Effects of Mg-load on the contraction of uterus in sponteneously diabetic mice

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First, we investigated effects of external magnesium (Mg) on the contractile tension of uterus under the condition of Mg-unload, using spontaneously diabetic mice (KK/tA9 and the non-diabetic mice (c57/BL). Both KK/Ta and C57/BL mice were divided into the folloowing three groups : a group fed with normal diet (N-group) ; a group fed with normal diet, accompanied by pohysical exercice (whirl cage running)(T-group) ; a group fed with restricted diet (a half of normal volume), accompanied by the physical exercice (TD-group). External Mg decreased the amplitude and frequency of the contractile tension of uttlerus in a concentration dependent manner. IC_<50> of Mg in the amplitude and frequency of the tension in both mice was the smallest in N-group, followed by T-group and TD-group in the order. This tendency was more eminent in diabetic mice. Secondly, the inhibitive effects of external Mg on the contractile tension of utterus were studied, using Mg-load mice, and compared with Mg-unload mice. Mg-load was done by 1. p. injection of 2M MgCl_2 for 2 to 3 weeks. In Mg-load KK/Ta mice, serum Mg concentration was significantly higher than in Mg-load C57/Bl, whereas uterus Mg concentration was almost same in both mice. In Mg-unload mice, C57/BL uterus was more sensitive to external Mg than KK/Ta uterus. In Mg-load mice, however, the difference between KK/Ta and C57/BL disappeared, suggesting the relation between the contractility and the Mg concentration in the tissue. In oxytocin-induced contracture, Mg-load suppressed the amplitude of the contracture and significantly shortened the time to the peak contracture. These resyults strogly suggested that Mg may affect the affinity between oxytocin and the receptors, and the density of the receptors.