Cervical ripening in guinea-pigs after a local application of nitric oxide

In humans cervical ripening is an inflammatory reaction accompanied by an infiltration of white blood cells and the remodelling of the extracellular matrix. Similar changes occur in guinea-pigs during cervical ripening. Nitric oxide (NO) is thought to be important in the maintenance of pregnancy because it is synthesized by the uterus and inhibits contractility. Previous studies in rats also demonstrated that an NO generating system is present in the cervix and is up-regulated during labour. We studied the local effect of the NO donor sodium nitroprusside (SNP) on cervical ripening in guinea-pigs during advanced pregnancy. SNP (5 mg/injection) was administered into the cervical canal in 0.2 mi phosphate-buffered saline containing 3% hydroxycellulose twice a day either for 1 [on day 42 post coitum (p.c.)] or 2 consecutive days (days 42-43 p.c.; term day 67+3 p.c.). The effects were assessed 24 h after treatment by both extensibility measurements (force resistance to incremental stretch) and morphological evaluation (light and electron microscopy after in-situ fixation). The control animals were treated with the vehicle. In another experiment, the guinea-pigs were subcutaneously (s.c.) treated on day 43 p.c. with either the progesterone antagonist onapristone (3 and 10 mg/animal s.c.) or with prostaglandin E-2 (PGE(2)) (1 and 3 mg/animal s.c.) and the PGE(2) analogue sulprostone (0.03 and 0.1 mg/animal s.c.). The cervical extensibility was measured 24 h later. One-day SNP treatment tended to reduce cervical resistance, but not significantly, whereas 2 day treatment with SNP led to a significant increase in cervical extensibility (P < 0.05) with little effect on cervical dilatation (indirect evidence of the absence of uterine contractions). The effects on cervical resistance were comparable to those achieved with 10 mg onapristone and high-dose prostaglandins (PG)s (3 mg PGE(2) and 0.1 mg sulprostone) treatment. An electron microscope study of the SNP-treated animals revealed a dissolution of collagen fibres, stromal oedema, arterial dilatation, and the infiltration of macrophages, lymphocytes and granulocytes. Numerous mast cells were also present. The morphological effects of SNP were similar to those observed during normal cervical ripening at term. We conclude that the local application of a NO donor effectively induces cervical ripening without inducing labour in pregnant guinea-pigs.