Oxytocin in Human Plasma: Correlation with Neurophysin and Stimulation with Estrogen*

Abstract

RIA for the measurement of oxytocin in human plasma is described. Extraction of oxytocin from larger peptides in plasma used acetone precipitation with a 75% +/- 2 SEM recovery of oxytocin. Nonspecific binding of the assay was less than 4%, and the minimum level of detection was 0.2 microunits/tube. No cross-reactivity was noted with neurophysins, arginine, or lysine vasopressin. The mean basal level (+/- SEM) of oxytocin in men was 1.80 +/- 0.07 microunits/ml and was not different in normal women (1.71 +/- 0.07 microunits/ml). Changes in posture had no effect on the levels of oxytocin. Samples obtained every 15 min over 4 h showed no pulsatile secretion of oxytocin. In women chronically receiving estrogen as an oral contraceptive, oxytocin was greater than normal, (4.59 +/- 0.51 microunits/ml; P less than 0.01). Estrogen-stimulated neurophysin was also elevated (8.45 +/- 1.99 ng/ml; P less than 0.005). Acute ingestion of estrogen caused an increase in the level of oxytocin in plasma by 12 h and a concomitant elevation of estrogen-stimulated neurophysin. When the neurophysin was isolated from plasma obtained from a subject after ingestion of estrogen, the neurophysin from plasma comigrated on a polyacrylamide gel with a human pituitary standard of estrogen-stimulated neurophysin. In the studies in which neurophysin was elevated, the correlation between the level of oxytocin and the level of estrogen-stimulated neurophysin in plasma was significant (P less than 0.01). The observation that estrogen administration stimulates the release of oxytocin and estrogen-stimulated neurophysin provides additional evidence that this neurophysin is the oxytocin-neurophysin of man.

Oxytocin is luteolytic in the rhesus monkey (Macaca mulatta)

“…) on Day 6 of the normal primate luteal phase did not alter corpus luteum function, suggesting a local rather than systemic effect of oxytocin. Auletta et al (1984) (Amico, Seif & Robinson, 1981), stimulate PGF-2a release from the corpus luteum-bearing ovary in the monkey (Auletta et al, 1978), and induce oxytocin receptors in the endometrium (Roberts, McCracken, Gavagan & Soloff, 1976;McCracken, 1980). These observations suggest that oestrogens may be the biochemical signal responsible for the synthesis and release of PGF-2a and oxytocin.…”

Osmotic Regulation of Estrogen Receptor-β in Rat Vasopressin and Oxytocin Neurons

“…A role for gonadal steroids in regulation of VP and OT secretion was suggested by early reports of gender differences in plasma VP and stimulation of OT release by estrogen (Skowsky et al, 1979;Amico et al, 1981). However, disparate results from gonadectomy and hormone replacement studies and opposite actions of gonadal steroids in vivo and in vitro made the role of gonadal steroids unclear (Crowley and Amico, 1993;Swenson and Sladek, 1997).…”

Correlation of Oxytocin (OTR) and Estrogen Receptor (ER) mRNA in the Canine Placenta with the Detected Circulating Levels of Oxytocin and Estrogen during Pregnancy and Parturition

“…In humans, plasma OT concentrations remain low throughout the pregnancy and birth. However, estradiol treatment results in increased circulating oxytocin levels (Mitchell et al, 1998;Amico et al, 1981). Klarenbeek et al (2007) demonstrated in dogs that OT concentrations remain low at the end of pregnancy, increasing only during the expulsion stage of vaginal labor.…”