Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, 601 Vernon Tharp Street, Columbus, OH 43210, USA.
Gen Comp Endocrinol. 2010 May 15;167(1):6-10. doi: 10.1016/j.ygcen.2010.02.022. Epub 2010 Mar 11.
Epithelial calcium transport occurs by paracellular and transcellular mechanisms. Transcellular transport in intestinal and renal epithelia involves several transport proteins, including transient receptor potential vanilloid member 5 (TRPV5), member 6 (TRPV6), calbindin D9k (CB9), calbindin D28k (CB28), sodium calcium exchanger 1 (NCX1), plasma membrane calcium ATPase 1 (PMCA1), and the vitamin D receptor (VDR). We are interested in the horse because of its unique calcium physiology (high blood calcium, high intestinal calcium absorption, high renal excretion of calcium, low vitamin D concentrations), and because horses often have dysregulated calcium balance with various diseases. We cloned the mRNA for equine TRPV5, TRPV6, CB9, CB28, NCX1, PMCA1, and VDR, performed comparative mRNA and protein sequence analysis, and quantified their mRNA expression in the kidney and gastrointestinal tract. Sequence homology for the mRNAs and proteins was high among mammals (>75%), with fish having the lowest homology (<75%). TRPV5, TRPV6, and CB9 expression was higher in the duodenum and proximal jejunum and followed a similar expression pattern. CB28 expression was greatest in the kidney. PMCA1 and NCX1 expression was similar throughout the intestine, but in the kidney PMCA1 expression was higher. Based on our findings, the proximal small intestine is the main site for transcellular calcium transport, with TRPV6 and CB9 serving as the main transport proteins. In the kidney, TRPV6, CB28, and PMCA1 are likely more important. The low VDR expression in the equine small intestine and kidney relative to the large intestine, together with the reported high intestinal absorption and renal excretion of calcium, and low vitamin D concentrations suggests that epithelial calcium transport in horses is not as dependent on vitamin D as in other species.
上皮细胞钙转运通过细胞旁和细胞内机制发生。肠道和肾脏上皮细胞的细胞内转运涉及几种转运蛋白，包括瞬时受体电位香草酸成员 5（TRPV5）、成员 6（TRPV6）、钙结合蛋白 D9k（CB9）、钙结合蛋白 D28k（CB28）、钠钙交换蛋白 1（NCX1）、质膜钙 ATP 酶 1（PMCA1）和维生素 D 受体（VDR）。我们对马感兴趣是因为它独特的钙生理学（高血钙、高肠道钙吸收、高肾钙排泄、低维生素 D 浓度），并且因为马经常因各种疾病而钙平衡失调。我们克隆了马 TRPV5、TRPV6、CB9、CB28、NCX1、PMCA1 和 VDR 的 mRNA，进行了比较 mRNA 和蛋白质序列分析，并定量了它们在肾脏和胃肠道中的 mRNA 表达。mRNA 和蛋白质的序列同源性在哺乳动物中较高（>75%），鱼类的同源性较低（<75%）。TRPV5、TRPV6 和 CB9 在十二指肠和近端空肠中的表达较高，且表达模式相似。CB28 在肾脏中的表达最高。PMCA1 和 NCX1 在整个肠道中的表达相似，但在肾脏中 PMCA1 的表达较高。根据我们的发现，近端小肠是细胞内钙转运的主要部位，TRPV6 和 CB9 是主要的转运蛋白。在肾脏中，TRPV6、CB28 和 PMCA1 可能更为重要。马小肠和肾脏中 VDR 的表达相对大肠较低，加上报道的高肠道钙吸收和肾钙排泄以及低维生素 D 浓度，表明马上皮细胞钙转运对维生素 D 的依赖性不如其他物种。
