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The isolated longitudinal or circular muscle strip of rat uterus on day 20 of pregnancy becomes quiescent when incubated with Krebs solution for about 2 hr. Electric stimulation gave rise to a twitch-like contraction with a small amplitude instead of the spontaneous phasic contraction observed during the early period of incubation. When saponin (30-500 micrograms/ml) was applied, the basal tension was elevated and spontaneous contractions having large amplitude were elicited. The amplitude was larger than the phasic contraction of the K-contracture observed prior to the saponin treatment. The membrane was depolarized by about 25 mV, and periodic burst discharge was generated at the onset of saponin treatment. Raising the external Mg concentration from 0 to 2.4 mM in steps caused a depression of contractions in a dose-dependent manner before treatment with saponin, whereas the depression by Mg was much reduced after saponin treatment. The saponin-treated muscle recovered to exhibit control activity when tissues were superfused with Krebs solution for hours, i.e. the membrane was repolarized, the duration of action potential was detracted, and the depressant effect of Mg was again potentiated. For comparison, the effects of saponin and Mg on the contractions of ileal longitudinal muscle of pregnant rat were studied. In view of the above observations, the following were discussed. 1. Saponin impaired the depressant effect of Mg on the generation of contractions. 2. The membrane once impaired by saponin recovered in vitro probably under the influence of the genomic effect of ovarian hormones, a phenomenon which appeared characteristic for pregnant uterus.

Effects of a brief treatment with saponin on the contractile and electrical activities of isolated uterine muscle of pregnant rat.