Influence of magnesium on the beta-inhibition of catecholamines in the uterine circular muscle of estrogen-treated rats.

The effects of Mg and isoprenaline on the uterine circular muscle of ovariectomized rats which were injected with estradiol for 2--6 days were investigated. The muscle strip exhibited an action potential that was composed of a spike and a plateau, and also exhibited a twitch-like contraction due to electrical stimulation. The duration of the action potential became longer when the treatment with estrogen was applied until the maximal effect was built up in 4--5 days. Both the duration of the action potential and the amplitude of contraction were reduced when isoprenaline was applied. When Mg (0.5 mM) was added to the bathing solution these beta-effects were progressively potentiated and the maximal effect was obtained in about 1 hr. An increase in membrane conductance was induced by the application of isoprenaline in a control Locke solution and it was further accelerated by exposure to Mg-containing solution. The possibility that the beta-action is potentiated by Mg ions which penetrated into the membrane is concluded.