Elevation of Oxytocin and the Oxytocin-Associated Neurophysin in the Plasma of Normal Women during Midcycle*

Abstract

A positive correlation was observed between the midcycle elevation of estrogen (E) and the level of oxytocin- and estrogen-stimulated neurophysin (ESN), the protein carrier of oxytocin, in the plasma of 5 of 6 women. The time of the maximal level of E was associated with a level of oxytocin significantly greater than that in either the early follicular or late luteal phase (P < 0.025). Likewise, the level of ESN at midcycle was greater than the level of ESN in the early follicular or late luteal phase (P < 0.01). Other than states of lactation or pregnancy, this is the only described cyclic secretion of oxytocin in humans. Since oxytocin chronologically correlates with a rise in the level of E at midcycle, a role for oxytocin in ovulation may be considered.