The mechanical effects of KCl, oxytocin and endothelin-1 on pregnant rat myometrium were examined using intact strips and beta-escin-treated skinned strips. Myometrial tissues from delivering rats were more sensitive to 10.7 mM K+ compared to mid and late gestation. Maximum contractions induced by K+ were obtained at concentrations of 118 mM at mid and late gestation and during delivery. The maximum amplitude of contractions induced by oxytocin and endothelin-1 compared to the 118 mM K(+)-induced contraction increased during the progress of gestation. Maximum contractions induced by oxytocin and endothelin-1 were greater than those induced by 118 mM K+ at delivery, and maximum contractions by oxytocin were larger than those by endothelin-1 during delivery. In 10 microM nifedipine and Ca(2+)-free (containing 2 mM EGTA) solutions, 118 mM K+ contractions were completely abolished; however, both oxytocin and endothelin-1 produced contractions. In Ca(2+)-free solutions, contractions by oxytocin were larger than those by endothelin-1. In skinned myometrial strips, guanosine 5'-O-thiotriphosphate (GTP, 1 microM-1 mM), guanosine 5'-O-(gamma-thiotriphosphate) (GTP gamma S, 0.1-100 microM) and oxytocin (1 nM-0.1 microM) with 10 microM GTP, but not endothelin-1 with 10 microM GTP increased Ca2+ sensitivity of contractile force.(ABSTRACT TRUNCATED AT 250 WORDS)