It has been reported that etomidate has the relaxant effects on vascular, tracheal, and non-pregnant uterine smooth muscle in vitro. The purpose of this study was to investigate the relaxant effects of etomidate on the contraction of the pregnant rat uterine smooth muscle.

Uterine muscle tissues were obtained from pregnant rats (n = 15). The uterine segments were mounted in organ baths filled with Krebs solution. After oxytocin-induced contractile activity had been established, etomidate in incremental concentrations (10^-7 to 10^-3 M) was added cumulatively to the bath, each administered 20 min apart, and resultant changes in contractile activity were continuously recorded. EC₅ (effective concentration of 5% reduction), EC₂₅, EC₅₀, EC₇₅, and EC₉₅ on active tension were calculated using a probit model.

Etomidate (10^-7 to 10^-3 M) induced dose-dependent decreases in amplitude and frequency of uterine contraction. The EC₅₀ of etomidate on active tension were 5.91 × 10^-5 M.

These results demonstrate that etomidate had inhibitory effects on pregnant rat uterine muscle at supraclinical concentration (5.91 × 10^-5 M).