Investigation Of The Myometrial Stimulatory Effect Of Amide Anaesthetics

Abstract

Obstetric anaesthesia is intended to provide optimal pain relief with reduced toxicity. Therefore an untoward effect on uterine contractility becomes of utmost importance. This study is aimed at examining the direct effects of local anaesthetics on uterine contractions. Lidocaine (0.05 -2.5 mM), bupivacaine (0.005 – 0.5 mM), procaine (0.1 – 1.0 mM), and benzocaine (0.05 – 500 μM) were tested on isolated non-pregnant uterine segments and compared to oxytocin (2.0 – 27 nM). The effects of lidocaine, bupivacaine, and oxytocin were further examined in calcium-free medium and in the presence of nifedipine (1 μM). Lidocaine and bupivacaine increased uterine contractions while procaine and benzocaine produced no significant increase in uterine contractility. Contractions induced by lidocaine and bupivacaine in calcium-free medium were completely abolished allowing only higher concentrations of oxytocin to elicit contraction. Nifedipine also completely abolished contractions induced by lidocaine, bupivacaine and oxytocin. It was also observed that the amide and amine groups are structural motifs that may be necessary for calcium channel interactions. These groups were found present in the amide anaesthetics used in this study. In conclusion, amide anaesthetics in contrast to the ester anaesthetics have been found in this study to stimulate myometrial contractility possibly through interaction with intracellular Ca²⁺ channels.

 Keywords: Amide anaesthetics; Uterus; Lidocaine; Bupivacaine; Calcium; Oxytocin; Procaine; Benzocaine