Effect of casein phospho peptides and Ca2+ on penetration of boar spermatozoa into pig oocytes matured in vitro.

To investigate the promoting effect of casein phospho peptides (CPP) on the penetration of ejaculated boar spermatozoa into in vitro-matured oocytes, preincubated spermatozoa were coincubated with in vitro-matured oocytes in fertilization medium with or without CPP for 60-240 min. Sperm penetration into zona-intact oocytes was observed within 90 min of coincubation. The penetration rate reached a maximum earlier in the presence of CPP than in the absence of CPP. Furthermore, spermatozoa preincubated in fertilization medium with CPP before coincubation with matured oocytes retained the ability to penetrate oocytes for a longer time than spermatozoa preincubated in fertilization medium without CPP. In fertilization medium containing CPP, Ca2+ uptake by spermatozoa increased within 90 min of incubation, with a peak of 113 nm/1 x 10(7) cells. On the other hand, in fertilization medium with no CPP, the intracellular Ca2+ concentration remained constant or gradually decreased until 180 min of incubation. When preincubated spermatozoa were coincubated with oocytes for 5 h in a fertilization medium containing CPP and/or EGTA, or with no addition, EGTA decreased the rate of oocytes penetrated by spermatozoa and the number of spermatozoa penetrating each oocyte. On the other hand, when CPP was added to the medium containing EGTA, the inhibitory effect of EGTA was neutralized. These results indicate that CPP promotes Ca2+ uptake of boar spermatozoa, resulting in a more rapid and longer-lasting shift of a subpopulation of spermatozoa that can penetrate oocytes; the findings also suggest that CPP neutralizes the inhibitory effect of EGTA on sperm penetration.