The inner ear is essential for maintaining balance and hearing predator and prey in the environment. Each inner ear contains three CaCO3 otolith polycrystals, which are calcified within an alkaline, K+-rich endolymph secreted by the surrounding epithelium. However, the underlying cellular mechanisms are poorly understood, especially in marine fish. Here, we investigated the presence and cellular localization of several ion-transporting proteins within the saccular epithelium of the Pacific Chub Mackerel (Scomber japonicus). Western blotting revealed the presence of Na+/K+-ATPase (NKA), carbonic anhydrase (CA), Na+-K+-2Cl−-co-transporter (NKCC), vacuolar-type H+-ATPase (VHA), plasma membrane Ca2+ ATPase (PMCA), and soluble adenylyl cyclase (sAC). Immunohistochemistry analysis identified two distinct ionocytes types in the saccular epithelium: Type-I ionocytes were mitochondrion-rich and abundantly expressed NKA and NKCC in their basolateral membrane, indicating a role in secreting K+ into the endolymph. On the other hand, Type-II ionocytes were enriched in cytoplasmic CA and VHA, suggesting they help transport HCO3− into the endolymph and remove H+. In addition, both types of ionocytes expressed cytoplasmic PMCA, which is likely involved in Ca2+ transport and homeostasis, as well as sAC, an evolutionary conserved acid–base sensing enzyme that regulates epithelial ion transport. Furthermore, CA, VHA, and sAC were also expressed within the capillaries that supply blood to the meshwork area, suggesting additional mechanisms that contribute to otolith calcification. This information improves our knowledge about the cellular mechanisms responsible for endolymph ion regulation and otolith formation, and can help understand responses to environmental stressors such as ocean acidification.

中文翻译：

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太平洋鲭鱼（Scomber japonicus）内耳上皮中两种离子细胞的免疫学特征

内耳对于保持平衡和聆听环境中的捕食者和猎物至关重要。每个内耳包含三个 CaCO3 耳石多晶，它们在由周围上皮分泌的碱性、富含 K+ 的内淋巴中钙化。然而，人们对潜在的细胞机制知之甚少，尤其是在海洋鱼类中。在这里，我们调查了太平洋鲭鱼（Scomber japonicus）的囊状上皮内几种离子转运蛋白的存在和细胞定位。蛋白质印迹显示存在 Na+/K+-ATPase (NKA)、碳酸酐酶 (CA)、Na+-K+-2Cl−-协同转运蛋白 (NKCC)、液泡型 H+-ATPase (VHA)、质膜 Ca2+ ATPase ( PMCA) 和可溶性腺苷酸环化酶 (sAC)。免疫组织化学分析确定了囊状上皮中两种不同的离子细胞类型：I 型离子细胞富含线粒体，并在其基底外侧膜中大量表达 NKA 和 NKCC，表明其在将 K+ 分泌到内淋巴中的作用。另一方面，II 型离子细胞富含细胞质 CA 和 VHA，表明它们有助于将 HCO3- 运输到内淋巴并去除 H+。此外，两种类型的离子细胞都表达细胞质 PMCA，这可能参与 Ca2+ 转运和体内平衡，以及 sAC，一种调节上皮离子转运的进化保守酸碱感应酶。此外，CA、VHA 和 sAC 也在为网状区域供血的毛细血管中表达，这表明导致耳石钙化的其他机制。