Intravenous hypnotics are used in pregnancy, labor and delivery. The aim of the present study was to investigate and compare the relaxant effects of propofol, thiopental, ketamine, and etomidate on isolated rat uterine smooth muscles.

Uterine smooth muscle preparations were obtained from non-pregnant female rats. The uterus of the rat was dissected and cut into 10 mm strips. The muscle strips were bathed in Krebs solution. After spontaneous uterine contractile activity had been accomplished, propofol, ketamine, thiopental, and etomidate in various concentrations were added cumulatively to the baths and resting tension, active tension, and frequency of contration were recorded at each concentration of agents. EC₅, EC₂₅, EC₅₀, EC₇₅, and EC₉₅ of each drug on active tension and frequency of contraction were calculated using a probit model.

Propofol, thiopental, and etomidate reduced uterine contractions in a concentration-dependent manner. Ketamine concentrations of 10^-7 to 10^-5 M augmented uterine contractions but ketamine concentrations of 10^-4 to 10^-3 M attenuated uterine contractions. The EC₅₀'s of propofol, thiopental, ketamine, and etomidate on active tension were 1.56 × 10^-5 M, 4.97 × 10^-5 M, 3.52 × 10^-4 M, and 2.73 × 10^-5 M, respectively.

All four intravenous hypnotics relaxed the uterine smooth muscle of rats except for ketamine in low concentrations (10^-7 to 10^-5 M). Propofol had the greatest relaxant effects on isolated rat uterine smooth muscle among these hypnotics. It seems that ketamine is a suitable obstetric hypnotic agent for hypovolemic parturients and propofol is a useful hypnotic agent for uterine relaxation during pregnancy.