Primary uterine inertia (PUI) is the most common type of dystocia in dogs. We hypothesized that PUI develops because of lower than normal expression of the basic contractile elements in the uterus, i.e., smooth muscle (SM) α- and γ-actin and SM-myosin, and that the expression of these proteins is influenced by the number of fetuses present in utero. Full-thickness inter-placental uterine biopsies were collected during Cesarean sections from dogs with PUI (n = 11), and from bitches with obstructive dystocia (OD) still presenting strong labor contractions (designated as the control group, n = 7). Relative gene expression was determined by semi-quantitative real-time (TaqMan) PCR, and protein localization by immunohistochemistry. Gene expression between PUI and OD bitches, and between PUI bitches carrying small, large, or average number of fetuses according to their breed, were compared. Uterine SM-γ-actin and SM-myosin mRNA levels were significantly higher in PUI than in OD dogs, while SM-α-actin did not differ. PUI bitches carrying large litters had lower uterine SM-γ-actin gene expression than those with small litters (P = 0.008). Immunostaining for SM-actin isoforms and SM-myosin was present in the myometrium, and localization pattern and staining intensity appeared similar in the PUI and OD groups. All proteins stained in blood vessels, and SM-γ-actin was also present in endometrial luminal and glandular epithelium. In conclusion, higher uterine SM-γ-actin and SM-myosin gene expression in PUI bitches, compared with OD dogs, might be an indication of abnormal progression with labor. Whether this is the cause of PUI due to an intrinsic error of the myometrium not becoming committed to labor, or the consequence of inadequate endocrine or mechanical stimuli, is not clear. Litter size was previously shown to be one of the risk factors for the development of uterine inertia in dogs, and our findings suggest possible differing uterine pathophysiology of PUI with respect to litter size.

中文翻译：

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平滑肌α-和γ-肌动蛋白以及平滑肌肌球蛋白在被诊断为子宫乏力和阻塞性难产的母犬中的子宫表达

原发性子宫惯性 (PUI) 是犬最常见的难产类型。我们假设 PUI 的发生是由于子宫中基本收缩元件（即平滑肌 (SM) α-和γ-肌动蛋白以及 SM-肌球蛋白）的表达低于正常水平，并且这些蛋白质的表达受数量的影响存在于子宫内的胎儿。在剖宫产期间从患有 PUI 的狗 (n = 11) 和患有阻塞性难产 (OD) 的母犬（被指定为对照组，n = 7）中收集全层胎盘间子宫活检。通过半定量实时 (TaqMan) PCR 确定相对基因表达，并通过免疫组织化学进行蛋白质定位。PUI 和 OD 母狗之间的基因表达，以及携带小、大、或根据其品种的平均胎儿数进行比较。PUI 中子宫 SM-γ-肌动蛋白和 SM-肌球蛋白 mRNA 水平显着高于 OD 犬，而 SM-α-肌动蛋白没有差异。携带大窝的 PUI 母犬的子宫 SM-γ-肌动蛋白基因表达低于那些小窝的母犬 (P = 0.008)。SM-肌动蛋白同种型和 SM-肌球蛋白的免疫染色存在于子宫肌层中，定位模式和染色强度在 PUI 和 OD 组中出现相似。所有蛋白质在血管中染色，SM-γ-肌动蛋白也存在于子宫内膜腔和腺上皮中。总之，与 OD 犬相比，PUI 母犬的子宫 SM-γ-肌动蛋白和 SM-肌球蛋白基因表达较高，可能表明分娩异常进展。这是否是由于子宫肌层的内在错误导致的 PUI 未致力于分娩，还是内分泌或机械刺激不足的后果，尚不清楚。窝产仔数先前已被证明是狗发生子宫惯性的危险因素之一，我们的研究结果表明 PUI 的子宫病理生理学可能因窝产仔数而异。