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Sperm intracellular Ca 2+  is crucial for the induction of sperm-egg interaction, but little is known about the significance of Ca 2+  maintenance prior to induction. In sperm of the newt  Cynops pyrrhogaster , intracellular Ca 2+  is localized to the midpiece during storage in the vas deferens, while extracellular Ca 2+  is influxed in modified Steinberg’s salt solution to promote a spontaneous acrosome reaction related to the decline of sperm quality. In the present study, sperm from the vas deferens were loaded with the Ca 2+  indicator Fluo8H, and changes in Ca 2+  localization in modified Steinberg’s salt solution were examined. Calcium ions expanded from the cytoplasmic area of the midpiece to the entire tail in most sperm during a 1-h incubation and localized to the principal piece in some sperm within 24 h. Similar changes in Ca 2+  localization were observed in reconstructed vas deferens solution that included ions and pH at equivalent levels to those in the vas deferens fluid. Sperm with Ca 2+  localization in the entire tail or the principal piece weakened or lost responsiveness to sperm motility-initiating substances, which trigger sperm motility for fertilization, but responded to a trigger for acrosome reaction. The change in Ca 2+  localization was delayed and transiently reversed by ethylene glycol tetraacetic acid or a mixture of Ca 2+  channel blockers including Ni 2+  and diltiazem. These results suggest that  C. pyrrhogaster  sperm localize intracellular Ca 2+  to the midpiece through Ca 2+  transport in the vas deferens to allow for responses to sperm motility-initiating substances.

Localization of sperm intracellular Ca2+ keeps fertilizability in the newt vas deferens