Journal of Dairy Science Vol. 85 No. 9 2308-2314
© 2002 by American Dairy Science Association ®
Nucleosides Are Efficiently Absorbed by Na+-Dependent Transport Across the Intestinal Brush Border Membrane in Veal Calves
Anja Theisinger,
B. Grenacher,
K. S. Rech and
E. Scharrer
Institute of Veterinary Physiology, University of Zurich Winterthurerstr. 260, CH-8057 Zurich Switzerland
Corresponding author:
E. Scharrer; e-mail:
scharrer{at}vetphys.unizh.ch.
 |
ABSTRACT
|
|---|
In previous work, a comparatively high capacity for Na+-dependent transport of nucleosides across the intestinal brush border membrane (BBM) was observed in dairy cows, which might be related to digestion of the large amount of nucleic acids present in ruminal microorganisms in the ruminant small intestine. If this were the case, the capacity for Na+-dependent intestinal nucleoside transport should be much lower in veal calves, in which only small amounts of nucleic acids, nucleotides, and nucleosides reach the small intestine via the milk replacer. To test this hypothesis, we investigated Na+-dependent transport of 3H-labeled thymidine and guanosine across the BBM using BBM vesicles (BBMV) isolated from the small intestine of veal calves. In the presence of a transmembrane Na+ gradient both substrates were transported against a concentration gradient. Inhibitory studies showed that thymidine and guanosine are transported by two different transporters with overlapping substrate specificity, one accepting predominantly pyrimidine nucleosides (N2) and one accepting particularly purine nucleosides (N1). Nucleoside transport was inhibited by glucose along the whole small intestine. Maximal transport rates similar to those in dairy cows were obtained for the proximal, mid-, and distal small intestine. These findings suggest that the high absorptive capacity for nucleosides is a genetically fixed property in the bovine small intestine, which is already present in the preruminant state of veal calves. It may contribute to the high digestibility of nucleic acids observed by others in veal calves receiving milk replacer supplemented with RNA. Its main function may be the efficient absorption of nucleosides resulting from the digestion of nucleic acids associated with desquamated enterocytes. Due to the limited de novo synthesis of nucleotides in enterocytes intracellular uptake of nucleosides across the BBM may contribute to nucleic acid synthesis in enterocytes and thus may have a trophic effect on the intestinal epithelium.
Key Words: absorption of nucleosides • brush border membrane • Na+-dependent nucleoside transport • inhibition of nucleoside transport by glucose
Abbreviation key: BBM = brush border membrane, BBMV = brush border membrane vesicles, N1 = Na+-dependent transporter for purine nucleosides, N2 = Na+-dependent transporter for pyrimidine nucleosides
|
INTRODUCTION
|
|---|
Large amounts of nucleic acids associated with ruminal microorganisms enter the duodenum of ruminants (Smith and McAllan, 1971; Stangassinger et al., 1995). They appear to be efficiently digested and absorbed, because 80 to 90% disappeared between the abomasum and the terminal ileum (McAllan, 1980). Nucleic acid degradation in the upper small intestine of steers was accompanied by a transient appearance of nucleosides, which generally completely disappeared by the terminal ileum. No pyrimidine bases and scarcely any purine bases appeared in the intestinal lumen during nucleic acid digestion (McAllan, 1980).
In former work, we showed by using isolated intestinal brush border membrane (BBM) vesicles (BBMV) that nucleosides are absorbed from the small intestine of cows by active Na+-dependent transport (Scharrer and Grenacher, 2001, 2002). Two Na+-dependent intestinal nucleoside transporters with overlapping substrate specificity were found, one transporting purine nucleosides (N1) and one transporting pyrimidine nucleosides (N2). The maximal transport capacity (Vmax) of both transporters was at least 10-fold higher than reported for BBMV isolated from rabbit and human small intestine. The affinity constants (Km-value) were also higher.
In experiments with isolated, perfused rat jejunum, nucleosides were split to a large extent to nucleobases and ribose phosphate by phosphorolysis after absorption. Purine bases were subsequently transformed to uric acid, while pyrimidine bases remained intact. Cytidine was not split (Stow and Bronk, 1993). With regard to absorption of purine nucleosides, the situation appears to be similar in cattle (Balcells et al., 1992). Since, as already noted, large amounts of microbial nucleic acids associated predominantly with rumen bacteria (about 100 to 200 g/d in dairy cows) are digested in the small intestine of ruminants (McAllan, 1980; Storm et al., 1983; Johnson et al., 1998, Scharrer and Grenacher, 2001), it is possible that the high maximal velocity of intestinal nucleoside transport by N1 and N2 in cows might be the result of an adaptation to the large amount of microbial nucleic acids digested in the small intestine (Scharrer and Grenacher, 2001). In that case, nucleoside transport across the intestinal BBM of veal calves should exhibit a lower activity, because microbial growth in the rumen is minimal due to the esophageal groove reflex in veal calves channeling the ingested milk or liquid milk replacer directly into the abomasum from where it is transferred into the duodenum (Titchen and Newhook, 1975). Moreover, milk or milk replacer only contains traces of nucleic acids, nucleotides, and nucleosides (Kobata et al., 1962; Gil Hernandez and Sanchez-Medina, 1981; Tiemeyer et al., 1984; Raezke et al., 1988). We therefore investigated intestinal nucleoside transport in veal calves using isolated BBMV. 3H-labeled thymidine and guanosine were used as transport substrates, because they appear to be selectively transported either by N1 (guanosine) or by N2 (thymidine) (Griffith and Jarvis, 1996; Patil and Unadkat, 1997; Scharrer and Grenacher, 2001). To clarify whether this also applies to veal calves the inhibitory potency of various nucleosides regarding thymidine and guanosine transport was first tested using BBMV isolated from the proximal and distal small intestine. Glucose was also included as an inhibitory substrate, because it inhibited Na+-dependent nucleoside transport across the BBM of cows especially in the proximal small intestine (Scharrer and Grenacher, 2002). Due to the higher intestinal glucose transport capacity in calves in comparison to cows (Wood et al., 2000), it was of interest to find out how glucose affects Na+-dependent nucleoside transport in veal calves. Finally we determined the substrate affinity and transport capacity of Na+-dependent thymidine and guanosine transport across the BBM in veal calves in order to compare them with the pertinent kinetic constants (Km-value, Vmax) in dairy cows obtained in former studies (Scharrer and Grenacher, 2001, 2002).
|
MATERIALS AND METHODS
|
|---|
Preparation of BBMV
The proximal jejunum (first 2 m of small intestine immediately caudal to the ligamentum duodenocolicum) and distal jejunum (first 2 m of intestine immediately proximal to the ligamentum ileocaecale) were obtained from freshly killed veal calves (age: 2 to 3 mo) at the local slaughterhouse. The intestine was removed about 10 min after death. The two segments of small intestine were opened along the mesenteric border, washed twice with chilled saline and transferred to the laboratory in ice-cold saline within 15 min after removal. BBMV were immediately prepared from mucosal scrapings according to the method of Kessler et al. (1978a). BBMV were prepared (per liter) with 300 mmol of D-mannitol, 100 mmol of KCl, 1 mmol of MgSO4, 35 mmol of N-2-hydroxyethylpiperazine-N-'2-ethanesulfonic acid (HEPES)/Tris, pH 7.4. Aliquots of 400 µl were stored under liquid nitrogen until use (Stevens et al., 1982). Purification of the BBM was tested by determining the activity of the BBM enzyme alkaline phosphatase (AP, EC 3.1.3.1) (test kit Unimate 3, Roche Diagnostics, 6343 Rotkreuz, Switzerland). The final BBM-fraction exhibited a 22.0 ± 3.2 (proximal small intestine) and 22.5 ± 1.9-fold (distal small intestine) enrichment in AP activity with respect to the original homogenate. Cross-contamination of BBM with basolateral membranes was negligible (Hopfer, 1977; Wolffram et al., 1990), as shown by the 0.9 ± 0.1 (proximal small intestine) and 1.3 ± 0.2-fold (distal small intestine) enrichment of the basolateral membrane marker enzyme Na+, K+-ATPase (EC 3.6.1.3). Protein content was measured applying the Bio-Rad protein assay kit with bovine albumin as standard (Bio-Rad Laboratories, Glattbrugg, Switzerland).
Measurement of Substrate Uptake into BBMV
Uptake of 3H-labeled thymidine ([methyl-3H] thymidine, Amersham Life Science, Little Chalford, UK) and guanosine ([8-3H] guanosine, American Radiolabeled Chemicals Inc., St. Louis, MO) was determined by a rapid filtration technique (Kessler et al., 1978b; Wolffram et al., 1989). Short-term incubations (1 to 5 s) were performed using a semi-automatic incubation device first described by Kessler et al. (1978b). Uptake was started by adding 10 µl of membrane suspension to 40 µl of the incubation medium and quenched by adding 3 ml of ice-cold stop solution of the same composition, as the final reaction mixture without the addition of substrate. The dilution mixture was immediately drawn through a nitrocellulose filter (0.45-µm pore size, Schleicher & Schüll, Feldbach, Switzerland) and rinsed twice with the stop solution. The composition of the reaction media is indicated in the legend of Figure 1
. The transmembrane potential difference was clamped at zero by adding valinomycin from an ethanolic stock to the thawed vesicle suspension to achieve a concentration of 20 µg of valinomycin per liter and 0.1% ethanol in the final reaction mixture (Wolffram et al., 1989). The 3H-activity remaining on the filters was determined by liquid scintillation counting in a beta-counter (Packard Scintillation Analyzer, 1600 TR).
View larger version (15K):
[in this window]
[in a new window]
|
Figure 1. Time course of uptake of 3H-labeled thymidine by brush border membrane vesicles isolated from proximal ( , •) and distal ( ,  ) small intestine of veal calves. Vesicles were preloaded with 300 mmol/LD-mannitol, 100 mmol/L KCl, 1 mmol/L MgSO4, 35 mmol/L N-2-hydroxyethylpiperazine-N-'2-ethanesulfonic acid (HEPES)/Tris, pH 7.4 and incubated in reaction media containing (final concentrations, mmol/L: 100 NaCl (, ) or choline chloride (•,), 100 D-mannitol, 100 KCl, 1 MgSO4, 0.002 3H-labeled thymidine respectively guanosine and 35 HEPES/Tris, pH 7.4. The values are means (M) ± SEM from four different vesicles preparations each determined in duplicate. * and +P < 0.05, ** and ++ P < 0.01, significantly higher than in the absence of Na+. # P < 0.05, ## P < 0.01, values are significantly higher compared to distal small intestine.
|
|
Statistical and Kinetic Analysis
Values are presented as means with the standard error of the mean. To verify the effects on nucleoside uptake of various inhibitors, data were subjected to ANOVA with subsequent Dunnett test. Differences between two means were statistically evaluated using the unpaired or when appropriate the paired t-test (Sachs, 1992). Statistical evaluations were performed on a personal computer using the program Graph Pad Instat V 3.00 (Graph Pad Software Inc., San Diego, CA). The apparent kinetic parameters Km (Michaelis constant, half-saturating substrate concentration) and Vmax (maximal transport capacity) of transport of nucleosides across the BBM were calculated from the values of uptake rates obtained in the presence of a transmembrane Na+ gradient corrected by uptake rates obtained in the presence of a choline gradient by nonlinear curve fitting based on the Michaelis-Menten equation (Preston et al., 1974). Kinetic analysis was performed on a personal computer using the program Graph Pad Prism V 3.00 (Graph Pad Software).
|
RESULTS
|
|---|
Time Course of Uptake of Nucleosides
The time course of uptake of thymidine and guanosine (concentration: 2 µmol/L) into BBMV isolated from the proximal and distal small intestine in the presence of an inwardly directed initial Na+ or choline+ gradient (Na+out or choline+out = 100 mmol/L, Na+in or choline+in = 0 mmol/L) is shown in Figures 1 and 2
. Initial uptake under Na+-gradient conditions significantly exceeded uptake under choline+ gradient conditions. A marked initial overshoot of Na+ gradient-dependent intravesicular thymidine and guanosine uptake in comparison to the 60 min uptake value occurred in BBMV isolated from the proximal small intestine, whereas this does not apply to the distal small intestine (Figures 1 and 2), where Na+ gradient-dependent uptake of both substrates was significantly smaller than in the proximal small intestine.
Inhibition of Uptake of Thymidine and Guanosine by Various Nucleosides and by Glucose
The inhibitory effect of various nucleosides and of glucose (concentration: 0.1 or 1 mmol/L) on uptake of 3H-labeled thymidine and guanosine (concentration: 2 µmol/L) in the presence of a Na+ gradient is given in Table 1. Thymidine, cytidine, uridine, and adenosine inhibited 3H-thymidine uptake concentration dependently at both intestinal sites, whereas guanosine, inosine, adenosine, and uridine inhibited 3H-guanosine uptake in a concentration-dependent mode. At the high concentration, guanosine and inosine inhibited 3H-thymidine uptake about 30%, whereas thymidine inhibited 3H-guanosine uptake 30 to 40%. Glucose at both concentrations inhibited 3H-thymidine and 3H-guanosine uptake about 50% in the proximal small intestine. In the distal small intestine at the low concentration, glucose inhibited the nucleoside uptake only about 20 to 30% but 50% at the high concentration.
View this table:
[in this window]
[in a new window]
|
Table 1. Effects of various purine and pyrimidine nucleosides and glucose on uptake of thymidine and guanosine (concentration: 2 µmol/L, incubation time: 5 s) into bovine intestinal brush border membrane vesicles.1
|
|
Similar to uptake of thymidine and guanosine (Table 1), Na+-dependent uptake of glucose into BBMV from the proximal small intestine exceeded that from the distal small intestine (Table 2).
Kinetics of Na+-Dependent Transport of Nucleosides
Uptake of thymidine and guanosine as a function of substrate concentrations (2 to 64 µmol/L) was determined. Uptake was corrected for Na+-independent uptake. Na+-dependent transport of both nucleosides increased in a curvilinear manner with mounting substrate concentration and reflected saturation kinetics (Figures 3 and 4). Apparent Km values were similar for the proximal (thymidine: 38 µmol/L, guanosine: 24 µmol/L) and distal (thymidine: 41 µmol/L, guanosine: 34 µmol/L) small intestine, whereas Vmax values for both nucleosides in proximal small intestine (thymidine: 104 pmol [mg protein]–1s–1, guanosine: 120 pmol [mg protein]–1s–1) exceeded those in the distal small intestine (thymidine: 50 pmol [mg protein]–1s–1, guanosine: 31 pmol [mg protein]–1s–1). The Vmax value of guanosine uptake was significantly (P < 0.01) higher in the proximal part of the small intestine. In an additional series of experiments (four calves) the kinetics of Na+-dependent guanosine transport was determined for the middle part of the small intestine (Km value: 20 µmol/L, Vmax: 74 pmol [mg protein]–1s–1). In those investigations the Vmax value was in between that of the proximal and distal small intestine, while the Km values for the three intestinal sites were similar.
View larger version (15K):
[in this window]
[in a new window]
|
Figure 4. Kinetics of Na+-dependent guanosine uptake by jejunal brush border membrane vesicles. Values are means ± of 18 determinations from six different vesicle preparations each determined in triplicate. Further conditions as described in legend of Figure 3. Kinetic constants proximal, (distal): Km = 24 µmol/L (34 µmol/L) and Vmax = 120 pmol [mg protein]–1s–1, (31 pmol [mg protein]–1s–1).
|
|
In Table 3, the values of the kinetic constants of Na+-dependent intestinal thymidine and guanosine transport in veal calves are compared with the pertinent values in dairy cows obtained in a previous investigation (Scharrer and Grenacher, 2001; 2002). Evidently Km and Vmax values in veal calves were similar to those in dairy cows.
View this table:
[in this window]
[in a new window]
|
Table 3. Kinetic constants of Na+-dependent transport of thymidine and guanosine across the brush border membrane of the proximal, mid-, and distal small intestine of veal calves and dairy cows.
|
|
|
DISCUSSION
|
|---|
The main finding of this study is the high capacity (Vmax) of Na+-dependent nucleoside transport across the small intestinal BBM in veal calves (Table 3), which was at least as high as in dairy cows (Scharrer and Grenacher, 2001, 2002). This observation is not in accordance with the hypothesis that the high transport capacity of N1 and N2 in cows results from an adaptation to the large amount of ruminal microbial nucleic acids (100 to 200 g/d in dairy cows) digested in the cow small intestine. The high transport capacity of N1 and N2 of the BBM rather seems to be a genetically fixed property of bovine small intestine, which is already expressed in the preruminant state of veal calves ingesting only very little roughage due to feeding of liquid milk replacer, which bypasses the rumen and therefore impedes microbial growth including nucleic-acid synthesis in the forestomach (Roy et al., 1971, Titchen and Newhook, 1975). However, it remains to be investigated at which developmental stage Na+-dependent nucleoside carriers are integrated in the bovine intestinal BBM. It has recently been shown that the nucleoside transporters N1 and N2 are already present in the human fetal intestinal BBM (Ngo et al., 2001).
Possibly the predominant function of N1 and N2 in the intestinal BBM of veal calves is cellular absorption of nucleosides released by degradation of endogenous nucleic acids associated with desquamated enterocytes (Parsons and Shaw, 1983). Due to the rapid turnover of enterocytes in combination with the large tissue mass of the epithelium of the small intestine a substantial amount of endogenous nucleic acids enters the intestinal lumen with desquamated enterocytes. For instance, in the adult rat, about 30 mg of endogenous nucleic acids enter the small intestine per day (Parsons and Shaw, 1983). Since de novo synthesis of nucleotides is limited in enterocytes (Le Leiko and Walsh, 1996), uptake of nucleosides deriving from endogenous nucleic acids across the intestinal BBM probably supports nucleic acid synthesis in enterocytes of preruminant calves and thus may have trophic effects on the intestinal epithelium (Nunez et al., 1990; Uany et al., 1990, Adjei et al., 1996; Le Leiko and Walsh, 1996). It should be noted in this context that due to these and further properties of nucleosides the European Commission has allowed supplementation of infant formula with nucleotides (Schlimme et al., 2000).
The inhibitory pattern of various nucleosides in regard to thymidine and guanosine transport on the whole is in accordance with the findings in cows (Scharrer and Grenacher, 2001, 2002) and also suggests that in veal calves nucleoside transport across the intestinal BBM occurs by N1 and N2. Deviating from the findings in cows, glucose markedly inhibited transport of thymidine and guanosine across the BBM not only in the proximal small intestine (Scharrer and Grenacher, 2002), but also in the distal small intestine. The greater inhibitory potency of glucose in veal calves may be related to their higher activity of Na+-dependent glucose transport across the BBM (Wood et al., 2000), which seems to compete with Na+-dependent nucleoside transport for the electrochemical transmembrane Na+-gradient (Scharrer and Grenacher, 2002). A close association of the Na+-dependent transporters of glucose and of nucleosides in the BBM may favor this competition (Roden et al., 1991). Similar interactions between Na+-dependent monosaccharide and amino acid transport across the intestinal BBM have also been described (Alvarado, 1970; Murer et al., 1975; Munck, 1980).
Interestingly, the proximal-to-distal gradient in nucleoside transport activity in the small intestine of veal calves was similar to that of cows (Scharrer and Grenacher, 2002) suggesting that this gradient also is not a direct consequence of digestion of large amounts of ruminal microbial nucleic acids in the proximal part of the small intestine of mature ruminants (McAllan, 1980). However, the high nucleoside transport activity in the proximal small intestine probably facilitates nucleic acid digestion in ruminants due to efficient removal of nucleic acid breakdown products.
Intracellular metabolism of nucleosides following uptake across the BBM may further enhance nucleoside removal from the intestinal lumen (Verbic et al., 1990; Barcells et al., 1992; Stow and Bronk, 1993).
The high Vmax values for Na+-dependent intestinal transport of nucleosides in veal calves may be related to the high digestibility of nucleic acids (98%) in veal calves observed in an investigation by Roth and Kirch-gessner (1979). In this study, the addition of 3.75% RNA (related to DM) to the milk replacer did not increase nucleic acid or N elimination via the feces, whereas urinary excretion of N and allantoine increased markedly.
Collectively, the findings presented show that in the preruminant state, bovine small intestine already possesses a very high activity of Na+-dependent nucleoside transport across the brush border membrane. Apparently this transport activity does not increase further in the maturing ruminant and, therefore, seems to be a genetically fixed property in the bovine species, which is already fully expressed in the preruminant state of veal calves.
|
ACKNOWLEDGEMENTS
|
|---|
This investigation was supported by the H. Wilhelm Schaumann Stiftung.
Received for publication December 19, 2001.
Accepted for publication March 26, 2002.
|
REFERENCES
|
|---|
Adjei, A. A., K. Yamauchi, Y. C. Chan, M. Konishi, and S. Yamamoto. 1996. Comparative effects of dietary nucleoside nucleotide mixture and its components on endotoxin induced bacterial translocation and small intestinal injury in protein deficient mice. Gut 38:531–537.[Abstract/Free Full Text]
Alvarado, F. 1970. Intestinal transport of sugars and amino acids: Independence or federalism. Am. J. Clin. Nutr. 23: 824–828.[Free Full Text]
Barcells, J., D. S. Parker, and C. J. Seal. 1992. Purine metabolite concentration in portal and peripheral blood of steers, sheep and rats. Comp. Biochem. Physiol. 101B:633–636.
Gil Hernandez, A., and F. Sanchez-Medina. 1981. The determination of acid-soluble nucleotides in milk by improved enzymic methods: A comparison with the ion-exchange column chromatography procedure. J. Sci. Food Agric. 32:1123–1131.[Medline]
Griffith, D. A., and S. M. Jarvis. 1996. Nucleoside and nucleobase transport systems of mammalian cells. Biochim. Biophys. Acta 1286:153–181.[Medline]
Hopfer, U. 1977. Isolated membrane vesicles as tools for analysis of epithelial transport. Am J. Physiol. 233:E445–E449.
Johnson, L. M., J. H. Harrison, and R. E. Riley. 1998. Estimation of the flow of microbial nitrogen to the duodenum using urinary uric acid or allantoin. J. Dairy Sci. 81:2408–2420.[Abstract]
Kessler, M., O. Acuto, C. Storelli, H. Murer, M. Müller, and G. Semenza. 1978a. A modified procedure for the rapid preparation of efficiently transporting vesicles from small intestinal brush border membranes. Biochim. Biophys. Acta 506:136–154.[Medline]
Kessler, M., V. Tannenbaum, and C. Tannenbaum. 1978b. A simple apparatus for performing short-time (1–2 seconds) uptake measurements in small volumes: Its application to D-glucose transport studies in brush border vesicles from rabbit jejunum and ileum. Biochim. Biophys. Acta 509:348–359.[Medline]
Kobata, A., S. Ziro, and M. Kido. 1962. The acid-soluble nucleotides of milk: I. Quantitative and qualitative differences of nucleotides constituents in human and cow’s milk. J. Biochem. 51:277–287.
Le Leiko, N. S., and M. J. Walsh. 1996. The role of glutamine, short-chain fatty acids, and nucleotides in intestinal adaptation to gastrointestinal disease. Pediatr. Clin. North Am. 43:451–469.[Medline]
McAllan, A. B. 1980. The degradation of nucleic acids in, and the removal of breakdown products from, the small intestine of steers. Br. J. Nutr. 44:99–112.[Medline]
Munck, B. G. 1980. Transport of sugars and amino acids across guinea pig small intestine. Biochchim. Biophys. Acta 597:411–417.[Medline]
Murer, H., K. Sigrist-Nelson, and U. Hopfer. 1975. On the mechanism of sugar and amino acid interaction in intestinal transport. J. Biol. Chem. 250:7392–7396.[Abstract/Free Full Text]
Ngo, L. Y., S. D. Patil, and J. D. Unadkat. 2001. Ontogenic and longitudinal activity of Na+-nucleoside transporters in the human intestine. Am. J. Physiol. 280:G475–G481.
Nunez, M. C., M. V. Ayudarte, D. Morales, M. D. Suarez, and A. Gil. 1990. Effect of dietary nucleotides on intestinal repair in rats with experimental chronic diarrhea. J. Parent. Ent. Nutr. 14:598–606.
Parsons, D. S., and M. I. Shaw. 1983. Use of high performance liquid chromatography to study absorption and metabolism of purines by rat jejunum in vitro. Q. J. Exp. Physiol. 100:1553–1562.
Patil, S. D., and J. D. Unadkat. 1997. Sodium-dependent nucleoside transport in the human intestinal brush-border membrane. Am. J. Physiol. 272:G1314–G1320.
Preston, R. L., J. F. Schaeffer, and P. F. Curran. 1974. Structure-affinity relationship of substrates for the neutral amino acid transport system in rabbit ileum. J. Gen. Physiol. 64:443–467.[Abstract/Free Full Text]
Raezke,K.-P., H. Frister, K. Pabst, and E. Schlimme. 1988. Ribonucleoside als minore Milchinhaltsstoffe. II. Untersuchung des Ribonucleosidmusters in Rohmilch während der zweiten Hälfte der Laktationsphase. Milchwissenschaft 43:294–298.
Roden, M., A. R. P. Patterson, and K. Turnheim, 1991. Sodium-dependent nucleoside transport in rabbit intestinal epithelium. Gastroenterology 100:1553–1563.[Medline]
Roth, F. X., and M. Kirchgessner. 1979. Verwertung alimentärer Ribonukleinsäure im N-Stoffwechsel des Kalbes. (Utilisation of alimentary ribonucleic acid in calves’ N-metabolism). Arch. Tierernährung 29:275–283.
Roy, J. H. B., I. J. F. Stobo, H. J. Gaston, P. Ganderton, S. M. Shotton, and S. Y. Thompson. 1971. The nutrition of the veal calf: 4. The effect of offering roughage on health and performance. Br. J. Nutr. 26:353–262.[Medline]
Sachs, L. 1992. Angewandte Statistik. 7th ed. Springer Verlag, Berlin.
Scharrer, E., and B. Grenacher. 2001. Active intestinal absorption of nucleosides by Na+-dependent transport across the brush-border membrane in cows. J. Dairy Sci. 84:614–619.[Abstract]
Scharrer, E., and B. Grenacher. 2002. Properties of Na+-dependent nucleoside transport in the proximal and distal small intestine of cows. J. Comp. Physiol. B. 172:191–196.[Medline]
Schlimme, E., D. Martin, and H. Meisel. 2000. Nucleosides and nucleotides: Natural bioactive substances in milk and colostrum. Br. J. Nutr. 84(Suppl. 1):S59–S68.
Smith, R. H., and A. B. McAllan. 1971. Nucleic acid metabolism in the ruminant. 3. Amounts of nucleic acids and total ammonia nitrogen in digesta from the rumen duodenum and ileum of calves. Br. J. Nutr. 25:181–190.[Medline]
Stangassinger, M., X. B. Chen, L. E. Lindberg, and D. Giesecke. 1995. Metabolism of purines in relation to microbial production. Pages 387–406 in Ruminant Physiology: Digestion, Metabolism, Growth and Reproduction. W. v. Engelhardt, S. Leonhardt-Marek, G. Breves and D. Giesecke, ed., Ferdinand Enke Verlag, Stuttgart.
Stevens, B. R., S. H. Wright, B. S. Hirayama, R. D. Ganther, H. J. Ross, V. Harms, E. Nord, I. Kippen, and E. M. Wright. 1982. Organic and inorganic solute transport in renal and intestinal membrane vesicles preserved in liquid nitrogen. Membrane Biochem. 4:271–282.
Storm, E., D. S. Brown, and E. R. Ørskov. 1983. The nutritive value of rumen micro-organisms in ruminants. 3. The digestion of microbial amino and nucleic acids in, and losses of endogenous nitrogen from the small intestine of sheep. Br. J. Nutr. 50:479–485.[Medline]
Stow, R. A., and J. R. Bronk. 1993. Purine nucleoside transport and metabolism in isolated rat jejunum. J. Physiol. 468: 311–324.[Abstract/Free Full Text]
Tiemeyer, W., M. Stohrer, and D. Giesecke. 1984. Metabolites of nucleic acids in bovine milk. J. Dairy Sci. 67:723–728.
Titchen, D. A., and I. C. Newhook. 1975. Physiological aspects of suckling and the passage of milk through the ruminant stomach. Pages 15–29 in Digestion and Metabolism in the Ruminant. I. W. McDonald and A. C. I. Warner, eds.,The University of New England Publishing Unit, Armidale, Australia.
Uauy, R., G. Stringel, R. Thoms, and R. Quan. 1990. Effect of dietary nucleotides on growth and maturation of the developing gut in the rat. J. Pediatr. Gastroent. Nutr. 10:497–505.[Medline]
Verbic, J., X. B. Chen, N. A. MacLead, and E. R. Orskov. 1990. Excretion of purine derivatives by ruminants. Effect of microbial nucleic acid infusion on purine derivative excretion by steers. J. Agric. Sci. (Cambridge) 114:243–248.
Wolffram, S., B. Bisang, B. Grenacher, and E. Scharrer. 1990. Transport of tri- and dicarboxylic acids across the intestinal brush border membrane of calves. J. Nutr. 120:767–774.
Wolffram, S., E. Eggenberger, and E. Scharrer. 1989. Kinetics of D-glucose transport across the intestinal brush border membrane in the cat. Comp. Biochem. Physiol. 94A:111–115.
Wood, I. S., J. Dyer, R. R. Hofmann, and S. P. Shirazi-Beechey. 2000. Expression of the Na+/glucose co-transporter (SGLT 1) in the intestine of domestic and wild ruminants. Pflügers Arch.-Eur. J. Physiol. 441:155–162.[Medline]
This article has been cited by other articles:
|
|
|
|
 
S. I. Kehoe, A. J. Heinrichs, C. R. Baumrucker, and D. L. Greger
Effects of Nucleotide Supplementation in Milk Replacer on Small Intestinal Absorptive Capacity in Dairy Calves
J Dairy Sci,
July 1, 2008;
91(7):
2759 - 2770.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
S. F. Liao, M. J. Alman, E. S. Vanzant, E. D. Miles, D. L. Harmon, K. R. McLeod, J. A. Boling, and J. C. Matthews
Basal Expression of Nucleoside Transporter mRNA Differs Among Small Intestinal Epithelia of Beef Steers and Is Differentially Altered by Ruminal or Abomasal Infusion of Starch Hydrolysate
J Dairy Sci,
April 1, 2008;
91(4):
1570 - 1584.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
K Katoh, K Yoshioka, H Hayashi, T Mashiko, M Yoshida, Y Kobayashi, and Y Obara
Effects of 5'-uridylic acid feeding on postprandial plasma concentrations of GH, insulin and metabolites in young calves
J. Endocrinol.,
July 1, 2005;
186(1):
157 - 163.
[Abstract]
[Full Text]
[PDF]
|
|
|
TOP
ABSTRACT
INTRODUCTION
