Isotopic analysis of calcium in blood plasma and bone from mouse samples by multiple collector-ICP-mass spectrometry

The biological processing of Ca produces significant stable isotope fractionation. The level of isotopic fractionation can provide key information about the variation in dietary consumption or Ca metabolism. To investigate this, we measured the 43Ca/42Ca and 44Ca/42Ca ratios for bone and blood plasma samples collected from mice of various ages using multiple collector-ICP-mass spectrometry (MC-ICP-MS). The 44Ca/42Ca ratio in bones was significantly (0.44-0.84 per thousand) lower than the corresponding ratios in the diet, suggesting that Ca was isotopically fractionated during Ca metabolism for bone formation. The resulting 44Ca/42Ca ratios for blood plasma showed almost identical, or slightly higher, values (0.03-0.2 per thousand) than found in a corresponding diet. This indicates that a significant amount of Ca in the blood plasma was from dietary sources. Unlike that discovered for Fe, there were no significant differences in the measured 44Ca/42Ca ratios between female and male specimens (for either bone or blood plasma samples). Similarity, the 44Ca/42Ca ratios suggests that there were no significant differences in Ca dietary consumption or Ca metabolism between female and male specimens. In contrast, the 44Ca/42Ca ratios of blood plasma from mother mice during the lactation period were significantly higher than those for all other adult specimens. This suggests that Ca supplied to infants through lactation was isotopically lighter, and the preferential supply of isotopically lighter Ca resulted in isotopically heavier Ca in blood plasma of mother mice during the lactation period. The data obtained here clearly demonstrate that the Ca isotopic ratio has a potential to become a new tool for evaluating changes in dietary consumption, or Ca metabolism of animals.