Release of endogenous and radioactive purines from the rabbit retina
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The adenine nucleotide pool of rabbit retina was labeled by an intravitreal injection in vivo of [3H]adenosine. Practically all the radioactivity was retained in the form of adenine nucleotides. The relative proportion of [3H]adenine nucleotides was the same as that of endogenous nucleotides. Potassium depolarization (43.6 mM) in vitro caused a rapid increase in the rate of release of radioactive purines. The radioactive material was composed of hypoxanthine, xanthine, inosine and trace amounts of adenine, adenosine and adenine nucleotides. The release of radioactive purines was delayed and reduced by the addition of the nucleoside inhibitor dipyridamole suggesting that the purines may be released in the form of nucleosides. Similarly, the addition of the ecto 5'-nucleotidase inhibitor alpha, beta-methylene ADP (AOPCP) did not alter the release of radioactivity or the composition of the released purines. Endogenous hypoxanthine, xanthine and inosine could be detected in the effluents, but there was only a very modest increase following potassium depolarization. There was a slight, but significant, decrease in the release of endogenous adenosine and increase in AMP after AOPCP. It is concluded that there is an intensive uptake and phosphorylation of adenosine in the rabbit retina. Depolarization induces release of radioactive purine nucleosides and bases. Most of these compounds appear to be released as such, but in addition there may be a small (maximally a few per cent of the total) fraction of the purines that are released as nucleotides.