IDENTIFICATION OF ADENOSINE RECEPTORS IN HUMAN SPERMATOZOA

1. The effects of adenosine receptor agonists and antagonists on human sperm motility have been studied. Specific binding sites for adenosine and its analogues on human sperm were also investigated. 2. Agonists stimulated human sperm motility in a dose‐dependent manner with a potency order of 5'‐N‐ethylcarboxamidoadenosine (NECA, EC₅₀= 0.3 μmol/L)>2‐[p‐(carboxyethyl)phenylethylamino]‐5'‐N‐ethylcarboxamidoadenosine(CGS‐21680, EC₅₀= 10 μmol/L)> adenosine (EC₅₀= 100 μmol/L). 3. NECA‐stimulated motility was competitively inhibited by various adenosine receptor antagonists. The potency order was 3,7‐dimethy‐l‐propargylxanthine>8‐(p‐sulfophenyl) theophylline > xanthine amino congener. 4. The radioligand [³H]‐NECA bound to sperm membrane in a saturable manner with a B_max of 21.3 pmol/mg protein and equilibrium K_d of 4 μmol/L. Adenosine agonists and antagonists competed for [³H]‐NECA binding with the same rank order of potency as for the stimulation of human sperm motility. 5. GTP_γs inhibited 63% of specific [³H]‐NECA binding with IC₅₀ value of 11 nmol/L. This suggests that the [³H]‐NECA binding sites may be coupled to one or more G proteins. 6. These results indicate the presence of adenosine A₂ receptors on human sperm which are responsible for adenosine‐mediated enhancement of sperm motility.