Rat Sperm are Mechanically Immobilized in the Caudal Epididymis by “lmmobilin, ” a High Molecular Weight Glycoprotein

In many mammals, sperm are immotile while stored in the caudal epididymis and do not become motile until ejaculation. We report here our investigation of the mechanism that initiates motility in mature rat epididymal sperm. We found that an external “activator ” is not required to initiate rat sperm motility since immotile sperm started to swim immedi-ately when exposed to solutions that contributed only osmotic support. Instead, we found that epididymal rat sperm are kept fully immobilized by a high molecular weight glyco-protein, “immobiin, ” that we have isolated from rat cauda epididymal (CE) fluid. Our results strongly support the hypothesis that immobiin inhibits sperm motility mechanically simply by creating a highly viscoelastic environment: 1) rat CE fluid inhibited the motility of such disparate cells as rat sperm, E. coli. and rabbit sperm (which are fully motile in rabbit CE fluid), 2) the degree to which a variety of enzymatic treatments or slight dilution of the CE fluid initiated sperm motility depended only on the degree to which the treat-ment reduced the viscoelastic drag of the fluid, and 3) centrifugation of CE fluid simul-taneously copurified the component of the fluid that immobilizes the sperm, the com-ponent that renders the fluid viscoelastic, and the glycoprotein immobiin