Passage of bovine serum albumin from the mother to rabbit blastocysts

Dutch belted rabbits were given single intravenous injections of 100 or 200 mg/kg doses of bovine serum albumin (BSA). BSA in serum and uterine fluid at various times after injection was estimated by a quantitative radial immunodiffusion test, which could measure a minimum of 40 ng. The presence of BSA in uterine fluid was confirmed by immunoelectrophoresis and double diffusion in agar. BSA passes readily into uterine fluid of non-pregnant rabbits, reaching a peak at 12 h after injection, when its concentration is 7–15% of that in serum. About 72 h seems to be required for equilibration of BSA between serum and uterine fluid, at which time the concentration in the former is about 5 times that in the latter. The kinetics of the process is discussed. Compared to the above, passage of BSA into uterine fluid of pregnant rabbits (5–7 days post coitum ) is restricted in the following ways. Significant amounts of BSA appear in the fluid only after a maternal dose of 200 mg/kg. BSA in uterine fluid reaches a peak at 24 h after injection, when it is only 4·5% of the serum level. The permeability rate seems to decrease with early gestation. Approximate rates of entry of BSA into uterine lumen of non-pregnant and pregnant rabbits are 0·4 and 0·25 μg/h. BSA seems to be treated like rabbit albumin in its passage across the uterine epithelium. There is no evidence of selection between these proteins.