Mechanisms governing protective pregnancy-induced adaptions of the pelvic floor muscles in the rat pre-clinical model
Authors: Rieger Mary M. , Wong Michelle , Burnett Lindsey A. , Sesillo Francesca Boscolo , Baynes Brittni B. , Alperin Marianna 
Published: August 2021

ABSTRACTBackgroundThe intrinsic properties of pelvic soft tissues in women who do and do not sustain birth injuries are likely divergent, however little is known about this. Rat pelvic floor muscles undergo protective pregnancy-induced structural adaptations, sarcomerogenesis and increase in intramuscular collagen content, that protect against birth injury.ObjectivesWe aimed to test the following hypotheses: 1) increased mechanical load of gravid uterus drives antepartum adaptations; 2) load-induced changes are sufficient to protect pelvic muscles from birth injury.Study DesignIndependent effects of load uncoupled from hormonal milieu of pregnancy were tested in 3- to 4-month-old Sprague-Dawley rats randomly divided into four groups, N=5- 10/group: (1) load-/pregnancy hormones- (controls); (2) load+/pregnancy hormones-; (3) reduced load/pregnancy hormones+; (4) load+/pregnancy hormones+. Mechanical load simulating a gravid uterus was simulated by weighing uterine horns with beads similar to fetal rat size and weight. Reduced load was achieved by unilateral pregnancy after unilateral uterine horn ligation. To assess acute and chronic phases required for sarcomerogenesis, rats were sacrificed at 4 hours or 21 days post bead loading. Coccygeus, iliocaudalis, pubocaudalis and non-pelvic tibialis anterior were harvested for myofiber and sarcomere length measurements. Intramuscular collagen content was assessed using hydroxyproline assay. Additional 20 load+/pregnancy hormones- rats underwent vaginal distention to determine whether load-induced changes are sufficient to protect from mechanical muscle injury in response to parturition-associated strains of various magnitude. Data, compared using two-way repeated measures analysis of variance/pairwise comparisons, are presented as mean ± standard error of mean.ResultsAcute increase in load resulted in significant pelvic floor muscle stretch, accompanied by acute increase in sarcomere length compared to non-loaded control muscles (coccygeus: 2.69±0.03 vs 2.30±0.06 µm, P<0.001; pubocaudalis: 2.71±0.04 vs 2.25±0.03 µm, P<0.0001; iliocaudalis: 2.80±0.06 vs 2.35±0.04 µm, P<0.0001). After 21 days of sustained load, sarcomeres returned to operational length in all pelvic muscles (P>0.05). However, the myofibers remained significantly longer in load+/pregnancy hormones- compared to load- /pregnancy hormones- in coccygeus (13.33±0.94 vs 9.97±0.26 mm, P<0.0001) and pubocaudalis (21.20±0.52 vs 19.52±0.34 mm, P<0.04) and not different from load+/pregnancy hormones+ (12.82±0.30 and 22.53±0.32mm, respectively, P>0.1), indicating that sustained load induced sarcomerogenesis in these muscles. Intramuscular collagen content in load+/pregnancy hormones- group was significantly greater relative to controls in coccygeus (6.55±0.85 vs 3.11±0.47µg/mg, P<0.001) and pubocaudalis (5.93±0.79 vs 3.46±0.52 µg/mg, P<0.05) and not different from load+/pregnancy hormones+ (7.45±0.65 and 6.05±0.62 µg/mg, respectively, P>0.5). Iliocaudalis required both mechanical and endocrine cues for sarcomerogenesis. Tibialis anterior was not affected by mechanical or endocrine alterations. Despite equivalent extent of adaptations, load-induced changes were only partially protective against sarcomere hyperelongation.ConclusionsLoad induces plasticity of the intrinsic pelvic floor muscle components that renders protection against mechanical birth injury. The protective effect, which varies between individual muscles and strain magnitudes, is further augmented by the presence of pregnancy hormones. Maximizing impact of mechanical load on pelvic floor muscles during pregnancy, such as with specialized pelvic floor muscle stretching regimens, is a potentially actionable target for augmenting pregnancy-induced adaptations to decrease birth injury in women who may otherwise have incomplete antepartum muscle adaptations.AJOG at a GlanceWhy was the study conducted?To determine the role of mechanical load, uncoupled from the hormonal milieu of pregnancy, in driving protective pregnancy-induced adaptations previously discovered in the rat pelvic floor muscles.What are the key findings?Mechanical load, in the absence of pregnancy hormones, induces sarcomerogenesis and extracellular matrix remodeling in rat pelvic floor muscles.Load-induced adaptations are partially protective against mechanical pelvic floor muscle injury consequent to parturition-associated strains.What does this study add to what is already known?The effect of sustained increased mechanical load, uncoupled from the hormonal milieu of pregnancy, on pelvic floor muscle plasticity has not been previously studied.Modulating pelvic floor muscles’ stretch antepartum, such as with specialized pelvic floor physical therapy regimens, could be a promising approach for augmentation of protective muscle adaptations in women.