Nutritional Calcium Supply Dependent Calcium Balance, Bone Calcification and Calcium Isotope Ratios in Rats

Serum calcium isotopes (δ(44/42)Ca) have been suggested as a non-invasive and sensitive Ca balance marker. Quantitative δ(44/42)Ca changes associated with Ca flux across body compartment barriers relative to the dietary Ca and the correlation of δ(44/42)Ca(Serum) with bone histology are unknown. We analyzed Ca and δ(44/42)Ca by mass-spectrometry in rats after two weeks of standard-Ca-diet (0.5%) and after four subsequent weeks of standard- and of low-Ca-diet (0.25%). In animals on a low-Ca-diet net Ca gain was 61 ± 3% and femur Ca content 68 ± 41% of standard-Ca-diet, bone mineralized area per section area was 68 ± 15% compared to standard-Ca-diet. δ(44/42)Ca was similar in the diets, and decreased in feces and urine and increased in serum in animals on low-Ca-diet. δ(44/42)Ca(Bone) was higher in animals on low-Ca-diet, lower in the diaphysis than the metaphysis and epiphysis, and unaffected by gender. Independent of diet, δ(44/42)Ca(Bone) was similar in the femora and ribs. At the time of sacrifice, δ(44/42)Ca(Serum) inversely correlated with intestinal Ca uptake and histological bone mineralization markers, but not with Ca content and bone mineral density by µCT. In conclusion, δ(44/42)Ca(Bone) was bone site specific, but mechanical stress and gender independent. Low-Ca-diet induced marked changes in feces, serum and urine δ(44/42)Ca in growing rats. δ(44/42)Ca(Serum) inversely correlated with markers of bone mineralization.