Relaxation of isolated human myometrial muscle by β2 -adrenergic receptors but not β1 -adrenergic receptors, ,

Objective: Human myometrium contains both β1 -adrenergic and β2 -adrenergic receptors. This study was designed to assess the importance of each β-adrenergic receptor subtype in relaxation of human myometrial muscle strips. Study Design: Radioligand binding studies were used to establish the presence of each β-adrenergic receptor subtype, whereas highly selective β1 -antagonists and β2 -antagonists were used to assess the contribution of β-adrenergic receptor subtypes to myometrial relaxation after exposure to (–)–isoproterenol. Results: Membranes prepared from myometrium contained 82% ± 4% β2 -adrenergic receptors. After contraction produced by exposure to potassium chloride (35 mmol/L), isoproterenol produced relaxation with half maximal effect at 0.02 μmol/L and a maximal relaxation of 52% ± 3%. β1 -Antagonist CGP-20712A had no significant effect, whereas β2 -antagonist ICI-118551 produced a characteristic rightward shift of the isoproterenol concentration-relaxation relationship. Conclusions: Although both β1 -adrenergic receptors and β2 -adrenergic receptors are present in human myometrial tissue at term, relaxation by nonselective β-agonist isoproterenol is mediated exclusively by β2 -adrenergic receptors. (Am J Obstet Gynecol 1998;179:895-8.)