Length tension relationships in the nonpregnant and pregnant rat uterus and the effect of antiprogestin

Length-tension relationships and the tissue composition of the corpus and the cervix uteri were investigated in a rat model. Four groups of rats were used: nonpregnant (n = 12); day 18 of pregnancy treated with vehicle (n = 8); day 18 of pregnancy treated with the antiprogestin ZK 98 299 (Onapristone(R)) for 19 h (n = 8); and day 22 of pregnancy during spontaneous labour (n = 8). Increased extensibility and maximal contractility in both corpus and cervix uteri were demonstrated with increased gestational age. The collagen concentration was reduced significantly in corporal preparations from pregnant rats compared with those from nonpregnant rats but not in specimens from the cervix. Antiprogestin treatment tended to increase the contractile ability.