Oxytocics reverse the tocolytic effect of glyceryl trinitrate on the human uterus

Gestational and Hormonal Effects on Magnesium Sulfate's Ability to Inhibit Mouse Uterine Contractility

Magnesium sulfate is used as a tocolytic, but clinical efficacy has been seriously questioned. Our objective was to use controlled ex vivo conditions and known pregnancy stages, to investigate how 2 key factors, hormones and gestation, affect magnesium's tocolytic ability. We hypothesized that these factors could underlie the varying clinical findings around magnesium's efficacy. Myometrial strips were obtained from nonpregnant (n = 10), mid-pregnant (n = 12), and term-pregnant (n = 11) mouse uterus. The strips were mounted in organ baths superfused with oxygenated physiological saline at pH 7.4 and 37 °C. The effect of different concentrations of MgSO₄ (2-20 mM) was examined on spontaneous and oxytocin-induced (0.5-1 nM) contractions. Contractile properties (amplitude, frequency, and area under the curve) were measured before and after application of magnesium. Magnesium sulfate had a dose-dependent inhibitory effect on both spontaneous and oxytocin-induced contractions but was less effective in the presence of oxytocin. In spontaneous contractions, magnesium was more potent as gestation progressed (P < .0001). In the presence of oxytocin, however, there were no significant gestational differences in its effects on contraction. The rapid onset and reversal of magnesium's effects suggest an extracellular action on calcium entry. Taken together, we conclude that magnesium's actions are influenced by both gestational state and hormones, such that, at least in mice, it is least effective in early gestation with oxytocin present and most effective at term in the absence of oxytocin. That magnesium is least effective preterm and oxytocin decreases its effectiveness throughout gestation, may explain its disappointing clinical effects as a tocolytic.

Investigation of uterotonic properties of Ananas comosus extracts

ETHNOPHARMACOLOGICAL RELEVANCE

In folklore medicine Ananas comosus (pineapple) is reputed to act as an abortifacient and in expectant women as a means of inducing labor. Several reports have claimed abortifacient property of A. comosus fruit (ripe or unripe). Ripe fruit has been used orally as traditional medicine in inducing abortion in Kerala state of India while the juice of unripe fruit was used for abortion in Bangladesh. However, scientific evidence supporting the efficacy of pineapple extracts in inducing uterine contractions is clearly lacking.

AIM OF THE STUDY

This study investigated the pharmacological effects of different fractions of pineapple extract with a range of maturities to identify the most potent uterotonic fraction.

MATERIALS AND METHODS

The ethanolic crude extracts of pineapple (edible part) were prepared and fractionated through a series of liquid-liquid partitions. Fractions were separately tested on isolated uterine muscle from pregnant SD rats and human pregnant myometrium, which were cut into strips along the longitudinal axis of uterus. The strips were mounted vertically in organ baths (37°C) and exposed to cumulative addition of fractions (0.1-10mgml^-1), serotonin (0.05-5µM) and different inhibitors to delineate the mechanism of action of the active ingredients of the extract.

RESULTS

Aqueous fraction (F4) possesses uterine stimulant property which was blocked by verapamil but unaffected by indomethacin, prazosin and atosiban. Notably, ketanserin (10µM) diminished the maximal contractile response induced by both F4 and 5HT by 74.3% and 92.1% respectively.

CONCLUSIONS

These results may indicate the presence of 5HT or 5HT-like compound(s) and serotonergic pathways may contribute to the uterotonic activity of pineapple extract.