ENDOGENOUS NUCLEOSIDES IN THE GUINEA-PIG EYE - ANALYSIS OF TRANSPORT AND METABOLITES

This study investigates the transport of endogenous nucleosides and de oxynucleosides from the capillaries of the eye into the aqueous humour and the lens using the in situ vascular eye perfusion technique in th e guinea-pig. The transport of [H-3] adenosine and [H-3] thymidine acr oss the blood-aqueous barrier proved to be very rapid with a volume of distribution after 4 minutes perfusion reaching 11.9 +/- 3.0% and 9.9 3 +/- 1.1%, respectively. However, the transport of [H-3] guanosine an d [H-3] cytidine was slower, with volumes of distribution reaching onl y 3.38 +/- 0.58% and 4.8 +/- 1.41%. The values for the entry of deoxya denosine and deoxyguanosine were not significantly different from the values obtained for corresponding ribonucleosides (adenosine and guano sine) so that a change in the pentose sugar does not change the affini ty of the nucleoside for the transport protein. Perfusion with a low s odium medium inhibited the transport of [H-3] adenosine and [H-3] thym idine into the aqueous humour. The presence of 800 nM NBTI also caused a decrease in adenosine transport into the aqueous humour, so that th e volume of distribution after 2 minutes reached only 3.78 +/- 1.87%. These findings suggest that the transfer of adenosine across the blood -aqueous barrier has both concentrative and equilibrative components. The presence of 0.1 mM thymidine had no effect on the [H-3] adenosine transport, whereas 0.1 mM of adenosine resulted in a marked decrease o n the [H-3] thymidine transport which suggests that the concentrative nucleotide transport is probably mediated by both cif and cit transpor t systems. The cellular uptake of nucleosides into the lens was very r apid and the volume of distribution of purine nucleosides was within t he range of 30-50% whereas that for thymidine uptake was somewhat lowe r, reaching 20-30%. HPLC analysis of the eye structures in the guinea- pig showed that lens, vitreous body and the rest of the eye do not con tain either free nucleosides or purine bases in detectable quantities, except for xanthine which was detected in aqueous humour at a concent ration of 2.51 +/- 0.51 mM. However, serum of the anaesthetised guinea -pig did not contain xanthine in detectable amount so it seems that th e metabolic degradation of the nucleosides in the guinea-pig eye progr esses as far as xanthine, which is then accumulated in the aqueous hum our. (C) 1998 Academic Press Limited.