The effect of labetalol on contractility of human myometrial preparations.

Because of results in animal experiments which demonstrated a partial beta 2-adrenoceptor activity of labetalol on rat uterine smooth muscle the present study was conducted in human preparations. The following results were obtained: 1. Rhythmic uterine contractions with a defined steady-state amplitude and frequency were elicited spontaneously and after methylergometrine. 2. Labetalol reduced amplitude of contractions dose-dependently after 3 h of incubation. Frequency was unaffected. 3. The tocolytic effect of labetalol is apparent only at high concentrations, above those used in the treatment of hypertension. 4. Neither beta 2-specific adrenoceptor blockade with ICI 118,551 nor alpha-blockade with phentolamine changed amplitude of contraction, either alone or in combination with labetalol. 5. Labetalol has little tocolytic effect on human myometrium in vitro. This effect is unrelated to alpha- or beta-antagonism, but seems to depend on a direct smooth muscle depressant effect. In conclusion, from the present in vitro experiments using human myometrial preparations, it seems unlikely that labetalol would interfere with the normal process of labor when used for the treatment of pregnancy hypertension.