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Smoking Enhances Oxytocin-Induced Rhythmic Myometrial Contraction¹

Department of Obstetrics and Gynecology, Kansai Medical University, Moriguchi, Osaka, 570-0074, Japan

Although smoking during pregnancy is one of the major risk factors of premature delivery, the underlying mechanism by which smoking causes premature delivery is unknown. In the present study, we examined the effects of smoking on uterine contractility induced by oxytocin and prostaglandin F₂. Rats inhaled either cigarette smoke or room air from Day 14 to Day 16 of pregnancy through an inhalation apparatus for experimental animals (type "Hamburg II"). After the rats were killed on Day 17 of pregnancy, the uterine contractile sensitivity and activity on exposure to oxytocin or prostaglandin F₂ were investigated. The expression levels of oxytocin-receptor mRNA and prostaglandin F₂ receptor mRNA in the uterus were investigated by reverse transcription-polymerase chain reaction. The contractile activity was assessed as the contractile force and the frequency of rhythmic contractions of myometrial strips that were treated with oxytocin or prostaglandin F₂. The contractile sensitivity to oxytocin was significantly higher in the smoking group than in the control group (P < 0.01). Although the contractile force of oxytocin-induced contractions did not differ between the smoking and control groups, the frequency of contractions was significantly higher in the smoking group than in the control group (P < 0.01). On the other hand, no significant differences were found in the contractile sensitivity and activity in response to prostaglandin F₂ between the smoking and control groups. The expression of oxytocin-receptor mRNA in the myometrium was significantly increased in the smoking group compared with the control group (P < 0.01). However, no significant difference was found in the level of expression of prostaglandin F₂-receptor mRNA between the two groups. These results suggest that smoking during pregnancy increases the contractile sensitivity and activity of the myometrium in response to oxytocin by up-regulating the expression of oxytocin-receptor mRNA. The effects of smoking on the contractile sensitivity and activity of the myometrium in response to oxytocin may increase the risk of premature delivery in smokers.

¹ Supported in part by grants from the Japan Smoking Research Foundation.

² Correspondence: Katsuhiko Yasuda, Department of Obstetrics and Gynecology, Kansai Medical University, 10-15 Fumizono-cho, Moriguchi, Osaka, 570-0074, Japan. FAX: 81 6 6992 3438; yasuda{at}takii.kmu.ac.jp

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