Is a Ca2+ -ATPase involved in Ca2+ regulation during capacitation and the acrosome reaction of guinea-pig spermatozoa?

The Ca2+-ATPase antagonists quercetin and ethacrynic acid accelerated the onset of the acrosome reaction in guinea-pig spermatozoa incubated in the continuous presence of Ca2+, whereas furosemide had no effect, and sodium orthovanadate only affected sperm motility. When spermatozoa were preincubated in a 'Ca2+-free' medium, quercetin and ethacrynic acid shortened capacitation time: spermatozoa incubated for 1 h in 100-200 microM-ethacrynic acid showed 60-80% acrosome reactions when Ca2+ was added. Such spermatozoa were able to fertilize zona-free hamster eggs. Our results therefore point to the possible involvement of a Ca2+-ATPase in the regulation of intracellular Ca2+ in spermatozoa. Cysteine and dithiothreitol, both disulphide reducing agents, prevented the effects of quercetin and ethacrynic acid, suggesting that sulphydryl groups may be important for the expression of Ca2+-ATPase activity. Lysophosphatidylserine (LS) also prevented the stimulatory effect of ethacrynic acid, an effect similar to that shown by LS on lysophosphatidylcholine (LC). It is argued that both LS and LC could exert their action through an effect on the Ca2+-ATPase.