Effects of Magnesium and Catecholamines on Spontaneous Contraction of Pregnant Human Isthmic Myometrium

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Abstract
The spontaneous isometric contraction of human myometrium was recorded with regard to the stage of pregnancy, the run of muscle bundles and the resected part of the muscle. Then, the effects of noradrenaline, terbutaline or magnesium and the relationship between β‐inhibition and Mg2+ were also studied. Electrical activities were recorded by the single sucrose‐gap method.Maximum tension, frequency and half‐duration of contractions were not significantly different among all specimens. Maximum tension was smaller than longitudinal muscle and larger than circular muscle of the term‐pregnant rat.Noradrenaline (10‐12 ‐ 10‐6 g/ml) exhibited α‐excitation and terbutaline (10‐8 ‐ 10‐6 g/ml) caused β‐inhibition in a dose dependent manner. Magnesium inhibited spontaneous contraction and decreased the gradient of pacemaker potential and the duration of each action potential. Furthermore, β‐inhibition of terbutaline was progressively potentiated with the increase in external Mg2+ concentration, and Mg2+ rich media shifted the dose‐response curve of terbutaline to the left.The results indicate that human myometrium might be composed of plural muscle bundles with different characteristics and have both α‐ and β‐ adrenoceptors (α‐predominant); they also suggest that Mg2+ inhibits spontaneous contraciton by affecting membrane activities and that it enhances the effect of terbutaline.

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