Postparturient hypocalcemia of dairy cows: a model for the study of the interdependence of Ca, Pi, and Mg homeostasis.

Disorders of calcium, phosphorus and magnesium homeostasis in ruminants provide natural models for the study of the physiology and pathophysiology of these minerals. The knowledge that can be acquired with a better understanding of the pathogenesis of these diseases could give useful clues in the puzzle of human osteoporosis. In the present study, the case of parturient paresis of dairy cows is reexamined with a newly developed technique for the measurements of serum ionized magnesium concentrations (Mg2+). The concentrations of total magnesium (Mgtot), ionized calcium (Ca2+), total calcium (Catot), and inorganic phosphate (Pi) were also determined in the sera of seventeen 3- to 16-year-old Brown Swiss and crossed Simmental/Red Holstein cows during the periparturient period. In each animal, a transient increase of Mg2+ and Mgtot serum concentrations was observed in association with the transient decrease after parturition of Ca2+, Catot and Pi serum concentrations. On average, throughout the study, serum Mg2+ concentrations were 68.5% of those of Mgtot whereas serum Ca2+ concentrations were 52% of those of Catot. The possible mechanisms involved in the transient increase of Mg2+ and Mgtot serum concentrations are discussed and the relevance of this data for osteoporosis is outlined.