EFFECTS OF MAGNESIUM ION AND TEMPERATURE ON THE PHASIC AND TONIC CONTRACTIONS OF THE POTASSIUM CONTRACTURE IN THE PREGNANT RAT MYOMETRIUM

Publisher Website
Google Scholar
Abstract
Effects of Mg2+, low temperature and 2,4-dinitrophenol on the K contracture were investigated in pregnant rat uteri. At 36-38.degree. C, the addition of Mg2+ increased the amplitude of tonic contraction when the isotonic K media contained 2.2 mM Ca (potentiating effect), whereas the same procedure decreased the amplitude of the tonic contraction when the media contained 0.22 mM Ca (inhibitory effect). When the muscle strip was loaded with Mg2+ by exposing it previously to a Locke-Ringer solution containing 10 mM Mg2+ for 20-30 min, the potentiating effect of Mg2+ was nearly abolished but the inhibitory effect persisted. The amplitude of the tonic contraction was reduced, and the potentiating effect of the Mg2+ was depressed at lower temperatures. The inhibitory effect persisted at low temperatures. The time course of relaxation of the phasic contraction consisted of 2 exponentials, the initial one fast and succeeding one slow. The effect of Mg on the relaxation during the initial phase was variable, but the succeeding one always became slower when Mg2+ were added. The initial phase of the relaxation became slower at lower temperatures. The succeeding phase became faster. The sites of Mg action were 2, 1 being at the outer surface of the membrane competing for occupation with Ca2+, 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.