Methylxanthines stimulate calcium transport and inhibit cyclic nucleotide phosphodiesterases in abalone sperm

Experiments were designed to determine the mechanism by which methylxanthines elevate abalone sperm cAMP concentrations and induce the acrosome reaction (AR). Theophylline or, more effectively, 1-methyl-3-isobutylxanthine (MIX) inhibit the cyclic nucleotide phosphodiesterase activities of abalone sperm homogenates. 45Ca2+ uptake by sperm is also stimulated by theophylline, and more effectively by MIX, and this stimulatory effect is blocked by KCN. Verapamil, a compound known to antagonize Ca2+ conductance, has no effect on the Ca2+ or MIX-induced cAMP elevation at concentrations up to 200 microM. However, verapamil reduces the sperm cAMP elevation caused by the addition of Ca2+ plus MIX. This inhibition is not complete, even at 200 microM verapamil. The AR induced by Ca2+ plus MIX is completely inhibited by 200 microM verapamil. The data suggest that these methylxanthines elevate abalone sperm cyclic nucleotide concentrations by inhibiting cyclic nucleotide phosphodiesterase activities. Furthermore, since sperm cAMP metabolism is modulated by Ca2+ flux, methylxanthines also appear to elevate abalone sperm cAMP concentrations by their effects on Ca2+ transport. The Ca2+-induced cAMP elevation occurs through a verapamil-insensitive mechanism, whereas the potentiation by MIX of the Ca2+ effect to elevate cAMP occurs through both verapamil-insensitive and -sensitive mechanisms. The methylxanthine-induced AR is mediated by a primary effect on Ca2+ transport and occurs through a verapamil-sensitive mechanism. Cyclic AMP may play a role in the methylxanthine-induced AR, but does not appear to act as the primary mediator of this exocytotic event.