Abstract
BackgroundIn many species, the small intestine is the major site of calcium (Ca2+) absorption. The horse differs considerably from most other species with regard to the physiology of its Ca2+ metabolism and digestion. Thus, this study was performed to get more information about the transcellular Ca2+ absorption in the horse.Two mechanisms of intestinal Ca2+ 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 Ca2+ absorption in horses, assessed by qualitative and quantitative RT-PCR, western blot, immunohistochemistry and the Ussing chamber technique.ResultsHighest 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 Ca2+ absorption only in the duodenum, but not in cecum and specific sites of the colon.ConclusionThe present findings suggest that TRPV6, CB and VDR may be involved in active intestinal Ca2+ absorption in horses, as described for other mammals. TRPV5 may not play a major role in this process. Furthermore, the expression patterns of these Ca2+ transport elements and the results of the Ussing chamber procedure indicate that a significant part of active intestinal Ca2+ absorption occurs in the duodenum in this species.
Highlights
IntroductionThe small intestine is the major site of calcium (Ca2+) absorption. The horse differs considerably from most other species with regard to the physiology of its Ca2+ metabolism and digestion
In many species, the small intestine is the major site of calcium (Ca2+) absorption
There were no significant differences between the vitamin D receptors (VDR) mRNA expression levels in the small and large intestine (p > 0.05) (Figure 2A)
Summary
The small intestine is the major site of calcium (Ca2+) absorption. The horse differs considerably from most other species with regard to the physiology of its Ca2+ metabolism and digestion. Two mechanisms of intestinal Ca2+ 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). In comparison with other mammals, the horse exhibits increased serum Ca2+ concentrations, low mean serum calcidiol and vitamin D concentrations and high intestinal Ca2+ absorption and urinary extrusions [18,19,24,25]. Rourke et al [19] investigated the mRNA expression of several Ca2+ transport elements in the gastrointestinal tract of horses and found that the small intestine seems to be the main site of transcellular Ca2+ absorption. The intestinal ex-vivo absorption of Ca2+ was measured using the Ussing chamber technique
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