Effect of progesterone on the post partum rat uterus

The reactivity of the uterus to estradiol (E2) and progesterone (P) has been examined in 44 post partum rats. Hormone treatment was initiated after surgical delivery at approximately 24 hours before the expected onset of labor in 10 animals and immediately after spontaneous labor in 34 rats. At the time of steroid treatment each animal was equipped with a pressure sensor for the sequential recording of intrauterine pressure (IUP) before and after the oxytocin test. Daily measurements during one week showed that post partum the IUP increases significantly in all animals not treated with P. The combined administration of E2 + P significantly reduces the maximum IUP (IUPm) if treatment is initiated approximately 24 hours before the spontaneous onset of labor. P treatment alone, initiated after spontaneous delivery, is ineffective, but it suppresses IUPm significantly if given in combination with E2 and during 3 days or longer. Apparently, shortly before and during labor the reactivity of the rat uterus to P changes markedly, a process which is inadequately understood.