Regulation of abalone sperm cyclic AMP concentrations and the acrosome reaction by calcium and methylxanthines

When Ca2+ is added to abalone sperm (Haliotis rufescens) in Ca2+-free artificial seawater (CaFASW) to a final concentration of 9.6 mM a 4-fold elevation in sperm cAMP occurs within 15-30 sec. The methylxanthines, theophylline and 1-methyl-3-isobutylxanthine (MIX), also elevate sperm cAMP concentrations. In CaFASW, either compound causes up to a 3-fold increase in cAMP within 15-30 sec. MIX (150 microM), added to sperm in the presence of 9.6 mM Ca2+, elevates sperm cAMP 100-fold within 15-30 sec and also triggers the acrosome reaction (AR) in the same period. Under identical conditions theophylline (1.67 mM) is much less potent at elevating cAMP and inducing the AR. The effects of methylxanthines on cAMP of sperm incubated in the presence of Ca2+ appear to represent a potentiation by these compounds of the action of Ca2+. Neither compound induces the AR in the absence of Ca2+. All of the observed effects on sperm cAMP and the AR are dependent on Ca2+ and methylxanthine concentrations. Added cyclic nucleotides or their derivatives do not induce the AR in either the absence or presence of Ca2+. Experiments with isolated sperm heads and flagella indicate that the dramatic stimulatory response of sperm cAMP to Ca2+ plus MIX is present in the head region (acrosome, nucleus, midpiece) of the cell. The data suggest that the dramatic elevation of cAMP by MIX in the presence of Ca2+ may occur directly by an inhibition of phosphodiesterase activity and indirectly by an increase in cellular Ca2+. A strong temporal correlation between the cAMP elevation and the abalone AR exists, although cAMP elevation by itself does not act as the primary mediator of this exocytotic event.