ABSTRACT. Plasma prothrombin levels in newborn hu-mRNA expression in the fetus and adult. We report here a mans are lower than in adults. The same is true of many comparison between mRNA for prothrombin in the fetal and newborn and fetal mammals, including the rabbit. To de-adult rabbit liver. termine if the lower levels are due to less expression of the protein, we have compared mRNA for prothrombin in fetal and adult rabbit liver. Northern blots were hybridized with MATERIALS AND METHODS a cDNA for rabbit prothrombin revealing a single mRNA Collection of animal materials. New Zealand White rabbits of approximately 2 kb in both adult and fetal animals. were used throughout. Blood was drawn from the ear artery of mRNA specific for prothrombin was quantitated by slot adult females and anticoagulated, separated, and stored as deblotting of RNA prepared from adults and fetuses aged 21 scribed previously (10). At appropriate intervals after timed d to term (31 d). Prothrombin-specific mRNA in fetuses matings, pregnant rabbits were anesthetized with i.v. nembutal was greater than 50% of that in adults even when the fetal and the fetuses were delivered by cesarean section. Blood was plasma prothrombin was only 15% of the adult level. This drawn from the jugular vein into 3.8% sodium citrate one part suggests that low plasma levels in the fetuses are not the to nine parts of blood. Plasma was obtained by centrifugation result of less transcription. Examination of liver sections and stored at -70°C until assayed. revealed that the predominant tissue in the fetus is hemaLivers were removed from anesthetized adult female rabbits, topoietic, not hepatic. In the youngest fetuses, less than rapidly cut into approximately 1-cm cubes, frozen immediately 20% of the liver consisted of hepatocytes, yet these fetuses in liquid nitrogen, and then stored at -70°C until use. Fetuses expressed more than 50% of the adult level of prothrombin-of anesthetized pregnant rabbits were delivered by cesarean secspecific mRNA. Thus, transcription of prothrombin mRNA tion and the livers removed, frozen, and stored in a similar may be proceeding at a greater rate in the fetal hepatocyte manner. Seven separate litters were studied, and litter sizes than in the adult, or hematopoietic cells may be expressing ranged from two to six fetuses. Liver and blood were not obtained the protein. We conclude that in fetal rabbit liver, pro-from the same fetus. This was because it was not technically thrombin is expressed at a high level relative to the hepa-possible to collect an unclotted blood sample for prothrombin tocyte content and that the cause of the low plasma levels assay and remove the liver before the fetus died. is posttranscriptional. (Pediatr Res 30: 266-269, 1991) Preparation of rabbit liver sections. Livers were fixed in 4% paraformaldehyde and paraffin-embedded, and sections were cut and stained with hematoxylin and eosin.Plasma prothrombin assay. Biologic assay was by a microtechnique described by Hurlet and Josso (1 1). RIA used an IZ5I...