Fetal and adult hemoglobin in the chronically catheterized sheep fetus.

The switch from fetal to adult hemoglobin (Hb) has been investigated in fetal and newborn lambs. In the period between 125 and 145 days gestation, the proportion of fetal cells containing adult Hb increased significantly in catheterized fetuses. This increase paralleled the rise in fetal plasma cortisol under several different experimental conditions. However, infusion of synthetic adrenocorticotropin (ACTH1-24) into the fetus, which increased fetal plasma cortisol concentrations to levels slightly higher than observed at term, did not bring about any increase in the proportion of cells containing adult Hb. Increasing fetal plasma prolactin (PRL) concentration either by infusion of PRL or by stimulating endogenous PRL release with thyrotropin-releasing hormone (TRH) did not influence the time or rate of the switch. The fetal reticulocyte count was significantly higher in blood samples taken at the time of catheterization in fetuses of 103-115 days gestation than at 116-128 days gestation. In one repeatedly sampled fetus of adult type Hb AB, Hb C was observed at the time when the fetus was anemic. At birth the number of cells containing adult Hb was significantly higher in catheterized lambs than in nonoperated controls.