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in Journal of the American Veterinary Medical Association
in Journal of the American Veterinary Medical Association


Objective—To establish reference values for protein-bound, ionized, and weak-acid complexed fractions of calcium and magnesium in equine serum and determine stability of ionized calcium (iCa) and ionized magnesium (iMg) in serum samples kept under various storage conditions.

Animals—28 clinically normal horses.

Procedure—Total calcium (tCa) and magnesium (tMg) in equine serum were fractionated by use of a micropartition system that allows separation of protein-bound calcium (pCa) and magnesium (pMg) and ultrafiltrable calcium (μCa) and magnesium (μMg) fractions. Serum concentrations of iCa and iMg were measured in the ultrafiltrate by use of selective electrodes. Serum concentration of complexed calcium (cCa) or magnesium (cMg) was calculated by subtracting iCa or iMg from μCa or μMg, respectively.

Results—Mean ±SE serum tCa concentration was 3.26 ± 0.06 mmol/L. Calcium fractions were as follows: pCa, 1.55 ± 0.03 mmol/L (47.4 ± 0.9%); iCa, 1.58 ± 0.03 mmol/L (48.5 ± 0.7%); and cCa, 0.13 ± 0.02 mmol/L (4.1 ± 0.9%). Serum tMg concentration was 0.99 ± 0.04 mmol/L. Magnesium fractions were as follows: pMg, 0.33 ± 0.04 mmol/L (33.3 ± 4.2%); iMg, 0.57 ± 0.02 mmol/L (57.6 ± 1.7%); and cMg, 0.09 ± 0.02 mmol/L (9.1 ± 1.9%). Refrigeration (4°C) did not affect iCa values, whereas iMg declined by 8% after 120 hours. Neither iCa nor iMg was affected by freezing (−20°C).

Conclusions and Clinical Relevance—In equine serum, iMg is less stable than iCa; thus, when serum samples are not going to be analyzed promptly, freezing may be preferable to refrigeration for storage.

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in American Journal of Veterinary Research
in Journal of the American Veterinary Medical Association



To determine effects of exercise on blood ionized calcium (Ca2+) and plasma parathyroid hormone (PTH) concentrations in horses and to compare the effects of exercise-induced and EDTA-induced hypocalcemia on PTH secretion.


17 horses entered in a show jumping competition and 5 horses given EDTA.


Blood Ca2+ and plasma PTH concentrations were measured before and after exercise in the 17 horses entered in the jumping competition. In the other 5 horses, concentrations were measured during infusion of EDTA IV.


Exercise resulted in a significant decrease in blood Ca2+ concentration and a significant increase in plasma PTH concentration, and blood Ca2+ concentration was correlated with plasma PTH concentration. Administration of EDTA resulted in hypocalcemia and an increase in PTH concentration. For the same decrease in Ca2+ concentration, magnitude of the exercise-induced increase in PTH concentration was similar to magnitude of the EDTA-induced increase.

Conclusions and Clinical Relevance

Results suggest that the increase in plasma PTH concentration in horses after exercise is secondary to hypocalcemia and that the increase in PTH concentration seems to be commensurate with the decrease in Ca2+ concentration. (Am J Vet Res 1998;59:1605-1607)

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in American Journal of Veterinary Research