Role of Carboxy-Extended Forms of Oxytocin in the Rat Uterus in the Process of Parturition1

Uterine tissue concentrations of extended forms of oxytocin were 5- to 30-fold greater than those of oxytocin itself.

• Extended forms of oxytocin increased progressively and significantly through late gestation.
• The ratio of oxytocin to its extended forms remained stable without significant change.
• Antagonists of estrogen or progesterone receptors significantly reduced concentrations of extended forms by over 90% and oxytocin by 50%.
• Extended forms of oxytocin were weak stimulants of uterine contractions and did not significantly affect oxytocin's concentration-response curves.
• Extended forms were two to three orders of magnitude less effective than oxytocin in displacing radiolabeled oxytocin from uterine binding sites.
• Carboxy-extended oxytocin prohormones may be primarily regulated by estrogen and progesterone, serving mainly as substrates for oxytocin synthesis.

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