning strategy using Xenopus laevis oocytes. A cDNA library derived from C6-BU-1 rat glioma cells was used as a source, because high expression of system L activity could be demonstrated with polyadenylated RNA isolated from these cells, when injected into Xenopus laevis oocytes [Bröer, Bröer and Hamprecht (1994) Biochim. Biophys. Acta 1192, 95-100]. A single clone (ILAT) was identified, the sense cRNA of which, on injection into Xenopus laevis oocytes, stimulated sodium-independent isoleucine transport by about 100-fold. Further characterization revealed that transport of cationic amino acids was also stimulated. Sequencing of the cDNA showed that the identified clone is the heavy chain of the rat 4F2 surface antigen, a marker of tumour cells and activated lymphocytes. Uptake of neutral and cationic amino acids was not stimulated by the presence of Na+ ions. Antisense cRNA transcribed from this clone or antisense oligonucleotides, when co-injected with polyadenylated RNA from C6-BU-1 rat glioma cells, completely suppressed system L-like isoleucine-transport activity. We conclude that ILAT is necessary for expression of system L-like amino acid-transport activity by polyadenylated RNA from C6-BU-1 rat glioma cells.