Received October 5, 2007
Revised October 31, 2007
Accepted after revision November 9, 2007

¹ School of Medicine, Belgrade
² King's College London
³ Kuwait University

^* To whom correspondence should be addressed. E-mail: redzic{at}hsc.edu.kw.

	Abstract

Sheep choroid plexus epithelium expresses equilibrative nucleoside transporters (ENT) 1 and 2 and concentrative nucleoside transporter 2 at the transcript level. This study was 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 [³H] adenosine and [³H] uridine was insensitive to 1 µM nitrobenzylthioinosine (NBTI), and the uptake of [³H] adenosine was reduced significantly when 10 µM NBTI was present in low Na⁺ Ringer. 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 was used for the in situ perfusion, the Na⁺ concentration in the venous effluent decreased to 14 mM; under these conditions the Umax of [³H] adenosine and [³H] uridine did not change significantly when compared to the Umax obtained when Ringer that contained 145 mM Na⁺ was used. Kinetic analysis revealed the apparent Michaelis constants (Km_app) for [³H] adenosine, [³H] inosine and [³H] thymidine cellular uptake of 1.2 ± 0.2, 15.7 ± 2.6 and 3.8 ± 0.9 µM, respectively. 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 Km values, it appears that under normal conditions inosine is more important nucleoside substrate for uptake by the basolateral membrane of the CP epithelium than other nucleosides.

Key Words: Adenosine, Blood-brain barrier, Choroid plexus