Mouse uterine muscle as a model for studying beta 2-adrenoreceptor mediated relaxation

Effects of isoprenaline, adrenaline and noradrenaline on contractility were studied in longitudinal strips of mouse and rat uteri in vitro. In mouse uteri, isoprenaline produced an inhibitory effect on spontaneous contractions which was antagonized by propranolol, a nonselective beta-blocker, but not by practolol, a selective beta 1-blocker. Thus, the inhibitory effect of isoprenaline is mediated by beta 2-adrenoreceptor activation. Adrenaline and noradrenaline also inhibited the spontaneous contractions of mouse uteri. In rat uteri, isoprenaline produced only an inhibitory effect whereas adrenaline initially inhibited the force and frequency of contraction but also had a secondary excitatory effect on the frequency which could be antagonized by phentolamine. Noradrenaline had an inhibitory effect on force but had only a negligible effect on frequency of rat uteri. Phenylephrine also reduced the spontaneous contractility of mouse uteri by a beta 2-mediated effect. From results generated in this study, and from the literature, it is concluded that beta 2-adrenoreceptors are predominant in nonpregnant uteri of rodents, especially the mouse, while alpha-adrenoreceptors are predominant in nonpregnant uteri of rabbit, monkey and human.