Nutritional secondary hyperparathyroidism in rabbits

The present study was designed to document the effect of a low (0.6%) calcium-high (1.2%) phosphorus (LCaHP) diet on the development of parathyroid gland hyperplasia in rabbits and to describe the dynamics of parathyroid function (PTH–Ca²⁺ curves) in rabbits with nutritional secondary hyperparathyroidism (N2HPT). Parathyroid gland weight, parathyroid cell proliferation (measured as percentage of cells in S-phase), and parathyroid calcium (CaRmRNA) and Vitamin D (VDRmRNA) receptor expression were measured in normal rabbits and in rabbits with N2HPT. The PTH–Ca²⁺ curve was studied in normal rabbits (Group I) and in rabbits with N2HPT at two stages: 2–3 weeks (Group IIA) and 5–6 weeks (Group IIB) after being fed LCaHP diet. An increase in parathyroid gland weight and percentage of cells in S-phase was detected in the course of N2HPT. After receiving a LCaHP diet for 6 weeks rabbits had decreased levels of CaRmRNA but VDRmRNA remained unchanged. A progressive increase in the concentrations of plasma PTH (Group IIA = 167 ± 14 pg/ml and Group IIB = 377 ± 54 pg/ml, P < 0.05 versus Group I = 27 ± 3 pg/ml) was detected in the rabbits fed a LCaHP diet. This was accompanied by an increase in maximal and minimal PTH, reductions in plasma Ca²⁺ and calcitriol and elevations in plasma phosphate and creatinine. In conclusion, feeding a LCaHPD results in a rapid induction of N2HPT in rabbits. After 6 weeks on the LCaHPD rabbits develop parathyroid hyperplasia characterized by increases in PTH secretion, glandular weight and proliferation and by a decrease in CaRmRNA.