The effect of pH on both motility and the acrosome reaction of guinea pig spermatozoa was examined by transferring fully capacitated spermatozoa from Ca2+ ‐free medium to Ca2+ ‐containing media with various pH values (8.2, 7.5, 7.0, 6.7, 6.4, and 6.1). When transferred to media of pH 7.5 or 8.2, the spermatozoa underwent their acrosome reactions to the maximum level within 15–20 min. Acrosome‐reacted spermatozoa were vigorously motile. The intensity of sperm motility and the incidence of the acrosome reaction declined with decreasing pH values. At pH 6.1, the motility of all the spermatozoa quickly became very weak. No acrosome reactions occurred at this low pH. The inhibition of motility and the acrosome reaction at pH 6.1 was reversed by transferring the spermatozoa back to the alkaline medium unless they were exposed to this low pH for longer than 3 hr. The inhibition of the acrosome reaction at low pH values (other than 6.1) was counteracted to some extent by increased concentrations of Ca2+ (4–10 mM). Possible mechanisms by which low pH reversibly inhibits sperm motility and the acrosome reaction are discussed.

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It is well established that extracellular Ca2+ and an alkaline pH are essential for the acrosome reaction of precapacitated guinea pig spermatozoa. In the present study, we investigated how other extracellular cations (Na+, K+, and Mg2+) and energy substrates (glucose, lactate, and pyruvate) affect the acrosome reaction of these spermatozoa. The effect of osmolality of the medium on the reaction was also examined. It was found that precapacitated spermatozoa could undergo the acrosome reaction in a medium with an osmolality of as low as 160 mOsmol or as high as 500 mOsmol. However, the osmolality range within which large proportions of spermatozoa underwent the acrosome reaction while retaining strong motility was 220 to 380 mOsmol. Extracellular Na+ was essential for the reaction to occur. A few spermatozoa could acrosome react in a medium with a very low concentration of Na+ (5 mM or less), but a higher concentration of Na+ (more than 60 mM) was needed to ensure the acrosome reaction in the majority of spermatozoa. High concentrations of extracellular K+ were not essential for the reaction, but the presence of K+ at physiological level ensures that the reaction will occur consistently at a high rate. Extracellular Mg2+ was not required for the acrosome reaction. The high concentrations of Mg2+ (up to 20 mM) and K+ (up to 60 mM) that were tested did not affect the incidence of the acrosome reaction. Exogenous substrates were not essential for the reaction. Although the majority of precapacitated spermatozoa could acrosome react in substrate‐free medium, the presence of substrates in the medium, particularly that of pyruvate, seemed to be beneficial to the reaction.

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