Uptake and metabolism of adenosine mediate a meiosis-arresting action on mouse oocytes

This study was carried out to evaluate the possible role of adenosine uptak e and metabolism in mediating the inhibitory actions of this nucleoside on spontaneous mouse oocyte maturation. Uridine blocked H-3-adenosine uptake b y oocyte-cumulus cell complexes (OCCs) and cumulus cell-enclosed oocytes (C EOs) by 82-85%, whereas uptake by denuded oocytes (DOs) was suppressed by 9 7%. Uridine had no effect on germinal vesicle breakdown (GVB) in CEOs when meiotic arrest was maintained with hypoxanthine or hypoxanthine plus adenos ine but reversed the combined inhibitory action of these purines in DOs. Fi ve of six adenosine analogs that bind to purinoceptors demonstrated meiosis -arresting activity but not in relation to their relative affinities for in hibitory or stimulatory adenosine receptors and only at high concentrations . Moreover, in DOs, uridine reversed the inhibitory effect of 2-chloroadeno sine and 5'-N-ethylcarboxamidoadenosine, two receptor agonists that are poo r substrates for adenosine-metabolizing enzymes. Results of experiments wit h adenosine kinase inhibitors showed that methylmercaptopurine riboside (MM PR) and tubercidin, but not 5'-amino-5'-deoxyadenosine, reversed meiotic ar rest maintained by hypoxanthine +/- adenosine, but this required an additio nal inhibitory action on de novo purine synthesis. Inhibition of de novo pu rine synthesis alone was not sufficient because azaserine failed to reverse meiotic arrest. MMPR was a very potent meiosis-inducing agent, completely reversing meiotic arrest in CEOs and DOs in the presence of a variety of me iotic inhibitors. The adenosine deaminase inhibitor deoxycoformycin had opp osite effects on oocyte maturation depending on the presence or absence of adenosine. the inhibitory action of hypoxanthine alone was bolstered, but t he meiosis-arresting action of adenosine was reversed. These data therefore indicate that at low adenosine concentrations phosphorylation predominates , but at higher adenosine concentrations deaminated products contribute to the meiotic inhibition. This idea was borne out by the ability of inosine t o mimic the synergistic interaction of adenosine with hypoxanthine. The act ion of adenosine is not due to deamination to inosine and conversion to nuc leotides through the hypoxanthine salvage pathway because adenosine-mediate d inhibition was not compromised in oocytes from mutant mice unable to salv age hypoxanthine. Mel. Reprod. Dev. 53:208-221, 1999. (C) 1999 Wiley-Liss, Inc.