The posterior pituitary gland

Plasma oxytocin in human pregnancy and parturition

Oxytocin concentrations were determined in serial peripheral plasma samples collected from clinically normal women during pregnancy and labor. Measurable concentrations of this hormone were detected in all maternal plasma samples during pregnancy, but there were wide differences in values between patients. Serial samples from individual patients revealed a pattern of gradual rise of oxytocin levels with advancing gestation and the increase in concentration was statistically significant. There were no significant differences in oxytocin levels at any stage of labor, with or without epidural analgesia. Oxytocin levels at the onset of the second stage did not differ statistically from those at crowning. Comparison of cross-sectional data showed no significant difference between the mean oxytocin concentration in early labor and in late pregnancy. Oxytocin surges occurred, but not in a regular pattern. Plasma oxytocin concentration did not increase after pelvic examination, sweeping of the membranes, low amniotomy or after cervical vibration. After spontaneous vaginal delivery, umbilical arterial plasma levels of oxytocin were consistently higher than plasma concentrations from the umbilical vein. The fetal arterio-venous difference was less pronounced at elective cesarean section. At spontaneous vaginal delivery, with and without epidural anesthesia, plasma levels from the umbilical artery were significantly higher than the maternal levels. After vaginal delivery, oxytocin levels in cord plasma were significantly higher than at elective abdominal delivery. Some methodological aspects with regard to blood sampling and to plasma oxytocin radioimmunoassay procedures are discussed. From the results presented it is concluded that the human fetus can be an important source of oxytocin and that neurohumoral birth reflexes described in animals do not occur systematically in man.

Individual neurophysin concentrations in the pituitary and circulation of humans

Specific, homologous human neurophysin I and II radioimmunoassays were established and used to measure the individual, immunoreactive neurophysin concentrations in human plasma. Circulating levels of human neurophysin I in normal individuals were less than 1 ng/ml and neurophysin II levels were 1-2 ng/ml. During dehydration, there was a significant rise in plasma neurophysin I, together with an increase in neurophysin II. Haemorrhage also was associated with a rise in plasma neurophysin I and II, but the percent increase was greater for I than II. In two subjects in whom nicotine inhalation caused a rise in plasma neurophysin I, there was no detectable increase in plasma neurophysin II. These stimuli which have been reported to release vasopressin from the posterior pituitary also are associated with the differential release of neurophysin I. Plasma neurophysin II levels could more clearly be shown to rise independently of plasma neurophysin I during events thought to be related to oxytocin release. Plasma neurophysin II levels were significantly elevated in women taking oral contraceptives. Similarly during pregnancy there was a progressive rise in plasma neurophysin II concentration which was proportional to the period of gestation. Plasma neurophysin II concentrations in seven of fifteen nursing women rose significantly during suckling. There was no detectable change in plasma neurophysin I concentrations during any of these events. Plasma neurophysin I and II levels were both significantly elevated in fourteen patients with chronic renal failure and rose over haemodialysis, suggesting that the kidney may be the major route of clearance of the neurophysins. In humans the independent release of neurophysin II was associated with stimuli thought to release oxytocin, but neurophysin I showed only a differential release compared to neurophysin II in vasopressin stimulated events.