Inhibition of forskolin- and HCO3− stimulated increase in the intracellular cAMP levels of human spermatozoa by adjudin. Sperm were separately incubated for 0 and 5 h in the HTF medium (containing 25 mM HCO3− ) with or without 3.6 µM forskolin and/or 10 μM adjudin. The results are expressed as the means ± SEM (fmol/2 × 106 cells; n = 5). The forskolin treatment (a) when compared with controls (b), (c) and (d) (HCO3− plus adjudin) after incubation of 0 h and 5 h. Adjudin significantly inhibits the increase in [cAMP]i level activated by both forskolin and HCO3− (a versus c, P < 0.001; a versus b, P < 0.05; b versus c, P < 0.01) or only HCO3− (d versus b, P < 0.01).