Calcium and Phosphorus Homeostasis in Horses

• Published in: The Journal of nutrition Vol. 104; no. 1; pp. 18 - 27
• Main Authors: Argenzio, R.A., Lowe, J.E., Hintz, H.F., Schryver, H.F.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.01.1974
• Subjects: Animal Nutritional Physiological Phenomena; Animals; Bone and Bones - metabolism; bone turnover; Calcium - blood; Calcium - metabolism; Calcium, Dietary - administration & dosage; compensation; Edetic Acid; Female; Homeostasis; Horse Diseases - diagnosis; Horses; Hypercalcemia - blood; Hypercalcemia - chemically induced; Hyperparathyroidism - diagnosis; Hyperparathyroidism - veterinary; Hyperplasia; Hypocalcemia - blood; Hypocalcemia - chemically induced; Male; nutritional secondary hyperparathyroidism; Parathyroid Glands - physiology; parathyroid hyperplasia; Phosphorus - administration & dosage; Phosphorus - blood; Phosphorus - metabolism; Thyroid Gland - physiology; Thyroidectomy; parathyroid hyperplasia; bone turnover; compensation; nutritional secondary hyperparathyroidism
• ISSN: 0022-3166
• DOI: 10.1093/jn/104.1.18

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.