Active absorption of hypoxanthine by lamb jejunum in vitro

In Vitro Drug Absorption Models. II. Salicylate, Cefoxitin, α-Methyldopa and Theophylline Uptake in Cells and Rings: Correlation with In Vivo Bioavailability

Isolated mucosal cells and everted intestinal rings have been examined as potential in vitro models for intestinal drug absorption. The uptake of salicylate, cefoxitin, α-methyldopa and theophylline was characterized on the basis of time, concentration and temperature dependence and compared to in vivo drug absorption. Theophylline was well absorbed in all systems. Biochemical studies supported a passive transport mechanism, although a significant temperature dependence was observed. Salicylate, cefoxitin and α-methyldopa demonstrated time- and concentration-dependent absorption. The uptake of α-methyldopa was temperature-dependent in both the isolated cell and ring studies. With all drugs, cellular uptake exhibited greater variability than drug accumulation in rings. A comparison of in vitro and in vivo absorption demonstrated a good correlation between the data from in vivo studies and intestinal rings. Cellular drug uptake did not completely mimic that observed in vivo. On the basis of technical aspects of preparation, reproducibility of results, and correlation with in vivo drug bioavailability, everted intestinal rings were judged to be the best in vitro model for intestinal drug absorption.

Na+ gradient-dependent transport of hypoxanthine by calf intestinal brush border membrane vesicles

The properties of hypoxanthine transport were investigated in purified brush border membrane vesicles isolated from calf proximal and distal jejunum. Hypoxanthine uptake in the vesicles was stimulated by a transmembrane Na(+) gradient and an inside negative potential resulting in a transient accumulation of intravesicular hypoxanthine, especially in the proximal jejunum. Na(+)-dependent hypoxanthine uptake at this site seemed to occur by two saturable transport systems, a high affinity (K(m)=0.33 micromol/l) and a low affinity (K(m)=165 micromol/l) transporter. Guanine, hypoxanthine, thymine and uracil inhibited intravesicular hypoxanthine uptake, whereas adenine and the nucleosides inosine and thymidine were without effect. These findings represent the first demonstration of active Na(+) gradient-dependent nucleobase transport in intestinal brush border membrane vesicles.

Active intestinal absorption of nucleosides by Na+-dependent transport across the brush border membrane in cows

Transport of 3H-labeled nucleosides across the bovine intestinal brush border membrane (BBM) was characterized with BBM vesicles (BBMV) isolated from mid-jejunum of cows because large amounts of nucleic acids are digested in the small intestine of ruminants. Two Na+-dependent electrogenic nucleoside transporters with overlapping substrate specificity were shown to be present in the jejunal BBM, one for pyrimidine nucleosides and one for purine nucleosides. As indicated by inhibitory studies, thymidine seemed to be a specific substrate for the pyrimidine nucleoside transporter, while this applied to guanosine and deoxyguanosine for the purine nucleoside transporter. Uridine and adenosine appear to have an affinity to both transporters. This also applies to deoxyadenosine and deoxyuridine. Nucleobases (uracil, hypoxanthine) did not affect transport of nucleosides. The kinetic constants (Km and Vmax) for Na-dependent thymidine and guanosine transport were 29 and 24 micromol/L and 78 and 51 pmol (mg protein)(-1) s(-1), respectively. These values are much higher than those reported for monogastric species.

Sodium-dependent nucleoside transport in rabbit intestinal epithelium

Cellular uptake of formycin B, a poorly metabolized analog of inosine, by the isolated epithelium of rabbit jejunum is three times higher in the presence of Na+ than without this cation. The Na(+)-dependent nucleoside transport system is located in the apical membrane of the enterocytes and is capable of uphill transport, as shown for formycin B and adenosine with brush border membrane vesicles. According to present and earlier evidence, nucleoside transport across the basolateral membrane appears to have the properties of facilitated diffusion. Na(+)-dependent formycin B transport activity in intestinal epithelium decreases from jejunum to ileum and is absent in descending colon. As with Na(+)-coupled cotransport systems for other organic solutes, apical entry of formycin B is driven by the electrochemical Na+ gradient into the cell. In contrast to the facilitated diffusion system for nucleosides, Na(+)-dependent formycin B transport is not inhibited by nitrobenzylthioinosine, but both carrier systems are sensitive to inhibitors of D-glucose transport. Natural purine nucleosides and uridine are strong inhibitors of Na(+)-dependent formycin B transport. Transepithelial flux measurements substantiated that the Na(+)-dependent transport mechanism for formycin B functions as an absorptive system.

Absorption and metabolism of purines by the small intestine of the chicken

1. Absorption of purines and their metabolism by the small intestine were estimated by using the everted gut sacs from the duodenum, jejunum and ileum of the chicken. 2. When no purine was added to the mucosal fluid, large amounts of uric acid, much less but appreciable adenine, hypoxanthine and xanthine and no detectable guanine were released from both sides of all segments of the small intestine, and these released amounts were largest in the duodenum. 3. Similar absorption rates of adenine from the jejunum and ileum were about 1.7-3.0 times as high as those of hypoxanthine and uric acid from these intestines and those of adenine and uric acid from the duodenum (P less than 0.05). 4. Guanine was not absorbed unchanged from any segments of the intestine and a little xanthine was absorbed only from the jejunum and ileum. 5. Guanine and xanthine seem to be absorbed in uric acid form, hypoxanthine in xanthine and uric acid forms and adenine in hypoxanthine form, from the small intestine especially from the jejunum. 6. Adenine, guanine, xanthine and hypoxanthine were greatly metabolized in the mucosa of the duodenum, and the conversions of hypoxanthine to xanthine and uric acid were most active.

Mechanism of acyclovir uptake in rat jejunum

The intestinal uptake mechanism of the purine analogue, acyclovir, was investigated in rat jejunum using in vitro and in situ methods. The pyrimidine, uracil, was used as a reference compound for carrier-mediated transport, while the purine analogue, caffeine, served as the reference compound for passive diffusion. With the in vitro intestinal ring method, acyclovir uptake was linear in the concentration range 0.01-5 mM. No significant competition for uptake was observed with uracil, 6-mercaptopurine, hypoxanthine, caffeine, or adenine. In addition, use of 2,4-dinitrophenol (DNP), ouabain, or K+ substituted buffer did not reduce the rate of acyclovir uptake. The in situ single-pass perfusion method yielded a wall permeability of approximately 0.2, which did not vary consistently with increasing concentration. Coperfusion of acyclovir with DNP did not decrease the wall permeability. None of the data provided evidence of a carrier-mediated transport system, and it was concluded that the uptake mechanism of acyclovir in the rat jejunum is predominantly via passive diffusion.

Stimulation of mucosal uptake of selenium from selenite by L-cysteine in sheep small intestine

The influence of cysteine (Cys) on mucosal uptake of 75Se-labeled selenite in sheep midjejunum was investigated using a short-term uptake technique. L-Cys (concn.: 1.0 mmol/L) significantly stimulated uptake of Se from selenite (concn.: 10 mumols/L). The stimulatory effect of L-Cys on mucosal uptake of Se from selenite was Na(+)- and pH-dependent. In the absence of Na+, or at an acidic pH (5.0), the stimulatory effect of L-Cys was abolished. L-alanine and L-lysine, but not L-glutamic acid inhibited uptake of Se from selenite in the presence of L-Cys. Preincubation of mucosal preparations with 10 mmol/L L-Cys produced enhanced mucosal uptake of Se from selenite. It is concluded from these results that L-Cys stimulates absorption of Se from selenite probably by generation of selenodicysteine and maybe cysteine selenopersulfide that are subsequently transported across the intestinal brush border membrane by Na(+)-dependent amino acid carriers. Furthermore, intracellular generation of selenodicysteine might contribute to the uptake of Se from selenite by maintaining the concentration gradient for diffusive uptake of selenite.

Hypoxanthine transport through the blood-brain barrier

The unidirectional influx of hypoxanthine across cerebral capillaries, the anatomical locus of the blood-brain barrier, was measured with an in situ rat brain perfusion technique employing [3H]hypoxanthine. Hypoxanthine was transported across the blood-brain barrier by a saturable system with a one-half saturation concentration of approximately 0.4 mM. The permeability-surface area product was 3 X 10(-4) sec-1 with a hypoxanthine concentration of 0.02 microM in the perfusate. Adenine (4 mM) and uracil and theophylline (both 10 mM), but not inosine (10 mM) or leucine (1 mM), inhibited hypoxanthine transfer through the blood-brain barrier. Thus, hypoxanthine is transported through the blood-brain barrier by a high-capacity, saturable transport system with a half-saturation concentration about 100 times the plasma hypoxanthine concentration. Although involved in the transport hypoxanthine from blood into brain, this system is not powerful enough to transfer important quantities of hypoxanthine from blood into brain.

Uptake of uric acid, xanthine and hypoxanthine by brush-border membrane vesicles from mouse small intestine

Using mouse small intestine brush-border membrane vesicles virtually free of xanthine oxidase (EC 1.2.3.2) and free of uricase (EC 1.7.3.3) the uptake of the purines uric acid, xanthine and hypoxanthine have been studied. The sodium-dependent overshoot phenomenon shown to exist for the uptake into the vesicles for D-glucose and L-phenylalanine was not observed with the purines. However, the uptake of the three purines in the presence of NaCl or KCl was greater than the uptake in the presence of either NaSCN or mannitol. Although 12.9% of the xanthine uptake and 17.6% of the hypoxanthine uptake was attributed to binding to the membranes, almost all the uric acid uptake was due to transport into an osmotically active space. The apparent intravesicular volume, calculated after 60 min incubation, for the three purines was consistently greater than the values obtained with D-glucose, L-glucose and L-phenylalanine equilibration, suggesting slow continuing penetration of purines associated with swelling or an apparent accumulation of purines within the vesicles associated with normal vesicle volume.

Effect of feeding a high protein diet on amino acid uptake into rat intestinal brush border membrane vesicles

Uptake of the neutral amino acid L-leucine into isolated rat intestinal brush border membrane (= BBM) vesicles and into a jejunal mucosa preparation as affected by the protein content of the diet was investigated. Adult rats fed either a high carbohydrate (HC) diet (11% protein) or a high protein (HP) diet (77% protein) for several weeks were used for the experiments. The time course of L-leucine uptake into BBM vesicles prepared from the small intestine of HC- or HP-rats was studied under conditions of an inwardly directed Na+-gradient and under Na+-equilibrium conditions. Furthermore, in one series of experiments the Na+-equilibrium was replaced by a K+-equilibrium. L-leucine uptake under Na+-gradient conditions displayed the overshoot phenomenon typically associated with Na+-gradient-dependent active transport processes in BBM vesicles and the overshoot in group HP exceeded that in group HC significantly. Under both Na+- and K+-equilibrium conditions L-leucine uptake into the BBM-vesicles also was faster in group HP. Finally L-leucine uptake into jejunal mucosa in group HP exceeded that in group HC, too. The results therefore indicate that Na+-dependent and Na+-independent transport of neutral amino acids across the intestinal brush border membrane adapts to the dietary protein level.

Active pyrimidine absorption by chicken colon

Pyrimidine absorption by chicken large intestine was investigated employing the everted sac and flux chamber techniques. 3H-labelled uracil was used as substrate. The small intestine and the colon unlike the caecum, transported uracil from the mucosal to the serosal surface against a concentration gradient in the everted sac experiments. Furthermore, there was a net transport of uracil from the mucosal to the serosal side of the colon and jejunum in the flux chamber experiments. Uracil transport by the everted colon sacs against a concentration gradient was inhibited when the purine hypoxanthine was present in the incubation medium. Uracil transport by the everted colon sacs was also inhibited under anaerobic conditions and when 2,4-dinitrophenol was present in the incubation medium. Replacing the Na+ ions of the incubation medium by Li+ ions also caused an inhibition of uracil transport. It is concluded from these results that uracil (and probably other pyrimidines) are absorbed from the chicken colon by a Na+ ion-dependent active transport process having also an affinity for purines.