The influence of acute and chronic hypercalcemia on the parathyroid hormone response to hypocalcemia in rabbits

Abstract

Objective: To investigate the influence of acute and chronic hypercalcemia on the parathyroid hormone (PTH) response to hypocalcemia. Design: The PTH response to hypocalcemia has been evaluated in three groups of rabbits: Group I, normal rabbits, Group II, normal rabbits subjected to an acute hypercalcemic clamp (induced by CaCl 2 infusion) and Group III, rabbits with chronic hypercalcemia (due to surgical reduction of renal mass). Results: In Group I (baseline Ca 2þ ¼ 1:69^0:02 mM), hypocalcemia resulted in stimulation of PTH secretion which reached a maximum (PTHmax) of 91:7^6:4 pg=ml: In rabbits from Group II, which also had normal baseline Ca 2+ ð1:70^0:02 mMÞ; plasma Ca 2+ was maintained at an elevated level for 2 h, at around 2.05 mM. The PTH response to hypocalcemia in Group II was attenuated and the PTHmax in these rabbits was 45:6^7:4 pg=ml: In rabbits from Group III, baseline Ca 2+ was elevated ð2:06^0:06 mMÞ for 1 month. The PTH response to hypocalcemia in Group III was esentially the same as in Group I and PTHmax reached levels of 94:8^9:9 pg=ml: Conclusions: A difference in PTH response to hypocalcemia has been found in rabbits after exposure to either acute or chronic hypercalcemia. After acute hypercalcemia, an attenuated PTH response to hypocalcemia has been identified. Chronic hypercalcemia, however, did not influence the PTH response to hypocalcemia.

The influence of the progression of secondary hyperparathyroidism on the set point of the parathyroid hormone-calcium curve

“…The rabbit model also has the advantage that extracellular Ca 2+ levels are consistently and markedly decreased in the course of 2 HPT, thus facilitating study of the influence of hypocalcemia on the set point of the PTH-Ca 2+ curve. In addition, rabbit PTH can be measured using the immunoradiometric assay for intact human PTH (Warren et al 1989, Bas et al 2002, 2003.…”

“…To avoid any influence of the hypercalcemic stimulus on the subsequent hypocalcemia (hysteresis) a week was allowed before performing the hypocalcemic part of the curve (AguileraTejero et al 1996, Bas et al 2002, 2003.…”

“…They were fed ad libitum on a balanced diet containing Ca=1·2% and P=0·6% (normal diet). The hypercalcemic and hypocalcemic parts of the PTH-Ca 2+ curve were obtained in the same animals with an interval of 1 week to avoid the phenomenom of hysteresis (Aguilera-Tejero et al 1996, Bas et al 2002, 2003.…”

“…Samples were immediately centrifuged. Plasma was separated and Ca 2+ and pH were measured using selective electrodes (Bayer Diagnostics, Barcelona, Spain) and then plasma was frozen at -70 C. PTH was measured on plasma samples within 3 months of collection using an immunoradiometric assay which has previously been validated for quantitation of rabbit PTH (Allegro Intact PTH; Nichols, San Juan Capistrano, CA, USA) (Warren et al 1989, Bas et al 2002, 2003.…”

Dynamics of Parathyroid Hormone Secretion in Health and Secondary Hyperparathyroidism

“…Some mammals, such as the rabbit and the horse, have higher serum calcium values than humans but have PTH concentrations similar to those in humans both in absolute values and as a percentage of maximal PTH (35)(36)(37)(38)(39).…”