Sperm motility-inhibiting factor in rat epididymis.

Rat spermatozoa collected by micropuncture from the cauda epididymidis are immotile when diluted in cell-free cauda epididymidal fluid but are vigorously motile when diluted in 0.154 M NaCl. Removal of proteins from cauda fluid by heat precipitation or enzymatic digestion reduces the ability of cauda fluid to keep cauda sperm immotile. Cauda sperm diluted in trypsin-treated cauda fluid have motility initiation scores that are not significantly different (P less than 0.05) from cauda sperm motility scores after dilution in 0.154 M NaCl. Cell-free lumen fluids from the proximal epididymis have a reduced ability to enforce quiesence on cauda spermatozoa, and rete testis fluid does not keep cauda sperm immotile. However, concentrated rete testis fluid does prevent the initiation of motility. It is proposed that a proteinaceous factor, possibly of testicular origin, is important in preventing the initiation of motility while sperm reside in the distal reproductive tract of the male rat.