Biomedical subjects

I Tamai

Publications and source records attributed to I Tamai.

At least 91 records · Page 5Linked to original sources

PubMed1996

Active secretion of drugs from the small intestinal epithelium in rats by P-glycoprotein functioning as an absorption barrier.

Because the significance of P-glycoprotein in the in-vivo secretion of beta-blockers in intestinal epithelial cells is unclear, the secretory mechanism for beta-blockers and other drugs has been evaluated. Uptake of the beta-blockers acebutolol, celiprolol, nadolol and timolol, and the antiarrhythmic agent, quinidine by the multidrug-resistant leukaemic cell line variant K562/ADM was significantly lower than that by drug-sensitive K562 cells, suggesting that these beta-blockers are transported by P-glycoprotein out of cells. The reduced uptake of acebutolol by the drug-resistant K562/ADM cells was reversed by treating the cells with anti-P-glycoprotein monoclonal antibody, MRK16, whereas no such alteration in uptake was observed for drug-sensitive K562 cells. Acebutolol uptake by K562/ADM cells was, moreover, markedly enhanced, in a concentration-dependent manner, in the presence of the specific P-glycoprotein inhibitors, MS-209 and cyclosporin. Caco-2 cells were used for evaluation of the role of P-glycoprotein in intestinal permeability to drugs in-vitro. Basolateral-to-apical transport of acebutolol was twice that in the reverse direction. A similar polarized flux was also observed in the transport of vinblastine, but not in that of acetamide or mannitol. When in-vivo intestinal absorption was evaluated by the rat jejunal loop method, with simultaneous intravenous administration of a P-glycoprotein inhibitor, cyclosporin, intestinal absorption of both acebutolol and vinblastine increased 2.6- and 2.2-fold, respectively, but no such enhancement was observed in the absorption of acetamide. The effect of cyclosporin on the intestinal absorption of several drugs was further examined, and the extent of the contribution of P-glycoprotein as an absorption barrier to those drugs was evaluated. ATP depletion by occlusion of the superior mesenteric artery resulted in a clear increase in epithelial permeability to vinblastine, but not to 3-O-methylglucose or acetamide, indicating that vinblastine is secreted by ATP-dependent P-glycoprotein into the lumen. These findings demonstrate that P-glycoprotein plays a role as an absorption barrier by transporting several drugs from intestinal cells into the lumen.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1996

Effect of polycarbophil on the absorption of nutrients.

The effects of polycarbophil on the absorption of various nutrients were evaluated by several in situ methods. Polycarbophil reduced the absorption of 3-O-methyl-D-glucose (3-OMG) and L-phenylalanine in the in situ loop and the in situ perfusion methods, but it did not affect the absorption of these nutrients in an open system, the in situ modified loop method, which is closer to physiological conditions. It also did not affect the absorption of vitamin A or phosphatidylcholine-L-alpha-dipalmitoyl in the latter system. These results indicate that the absorption of nutrients is probably not altered by polycarbophil under physiological conditions.

3-O-Methylglucose↗

PubMed1995

cDNA cloning and functional characterization of rat intestinal monocarboxylate transporter.

A cDNA clone which encodes a monocarboxylate transporter (ratMCT1) was isolated from a rat small intestinal cDNA library, which was screened by using full-length MCT1 cDNA of Chinese hamster ovary cells. The ratMCT1 cDNA was sequenced and predicted a protein of 494 amino acids with twelve potential transmembrane domains. The amino acid sequence showed 93.1% and 84.6% identity to the hamster and human monocarboxylate transporters, respectively. When expressed in Xenopus laevis oocytes, the ratMCT1 cRNA caused a significant increase in the uptake of radiolabeled lactic acid. Poly(A)+ RNA transcripts hybridizing to the ratMCT1 cDNA were detected in rat brain, heart, kidney, lung, muscle and brain capillaries. These results indicate that MCT1 contributes to pH-dependent and carrier-mediated transport of monocarboxylic acids in many tissues, not just in the small intestine.

Amino Acid Sequence↗

PubMed1995

Na(+)- and Cl(-)-dependent transport of taurine at the blood-brain barrier.

The characteristics of carrier-mediated transport of taurine at the blood-brain barrier (BBB) were studied by using primary cultured bovine brain capillary endothelial cells (BCECs), in situ brain perfusion and brain capillary depletion methods in rats. The uptake of [3H]taurine by cultured cells showed that the active transporter functions on both the luminal and antiluminal membranes of BCECs. The kinetic parameters for the saturable transport of taurine were estimated to be: for the luminal uptake, the Michaelis constant, Kt, was 12.1 +/- 0.5 microM, and the maximum uptake rate, Jmax, was 4.32 +/- 0.05 nmol/30 min/mg protein; for the antiluminal uptake, Kt was 13.6 +/- 2.4 microM and Jmax was 2.81 +/- 0.22 nmol/30 min/mg protein. The luminal and antiluminal uptakes of [3H]taurine were each dependent on both Na+ Cl-. Stoichiometric analyses suggest that two Na+ and one Cl- are associated with the luminal uptake of one taurine molecule. beta-Amino acids such as beta-alanine and hypotaurine strongly inhibited the uptake of [3H]taurine, whereas alpha- and gamma-amino acids had little or no effect. Furthermore, by in situ brain perfusion and in vivo brain capillary depletion methods, the carrier-mediated transport found by in vitro experiments was confirmed to function for the translocation of the taurine molecule from the vascular space into the brain. From these results, it was concluded that a Na+ and Cl- gradient-dependent transport (uptake) system for taurine exists in both the luminal and the antiluminal membranes of BCECs.

Amino Acids↗

PubMed1995

Participation of a proton-cotransporter, MCT1, in the intestinal transport of monocarboxylic acids.

A molecular mechanism for the intestinal monocarboxylic acid transport was characterized by using a proton/monocarboxylate transporter, MCT1, in Chinese hamster ovary (CHO) cells, first found by Garcia et al. (Cell, 76, 865-873, 1994). Northern blotting analysis showed that MCT1-isomers exist in the rat and rabbit intestinal enterocytes and Caco-2 cells. The expression of [14C]lactic acid uptake by Xenopus laevis oocytes injected with rabbit intestinal mRNA was reduced by hybridizing the mRNA with a MCT1 cDNA of CHO cells before microinjection used as the antisense DNA. [14C]Lactic acid uptake by CHO cells was pH dependent, saturable, stereospecific, and reduced in the presence of acetic acid, benzoic acid, S- and R-ibuprofen, S- and R-mandelic acid, nicotinic acid, pravastatin, propionic acid and valproic acid. In addition, several monocarboxylic acids were transported in pH-dependent and saturable manners. These results suggest that the intestinal MCT1-related protein contributes to a carrier-mediated absorption for organic weak acid compounds.

Animals↗

PubMed1995

In vivo and in vitro evidence for ATP-dependency of P-glycoprotein-mediated efflux of doxorubicin at the blood-brain barrier.

We investigated the role of ATP in the active efflux of doxorubicin (DOX) mediated by P-glycoprotein (P-gp), the multidrug-resistance (MDR) gene product, at the blood-brain barrier. In transient brain ischemic rats prepared with 4-vessel occlusion of vertebral and common carotid arteries for 20 min, a procedure that depleted their brain ATP content to 3% that of normal rats, the estimated permeability coefficient of DOX was increased 17-fold (to 243 +/- 2.5 microL/min/g brain). When the ATP content recovered to a normal level by means of 30-min and 24-hr cerebral recirculation of blood, the permeability coefficient recovered to 14.0 +/- 5.0 and 18.4 +/- 2.3 microL/min/g brain (mean +/- SEM, N = 3-6), respectively, very close to the control permeability (14.3 +/- 1.5 microL/min/g brain). The uptake of DOX by primary cultured brain capillary endothelial cells expressing P-gp at the luminal membrane was increased significantly (up to 2-fold), which correlated well with the decrease of cellular ATP contents caused by treating the cells with metabolic inhibitors. Evidence for the ATP-dependent transport of DOX obtained from the present in vivo and in vitro studies strongly indicates that P-gp in the brain capillaries functions actively as an efflux pump in the physiological state, providing a major mechanism to restrict the transfer of DOX into the brain.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1995

Hepatic uptake of octreotide, a long-acting somatostatin analogue, via a bile acid transport system.

The hepatic transport mechanism of octreotide (Sandostatin), a somatostatin analogue, was studied using freshly prepared rat hepatocytes. The initial uptake rate of octreotide represented exclusively a saturable transport process. The half-saturation constant, Kt, and the maximum uptake-rate, Jmax, for the uptake of octreotide were 91.1 +/- 28.4 microM and 104.6 +/- 19.7 pmol/mg protein/min, respectively. An energy requirement was demonstrated for [14C]octreotide uptake since metabolic inhibitors (DNP, rotenone, antimycin and NaCN) significantly reduced the initial uptake rate. [14C]octreotide uptake was also significantly inhibited by ouabain. [14C]octreotide uptake was reduced in the absence of Na+ in the uptake medium. [14C]octreotide uptake was significantly inhibited by bile acids, iodipamide, d-tubocurarine, whereas it was not inhibited by bilirubin, TEMA and insulin. Competitive inhibition of taurocholic acid was observed for octreotide uptake with the inhibition constant, Ki, of 82 +/- 17 microM. Moreover, a significant inhibitory effect of octreotide was observed for the Na+ dependent uptake of [14C]taurocholic acid. These results suggest that octreotide is transported into hepatocytes via a bile acid carrier-mediated system.

Amino Acid Sequence↗

PubMed1995

Proton-cotransport of pravastatin across intestinal brush-border membrane.

PURPOSE: The purpose of the present study is to clarify the intestinal brush-border transport mechanism of a weak organic acid, pravastatin, an HMG-CoA reductase inhibitor. METHODS: The transport of pravastatin was studied by using intestinal brush-border membrane vesicles prepared from rabbit jejunum, and uptake by the membrane vesicles was measured using rapid filtration technique. RESULTS: The initial uptake of [14C]pravastatin was markedly increased with decreases in extravesicular pH and showed a clear overshoot phenomenon in the presence of a proton gradient (pHin/out = 7.5/5.5). A protonophore, carbonylcyanide p-trifluoromethoxyphenylhydrazone, significantly reduced the uptake of [14C]pravastatin. In addition, an ionophore for sodium, potassium and proton, nigericin, stimulated the uptake of [14C]pravastatin in the presence of a potassium gradient ([K+]in/[K+]out = 0/145 mM). On the other hand, neither the imposition of an inwardly directed sodium gradient nor an outwardly directed bicarbonate gradient stimulated the uptake of [14C]pravastatin. In the presence of a proton gradient (pHin/out = 7.5/5.5), the initial uptake of pravastatin was saturable with the apparent Kt of 15.2 +/- 3.2 mM and Jmax of 10.6 +/- 1.21 nmol/mg protein/10 sec. The uptake of pravastatin was significantly inhibited by monocarboxylic acid compounds such as acetic acid and nicotinic acid in a competitive manner but not by di- or tricarboxylic acids, or acidic amino acid. CONCLUSION: It was concluded that a pH-dependent transport of pravastatin across the brush-border membrane occurs by a proton-gradient dependent carrier-mediated mechanism rather than by simple diffusion of its unionized form.

Animals↗

PubMed1995

Functional expression of transporter for beta-lactam antibiotics and dipeptides in Xenopus laevis oocytes injected with messenger RNA from human, rat and rabbit small intestines.

A heterologous gene expression system, Xenopus laevis oocytes, was used to prove the intestinal absorption of various beta-lactam antibiotics mediated by an H(+)-dipeptide cotransport system in rat, rabbit and human small intestines. The microinjection of mRNA (messenger RNA) from rat intestine into Xenopus laevis oocytes led to significantly higher uptakes of p.o. active cephalosporins including zwitter-ionic derivatives (cephalexin, cephradine and cefadroxil) and dianionic derivatives (cefixime and ceftibuten) in comparison with oocytes injected with water, whereas the uptake of cefazolin, a parenterally administered derivative, was negligible in both mRNA- and water-injected oocytes. The uptake of cefadroxil was reduced significantly in the presence of dipeptide and various beta-lactam antibiotics, but not in the presence of an amino acid. After sucrose density gradient centrifugation of mRNA, the highest expression of transport activities of both cefadroxil and ceftibuten was observed in the same mRNA fraction with a size of 2.20 to 3.75 kilobases. mRNA-injected oocytes showed a marked pH-dependency in the uptakes of cefadroxil and ceftibuten, whereas water-injected oocytes exhibited only modes uptakes. The most stimulated uptakes of cefadroxil and ceftibuten were observed at an external pH of 5.5 and 5.0, respectively. Furthermore, injection of mRNA isolated from either rat rabbit or human small intestine into oocytes produced significantly higher uptake of cefadroxil and ceftibuten compared with those by oocytes injected with water. Thus, intestinal absorption of p.o. active beta-lactam antibiotics was confirmed to be mediated by an H+ gradient-dependent transport system across the brush-border membrane of rats, rabbits and humans. The carrier-protein for this process is likely a dipeptide transport system.

Animals↗

PubMed1994

In vivo evidence for ATP-dependent and P-glycoprotein-mediated transport of cyclosporin A at the blood-brain barrier.

To evaluate the significance of P-glycoprotein (P-gp)-mediated active efflux on the blood-brain barrier (BBB) permeability of cyclosporin A (CsA) in vivo, we investigated the effects of ATP depletion in the brain and of a multidrug-resistant (MDR) reversing agent on the transport of CsA across the BBB. Using transient brain ischemia obtained by 4-vessel occlusion of vertebral and common carotid arteries in rats to deplete ATP content in the brain, the estimated permeability surface area product (PS) value of [3H]CsA was increased 2.7-fold compared with that in normal rats, whereas the PS value of [14C]sucrose was not altered. Additionally, when quinidine hydrochloride (QND) was infused into the brain through a microdialysis probe implanted in the rat hippocampus, the extravascular extraction of CsA was increased to approximately 2.5-fold of the control, whereas no difference in the extravascular extraction between control and normal rats having no implanted dialysis probe was observed. Furthermore, the efflux rate from brain to blood of CsA was decreased remarkably to 5% of control at steady-state by co-administration of CsA with QND directly into the brain through the dialysis probe. The ATP-dependent and QND-sensitive efflux of CsA from the brain strongly indicates that P-gp in the brain capillary endothelial cells functions as an efflux pump under the physiological state, and that P-gp-mediated efflux of CsA is a major mechanism of the restricted transfer from blood into the brain.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1994

Functional expression of intestinal dipeptide/beta-lactam antibiotic transporter in Xenopus laevis oocytes.

An intestinal active transport system specific to small peptides and peptide-like drugs such as beta-lactam antibiotics was functionally expressed in Xenopus laevis oocytes after microinjection of messenger RNA (mRNA) derived from rat intestinal mucosal cells. The transport activity was evaluated by measuring the uptake of a tripeptide-like cephalosporin antibiotic, ceftibuten, which has high affinity for the intestinal peptide/H+ co-transporter and is resistant to peptidases. Ceftibuten transport in mRNA-injected oocytes was pH dependent (a proton gradient is the driving force), stereo selective (uptake of the cis-isomer of ceftibuten was about 4-fold higher than that of the trans-isomer), saturable and temperature dependent. Furthermore, various dipeptides showed cis-inhibitory and trans-stimulatory effects on the uptake of ceftibuten by mRNA-injected oocytes, suggesting that ceftibuten and dipeptides are transported by a common carrier protein. These results are in accordance with the functional properties of native proton-coupled peptide transporter previously clarified by studies with isolated intestinal brush-border membrane vesicles and other experimental systems. A protein with a molecular mass of about 130 kDa expressed in the membrane of mRNA-injected oocytes was identified as the transport protein by specific labeling with a photoreactive beta-lactam antibiotic, [3H]benzylpenicillin, followed by SDS-PAGE analysis of the radiolabeled protein. Furthermore, an experiment with mRNA size-fractionated by sucrose density gradient centrifugation indicated that the peptide transporter is encoded by mRNA of between 1.8 and 3.6 kb. These results, obtained using a heterologous gene expression technique, confirm that intestinal absorption of beta-lactam antibiotics occurs through a carrier-mediated mechanism and show that biologically stable beta-lactam antibiotics can be useful probes for molecular analysis of intestinal peptide transporter.

Affinity Labels↗

PubMed1994

N-(p-azido-3-[125I]iodophenethyl)spiperone binds to specific regions of P-glycoprotein and another multidrug binding protein, spiperophilin, in human neuroblastoma cells.

P-glycoprotein (P-gp) is an energy-dependent drug extrusion pump with broad specificity for diverse hydrophobic anticancer agents and compounds known to reverse multidrug resistance (MDR). Among MDR reversing agents, phenothiazines (PTZs) and related compounds may sensitize MDR by interacting with a specific binding site(s) on P-gp and by other mechanisms. In order (1) to identify a binding site for PTZs and related compounds on P-gp, (2) to examine whether these compounds and other MDR modulators bind to the same domains of P-gp, and (3) to identify proteins with high specificity for these neuroleptic agents and other MDR modulators, we used a butyrophenone D2-dopamine receptor photoaffinity probe, N-(p-azido-3-[125I]iodophenethyl)spiperone ([125I]NAPS). [125I]NAPS was actively effluxed from vincristine (VCR)-resistant SH-SY5Y/VCR human neuroblastoma cells, and nonradioactive I-NAPS was a potent chemosensitizing agent. After photolabeling, the probe bound specifically and with high efficiency to P-gp and to another multidrug binding 17-kDa membrane-bound protein, spiperophilin, in these cells. The efficiency of [125I]NAPS binding to P-gp was 5-6-fold more than [3H]azidopine and [125I]arylazidoprazosin ([125I]AAP), known photoaffinity analogs for P-gp. [125I]NAPS photolabeling of P-gp was preferentially competed by MDR-related drugs, with vinblastine > VCR > colchicine > doxorubicin > actinomycin D. Many drugs that are known to reverse MDR were potent inhibitors of [125I]NAPS binding to P-gp. While PTZs and related compounds were potent inhibitors of [125I]NAPS binding to P-gp, most of them enhanced the binding of [125I]AAP significantly. cis-Flupentixol increased the binding of [125I]AAP to P-gp 9-fold more than did trans-flupentixol, but both were potent inhibitors of [125I]NAPS binding, suggesting their stereoselective effect on the [125I]AAP binding site. Proteolysis of [125I]NAPS-bound P-gp with Staphylococcus aureus V8 protease revealed that this probe binds to two major peptides, 6 and 8 kDa, and a number of minor ones, while [125I]AAP binds to only an 8-kDa peptide. These results suggest that modulators of MDR may interact with separate or overlapping domains. Furthermore, most MDR modulators, dopaminergic drugs, and beta-adrenergic antagonists used also inhibited binding of [125I]-NAPS to spiperophilin, suggesting that this protein may be a target for these drugs.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1994

Transcellular transport of benzoic acid across Caco-2 cells by a pH-dependent and carrier-mediated transport mechanism.

The pH-dependent transcellular transport of [14C]benzoic acid across a Caco-2 cell monolayer is shown to be mediated by a monocarboxylic acid-specific carrier-mediated transport system, localized on the apical membrane. Evidence for the carrier-mediated transport of benzoic acid includes (a) the significant temperature and concentration dependence, (b) the metabolic energy dependence, (c) the inhibition by unlabeled benzoic acid and other monocarboxylic acids, (d) countertransport effects on the uptake of [14C]benzoic acid, and (e) effects of a proteinase (papain) and amino acid-modifying reagents. Furthermore, since carbonylcyanide p-trifluoromethoxyphenylhydrazone and nigericin significantly inhibited the transport of [14C]benzoic acid, the direct driving force for benzoic acid transport is suggested to be the inwardly directed proton gradient. From these results, together with previous observations using intestinal brush border membrane vesicles, the pH dependence of the transcellular transport of certain organic weak acids across Caco-2 cells is considered to result mainly from a proton gradient-dependent, carrier-mediated transport mechanism, rather than passive diffusion according to the pH-partition theory.

Acetates↗

PubMed1994

In vivo and in vitro blood-brain barrier transport of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors.

Among the HMG-CoA reductase inhibitors, lovastatin and simvastatin have central nervous system (CNS) side effects, such as sleep disturbance, whereas pravastatin does not. This difference in CNS side effects may be due to a difference in blood-brain barrier (BBB) permeability among these inhibitors. To test this hypothesis, we compared the BBB transport ability of HMG-CoA reductase inhibitors by using an in vivo brain perfusion technique in rats and an in vitro culture system of bovine brain capillary endothelial cells. The in vivo BBB permeability coefficients of the lipophilic inhibitors, [14C]lovastatin and [14C]simvastatin, were high. In contrast, that of the hydrophilic inhibitor, [14C]pravastatin, was low and not significantly different from that of [14C]sucrose, an extracellular space marker. Similarly, the in vitro BBB permeability coefficients of [14C]lovastatin and [1C]simvastatin were high, while that of [14C]-pravastatin was low. The in vivo and in vitro transcellular permeabilities obtained for HMG-CoA reductase inhibitors were comparable. This study shows that the BBB permeability correlates with the CNS side effects of the HMG-CoA reductase inhibitors.

Animals↗

PubMed1994

pH-dependent and carrier-mediated transport of salicylic acid across Caco-2 cells.

The transport of monocarboxylic acid drugs such as salicylic acid was examined in the human colon adenocarcinoma cell line, Caco-2 cells that possess intestinal epithelia-like properties. [14C]Salicylic acid transport was pH-dependent and appeared to follow the pH-partition hypothesis. However, 10 mM unlabelled salicylic acid significantly reduced the permeability coefficient of [14C]salicylic acid. Kinetic analysis of the concentration dependence of the permeation rate of salicylic acid across Caco-2 cells showed both saturable (Kt = 5.28 +/- 0.72 mM Jmax = 36.6 +/- 3.54 nmol min-1 (mg protein)-1) and nonsaturable (kd = 0.37 +/- 0.08 microL min-1 (mg protein)-1) processes. The permeation rate of [14C]salicylic acid was competitively inhibited by both acetic acid and benzoic acid, which were demonstrated in our previous studies to be transported in the carrier-mediated-transport mechanism which is responsible for monocarboxylic acids. Furthermore, certain monocarboxylic acids significantly inhibited [14C]salicylic acid transport, whereas salicylamide and dicarboxylic acids such as succinic acid did not. From these results, it was concluded that the transcellular transport of [14C]salicylic acid across Caco-2 cells is by the pH-dependent and carrier-mediated transport mechanism specific for monocarboxylic acids.

Acetates↗

PubMed1993

Restricted transport of cyclosporin A across the blood-brain barrier by a multidrug transporter, P-glycoprotein.

The blood-brain barrier permeability of cyclosporin A (CsA), an immunosuppressive cyclic peptide, is restricted despite its highly lipophilic nature. In this study, the uptake of CsA by primary cultured bovine brain capillary endothelial cells (BCEC) was investigated in order to clarify whether P-glycoprotein (P-gp), an ATP-dependent drug efflux pump expressed in the luminal surface of BCEC, causes the decreased transport of CsA into the brain. P-gp, having a molecular mass of 130-140 kDa, was detected with anti-P-gp monoclonal antibody, C219, using western blot analysis of the membrane fraction isolated from the bovine brain capillary. The uptake of CsA by primary cultured bovine BCEC was time-dependent, and the steady-state uptake of CsA was increased in the presence of several multidrug resistance reversing agents including verapamil and steroid hormones and the substrate of P-gp in BCEC, vincristine. The steady-stage uptake was increased significantly to approximately 4-fold when cellular ATP was depleted by treating with 2,4-dinitrophenol, suggesting that the efflux process is ATP dependent. Furthermore, in the presence of an anti-P-gp monoclonal antibody, MRK16, at a 10 micrograms/mL concentration, the uptake of CsA was increased approximately 3-fold. These results suggest that the low permeability of CsA into the brain is caused by the active efflux from BCEC by P-gp present in the luminal surface of cells.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1993

Intestinal brush-border transport of the oral cephalosporin antibiotic, cefdinir, mediated by dipeptide and monocarboxylic acid transport systems in rabbits.

Intestinal absorption of the orally active cephalosporin, cefdinir, was investigated using brush-border membrane vesicles prepared from rabbit small intestine. The initial uptake of cefdinir was pH-dependent, with increased uptake at acidic pH, and was not influenced by either sodium gradient or membrane potential difference. Cefdinir uptake was saturable with an apparent Michaelis constant of 8.1 mM. Initial uptake of cefdinir was inhibited by dipeptides (glycyl-L-proline and glycylsarcosine), beta-lactam antibiotics (cephradine, cefixime and penicillin V), and monocarboxylic acids (acetic acid and L-lactic acid), whereas the uptake of cephradine and cefixime was not inhibited by monocarboxylic acids. Cefdinir significantly inhibited the initial uptake of cephradine, cefixime and [3H]acetic acid. From these results, it was suggested that cefdinir was transported across brush-border membranes by both dipeptide and monocarboxylic acid carriers.

Acetates↗

PubMed1993

Expression of the Na+ dependent uridine transport system of rabbit small intestine: studies with mRNA-injected Xenopus laevis oocytes.

Xenopus laevis oocytes were used as an expression system to prove and characterize the carrier-mediated transport of uridine in the small intestine. Significant Na+ dependency was observed for the uptake of [3H]uridine by Xenopus laevis oocytes injected with poly(A)+RNA prepared from rabbit small intestinal mucosa. By contrast, the uptake of [3H]uridine was negligible in water-injected oocytes. There was no significant difference in the Na+ dependent uptake rates of [3H]uridine among oocytes expressed by using mRNA prepared by three different methods. The uptake of [3H]uridine by mRNA-injected oocytes was enhanced by increasing the culturing time after mRNA injection. Concentration dependency for uridine transport was observed with the Michaelis constant of 8.27 microM, which was comparable to that reported in the study using the brush-border membrane vesicles from rabbit small intestine (6.4 microM). Furthermore, the uptake of [3H]uridine was significantly inhibited by adenosine and thymidine, but not by adenine and uracil. Consequently, the transport system of uridine expressed in mRNA-injected oocytes is clarified to be similar to that functioning in the brush-border membrane of the small intestine.

Animals↗