The effects of opioids, local anaesthetics and adjuvants on isolated pregnant rat uterine muscles

Jul 5, 2007Methods and findings in experimental and clinical pharmacology

How opioids, local anesthetics, and additives affect isolated pregnant rat uterus muscles

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Abstract

Alfentanyl, meperidine, remifentanyl, bupivacaine, ropivacaine, and midazolam significantly decreased uterine muscle contractions induced by oxytocin.

Alfentanyl, meperidine, remifentanyl, bupivacaine, ropivacaine, and midazolam were shown to significantly reduce the contractile responses of isolated pregnant rat uterine muscle.
Mepivacaine increased oxytocin-induced contractions by 33.1% ± 7.2%, indicating a unique effect compared to other agents tested.
Clonidine did not have a significant effect on uterine contractions in this study.
The sensitivity of uterine muscle to the tested local anesthetics and opioids did not show significant differences.
These findings suggest that certain local anesthetics, opioids, and adjuvants may specifically alter uterine contractility, which could be relevant for managing labor pain.

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Local anaesthetics, opioids and adjuvants are often used for managing labor pain. Some others of these agents are reported to cause alterations on uterine contractility during labor. However, there are controversies and the effects of some others are unknown. In the present study, we aimed to elucidate the effects of opioids such as alfentanyl, meperidine, remifentanyl; local anesthetics such as mepivacaine, ropivacaine, bupivacaine; and adjuvants such as clonidine and midazolam on isolated pregnant rat uterine muscle. Strips of longitudinal uterine smooth muscle obtained from rats pregnant for 18-21 days were suspended in 20 ml organ baths. Isometric tension was continuously measured with an isometric force transducer connected to a computer-based data acquisition system. The effects of cumulative concentrations of alfentanyl, meperidine, remifentanyl, mepivacaine, ropivacaine, bupivacaine, clonidine and midazolam (10(-8) - 10(-4) M, for all) on contractions induced by oxytocin (1 mU/ml) were studied. Alfentanyl (10(-5) M), meperidine (10(-5) M), remifentanyl (10(-4) M), bupivacaine (10(-4) M), ropivacaine (10(-4) M) and midazolam (3 x 10(-5) M) caused significant decreases in contractile responses of uterine strips to oxytocin. Contrastingly, mepivacaine increased (33.1% +/- 7.2%) oxytocin-induced contractions of uterine strips while clonidine exerted no significant effect. The sensitivity of myometrial preparations to tested local anesthetics or opioids did not differ significantly. The findings of the present study demonstrated that some local anesthetics, opioids and adjuvants caused significant and agent-specific alterations on contractility of the pregnant rat myometrium. Therefore, they seemed to have a potential to influence uterine contractility during clinical management of pain during labor. However, further research is needed to extrapolate these finding to clinical practice.

The effects of opioids, local anaesthetics and adjuvants on isolated pregnant rat uterine muscles

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