Calcium and phosphorus homeostasis in horses

Abstract

A study with horses was undertaken to determine whether 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.