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Kinetics of nucleoside uptake by the basolateral side of the sheep choroid plexus epithelium perfused in situ
Author(s) -
Markovic Ivanka,
Segal Malcolm,
Djuricic Bogdan,
Redzic Zoran
Publication year - 2008
Publication title -
experimental physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.925
H-Index - 101
eISSN - 1469-445X
pISSN - 0958-0670
DOI - 10.1113/expphysiol.2007.040980
Subject(s) - inosine , choroid plexus , nucleoside , adenosine , uridine , thymidine , chemistry , epithelium , transporter , biochemistry , nucleoside transporter , biophysics , microbiology and biotechnology , biology , endocrinology , in vitro , rna , genetics , gene , central nervous system
Sheep choroid plexus epithelium expresses equilibrative nucleoside transporters (ENT) 1 and 2 and concentrative nucleoside transporter 2 at the transcript level. This study aimed to explore the kinetics and functional role of these transporters at the basolateral side of the sheep choroid plexus epithelium perfused in situ . The cellular uptake of [ 3 H]adenosine and [ 3 H]uridine was insensitive to 1 μ m nitrobenzylthioinosine (NBTI), and the uptake of [ 3 H]adenosine was reduced significantly when 10 μ m NBTI was present in low‐Na + Ringer solution. This might suggest that ENT2, a transporter sensitive to micromolar NBTI, is functionally active at the basolateral side of the choroid plexus epithelium while ENT1, a transporter sensitive to nanomolar NBTI, is not active. When low‐Na + Ringer solution was used for the in situ perfusion, the Na + concentration in the venous effluent decreased to 14 m m ; under these conditions the maximal uptake ( U max ) of [ 3 H]adenosine and [ 3 H]uridine did not change significantly when compared with the U max obtained when Ringer solution that contained 145 m m Na + was used. Kinetic analysis revealed apparent Michaelis–Menten constants ( K m,app ) for cellular uptake of [ 3 H]adenosine, [ 3 H]inosine and [ 3 H]thymidine of 1.2 ± 0.2, 15.7 ± 2.6 and 3.8 ± 0.9 μ m , respectively. The HPLC and HPLC–fluorometric analysis of the sheep plasma and cerebrospinal fluid revealed nanomolar concentrations of adenosine and thymidine and micromolar levels of inosine and nucleobases. Considering the estimated K m,app values, it appears that under normal conditions inosine is the more important nucleoside substrate for uptake by the basolateral membrane of the choroid plexus epithelium than other nucleosides.