Requirement of Serum Factors for Capacitation and the Acrosome Reaction of Guinea Pig Spermatozoa in Buffered Medium Below pH 7.81

In a protein-deficient minimal culture medium containing various buffers at pH 8.3–8.4, acrosome reactions of motile guinea pig spermatozoa were first evident after 30 min and maximal by 2 h. If the pH of the culture medium was adjusted to pH 7.4, however, spermatozoa did not undergo acrosome reactions within 8 h. Subsequent experiments showed that acrosome reactions were not spontaneously induced below pH 7.8, whereas at pH 10.5 ∿85% of the motile spermatozoa showed acrosome reactions within 5 min. Similarly, acrosome reactions could be maximally induced within 40 min in spermatozoa incubated at pH 9.5. The induction of acrosome reactions at high pH required the presence of Ca2+ since no acrosome reactions were observed when 1 mM Mg2+ and 0.2 mM ethylcne-glycol-bis (β-aminoethyl ether) N,N'-tetraacetic acid (EGTA) were substituted for Ca2+. Acrosome reactions occurred at pH values less than 7.8, however, if 20% (v/v) of heat-treated guinea pig serum or human plasma was added to the culture medium. The serum or plasma was fractionated by ultrafiltration or dialysis, and both a low molecular weight fraction (<1000) and a larger molecular weight fraction (MW 30,000–100,000) appeared required for induction of the acrosome reaction. Spermatozoa capacitated in Ca2+-free medium at pH 8.3 and then transferred to Ca2+-free medium at pH 7.4 reponded to the addition of Ca2+ with immediate acrosome reactions in the absence of biological factors, but if cells were incubated 3 h at pH 7.4 in the absence of biological factors and then transferred to medium at pH 8.4, the normal capacitation time was still required. These results demonstrate that although guinea pig spermatozoa can capacitate and respond to added Ca2+ with acrosome reactions in the absence of added biological factors at alkaline pH values (>7.8), added biological factors are required to capacitate cells at lower pH values.