Inhibition of the guinea-pig sperm acrosome reaction by a low molecular weight factor(s) in epididymal fluid and serum.

In protein-deficient minimal culture medium, acrosome reactions of motile guinea-pig spermatozoa were first evident after 30 min and maximal by 2 h. Addition of 5% (v/v) of guinea-pig serum filtrate or human plasma filtrate, obtained by passing these fluids through an Amicon UM-2 ultrafiltration membrane, prevented the sperm acrosome reaction during a 4-h incubation, but did not inhibit sperm motility. A similar inhibitory effect was found in porcine epididymal fluid. The factor(s) in porcine epididymal fluid effectively inhibited acrosome reactions if it was added to uncapacitated spermatozoa but failed to decapacitate sperm cells capacitated in Ca2+-free medium. Preliminary characterization of the factor(s) in porcine epididymal fluid indicate that it is a small organic molecule, stable to heat (90 degrees C), soluble in methanol, sparingly soluble in ethanol and insoluble in ether, chloroform or acetone; it also appears to have no net charge at pH values between 4 and 10.