Neurohypophyseal peptides in the developing rat fetus.

The quantitative changes in the content of the neurohypophyseal peptides, neurophysins, oxytocin, and vasopressin, were determined in the developing rat fetus. No neurohypophyseal peptides were found in 12-day fetuses. Neurophysins were first detected at day 13 and increased dramatically on day 14. The hormones vasopressin and oxytocin were not detected at day 13 and were measured at low levels at day 14. A 350-fold molar excess of neurophysin to hormones existed at day 14. From day 14 to day 19 the total content of neurophysin decreased while the content of vasopressin and oxytocin slowly increased. At day 19 there was a near molar equivalency between the content of neurophysin and that of the neurohypophyseal hormones. From day 18 to day 22 there was a sharp increase in the content of vasopressin while the content of neurophysin and oxytocin increased less dramatically. At term there was a molar excess of vasopressin, and the molar ratio of neurophysin to hormone at the time of delivery was 0.12. Measurement of vasopressin by different radioimmunoassays and by bioassay indicated no contribution of arginine-vasotocin to the measured vasopressin.