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

ABSTRACT

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 (δ^44/40Ca = [(⁴⁴Ca/⁴⁰Ca)_sample/(⁴⁴Ca/⁴⁰Ca)_reference]−1) of diet, faeces, blood, bones and urine from Göttingen 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 δ^44/40Ca_faeces values (0.39 ± 0.13‰ and 0.28 ± 0.08‰, respectively) tend to be lower than their diet (0.47 ± 0.02‰), δ^44/40Ca_faeces of −CaD (−0.27 ± 0.21‰) was significantly lower than their δ^44/40Ca_diet (0.37 ± 0.03‰), but also lower than δ^44/40Ca_faeces of Con and GIO. We suggest that the low δ^44/40Ca_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 δ^44/40Ca_faeces of Con and GIO. The δ^44/40Ca_urine of all groups are high (>2.0‰) reflecting preferential renal reabsorption of light Ca isotopes. In Göttingen minipigs we found a Ca isotope fractionation between blood and bones (Δ^44/40Ca_blood-bone) of 0.68 ± 0.15‰.

KEYWORDS:

• Animals
• calcium-40
• calcium-44
• calcium metabolism
• isotope application in medicine
• isotope ecology
• stable isotope tracer techniques

Acknowledgements

We thank A. Kolevica for lab support and K. Gonda for her analytical assistance (sample digestions and chemical Ca analyses). J. Kunze, H. Fischer and D. Siewertsen are acknowledged for their thoroughly performed animal care. We are thankful for the constructive and helpful input given by two anonymous reviewers. The editor is thanked for the handling of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study was supported by the Deutsche Forschungsgemeinschaft (DFG) Project No. Ei-272/23-1 and Schr 430/5-1,5-2 and by GEOMAR internal funding.