Growth-promoting effects of relaxin and related compositional changes in the uterus, cervix, and vagina of the rat.

Although the precise role of relaxin has yet to be elucidated, it has been implicated in the regulation of physiological and biochemical processes in the reproductive tract during pregnancy and parturition. In this study, the growth-promoting effects of relaxin and related compositional changes in the uterus, cervix, and vagina of immature ovariectomized estrogen-primed rats were examined. Relaxin increased the wet weight of the uterus, cervix, and vagina in a significant and linear manner over the log of the dose range (1-30 micrograms; 6 h). The increase in uterine weight was due to increases in both dry weight and water content at all doses. A dose of 1 microgram relaxin induced maximal increases in dry weights in the cervix and vagina over control values; higher doses increased wet weight, but these changes were due solely to increases in water content. Thirty micrograms of relaxin were found to increase total soluble protein and glycogen content of the vagina above control values after 6 h. Relaxin did not alter the total collagen levels of the uterus or cervix, and collagen concentrations were significantly reduced in these organs 6 and 24 h after treatment. Total glycosaminoglycan levels were elevated by relaxin in the uterus (6 h) and cervix (24 h). Total vaginal collagen was increased 24 h after relaxin injection, but the collagen concentration was decreased over the time interval studied, and glycosaminoglycan levels in the vagina were unaltered. In summary, relaxin stimulates growth of the uterus, cervix, and vagina by increasing water content and tissue mass. The increases in distensibility that relaxin induces in these organs appear to be related to changes in the fluid matrix and proteoglycan metabolism rather than alterations in collagen concentration, at least 6-24 h after a single injection. These results support the hypothesis that relaxin plays a significant role in the maintenance of pregnancy through its contribution to fetal accommodation and in the facilitation of parturition through expansion of the entire birth canal.