Exclusion of the fetal brain as the main source of rat and human amniotic fluid oxytocin

The effect of early labour, maternal analgesia and fetal acidosis on fetal plasma oxytocin concentrations

OBJECTIVE

To determine the effect of early labour, maternal analgesia and fetal hypoxia on circulating fetal oxytocin concentrations.

DESIGN

Prospective observational study.

SETTING

Delivery suite in a District General Hospital.

SUBJECTS

Fifty women at term who did not require oxytocin administration or more than one form of analgesia. Study groups: vaginal delivery with (1) no analgesia, (2) pethidine, or (3) epidural analgesia. Caesarean section under regional analgesia (4) prior to, and (5) after the onset of labour.

INTERVENTIONS

Samples of blood were collected from the umbilical artery (UA) and umbilical vein (UV) immediately after fetal delivery prior to placental separation or oxytocic administration.

MAIN OUTCOME MEASURES

Plasma oxytocin (OT) concentration, umbilical vein pH, cystine aminopeptidase activity.

RESULTS

The geometric mean UA-OT was significantly greater than UV-OT in all groups and was not altered by pethidine; however, epidural administration increased the UA-UV difference. The UA-UV difference at caesarean section was not significantly altered by the onset of labour. There was no correlation between UV pH and UA-UV plasma oxytocin. Cystine aminopeptidase activity was not detectable in UA and UV plasma.

CONCLUSIONS

Fetal OT production is increased by epidural but not by pethidine analgesia. It is not influenced by the onset of labour or fetal hypoxia.

Amniotic oxytocin and vasopressin in relation to human fetal development and labour

Previous experiments in rats revealed increased amniotic oxytocin (OXT) levels in the course of normal development and increased vasopressin (AVP) levels in retarded fetal growth. In order to see whether similar changes would also occur in human, OXT and AVP levels were determined in amniotic fluid, maternal and fetal plasma before or during labour. Positive correlations were found between umbilical AVP levels on the one hand and gestation length, birth weight, placental weight and brain size on the other. This suggests a maturation process taking place in the AVP-producing system. A marked increase in maternal plasma OXT and umbilical AVP was found during labour. Increased amniotic fluid AVP levels were found in conditions indicative of fetal stress (viz., lower pH, higher PCO2) without any correlation to fetal growth parameters, while increased amniotic OXT levels were correlated with higher PO2. Furthermore, fetal brain size correlated positively with amniotic OXT levels. In conclusion, a rise in amniotic OXT levels seems to reflect normal development of the human fetus, similarly to what has been found in the rat. Increased amniotic AVP levels seem indicative of fetal acidosis.

Circulating neurohypophyseal hormones in anencephalic infants

Human anencephalic infants have always been considered not to have circulating levels of oxytocin or vasopressin. However, this article shows that amniotic oxytocin levels in anencephalic infants without hydramnios fall within the control range. In addition, low levels of both oxytocin and vasopressin are present in the umbilical circulation. These peptides are probably derived from fetal sources other than the fetal brain, for example, the fetal adrenal cortex.

Oxytocin levels in maternal and fetal plasma, amniotic fluid, and neonatal plasma and urine

Oxytocin was measured in maternal and fetal plasma, amniotic fluid and neonatal plasma and urine using a specific radioimmunoassay, following extraction procedures with Florisil. Maternal oxytocin levels rose progressively with advancing gestation, but there were no significant differences between oxytocin levels around the onset of labor. No diurnal rhythm of oxytocin was evident in maternal plasma during the third trimester. Maternal and umbilical plasma oxytocin levels at spontaneous delivery were significantly higher than those at elective cesarean section. Maternal oxytocin levels in four cases of post-term delivery were lower than those during normal late pregnancy; all four cases experienced uterine inertia. All amniotic fluid samples had detectable oxytocin levels and there were no significant differences between oxytocin levels in the second trimester and those in the third trimester. Oxytocin levels in neonatal urine were higher than levels in amniotic fluid and lower than in the umbilical artery. Neonatal plasma oxytocin levels gradually decreased and oxytocin levels of 7-day-old infants were significantly lower than those in the umbilical artery, but higher than those in adults. In conclusion, it appears that maternal oxytocin levels may not be involved in triggering the onset of labor but may play a role in the maintenance and reinforcement of labor.

Localisation of oxytocin, vasopressin and parts of precursors in the human neonatal adrenal

Being a possible alternative source for the production of vasopressin (AVP) and oxytocin (OXT), a study was undertaken of the fetal adrenal. The concentrations of these peptides within the fetal adrenal turned out to be low, viz., approx. 1 pg/mg in the rat and within the pg/g range in the human. Immunocytochemistry was performed either on conventional autopsy material kept till 12 years in paraffin blocks, or on more recently obtained formalin or glutaraldehyde-paraformaldehyde fixed material. In both types of material staining was good. In order to localize AVP cells, anti-AVP, an antibody against its associated neurophysin (anti-NSN) or an antibody raised against the c-terminal glycopeptide part of the AVP precursor (anti-GP) was used. OXT cells were localized by means of anti-OXT or an auto-antibody of a multiple sclerosis patient (auto-MS) probably recognizing OXT-neurophysin. The antibodies were characterized on human and rat brain material. In the external zone of the definitive cortex, apart from parenchyma cells, anti-AVP, anti-NSN and anti-GP stained fibre-like structures running in the connective tissue septa and around parenchyma cells and the cytoplasma of these cells. Anti-OXT and auto-MS stained droplets in the cytoplasm of the fetal zone cells. Similar distinct staining patterns for AVP and OXT cells were obtained in human anencephalics. These observations show that the peptides are not derived from the fetal brain, but are rather produced in the fetal adrenal cortex.(ABSTRACT TRUNCATED AT 250 WORDS)