Calcium and Phosphorus Homeostasis in Horses

Abstract

A study with horses was undertaken to determine if an abnormal calcium homeostasis, due to nutritional or hormonal imbalance, would be present following induction of either hypo- or hypercalcemia. This response was studied in 12 horses fed diets containing four different ratios of calcium to phosphorus (Ca:P). Animals fed the high phosphate diet (Ca:P = 0.5) demonstrated an impairment in returning serum Ca to normal following Ca infusion but recovered from ethylene-diaminetetraacetate (EDTA) induced hypocalcemia significantly faster than control animals (Ca:P = 1.2). Thyroidectomized horses showed a significant impairment in recovering normocalcemia from both a hyper- and hypocalcemic induction. However, the responses were completely corrected with thyroxine treatment. The response to Ca infusion was examined in two ponies during the stage of compensatory adaptation to a higher level of phosphate (Ca:P = 0.2) than used in the first study. Serum Ca and P and the Ca tolerance times were acutely affected and only began to return towards normal after the ponies had been fed the diet for 6 weeks. Biopsies taken from the frontal bone indicated increased osteoblastic and osteolytic activity, suggesting an increased bone turnover. It is suggested that Ca infusion studies may be useful in establishing a diagnosis of nutritional secondary hyperparathyroidism (NSH) in horses. Possible roles of the thyroid and parathyroid gland in NSH are discussed.