A sustained increase in intracellular Ca2+ is required for the acrosome reaction in sea urchin sperm

The acrosome reaction (AR), necessary for fertilization in many species, requires an increase in intracellular Ca2+ ([Ca2+](i)). In sea urchin sperm, the AR is triggered by an egg-jelly factor: the associated [Ca2+](i) elevation lasts minutes and involves two Ca2+ permeable channels. Both the opening of the second channel and the onset of the AR occur similar to5 s after treatment with egg factor, suggesting that these events are linked. in agreement, removal of Ca2+ from sea water or addition of Ca2+ channel blockers at the time when opening of the second channel is first detected inhibits AR and causes a "rapid" (t(1/2) = 3-15 s) decrease in [Ca2+](i) and partial inhibition of the intracellular pH change associated with the AR. Simultaneous addition of NH4Cl and either EGTA, Co2+, or Ni2+ 5 s after egg factor prevents the partial inhibition of the evoked pH(i) change observed but does not reverse AR inhibition. Therefore, the sustained increase in [Ca2+](i) caused by the second Ca2+ channel is needed for the sperm AR. Experiments with agents that induce capacitative Ca2+ uptake (thapsigargin and cyclopiazonic acid) suggest that the second channel opened during the AR could be a store-operated Ca2+ channel. (C) 2001 Academic Press.