Effects of magnesium on the membrane activity and contraction of the circular muscle of rat myometrium during late pregnancy.

Abstract

Comparative effects of Mg were studied on circular muscle strips of pregnant rat myometrium near term and during delivery. Frequency and amplitude of spontaneous contraction were markedly depressed by adding 0.6 mM Mg to Mg-free Krebs solution in the muscle strips taken in the morning of Days 19 and 22 of pregnancy. Frequency decreased, whereas the amplitude was not depressed by 0.6 mM Mg in muscle strips taken during delivery. Plateau potential dominated in the circular muscle taken in the morning on Days 19 and 22, and it was depressed by Mg, leaving the evoked spike potential nearly unaffected. Burst of spike potentials dominated in the circular muscle during delivery, and 0.6 mM Mg did not affect the burst discharge. In contrast to the variable effects of 0.6 mM Mg on the polarized muscle strips of different pregnant stages, application of Mg (0.6-3.6 mM) caused commonly a potentiation of tonic contraction of K-contracture generated in muscle strips of any pregnant stage. Prolonged perfusion with Mg-free Krebs solution (several hours) of the muscle strip taken in the morning on Day 22 caused a transformation of membrane activity from a plateau dominant type to a spike dominant one. With the same procedure, the depressant effect of Mg on phasic contractions was progressively decreased.

Postpartum changes in the mechanical responses of the circular muscle of rat uterus.

“…In some preparations, the muscle strip underwent a persistent contracture. This feature strongly contrasted to that observed in the prepartum preparation (OSA and OGASAWARA, 1983).…”

Changes in the adrenergic effects and membrane activity of the circular muscle of rat uterus during late pregnancy and postpartum.

“…Rather, these features suggest adrenergic effects tended to be transformed to those of the following days. In a previous report (OsA and OGASAWARA, 1983), it was shown that the contractile inhibition was caused by ap- Vol. 34, No.…”

“…It was recently reported (OsA and OGASAWARA, 1983) that the amplitude of contraction was markedly depressed by adding 0.6 mM Mg to Mg-free Krebs solution to the circular muscle strips taken in the morning on Day 22 of pregnancy, however it was not depressed in the circular muscle strips taken during delivery. The depressant effect of Mg on phasic contraction of the muscle strip taken in the morning on Day 22 was progressively decreased by prolonged perfusion with Mg-free Krebs solution.…”

Changes in Serum Calcium, Magnesium, Cyclic AMP and Monoamine Oxidase Levels during Pregnancy and under Prolonged Ritodrine Treatment for Preterm Labor

“…Moreover, calcium has been known to play a dual role in the electrical activity of uterine smooth muscle of small animals, one being an excitatory role in spike potential [3,4], and the other an inhibitory role in membrane sta bilization [3,5]. In human myometrium, spontaneous contractions were strongly affected by the concentration of external calcium; 2.5 mM ionic calcium in Krebs solu tion was the most efficient concentration and both ex cess and low ionic calcium levels suppressed contraction [6], On the other hand, the effect of external magnesium on spontaneous electrical and mechanical activities of the circular muscle of term pregnant rat uterus was investigated and it was indicated that the contractile depression was largely due to suppression of the plateau potential [7], In pregnant human myometrium, ionic magnesium suppressed spontaneous contractions, espe cially in frequency, by suppressing the gradient of pace maker potential and the duration of each action poten tial [8], As mentioned above, it is obvious that sponta neous contraction of the uterus is regulated by mem brane activity based on ionic currents. Furthermore, in rat myometrium, P-adrenergic action of catecholamine indicated that a hyperpolarization of the membrane due to an increase in membrane conductance [9,10] and an increase in cAMP [11] underlay the depression of spon taneous contraction, in which ionic calcium might play an important role.…”