Aim: To clarify the relationship between bicarbonate and cAMP in the promoting effects on the sperm agglutination. Methods: Spermatozoa were collected from mature boars, washed and resuspended in a modified Krebs-Ringer HEPES lacking calcium chloride (mKRH). The sperm suspensions were incubated in a water bath (38.5 ) for 60 min and then the percentage of head-to-head agglutinated spermatozoa was determined. Results: Supplementation of the mKRH with sodium bicarbonate (5-10 mM) significantly raised the percentage of head-to-head agglutinated spermatozoa in the samples. The addition of selective inhibitors for calcium/calmodulin-dependent phosphodiesterases (type 1: 8-methoxymethyl-IBMX and vinpocetine, 25-50 mM) or for cAMP-specific phosphodiesterases (type 4: Ro20-1724 and rolipram, 25-50 mM) enhanced the effect of bicarbonate on sperm agglutination as highly as did the addition of non-selective inhibitors for phosphodiesterases (IBMX and papaverine, 25-50 mM). A calmodulin antagonist (W-7, 2 mM), that potentially blocks the stimulator of the calcium/calmodulin-dependent phosphodiesterases, significantly enhanced the effect of bicarbonate on sperm agglutination. Moreover, a phosphodiesterase-resistant cAMP analogue (cBiMPS, 0.1 mM) markedly induced agglutination in more spermatozoa (76%) after the incubation without bicarbonate and phosphodiesterase inhibitors than did a less potent cAMP analogue (dibutyryl cAMP, 1 mM) (21%), while three kinds of cGMP analogues (0.1-1 mM) had no effect on sperm agglutination. In addition, a cAMP antagonist (Rp-cAMPS, 1 mM) significantly reduced the sperm agglutination resulting from the actions of bicarbonate and IBMX. On the other hand, the effect of bicarbonate was abolished by a change of incubation temperature from 38.5 to 25. Conclusion: These findings demonstrate that the bicarbonate-induced agglutination of boar spermatozoa is controlled via the cAMP-mediated, temperature-dependent signaling cascade. This cascade is suppressed by the action of the phosphodiesterase (at least types 1 and 4).

Keywords: cAMP; capacitation; phosphodiesterase; calmodulin

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