title: 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(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.
authors.affiliation: Department of Veterinary Biosciences, The Ohio State University, Columbus, OH 43210, USA. toribio.1@osu.edu
authors.forename: Ramiro E
authors.initials: RE
authors.lastname: Toribio
authors.forename: Catherine W
authors.initials: CW
authors.lastname: Kohn
authors.forename: Richard A
authors.initials: RA
authors.lastname: Sams
authors.forename: Charles C
authors.initials: CC
authors.lastname: Capen
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Rosol
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D019540:Area Under Curve; D002118:Calcium / Q000097:blood* / Q000493:pharmacokinetics; D005260:Female; D006706:Homeostasis / Q000502:physiology; D006736:Horses / Q000097:blood*; D006934:Hypercalcemia / Q000097:blood* / Q000662:veterinary; D006996:Hypocalcemia / Q000097:blood* / Q000662:veterinary; D008297:Male; D010281:Parathyroid Hormone / Q000097:blood* / Q000378:metabolism; D012016:Reference Values; D013997:Time Factors
chemical_list: D010281:Parathyroid Hormone; D002118:Calcium