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

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 1, normal rabbits, Group II, normal rabbits subjected to an acute hypercalcemic clamp (induced by CaCl2 infusion) and Group III, rabbits with chronic hypercalcemia (due to surgical reduction of renal mass). Results: In Group I (baseline Ca2+ = 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 11, which also had normal baseline Ca2+ (1.70 +/- 0.02 mM), plasma Ca2+ was maintained at an elevated level for 2 h, at around 2.05 mM. The PTH response to hypocalcemia in Group 11 was attenuated and the PTHmax in these rabbits was 45.6 +/- 7.4 pg/ml. In rabbits from Group III, baseline Ca2+ was elevated (2.06 +/- 0.06 mM) for I 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.