Induction of the Acrosome Reaction in Starfish: acrosome reaction/starfish sperm/egg jelly/Ca‐channel/ionophore

In the starfish, Asterias amurensis, at least two distinct components of the egg jelly are required for inducing the acrosome reaction: a sulfated glycoprotein named acrosome reaction‐inducing substance (ARIS) and a diffusible organic substance(s) named Co‐ARIS. The following evidence suggested that ARIS and Co‐ARIS cooperatively activate CA‐channels of the sperm plasma membrane and eventually induce dramatic changes in sperm morphology, the acrosome reaction. 1) Pronase digest of ARIS (P‐ARIS) and Co‐ARIS, either as a pure or a crude preparation (Fraction M₈), were fully effective in combination for induction of the acrosome reaction in normal sea water, although they were not effective individually. P‐ ARIS alone induced the acrosome reaction fully in high Ca²⁺ sea water and markedly at high pHs, whereas Fraction M₈ alone did not induce the reaction even in these conditions. The reaction was not induced by increase in either the Ca²⁺ concentration or the pH of sea water, but was markedly induced in the absence of jelly components by raising both the pH and Ca²⁺ concentration together. 2) The ionophore A23187 induced the acrosome reaction appreciably when present alone and fully in the presence of monensin or Fraction M₈. Monesin alone was ineffective. 3) The jelly or a combination of ARIS and Fraction M₈ caused abrupt Ca²⁺ ‐uptake by the sperm. The Ca‐channel blockers verapamil and diltiazem inhibited the jelly‐induced acrosome reaction.