Inhibition of uterine contractility in labor

Uterine contractility was recorded by the direct electronic method in 4 groups of 12 women in labor, in order to study the inhibitory effect of: (1) pregnenolone sulfate, precursor of placental progesterone, expected to block the sensitivity of the myometrial cell; (2) ethanol, known to inhibit the release of oxytocin; (3) rapid infusion of 1,000 ml. 5 per cent dextrose, used as the vehicle; and to compare these effects with that of all these factors, acting concomitantly through different mechanisms. Pregnenolone sulfate depressed the frequency of contractions to 67 per cent ± 5.7∗ and their activity to 75 per cent ± 8.0. The effect was immediate but transient. The effect of ethanol was delayed but prolonged and more marked (62 per cent ± 6.9 and 69 per cent ± 1.38, respectively). Rapid infusion of 5 per cent dextrose had a slight but prolonged inhibitory effect (93 per cent ± 5.7 and 83.1 per cent ± 4.1), attributed to the Henry-Gauer reflex which blocked oxytocin release. Neither of these treatments affected the intensity of contractions. The most marked synergistic effect was obtained by the combined pregnenolone-ethanol-dextrose treatment. It was immediate like pregnenolone and prolonged like ethanol; it inhibited not only the frequency (63 per cent ± 7.0) and activity (45 per cent ± 6.3) but also the intensity of uterine contractions (72 per cent ±8.6), although the latter effect tended to recover rapidly. Inhibited uterine activity was related to the elevated plasma progesterone level only during the combined treatment, not during other treatments, when inhibited uterine contractility was related to elevated plasma pregnenolone or ethanol levels, while the progesterone level remained unchanged.