Evidence that cellular proliferation contributes to relaxin-induced growth of both the vagina and the cervix in the pregnant rat.

It is well established that cervical growth during rat pregnancy is relaxin dependent. The first objective of this study was to determine if relaxin also promotes vaginal growth in the pregnant rat. Finding that this is the case, the second objective of this study was to determine if cell proliferation accompanies relaxin-dependent vaginal and cervical growth during rat pregnancy. Primiparous pregnant rats were ovariectomized (O) or sham ovariectomized (group C) on day 9 (D9) of pregnancy, before relaxin (R) is detectable in the peripheral circulation. After ovariectomy, rats were treated continuously with progesterone (P) and estrogen (E, group OPE), or P, E, and porcine R (group OPER) in doses that restored normal pregnancy and parturition parameters. P and E were administered via silicon tubing implants. R was administered from miniature osmotic pumps. Vaginas and cervices were collected on D9 and D22 from group C, and on D22 from groups OPE and OPER (n = 6/group). Vaginas and cervices were weighed, frozen, and lyophilized until dry. Dried tissues were weighed, homogenized, and their DNA contents were determined. In sham-operated controls (group C), the wet weight, dry weight, and DNA contents of both the vagina and cervix increased 50-300% from D9-D22. On D22, vaginal and cervical wet and dry weights were significantly lower than controls in R-deficient group OPE; whereas, they were greater than controls in group OPER. Similarly, on D22, vaginal and cervical DNA content did not differ from D9 controls in group OPE; whereas they exceeded D22 controls in group OPER. In conclusion, this study demonstrates that vaginal growth during the second half of rat pregnancy is R dependent. Additionally, this study provides evidence that R may contribute to both vaginal and cervical growth by promoting cellular proliferation.