Sodium-adenosine cotransport in brush-border membranes from rabbit ileum.
To determine the mechanism(s) of transcellular adenosine transport in epithelial tissues that possess an adenosine receptor response, we studied [3H]adenosine uptake using vesicles prepared from isolated brush-border and basolateral membranes of the rabbit ileum. In the presence of the adenosine deaminase inhibitor deoxycoformycin uptake of [3H]adenosine into brush-border membrane vesicles is stimulated fivefold by an inwardly directed Na gradient. Na-dependent [3H]adenosine uptake is enhanced and concentrative under conditions that increase inside negativity of vesicles, thus providing evidence for an electrogenic carrier. Na-dependent adenosine uptake is a saturable function of adenosine concentration with a Michaelis-Menten constant of 17.3 +/- 7.1 microM and maximum transport rate of 216.9 +/- 20.2 pmol.min-1.mg protein-1. Both uridine and inosine inhibit [3H]adenosine uptake, suggesting that the Na-dependent transporter has broad substrate specificity for both purine and pyrimidine ribonucleosides. Na-dependent adenosine uptake is inhibited by dipyridamole but is insensitive to 6-(4-nitrobenzyl)thio-9-beta-D-ribofuranosylpurine. We conclude that adenosine is transported across ileal brush-border membranes by a Na-ribonucleoside cotransport system. In contrast, adenosine uptake in basolateral membranes is not stimulated by a Na gradient. These studies show asymmetry in the distribution of transport systems for adenosine in polarized intestinal epithelia.
Section of Nephrology, Yale University School of Medicine, New Haven, Connecticut 06510.
S L
SL
Betcher
J N
JN
Forrest
R G
RG
Knickelbein
J W
JW
Dobbins
D000241:Adenosine / Q000378:metabolism*; D058892:Adenosine Deaminase Inhibitors; D000818:Animals; D000838:Anions; D001692:Biological Transport / Q000187:drug effects; D004848:Epithelium / Q000378:metabolism; D007082:Ileum / Q000378:metabolism*; D007288:Inosine / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D008871:Microvilli / Q000378:metabolism*; D009130:Muscle, Smooth / Q000378:metabolism; D011817:Rabbits; D011983:Receptors, Purinergic / Q000187:drug effects / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D014529:Uridine / Q000494:pharmacology
D058892:Adenosine Deaminase Inhibitors; D000838:Anions; D011983:Receptors, Purinergic; D007288:Inosine; D012964:Sodium; D000241:Adenosine; D014529:Uridine