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Departamento de Medicina y Cirugía Animal, Universidad de Córdoba, Campus de Rabanales, Ctra Madrid-Cádiz km 396, 14014 Córdoba, Spain
¹ Departamento de Nefrología y Unidad de Investigación, Hospital Universitario Reina Sofía, Avda Menéndez Pidal s/n, 14004 Córdoba, Spain

(Requests for offprints should be addressed to E Aguilera-Tejero; Email: pv1agtee{at}lucano.uco.es)

The influence of secondary hyperparathyroidism (2 HPT) on the set point of the parathyroid hormone (PTH)-Ca²⁺ curve is controversial. In vitro experiments have shown an increase in the set point. However, clinical studies with hemodialysis patients have provided a variety of results (increases, decreases and no changes in the set point have been reported). The present study was designed to investigate the influence of the progression of 2 HPT on the set point of the PTH-Ca²⁺ curve. The PTH-Ca²⁺ curve and the expression of parathyroid calcium receptor (CaR mRNA) and vitamin D receptor (VDR mRNA) have been studied in normal rabbits (group I, n=9) and in nephrectomized rabbits (group II, n=18) at two stages after inducing 2 HPT: 2–3 weeks (group IIA) and 5–6 weeks (group IIB). In group I, the set point of the PTH-Ca²⁺ curve was 1.63±0.03 mM. A progressive hypocalcemia was detected during the evolution of 2 HPT (groups IIA and IIB). Rabbits from group IIA had a significant (P<0.001) decrease in the set point to values of 1.45±0.02 mM. However, the set point increased significantly in group IIB (P<0.001) to 1.56±0.03 mM. CaR mRNA was similarly decreased in groups IIA (39±12%) and IIB (48±7%). No changes were detected in VDR mRNA. In conclusion, a reduction in the set point of the PTH-Ca²⁺ curve in response to decreased extracellular Ca²⁺ was detected in the early phases of 2 HPT. However, with the progression of 2 HPT the set point tended to increase even though extracellular Ca²⁺ was markedly decreased. The increase in the set point in the course of 2 HPT seems to be a complex process that cannot be fully explained by changes in parathyroid CaR mRNA or VDR mRNA.

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