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.