Increased sensitivity to 1,25(OH)2D3 in bone from genetic hypercalciuric rats

Abstract

As a model of human hypercalciuria, we have selectively inbred genetic hypercalciuric stone-forming (GHS) Sprague-Dawley rats whose mean urine calcium excretion is eight to nine times greater than that of controls. A large component of this excess urine calcium excretion is secondary to increased intestinal calcium absorption, which is not due to an elevation in serum 1,25(OH)2D3, but appears to result from an increased number of intestinal 1,25(OH)2D3 receptors (VDR). When GHS rats are fed a low-calcium diet, the hypercalciuria is only partially decreased and urine calcium excretion exceeds intake, suggesting that an additional mechanism contributing to the hypercalciuria is enhanced bone demineralization. To determine if GHS rat bones are more sensitive to exogenous 1,25(OH)2D3, we cultured calvariae from neonatal (2- to 3-day-old) GHS and control rats with or without 1,25(OH)2D3 or parathyroid hormone (PTH) for 48 h at 37 degrees C. There was significant stimulation of calcium efflux from GHS calvariae at 1 and 10 nM 1,25(OH)2D3, whereas control calvariae showed no significant response to 1,25(OH)2D3 at any concentration tested. In contrast, PTH induced similar bone resorption in control and GHS calvariae. Immunoblot analysis demonstrated a fourfold increase in the level of VDR in GHS calvariae compared with control calvariae, similar to the increased intestinal receptors described previously. There was no comparable change in VDR RNA levels as measured by slot blot analysis, suggesting the altered regulation of the VDR occurs posttranscriptionally. That both bone and intestine display an increased amount of VDR suggests that this may be a systemic disorder in the GHS rat and that enhanced bone resorption may be responsible, in part, for the hypercalciuria in the GHS rat.

Molecular Mechanisms of Primary Hypercalciuria

“…The increased sensitivity to 1,25(OH) 2 D 3 observed in enterocytes, which is presumably due to the increase in number of vitamin D receptors (22), may be important in bone, as well. To determine if GHS rat bones are more sensitive to exogenous 1,25(OH) 2 D 3 compared with bone from control rats, we cultured calvariae from neonatal GHS and control rats with or without 1,25(OH) 2 D 3 or PTH for 48 h (25). There was significant stimulation of calcium efflux from GHS calvariae at 1 and 10 nM 1,25(OH) 2 D 3 , while control calvariae showed no significant response to 1,25(OH) 2 D 3 at any concentration tested.…”

“…We have shown evidence for both primary bone dissolution apparently due to augmented vitamin D receptor number in GHS rat osteoblasts leading to enhanced bone resorption and a primary defect in renal tubular reabsorption of calcium. Though the renal tubular defect would necessitate bone resorption to maintain normal serum calcium levels, both the renal and bone defects have been shown to be independent of each other (25,28,29). The independent defects in calcium handling by the intestine, kidney, and bone suggest a systemic defect in calcium handling resulting in hypercalciuria.…”

Expression of Fibroblast Growth Factor 23, Vitamin D Receptor, and Sclerostin in Bone Tissue from Hypercalciuric Stone Formers

“…Genetic hypercalciuric stone-forming (GHS) rats exhibit increased VDR coupled with normal levels of 1,25(OH) 2 D 3 , resulting in increased functional VDR-1,25(OH) 2 D 3 complexes (16). Finally, increased bone resorption has been observed after calcitriol exposure in GHS cultured calvarias (17).…”

Thiosulfate Reduces Calcium Phosphate Nephrolithiasis

“…17 The pathophysiology of the hypercalciuria seems similar to that in humans in that it involves intestinal hyperabsorption, 18,19 reduced renal tubular reabsorption, 20 and increased bone mineral lability. 21,22 Virtually all of the GHS rats form kidney stones, whereas control rats have no evidence of stone formation. 23 On a standard rat diet, the kidney stones formed contain only calcium and phosphate.…”