Development and validation of a radioimmunoassay for prostaglandin E2 metabolite levels in plasma.

Measurements of primary prostaglandins (PGs) in plasma are unreliable, and determinations of their metabolite levels are to be preferred. The principal circulating metabolite of PGE2 [13,14-dihydro-15-keto-PGE2 (PGEM)] is itself unstable however, and this has hindered development of PGE2 metabolite assays. Alkalinization of plasma uniformly converts PGEM and its degradation products to a stable bicyclo compound 11-deoxy-13,14-dihydro-15-keto-11 beta, 16 epsilon-cyclo PGE2 (bicyclo-PGEM). Measurement of bicyclo-PGEM, therefore, enables the PGEM instability problem to be circumvented. This paper describes a specific and sensitive RIA for bicyclo-PGEM in plasma. The assay uses a rabbit antiserum against bicyclo-PGEM and a radiolabel produced by alkalinization of tritiated PGEM. The assay is direct, employing neither chromatographic nor extraction steps. The least detectable mass of bicyclo-PGEM is 1 pg, and the mean mass of added bicyclo-PGEM required to displace zero point binding by 50% is 15 pg. Cross-reactivities of the antiserum against prostanoids are less than 1%. The assay is both accurate and reproducible. Intra- and interassay coefficients of variation are 9.8% and 15.3%, respectively. Plasma concentrations of bicyclo-PGEM in man, rhesus monkey, sheep, and rabbit are reported. This assay allows for reliable measurements of plasma PGE2 metabolites and opens the way for in vivo studies on the significance of PGE2 in health and disease.

Is oxytocin involved in parturition?

Peripheral plasma levels of oxytocin were determined in women during pregnancy and labour. There was a significant increase in maternal plasma oxytocin with advancing gestation but no significant change in levels at any stage of labour. Oxytocin concentrations in umbilical cord plasma were measured after spontaneous vaginal delivery and at elective caesarean section. A significant arteriovenous difference, with higher arterial levels, was found in both groups and the values after labour were significantly higher tha at elective caesarean section. These findings suggest that the fetus is able to produce oxytocin which may play a role in human parturition.

Plasma oxytocin concentrations during the menstrual cycle in the rhesus monkey, Macaca mulatta.

Plasma samples were obtained from 8 conscious rhesus monkeys at 3-4 day intervals throughout the menstrual cycle. Oxytocin concentrations were significantly higher in mid-cycle (Days 10-11) than at Days 4-5 (P < 0.01) and Days 28-29 (P < 0.02, Wilcoxon signed rank test). No significant correlations between oxytocin and oestradiol or progesterone were found.

Concentrations of oxytocin in the plasma and amniotic fluid of rhesus monkeys (Macaca mulatta) during the latter half of pregnancy.

A specific radioimmunoassay for oxytocin has been established with a sensitivity of 0.8 pg/tube. This assay has been applied to the measurement of oxytocin in serial samples of peripheral plasma and amniotic fluid from pregnant rhesus monkeys, collected at weekly intervals by venepuncture and amniocentesis. Concentrations of oxytocin in both fluids were generally low and showed no trends throughout the latter half of gestation.