Tramadol inhibits the contractility of isolated human myometrium

1. This study was conducted to determine whether the atypical opioid analgesic tramadol inhibits the contractility of isolated non-pregnant human myometrium. Ten strips of non-pregnant human myometrium stimulated with 55 mm potassium chloride (KCl) were treated with three concentrations (30, 100 and 300 μm) of tramadol to test for any inhibitory effect of tramadol. The effects of concurrent administration of the ß adrenoceptor antagonist propranolol (1 μm), the guanylyl cyclase and nitric oxide synthase inhibitor methylene blue (20 μm) and the opioid receptor antagonist naloxone (100 μm) with tramadol were also studied.
2. Tramadol caused a concentration-dependent inhibition of KCl-induced myometrial contractility, which was statistically significant at all three concentrations of tramadol used. Propranolol significantly reversed the inhibitory effect of 100 μm tramadol on KCl-induced myometrial contractility but not that of 300 μm tramadol. Neither methylene blue nor naloxone reversed the inhibitory effect of tramadol on KCl-induced myometrial contractility.
3. These results suggest that tramadol inhibits KCl-induced contractility of isolated human myometrium. They also suggest that tramadol relaxes the myometrium due to stimulation of ß₁ adrenoceptors. However, the concentrations of tramadol required to relax the myometrium were high and likely to be attained at toxic doses, rather than therapeutic doses, of tramadol.