THE EFFECT OF OXYTOCIN RECEPTOR BLOCKADE ON PARTURITION IN GUINEA-PIGS

The mechanism of the onset of labor is unknown in humans and guinea pi gs. Contrary to most other species, progesterone withdrawal appears no t to precede the onset of labor. To elucidate the role of oxytocin in the onset and maintenance of labor, guinea pigs were fitted with vascu lar catheters, an intraabdominal pressure catheter and an array of ute rine electromyogram electrodes. An oxytocin antagonist (des-Gly(9)-[D- Trp(2),Thr(4),Orn(8)]dC(6)-oxytocin 20 mu g/kg per h, n = 11) or salin e solution (n = 12) was infused starting on day 66 of gestation (term is 69 d). Oxytocin receptor blockade resulted in decreased uterine act ivity and a prolonged expulsive phase (second stage) of labor. Fetal d elivery was delayed and fetal mortality was increased. The onset of th e expulsive phase of labor was delayed but maximum uterine activity oc curred in time together with a timely change in uterine electromyogram activity from a prepartum to a postpartum pattern following an unalte red progressive increase in baseline uterine activity. This indicates that oxytocin is requisite for the normal progress of the first and se cond stage of labor, but has no involvement in the mechanism of the on set and the timing of labor.