The kinetics of hypoxanthine transport across the perfused choroid plexus of the sheep.

The uptake of principal salvageable nucleobase hypoxanthine was investigated across the basolateral membrane of the sheep choroid plexus (CP) perfused in situ. The results suggest that hypoxanthine uptake was Na+-independent, which means that transport system on the basolateral membrane can mediate the transport in both directions. Although the unlabelled nucleosides adenosine and inosine markedly reduce the transport it seems that this inhibition was due to nucleoside degradation into nucleobases in the cells, since non-metabolised nucleoside analogue NBTI did not inhibit the transport. The presence of adenine also inhibits hypoxanthine uptake while the addition of the pyrimidines does not show any effect, so it seems that the transport of purine nucleobases through basolateral membrane is mediated via a common transporter which is different from the nucleoside transporters. The inclusion of allopurinol in the perfusion fluid did not change the value and general shape of the curve for the uptake which suggest that degradation of hypoxanthine into xanthine and uric acid does not occur in the CP. The capacity of the CP basolateral membrane to transport hypoxanthine is high (90.63+/-3.79 nM/min/g) and close to the values obtained for some essential amino acids by the CP and blood-brain barrier, while the free diffusion is negligible. The derived value of Km (20.72+/-2.42 microM) is higher than the concentration of hypoxanthine in the sheep plasma (15.61+/-2.28 microM) but less than a half of the concentration in the CSF, which indicates that the transport system at basolateral membrane mostly mediates the efflux of hypoxanthine from the cerebrospinal fluid in vivo.