Intracellular pH Changes of Starfish Sperm Upon the Acrosome Reaction

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Abstract
The acrosome reaction is accompanied by ionic changes such as increases in intracellular Ca2+ and intracellular pH (pHi). Since the two jelly components essential for inducing the acrosome reaction, ARIS and Co-ARIS, were shown to activate Ca-channels (accompanying paper), we examined the jelly components to determine which was responsible for the pHi-increase using 9-aminoacridine as a probe of pHi. This paper presents evidence that an oligopeptide(s) is responsible for the pHi-increase. The pHi of swimming sperm is 7.4-7.5. Within 20 sec after the addition of jelly, their pHi increased rapidly by 0.06 pH unit, then decreased by 0.2-0.3 pH unit, and reached a plateau level within 3 min. Similar changes in pHi were observed on addition of a Pronase digest of ARIS (P-ARIS) and a diffusible fraction of jelly (Fraction M8) together. Fraction M8, but not ARIS or Co-ARIS increased the pHi, and activated sperm respiration in sea water at pH 6.5. The two activities of Fraction M8 depended upon Na+ but not Ca2+, and wer susceptible to Pronase digestion. Fraction M8 is also known to enhance induction of the acrosome reaction by the Ca-ionophore A23187. These results suggest that the egg jelly contains a peptide(s) that is not obligatory for the acrosome reaction but facilitates the reaction by increasing the pHi of the sperm. The significance of the pHi-increase upon the acrosome reaction is discussed.