Influence of different calcium supplies and a single vitamin D injection on vitamin D receptor and calbindin D9k immunoreactivities in the gastrointestinal tract of goat kids

The purpose of this study was to investigate whether diets differing in Ca concentration would have an influence on vitamin D (VitD) receptor (VDR) and calbindin D9k (Calb9k) immunoreactivities in the gastrointestinal tract of growing goats. In addition, the effect of a single VitD injection was studied, to clarify whether exogenous VitD would further increase the active Ca absorption mechanisms. The hypothesis of the study was that reduced Ca intake would lead to greater active Ca absorption, and with that, to greater amounts of VDR and Calb9k immunoreactivities. The normal Ca kid group (according to age requirements) received 2.5 to 6 g of Ca/d, whereas the lesser Ca kid group (less than requirements) received 1.5 to 4 g of Ca/d from wk 6 (weaning) to 15 (slaughter). In addition, 5 and 6 goat kids, respectively, of each group (normal Ca kid group, lesser Ca kid group), were injected with VitD (0.05 mg of cholecalciferol/kg of BW) in wk 14 of life. Blood samples were taken in wk 14 and 15. Calcium and VitD (25-hydroxyvitamin D and 1,25-dihydroxyvitamin D) concentrations were determined in serum. Immediately after slaughter, the duodenum (DD) and rumen (RU) were mounted in conventional Ussing chambers. Unidirectional flux rates of Ca across gastrointestinal tissues were measured. Additionally, tissue specimens of the gastrointestinal tract were collected, and formaldehyde-fixed paraffin sections were used for VDR and Calb9k immunohistochemistry. In all kid groups, a net absorption in the RU and a net secretion of Ca in the DD were observed. Immunoreactions of VDR were greatest in the duodenal mucosa, whereas Calb9k immunoreactions were observed in the forestomach and intestinal tissues. The greatest expression was observed in the duodenal surface epithelium. Additionally, in the VitD-injected groups, an immuno-reaction occurred in the jejunal superficial and basal glands and the ileal superficial epithelium. In contrast, the other groups showed no Calb9k immunoreactions at these sites. In conclusion, there is clear evidence for the RU as a main site for Ca absorption. The results of this study also indicate that VDR and Calb9k are highly expressed in the duodenal mucosa. The active absorption may not have such an important role in the DD because active transport was also evident in the RU. However, Calb9k expression seems to be stimulated by VitD administration.