Inhibition of Nitric Oxide Synthase Activity Diminishes the Acute Effects of Relaxin on Growth, But Not Softening, of the Cervix in the Rat

Abstract

Relaxin promotes growth and softening of the cervix during pregnancy in the rat. This study examined the hypothesis that nitric oxide (NO) mediates the effects of relaxin on the rat cervix. To test that hypothesis, N-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 ovariectomized when they were 78 days old (day 1 of treatment). At ovariectomy each animal was fitted with silicon tubing implants containing progesterone (P) and estrogen (E) in doses that provide blood levels similar to those during late pregnancy. Rats were assigned to three treatment groups. The control group OPE (n ϭ 6 rats) received 0.5 ml L-NAME vehicle (PBS) sc at 6-h intervals from 0600 h on day 7 through 1200 h on day 8 and 0.5 ml relaxin vehicle (PBS) sc at 0600 and 1200 h on day 8. Group OPER (n ϭ 6 rats) was treated in the same way as group OPE, except that 20 g porcine relaxin were administered. Group OPERI (n ϭ 7 rats) was treated in the same way as group OPER, except that L-NAME was administered at a dose of 100 mg/kg⅐6 h. Between 1400 -1500 h on day 8, the cervices were removed and weighed. Cervical wet weight and extensibility were markedly greater (P Ͻ 0.01) in relaxintreated group OPER rats than in group OPE controls. Treatment with L-NAME diminished relaxin's effects on cervical wet weight, but not cervical extensibility. In conclusion, this study provides evidence that NO contributes to the acute effects of relaxin on the growth, but not the softening, of the rat cervix. (Endocrinology 141: 2458 -2464, 2000) R ELAXIN is produced and secreted by the corpora lutea during the second half of pregnancy in the rat (1). One vital physiological role of relaxin throughout this period is to promote the progressive and marked growth and softening of the cervix that is required for rapid and safe delivery (2-5). Studies at the light microscope level demonstrated that relaxin increases the accumulation of new epithelial and stromal cells (6, 7), reduces the organization and density of collagen fiber bundles (6), reduces the length of elastin fibers (6), and increases the cross-sectional area of arteries (6).The molecular mechanisms that mediate the effects of relaxin in the cervix are poorly understood. The free radical molecule nitric oxide (NO) is known to play a key role in the regulation of mammalian physiology through its effects in cardiovascular, nervous, renal and other systems (8 -10). NO is produced from one of the two guanidino moieties of the amino acid l-arginine in a reaction catalyzed by the oxidoreductase enzyme NO synthase (NOS) with l-citrulline as coproduct. There are three isoforms of NOS: the so-called constitutive isoforms, endothelial NOS (eNOS or NOS-III) and neuronal NOS (nNOS or NOS-I), and inducible NOS (iNOS or NOS-II). All three NOS isoforms are expressed in the rat cervix during pregnancy (11, 12). There are reasons to suspect that the effects of relaxin on the rat cervix are mediated at least in part...

Relaxin Depresses Small Bowel Motility Through a Nitric Oxide-Mediated Mechanism. Studies in Mice1

“…These findings indicate that the L-arginine NO pathway is involved in the mechanism of action of relaxin on ileal smooth muscle cells, similar to what occurs in vascular and myometrial smooth muscle cells, which have been also found to be targets for relaxin [23,24]. The current findings also provide further evidence that relaxin acts as a stimulator of intrinsic NO biosynthesis on its targets, which includes other, different types of cells and organs [40][41][42][43][44][45][46] besides smooth muscle cells.…”

Relaxin’s Physiological Roles and Other Diverse Actions

“…Blocking the synthesis of nitric oxide with the nitric oxide synthase inhibitor N-nitro larginine methyl ester restrained cervical softening, and local application of nitric oxide donors promoted cervical softening in pregnant rats (268,272). Two studies aimed at determining whether nitric oxide mediates relaxin's effects on cervical softening in rats are not consistent (268,273). There is limited evidence that nitric oxide may contribute to relaxin's effects on cervical wet weight (273).…”

“…Two studies aimed at determining whether nitric oxide mediates relaxin's effects on cervical softening in rats are not consistent (268,273). There is limited evidence that nitric oxide may contribute to relaxin's effects on cervical wet weight (273).…”

Clinical Use of Relaxin to Facilitate Birth: Reasons for Investigating the Premise

“…Seven days before relaxin treatment, nonpregnant rats were ovariectomized and fitted with silicon tubing implants containing progesterone and estrogen in doses that provided blood levels that were similar to those of late pregnancy. Beginning on the morning of day 8 of treatment, subcutaneous (sc) injection of highly purified porcine relaxin26 to steroid‐treated rats at 6‐h intervals for periods ranging from 42 h27 to as few as 8 h28 promoted a marked increase in both the wet weight and the extensibility of the cervix. Intramuscular (im) injection of porcine relaxin alone every 6 h for 54 h to intact29,30 or ovariectomized31 prepubertal gilts increased both the wet weight30 and the distensibility29,31 of the cervix, and its effects were greater when relaxin was given in combination with estrogen or estrogen plus progesterone 31…”