Effects of local anesthetics on contractions of pregnant and non-pregnant rat myometrium in vitro

In order to determine whether local anesthetics directly affect the propagation and strength of myometrial contractions, we compared the effects of bupivacaine, ropivacaine, lidocaine and tetracaine on the contractions of myometrium isolated from pregnant and non-pregnant rats. Full-thickness myometrial strips were obtained from 18-to 21-day pregnant and non-pregnant Sprague-Dawley rats and incubated in an organ bath. When spontaneous contractions became regular, strips were exposed to cumulative concentrations of the four local anesthetics ranging from 0.01 to 300 mu mol/L and the amplitude and frequency of contraction were recorded. All four compounds caused a concentration-dependent inhibition of the contractility of pregnant and non-pregnant uterine muscle. In pregnant myometrium, the concentration that caused 50% inhibition (IC50) was 100 mu mol/L for bupivacaine, 157 mu mol/L for ropivacaine, > 1000 mu mol/L for lidocaine, and 26.3 mu mol/L for tetracaine. In non-pregnant myometrium, the IC50 was 26.9 mu mol/L for bupivacaine, 40 mu mol/L for ropivacaine, 384 mu mol/L for lidocaine, and 7.4 mu mol/L for tetracaine. These results suggested that local anesthetics do inhibit myometrial contractions in pregnant and non-pregnant rats in a concentration-dependent manner.