Effect of cAMP on Acrosome Reaction and Fertilization

Abstract

The effect of cAMP on acrosome reaction and fertilization was determined by altering the levels of intracellular cAMP with the phosphodiesterase (PDE) inhibitors caffeine, theophylline and isobutylmethylxanthine (MIX), with the PDE stimulator, imidazole, and with the cAMP analog dibutyryl cyclic-AMP. Caffeine, theophylline and MIX, tested at concentrations of 1-10 mM, effectively inhibited guinea pig acrosome reaction to levels of less than 10% at concentrations of 5 mM, 7 mM and 1 mM, respectively. The percentage of guinea pig acrosome reaction was significantly stimulated (80% vs 50% in controls) at concentrations of 6 mM imidazole or higher. Addition of dbcAMP to guinea pig sperm produced results similar to the PDE inhibitors: that is, significant inhibition of acrosome reaction at concentrations above 8 mM. Fertilization studies with hamster spermatozoa showed a significant inhibition when caffeine, theophylline, MIX or dbcAMP was incorporated in the medium from the outset of incubation. If, however, the PDE inhibitors or the dbcAMP was added to the sperm after acrosome reaction had occurred, there was no inhibition of fertilization of zona-intact or zona-free eggs by hamster and guinea pig spermatozoa, respectively. This indicated that the inhibitory effect of cAMP is at the level of capacitation and/or acrosome reaction and not at the level of zona penetration or vitelline membrane fusion. These results suggest that reduction of intracellular cAMP may be a part of the capacitation and acrosome reaction mechanism since elevated levels of cAMP inhibit and reduced levels stimulate acrosome reaction and fertilization.