Monoclonal antibodies specific for rat relaxin. IX. Evidence that endogenous relaxin promotes growth of the vagina during the second half of pregnancy in rats.

It is established that endogenous relaxin promotes the growth and development of the cervix, mammary glands, and nipples in pregnant rats. Additionally, the observation that porcine relaxin promotes growth of the vagina in both nonpregnant and pregnant rats provides indirect evidence that endogenous relaxin may effect growth of the vagina during rat pregnancy. The purpose of this study was to determine whether endogenous relaxin promotes growth of the vagina in pregnant rats. To that end, a monoclonal antibody, specific for rat relaxin, designated MCA1, was used to passively neutralize endogenous relaxin throughout the second half of pregnancy in intact rats. Five milligrams of highly purified MCA1 were injected iv to rats daily from days 12-22 of pregnancy. Controls received either a monoclonal antibody for fluorescein or PBS. The vaginal wet weight, dry weight, length, diameter, inner surface area, DNA content, and percent water content were determined. No differences were found between monoclonal antibody for fluorescein and PBS controls for any of the measured vaginal parameters. In contrast, values for all physical parameters, except percent water content, were significantly lower in MCA1-treated rats than in controls. Vaginal DNA content was also lower in MCA1-treated rats than in controls; and this observation supports the view that relaxin induces vaginal growth at least in part by promoting cell proliferation. To examine the mechanism of relaxin's apparent action on the vagina, specific relaxin-binding sites were localized immunohistochemically. Relaxin-binding sites were found in epithelial and smooth muscle cells, and the binding was specific for relaxin. We conclude that endogenous relaxin promotes growth of the vagina in pregnant rats.