[Changes of the contraction of the circular and longitudinal muscles of pregnant rat myometrium during pregnancy (author's transl)].

The different reactions caused by 4 drugs (terbutaline sulphate, noradrenaline, PGF2 alpha and oxytocin) and Ca on the longitudinal and circular muscles of a pregnant rat myometrium as well as their electrical activity and the degree of their co-ordinated contractions during pregnancy were studied in vitro. The results were as follows; 1. The circular muscle showed plateau potential in the middle, but spike potential became dominant in the late stage. The longitudinal muscle, however, showed spike potential throughout. 2. Well co-ordinated contraction between 1) the neighboring longitudinal muscles, 2) the neighboring circular muscles, and 3) the longitudinal and circular muscles were evident in the late stage. 3. The circular muscle was stimulated by noradrenaline in the middle stage, but inhibited by it in the late stage, whereas the longitudinal muscle was inhibited throughout. 4. The circular muscle developed high sensitivity to terbutaline sulphate in the late stage, whereas the longitudinal muscle showed constant sensitivity throughout. 5. PGF2 alpha failed to cause contracture of the circular muscle in the late stage. 6. The high resistance of the circular muscle to low Ca in the middle stage weakened in the late stage, whereas the longitudinal muscle resistance was low in both stages. In conclusion, in the final stage of pregnancy, the action of the circular muscles resembles that of the longitudinal ones, however, the contracture of the circular muscles is difficult to notice. For the purpose of the expulsion of the uterine contents, the longitudinal muscle may play main role. While the longitudinal muscle is in the contracture state, the circular muscle may show repeated coordinated contractions, thereby assisting the expulsion of the contents.