Hysteresis and calcium set-point for the calcium parathyroid hormone relationship in healthy horses

Abstract

Abnormalities in calcium (Ca 2+) homeostasis are reported in horses with several pathological conditions; however, there is little information on Ca 2+ regulation in horses. The objectives of the present study were to determine the Ca 2+ set-point in healthy horses, to determine whether the Ca 2+/parathyroid hormone (PTH) response curves were characterized by hysteresis, and to determine if the order of experimentally induced hypocalcemia or hypercalcemia had an effect on PTH secretion. The Ca 2+ set-point and hysteresis were determined in 12 healthy horses by infusing Na 2EDTA and calcium gluconate. The Ca 2+ set-point was 1.37±0.05 mmol/ L , which is higher than values reported for humans and dogs (1.0–1.2 mmol/L). Hysteresis was present during hypocalcemia and hypercalcemia. Horses in which hypocalcemia was followed by hypercalcemia secreted more PTH (7440±740 pmol min / L) than horses in which hypercalcemia was followed by hypocalcemia (5990±570 pmol min / L) . This study has demonstrated that the Ca 2+ set-point in the horse is higher than in other domestic animals and man. We have shown that the Ca 2+/PTH relationship in horses is sigmoidal and displays hysteresis during both hypocalcemia and hypercalcemia, and that extracellular Ca 2+ concentrations may affect the response of the parathyroid gland to hypocalcemia.