Abstract
Acta Obstet Gynecol Scand 2002; 81: 825-830. © Acta Obstet Gynecol Scand 2002 Background. Magnesium sulfate, isradipine, and ritodrine are smooth muscle relaxants used for treating preterm labor. The aim of this study is to investigate the action profile of these drugs on the amplitude and frequency of spontaneous contractions of isolated myometrial strips of pregnant human and rat. Material and methods. Isolated myometrial strips were obtained from humans undergoing elective cesarean section at term pregnancy and Wistar albino rats on gestational days 19-21. These strips were mounted in organ baths for recording of isometric tensions. The effect of magnesium sulfate, isradipine, and ritodrine on the amplitude and frequency of spontaneous contractions was compared with theanova test. Results. Ritodrine (10 −8 −10 −5 M) concentration-dependently inhibited the frequency and amplitude of spontaneous contractions of myometrial strips. At 10 −4 M, tachyphylaxis of ritodrine occurred and contractions started again. Magnesium sulfate (10 −7 −10 −4 M) inhibited the frequency but did not change the amplitude of the spontaneous contractions. Isradipine (10 −7 −10 −4 M) had a concentration-related inhibitor effect on both the frequency and amplitude of the spontaneous contractions. The effects of magnesium sulfate, isradipine, and ritodrine were considerably similar in myometrium strips obtained from pregnant rats and humans. Conclusions. Because of tachyphylaxis, the effectiveness of ritodrine is limited for inhibition of myometrial contractions. Magnesium sulfate reduced the frequency of spontaneous contractions without affecting the amplitude. Isradipine may be an interesting therapeutic alternative in situations where inhibition of uterine activity is desirable.