Phosphate depletion in the rat: Effect of bisphosphonates and the calcemic response to PTH.

The removal of phosphate from the diet of the growing rat rapidly produces hypercalcemia, hypophosphatemia, hypercalciuria, and hypophosphaturia. Increased calcium efflux from bone has been shown to be the important cause of the hypercalcemia and hypercalciuria. It has been proposed that the increased calcium efflux from bone is osteoclast mediated. Because bisphosphonates have been shown to inhibit osteoclast-mediated bone resorption, this study was performed to determine whether bisphosphonate-induced inhibition of osteoclast function changed the biochemical and bone effects induced by phosphate depletion.

Four groups of pair-fed rats were studied: (a) low-phosphate diet (LPD; phosphate less than 0.05%), (b) LPD plus the administration of the bisphosphonate Pamidronate (APD; LPD + APD), (c) normal diet (ND, 0.6% phosphate), and (d) ND + APD. All diets contained 0.6% calcium. A high dose of APD was administered subcutaneously (0.8 mg/kg) two days before the start of the study diet and on days 2, 6, and 9 during the 11 days of the study diet. On day 10, a 24-hour urine was collected, and on day 11, rats were either sacrificed or received an additional APD dose before a 48-hour parathyroid hormone (PTH) infusion (0.066 μg/100 g/hr) via a subcutaneously implanted miniosmotic pump.

Serum and urinary calcium were greater in the LPD and LPD + APD groups than in the ND and ND + APD groups [serum, 11.12 ± 0.34 and 11.57 ± 0.45 vs. 9.49 ± 0.17 and 9.48 ± 0.15 mg/dl (mean ± se), P < 0.05; and urine, 8.78 ± 2.74 and 16.30 ± 4.68 vs. 0.32 ± 0.09 and 0.67 ± 0.28 mg/24 hr, P < 0.05]. Serum PTH and serum and urinary phosphorus were less in the LPD and LPD + APD than in the ND and ND + APD groups (P < 0.05). The calcemic response to PTH was less (P < 0.05) in the LPD and LPD + APD groups than in the ND group and was less (P = 0.05) in the LPD + APD than in the ND + APD group. Bone histology showed that phosphate depletion increased the osteoblast and osteoclast surface, and treatment with APD reduced the osteoblast surface (LPD vs. LPD + APD, 38 ± 4 vs. 4 ± 2%, P < 0.05, and ND vs. ND + APD, 20 ± 2 vs. 5 ± 2%, P < 0.05) and markedly altered osteoclast morphology by inducing cytoplasmic vacuoles.

(a) Phosphate depletion induced hypercalcemia and hypercalciuria that were not reduced by APD administration. (b) The calcemic response to PTH was reduced in phosphate-depleted rats and was unaffected by APD administration in normal and phosphate-depleted rats, and (c) APD administration markedly changed bone histology without affecting the biochemical changes induced by phosphate depletion.