Egg Water from the Amphibian Bufo arenarum Modulates the Ability of Homologous Sperm to Undergo the Acrosome Reaction in the Presence of the Vitelline Envelope.

KRAPF, DARIO; O'BRIEN, EMMA; CABADA, MARCELO OSCAR; VISCONTI, P.; ARRANZ, SILVIA

BIOLOGY OF REPRODUCTION

Año: 2008

0006-3363

Sperm from the toad Bufo arenarum must penetrate the egg jelly before reaching the vitelline envelope (VE) where the acrosome reaction is triggered. When the jelly coat is removed, sperm still bind to the VE but acrosomal exocytosis is not promoted. Our previous work demonstrated that diffusible substances of the jelly coat, named egg water (EW), triggered capacitation-like changes in Bufo sperm, promoting the acquisition of a transient fertilizing capacity. In the present work we have correlated this fertilizing capacity with the sperm ability to undergo the acrosome reaction, further substantiating the role of the jelly coat in fertilization. When sperm where exposed to VE, only those preincubated in EW for 5 or 8 min underwent an increase of the intracellular calcium concentration ([Ca(2+)]i) that led to the acrosomal exocytosis. Responsiveness to VE was not acquired upon preincubation in EW for 2 or 15 min, or in Ringer medium regardless of the preincubation time. In contrast, depletion of intracellular calcium stores (thapsigargin), promoted both [Ca(2+)]i rise and acrosome reaction even in sperm that were not exposed to EW. Acrosomal exocytosis was blocked by the presence of Ca(2+) chelators, independently of whether a physiological or pharmacological stimulus was used. However, Ni(2+) and mibefradil prevented both [Ca(2+)]i rise and acrosome reaction of sperm exposed to VE but not to thapsigargin. These data suggest that the acrosomal responsiveness of Bufo sperm, present during a narrow time period, is acquired during EW incubation and involves the modulation of a voltage dependant Ca(2+) channel.