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Studies of cervical ripening in pregnant rats: effects of various treatments.

The exact mechanisms that regulate cervical softening or ripening during pregnancy are not completely understood. The aim of this study was to estimate the effects of various agents on cervical softening during pregnancy in rats. Cervical resistance was examined after treatment with nitric oxide (NO) donors and inhibitors and different hormonal agents. Cervical resistance was significantly reduced (P< 0.05) in rats treated with the NO donors: sodium nitroprusside, molsidomine and prostaglandin E(2). However, treatments with the NO synthase (NOS) inhibitors N(omega)-nitro-L-arginine methyl ester (L-NAME) and L-N(6)-1-iminoethyl-lysine (L-NIL), or the prostaglandin synthesis inhibitor, indomethacin, significantly increased resistance (P<0.05). The antiprogesterone, onapristone, reduced cervical resistance and its effects were only partially blocked by the progesterone agonist, promegestone. Relaxin reduced cervical resistance and NOS inhibitors partially blocked the effect of relaxin. These studies demonstrate that NO regulates cervical ripening. Relaxin also softens the cervix and may act by stimulating NO synthesis. Progesterone seems important in the control of cervical ripening, but its role appears complex. NO and prostaglandin pathways may independently control ripening by acting in parallel or synergistically.