Multiparity is considered as the reproductive experience that includes at least two episodes of mating, pregnancy, parturition and lactation. The parturition is a mechanical and hormonal event, which has been linked to alterations in the lower urogenital tract, the pelvic floor muscles, and its innervation. Thus, multiparity has effects in both the anatomy and physiology of these structures. Studies in female rabbits have showed that multiparity alters the histological characteristics of the vagina and urethra, and the activity of pelvic/perineal striated muscles during the micturition. In addition to these changes, multiparity modifies fiber type composition and the contractile properties of striated muscles affecting vaginal pressure, as well as differential damage in some pelvic/perineal striated muscles. All these changes in pelvic floor components indicate plastic adjustments and damage that may affect excretory and reproductive functions in female mammals. The striated musculature is constituted by a large amount of pelvic/perineal striated muscles, and considering the damaging effects caused by multiparity it is important to determine whether there is damage to muscles directly related to the pelvic vagina and urethra as the obturator internus (OI) and bulboglandularis (Bg) muscles. Therefore, it is relevant to understand the form and temporality in which each muscle contributes to the function of the urogenital tract generating pressure on the vagina and urethra, with a particular contractile force. Also, to determine if current changes are due to the effect of multiparity. The aim of this study was to evaluate, in young nulliparous and young multiparous rabbits, the contractile force in isometric conditions and the pressure generated on the pelvic vagina and urethra by the OI and Bg muscles.

Twenty-four Chinchilla-breed female rabbits (10 ± 2 months old) were used in this study. Animals were divided in two groups: young virgin nulliparous (n=12) and young multiparous (n=12). Multiparous female rabbits started to copulate at 6 months old. On the day of the fourth delivery, neonate pups were euthanized to prevent lactation, and to give female rabbits the opportunity to reach a hormonal condition similar to the one maintained in nulliparous rabbits.  Urethane-anesthetized (1.5g/Kg) nulliparous and multiparous rabbits were fixed in a dorso-supine position. Physiological recordings were performed at 20 days subsequent to the fourth delivery. Each condition (nulliparous, N; multiparous, M) was randomly distributed into two experimental groups. In the first group, we evaluated the pelvic intravaginal pressure (IVP) and intraurethral pressure (IUP) generated by the contraction of the OI and Bg muscles (n=6 per group). In the second group we evaluated the contractile force developed by the OI muscle (n=6 per group). Data were expressed as mean ± standard error (SE), the significant differences (P<0.05) between groups, unpaired Student t tests were carried out.

We observed that at 100 Hz, the contraction of the OI muscle was able to generate a greater IVP in nulliparous than in multiparous female rabbits. At the level of the urethra, the pressure generated by the OI muscle did not show significant differences between groups. During the contraction of the RE muscle at 100 Hz, the IVP generated was greater in female nulliparous when compared to multiparous rabbits. At the level of urethra, the pressure generated by the contraction of the RE muscle in nulliparous was greater in comparison to multiparous. The contractile force generated by the OI muscle was not significantly different between group.

These results suggest that both the OI and the RE muscles contribute in a differential way to the generation of IVP and IUP. Likewise, multiparity affects muscles differently, mainly because the contractile properties of the OI muscle were unchanged. It is possible that the differences in urethra and vaginal wall tissue contribute to the observed changes in the IVP and IUP from multiparous rabbit.

In turn, this situation could modify reproductive and non-reproductive processes.