Neurohypophysial hormones in fetal life and pregnancy I. Pharmacological studies in the sheep ( Ovis aries)

The changes in the oxytocic, vasopressor and antidiuretic activities of the posterior pituitary of the sheep were followed through development and pregnancy. In the fetus, the oxytocic activity of the lyophilised pituitary was always low. It rose slowly from midterm to term. but the activity (mU/mg dry tissue) never exceeded 10.4% of the adult value; most development took place after birth. In contrast, vasopressor activities were always higher, and reached approximately adult levels close to birth. However, there was a marked reduction (54%) during the neonatal period; this change probably originated during birth. Antidiuretic assays confirmed the changes seen in vasopressor activity. Acetone-dried glands from a different breeding season showed closely similar changes. Vasopressin (V) predominated over oxytocin (O) throughout fetal and neonatal life. However, the high V/O ratios fell steadily with development. Midterm, late-term, term, neonatal and adult ratios were 16.3, 10.7, 5.4, 5.2 and 0.93, respectively, for lyophilised glands, and 15.3, 9.5, 4.6, 3.6 and 0.62 for acetone-dried glands. The close similarity in the two series showed that the high ratios were not artifacts produced by acetone-drying, but true aspects of development. In the pregnant sheep, all the activities in the gland appeared depressed; however, there was a tendency to recover close to term, and this was particularly noticeable for oxytocin. The main results of this study suggested that the vasopressin-producing system was particularly well developed during fetal life.