Inhibition of nitric oxide synthase activity diminishes the acute effects of relaxin on growth, but not softening, of the cervix in the rat

Relaxin promotes growth and softening of the cervix during pregnancy in the rat. This study examined the hypothesis that nitric oxide (NO) mediates th e effects of relaxin on the rat cervix. To test that hypothesis, Nw-nitro-L -arginine methyl ester (L-NAME) was used to inhibit NO synthase, the enzyme that converts arginine to NO and L-citrulline. Nonpregnant rats were ovari ectomized when they were 78 days old (day 1 of treatment). At ovariectomy e ach animal was fitted with silicon tubing implants containing progesterone (P) and estrogen (E) in doses that provide blood levels similar to those du ring late pregnancy. Rats were assigned to three treatment groups. The cont rol group OPE (n = 6 rats) received 0.5 ml L-NAME vehicle (PBS) sc at 6-h i ntervals from 0600 h on day 7 through 1200 h on day 8 and 0.5 ml relaxin ve hicle (PBS) sc at 0600 and 1200 h on day 8. Group OPER (n = 6 rats) was tre ated in the same way as group OPE, except that 20 mu g porcine relaxin were administered. Group OPERI (n = 7 rats) was treated in the same way as grou p OPER, except that L-NAME was administered at a dose of 100 mg/kg.6 h. Bet ween 1400-1500 h on day 8, the cervices were removed and weighed. Cervical wet weight and extensibility were markedly greater (P < 0.01) in relaxin-tr eated group OPER rats than in group OPE controls. Treatment with L-NAME dim inished relaxin's effects on cervical wet weight, but not cervical extensib ility. Tn conclusion, this study provides evidence that NO contributes to t he acute effects of relaxin on the growth, but not the softening, of the ra t cervix.