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.

Abstract

The purpose of this study was to investigate whether diets differing in calcium (Ca) concentration 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 even more increase active Ca absorption mechanisms. The hypothesis of the study was that lower Ca intake leads to higher active Ca absorption, and with that, to higher amounts of VDR and Calb9k immunreactivities. One group (Ca-nk, according to age requirements) received 2.5 to 6 g Ca/d, whereas a second group (Ca-lk, lower than requirements) received 1.5 to 4 g Ca/d from the 6(th) wk (weaning) until 15(th) wk (slaughtering). In addition, 5 and 6 goat kids, respectively, of each group (Ca-nk, Ca- lk), were injected with VitD (0.05 mg/kg BW cholecalciferol) in the 14(th) wk of life. Blood samples were taken in the 14(th) and 15(th) wk. Calcium and VitD (25-hydroxyvitamin D and 1,25-dihydroxyvitamin D) concentrations were determined in serum. Immediately after slaughter, 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 RU and a net secretion of calcium in DD were observed. Immunoreactions of VDR were highest in the duodenal mucosa, whereas Calb9k immunoreactions were observed in the forestomach and intestinal tissues. The highest expression was observed in the duodenal surface epithelium. Additionally, in the VitD injected groups, an immunoreaction occurred in the jejunal superficial and basal gland 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 play such an important role in the DD as there was also active transport evident in the RU. However, Calb9k expression seems to be stimulated by VitD administration.