Further studies on the gonadotrophin-induced inhibition of respiration in the preovulatory rat cumulus oophorus

Adenosine as substrate and receptor agonist in the ovary

In the present study the possible dual effects of adenosine as substrate and adenosine receptor agonist in rat granulosa cells, cumulus-oocyte complexes, luteal cells and ovarian membranes are discussed. Adenosine is an indispensable compound in cell energy metabolism, as precursor to cofactors, second messenger and nucleic acids. Adenosine is also an agonist to adenosine receptors. The adenosine receptor can either inhibit (A1) or stimulate (A2) adenylate cyclase. Alternatively, in some cells adenosine receptor activation is linked to other cellular events like inhibition of Ca2+ fluxes. Adenosine is taken up by isolated preovulatory granulosa and luteal cells from pregnant mare serum gonadotropin-treated immature rats, but follicle stimulating hormone (FSH) decreases the uptake by granulosa cells. Adenosine, but not the non-metabolizable adenosine analogs 5'-(N-ethyl)carboxamide-adenosine (NECA), 2-chloro-adenosine (2-Clado), N6-(R-phenyl-isopropyl)-adenosine (R-PLA) and N6-(S-phenyl-isopropyl)-adenosine (S-PLA), increase granulosa cell ATP levels. FSH and luteinizing hormone (LH) decrease granulosa cell ATP levels in the presence or absence of adenosine. It has previously been shown that FSH and LH decrease oxygen consumption by cumulus-oocyte complexes and increase their lactate production. These effects have been suggested to be due to a competition of cofactors (e.g. ADP) common to glycolysis and the respiratory chain. The fact that adenosine reverse the gonadotropin-induced effects on oxygen consumption and lactate production support this theory. Adenosine and its analogs increase cAMP accumulation in luteal and granulosa cells only in the presence of gonadotropins, and this effect is antagonized by the adenosine receptor antagonist 8-phenyl-theophylline (8-PHT). Furthermore, adenylate cyclase is stimulated by adenosine analogs in membranes from non-luteinized and luteinized ovarian membranes and in luteal cell homogenates. The effect of NECA is antagonized by 8-PHT. In the membranes, the rank order of potency was NECA greater than 2-Clado greater than R-PLA greater than S-PLA, suggesting adenosine A2 receptors. In summary, it is suggested that adenosine can act both as a substrate to intracellular metabolism and as an adenosine A2 receptor agonist in granulosa and luteal cells. A paracrine short loop positive feedback model is proposed where extracellular adenosine, derived from a gonadotropin-induced extracellular increase in cAMP and a decrease in cellular ATP, enhances gonadotropin stimulation in granulosa and luteal cells.