Expression patterns of intestinal calcium transport factors and ex-vivo absorption of calcium in horses

BACKGROUND In many species, the small intestine is the major site of calcium (Ca(2+)) absorption. The horse differs considerably from most other species with regard to the physiology of its Ca(2+) metabolism and digestion. Thus, this study was performed to get more information about the transcellular Ca(2+) absorption in the horse.Two mechanisms of intestinal Ca(2+) absorption are described: the passive paracellular pathway and the active, vitamin D-dependent transcellular pathway. The latter involves the following elements: vitamin D receptors (VDR), transient receptor potential vanilloid channel members 5 and 6 (TRPV5/6), calbindin-D9k (CB), the Na/Ca exchanger (NCX1) and the plasma membrane Ca-ATPase (PMCA). The aim of the present study was to investigate the protein and mRNA expression patterns of VDR, CB and TRPV6 and the ex-vivo Ca(2+) absorption in horses, assessed by qualitative and quantitative RT-PCR, western blot, immunohistochemistry and the Ussing chamber technique. RESULTS Highest CB and TRPV6 mRNA levels were detected in the duodenum as compared to the middle parts of the jejunum and ileum and several sites of the large intestine. VDR mRNA levels did not change significantly throughout the intestine. TRPV5 mRNA was not detectable in the horse intestine. The highest VDR and CB protein levels were measured in the duodenum. Ussing chamber studies revealed ex-vivo Ca(2+) absorption only in the duodenum, but not in cecum and specific sites of the colon. CONCLUSION The present findings suggest that TRPV6, CB and VDR may be involved in active intestinal Ca(2+) absorption in horses, as described for other mammals. TRPV5 may not play a major role in this process. Furthermore, the expression patterns of these Ca(2+) transport elements and the results of the Ussing chamber procedure indicate that a significant part of active intestinal Ca(2+) absorption occurs in the duodenum in this species.