Biological fractionation of stable Ca isotopes in Gottingen minipigs as a physiological model for Ca homeostasis in humans

In order to investigate fractionation of calcium (Ca) isotopes in vertebrates as a diagnostic tool to detect Ca metabolism dysfunction we analyzed the Ca isotopic composition (Ca-44/40=[(Ca-44/Ca-40)(sample)/(Ca-44/Ca-40)(reference)]-1) of diet, faeces, blood, bones and urine from Gottingen minipigs, an animal model for human physiology. Samples of three groups were investigated: 1. control group (Con), 2. group with glucocorticosteroid induced osteoporosis (GIO) and 3. group with Ca and vitamin D deficiency induced osteomalacia (-CaD). In contrast to Con and GIO whose average Ca-44/40(faeces) values (0.39 +/- 0.13 parts per thousand and 0.28 +/- 0.08 parts per thousand, respectively) tend to be lower than their diet (0.47 +/- 0.02 parts per thousand), Ca-44/40(faeces) of -CaD (-0.27 +/- 0.21 parts per thousand) was significantly lower than their Ca-44/40(diet) (0.37 +/- 0.03 parts per thousand), but also lower than Ca-44/40(faeces) of Con and GIO. We suggest that the low Ca-44/40(faeces) of -CaD might be due to the contribution of isotopically light Ca from gastrointestinal fluids during gut passage. Assuming that this endogenous Ca source is a common physiologic feature, a fractionation during Ca absorption is also required for explaining Ca-44/40(faeces) of Con and GIO. The Ca-44/40(urine) of all groups are high (>2.0 parts per thousand) reflecting preferential renal reabsorption of light Ca isotopes. In Gottingen minipigs we found a Ca isotope fractionation between blood and bones (Ca-44/40(blood-bone)) of 0.68 +/- 0.15 parts per thousand.