Effects of Magnesium Ion and Temperature on the Phasic and Tonic Contractions of the Potassium Contracture in the Pregnant Rat Myometrium

Effects of Mg ion, low temperature and 2,4-dinitrophenol on the Kcontracture were investigated in pregnant rat uteri. At 36-38degrees C, the addition of Mg ions increased the amplitude of tonic contraction when the isotonic K media contained 2.2 mM Ca (inhibitory effect). When the muscle strip was loaded with Mg ion by exposing it previously to a Locke-Ringer solution containing 1as nearly abolished but the inhibitory effect persisted. The amplitude of the tonic contraction was reduced, and the potentiating effect of the Mg ion was depressed at lower temperatures. The inhibitory effect persisted at low temperatures. The time course of relaxation of the phasic contraction consisted of two exponentials, the initial one fast and succeeding one slow. The effect of Mg on the relaxation during the initial phase was variable, butthe succeeding one always became slower when Mg ions were added. The initial phase of the relaxation became slower at lower temperatures. However, the succeding phase became faster. The above experimental results were interpreted to suggest that the sites of Mg action were two, one being at the outer surface of the membrane competing for occupation with Ca ions, and the other within or at the inner surface of the membrane regulating the amount of bound Ca which is in turn utilized to evoke contraction. The latter is probably mediated by metabolic processes.