Relationship between plasma membrane Ca(2+)-ATPase activity and acrosome reaction in guinea pig sperm

The results obtained by biochemical measurement demonstrated for the first time that significant decrease of the plasma membrane Ca(2+)-ATPase activity occurred during capacitation and acrosome reaction of guinea pig sperm. Ethacrynic acid, one kind of Ca(2+)-ATPase antagonists, inhibited the plasma membrane Ca(2+)-ATPase activity, but calmodulin (50 micrograms/mL) and trifluoperazine (200-500 mumol/L) did not, suggesting that calmodulin is not involved in ATP-driven Ca2+ efflux from sperm. However, calmodulin is involved in the control of Ca2+ influx. TFP, one kind of calmodulin antagonists, accelerated the acrosome reaction and Ca2+ uptake into sperm cells significantly. Ca(2+)-ATPase antagonists, quercetin, sodium orthovandate, furosemide and ethacrynic acid promoted the acrosome reaction, but inhibited Ca2+ uptake, which cannot be explained by their inhibitory effects on the plasma membrane Ca(2+)-ATPase activity. It is speculated that this phenomenon might be caused by simultaneous inhibitions of the activities of Ca(2+)-ATPase present in the plasma membrane, the outer acrosome membrane and the outer mitochondrion membrane resulting in Ca2+ accumulation in the cytoplasm, which in turn blocks further Ca2+ entry through some negative feedback mechanism(s). The inhibitory effect of Ca(2+)-ATPase antagonist on glycolytic activity may also be the reason for Ca2+ accumulation in cytoplasm and inhibition of Ca2+ uptake.