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

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(2)EDTA 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.