Fibroblast growth factor 23 (FGF23) significantly increases with declining renal function, leading to reduced renal tubular phosphate reabsorption, decreased 1,25-dihydroxyvitamin D, and increased left ventricular hypertrophy. Elevated FGF23 is associated with increased mortality. FGF23 is synthesized in osteoblasts and osteocytes; however, the mechanisms by which it is regulated are not clear. Patients with chronic kidney disease have decreased renal acid excretion leading to metabolic acidosis, which has a direct effect on bone cell activity. We hypothesized that metabolic acidosis would directly increase bone cell FGF23 production. Using cultured neonatal mouse calvariae, we found that metabolic acidosis increased medium FGF23 protein levels as well as FGF23 RNA expression at 24 h and 48 h compared with incubation in neutral pH medium. To exclude that the increased FGF23 was secondary to metabolic acidosis-induced release of bone mineral phosphate, we cultured primary calvarial osteoblasts. In these cells, metabolic acidosis increased FGF23 RNA expression at 6 h compared with incubation in neutral pH medium. Thus metabolic acidosis directly increases FGF23 mRNA and protein in mouse bone. If these results are confirmed in humans with chronic kidney disease, therapeutic interventions to mitigate acidosis, such as bicarbonate administration, may also lower levels of FGF23, decrease left ventricular hypertrophy, and perhaps even decrease mortality.osteoblasts; chronic kidney disease THE LEVEL OF THE phosphaturic hormone fibroblast growth factor 23 (FGF23) increases incrementally with declining renal function (29, 33), which results in decreased renal tubule inorganic phosphate (P i ) reabsorption (27, 33), a reduction of 1,25-dihydroxyvitamin D [1,25(OH) 2 D] (27, 33), and subsequent reduced intestinal P i absorption (33). Elevated levels of FGF23 induce left ventricular hypertrophy (6) and are associated with a significant increase in mortality (12).FGF23 is produced in osteocytes and osteoblasts (27); however, the mechanisms by which FGF23 is regulated are not clear. As patients progress from chronic kidney disease (CKD) stage 1 to stage 5, there is a significant incremental increase in FGF23, and patients on dialysis have the highest levels of FGF23 (29). The increase in both P i retention and serum P i during CKD may increase serum FGF23 (31). However, P i may not be a primary regulator of FGF23 since there is little correlation between serum P i and serum FGF23 in patients without CKD (26). 1,25(OH) 2 D has also been shown to directly increase FGF23 production and has been suggested as a "counterregulatory" hormone (24). However, in CKD, where levels of 1,25(OH) 2 D are low (23), 1,25(OH) 2 D is unlikely to be the proximate stimulus for the elevated FGF23 serum levels. Parathyroid hormone (PTH), which is elevated in patients with CKD with secondary hyperparathyroidism (23), stimulates FGF23 expression in osteoblasts (21). However, a large scale study in humans with CKD suggests that levels of FGF23 increase ...

We have bred a strain of rats to maximize urine (U) calcium (Ca) excretion and model hypercalciuric nephrolithiasis. These genetic hypercalciuric stone-forming (GHS) rats excrete more UCa than control Sprague-Dawley rats, uniformly form kidney stones and, similar to patients, demonstrate lower bone mineral density. Clinically thiazide diuretics reduce UCa and prevent stone formation; however, whether they benefit bone is not clear. We used GHS rats to test the hypothesis that the thiazide diuretic chlorthalidone (CTD) would have a favorable effect on bone density and quality. Twenty GHS rats received a fixed amount of a 1.2% Ca diet and half were also fed CTD (4–5 mg/kg/day). Rats fed CTD had a marked reduction in UCa. The axial and appendicular skeletons were studied. An increase in trabecular mineralization was observed with CTD compared to controls. CTD also improved the architecture of trabecular bone. Using µCT, trabecular bone volume (BV/TV), trabecular thickness and trabecular number were increased with CTD. A significant increase in trabecular thickness with CTD was confirmed by static histomorphometry. CTD also improved the connectivity of trabecular bone. Significant improvements in vertebral strength and stiffness were measured by vertebral compression. Conversely, a slight loss of bending strength was detected in the femoral diaphysis with CTD. Thus, results obtained in hypercalciuric rats suggest that CTD can favorably influence vertebral fracture risk. CTD did not alter formation parameters suggesting that the improved vertebral bone strength was due to decreased bone resorption and retention of bone structure.

Potassium citrate is prescribed to decrease stone recurrence in patients with calcium nephrolithiasis. Citrate binds intestinal and urine calcium and increases urine pH. Citrate, metabolized to bicarbonate, should decrease calcium excretion by reducing bone resorption and increasing renal calcium reabsorption. However, citrate binding to intestinal calcium may increase absorption and renal excretion of both phosphate and oxalate. Thus, the effect of potassium citrate on urine calcium oxalate and calcium phosphate supersaturation and stone formation is complex and difficult to predict. To study the effects of potassium citrate on urine supersaturation and stone formation, we utilized 95th-generation inbred genetic hypercalciuric stone-forming rats. Rats were fed a fixed amount of a normal calcium (1.2%) diet supplemented with potassium citrate or potassium chloride (each 4 mmol/d) for 18 weeks. Urine was collected at 6, 12, and 18 weeks. At 18 weeks, stone formation was visualized by radiography. Urine citrate, phosphate, oxalate, and pH levels were higher and urine calcium level was lower in rats fed potassium citrate. Furthermore, calcium oxalate and calcium phosphate supersaturation were higher with potassium citrate; however, uric acid supersaturation was lower. Both groups had similar numbers of exclusively calcium phosphate stones. Thus, potassium citrate effectively raises urine citrate levels and lowers urine calcium levels; however, the increases in urine pH, oxalate, and phosphate levels lead to increased calcium oxalate and calcium phosphate supersaturation. Potassium citrate induces complex changes in urine chemistries and resultant supersaturation, which may not be beneficial in preventing calcium phosphate stone formation.

Frick KK, Asplin JR, Favus MJ, Culbertson C, Krieger NS, Bushinsky DA. Increased biological response to 1,25(OH)2D3 in genetic hypercalciuric stone-forming rats. Am J Physiol Renal Physiol 304: F718-F726, 2013. First published January 23, 2013 doi:10.1152/ajprenal.00645.2012.-Genetic hypercalciuric stone-forming (GHS) rats, bred to maximize urine (U) calcium (Ca) excretion, have increased intestinal Ca absorption and bone Ca resorption and reduced renal Ca reabsorption, leading to increased UCa compared with the Sprague-Dawley (SD) rats. GHS rats have increased vitamin D receptors (VDR) at each of these sites, with normal levels of 1,25(OH)2D3 (1,25D), indicating that their VDR is undersaturated with 1,25D. We tested the hypothesis that 1,25D would induce a greater increase in UCa in GHS rats by feeding both strains ample Ca and injecting 1,25D (25 ng · 100 g body wt Ϫ1 · day Ϫ1 ) or vehicle for 16 days. With 1,25D, UCa in SD increased from 1.7 Ϯ 0.3 mg/day to 24.4 Ϯ 1.2 (⌬ ϭ 22.4 Ϯ 1.5) and increased more in GHS from 10.5 Ϯ 0.7 to 41.9 Ϯ 0.7 (⌬ ϭ 29.8 Ϯ 1.8; P ϭ 0.003). To determine the mechanism of the greater increase in UCa in GHS rats, we measured kidney RNA expression of components of renal Ca transport. Expression of transient receptor potential vanilloid (TRPV)5 and calbindin D28K were increased similarly in SD ϩ 1,25D and GHS ϩ 1,25D. The Na ϩ /Ca 2ϩ exchanger (NCX1) was increased in GHS ϩ 1,25D. Klotho was decreased in SD ϩ 1,25D and GHS ϩ 1,25D. TRPV6 was increased in SD ϩ 1,25D and increased further in GHS ϩ 1,25D. Claudin 14, 16, and 19, Na/K/2Cl transporter (NKCC2), and secretory K channel (ROMK) did not differ between SD ϩ 1,25D and GHS ϩ 1,25D. Increased UCa with 1,25D in GHS exceeded that of SD, indicating that the increased VDR in GHS induces a greater biological response. This increase in UCa, which must come from the intestine and/or bone, must exceed any effect of 1,25D on TRPV6 or NCX1-mediated renal Ca reabsorption. vitamin D; calcium; kidney stones; reabsorption HYPERCALCIURIA IS THE MOST common metabolic abnormality in patients with nephrolithiasis(19). Hypercalciuria leads to increased urine (U) supersaturation (SS) with respect to the solid phases of calcium hydrogen phosphate (CaHPO 4 , brushite) and calcium oxalate (CaOx), enhancing the probability of nucleation and growth of crystals into clinically significant stones (19). In humans, idiopathic hypercalciuria (IH) is characterized by increased intestinal calcium (Ca) absorption and/or decreased renal tubule Ca reabsorption resulting in hypercalciuria with normal serum (S) Ca, normal or elevated S1,25(OH) 2 D 3 (1,25D), normal S parathyroid hormone (PTH), normal or low S phosphate (P), and low bone mass (19,70,85). The familial pattern of IH is consistent with a polygenic mode of inheritance (70,71,85).To study the pathophysiology of hypercalciuria and stone formation, we have established a strain of hypercalciuric rats by selectively inbreeding Sprague-Dawley (SD) rats for increased UCa excretion (3-5, 16 -18, 20 -25, 27-29, 31, 3...

(1,25D) increases urine (U) Ca to a greater extent in GHS than in controls [Sprague-Dawley (SD)]. The additional UCa may result from an increase in intestinal Ca absorption and/or bone resorption. To determine the source, we asked whether 1,25D would increase UCa in GHS fed a low-Ca (0.02%) diet (LCD). With 1,25D, UCa in SD increased from 1.2 Ϯ 0.1 to 9.3 Ϯ 0.9 mg/day and increased more in GHS from 4.7 Ϯ 0.3 to 21.5 Ϯ 0.9 mg/day (P Ͻ 0.001). In GHS rats on LCD with or without 1,25D, UCa far exceeded daily Ca intake (2.6 mg/day). While the greater excess in UCa in GHS rats must be derived from bone mineral, there may also be a 1,25D-mediated decrease in renal tubular Ca reabsorption. RNA expression of the components of renal Ca transport indicated that 1,25D administration results in a suppression of klotho, an activator of the renal Ca reabsorption channel TRPV5, in both SD and GHS rats. This fall in klotho would decrease tubular reabsorption of the 1,25D-induced bone Ca release. Thus, the greater increase in UCa with 1,25D in GHS fed LCD strongly suggests that the additional UCa results from an increase in bone resorption, likely due to the increased number of VDR in the GHS rat bone cells, with a possible component of decreased renal tubular calcium reabsorption. vitamin D; calcium; kidney stones; intestinal absorption THE MOST COMMON METABOLIC abnormality in patients who form calcium-based kidney stones is hypercalciuria (14, 50). The increased urine (U) calcium (Ca) excretion enhances nucleation and growth of calcium hydrogen phosphate (CaHPO 4 ; brushite) and/or calcium oxalate (CaOx) crystals into stones (14). Idiopathic hypercalciuria (IH) typically manifests as hypercalciuria with normal serum (S) Ca, normal or elevated S1,25(OH) 2 D 3 (1,25D), normal or elevated S parathyroid hormone (PTH), normal or low S phosphate (P), and low bone mass (14,48,55) and is polygenic (48,49,55).To study the pathophysiology of hypercalciuria and stone formation, we established a strain of hypercalciuric rats by selectively inbreeding Sprague-Dawley (SD) rats for increased UCa excretion (3-5, 11-13, 15-19, 21-24, 27, 32, 35, 36, 40, 41, 43, 45, 52, 57, 61, 62). After more than 80 generations, each rat consistently excretes ϳ8-to 10-fold more UCa than SD controls (3-5, 11-13, 15-19, 21-24, 27, 32, 35, 36, 40, 41, 43, 45, 52, 57, 61, 62) and forms kidney stones (3,17,18,22); these animals are termed genetic hypercalciuric stone-forming (GHS) rats (3-5, 11-13, 15-19, 21, 23, 24, 27, 32, 35, 36, 40, 41, 43, 45, 52, 57, 61, 62).GHS rats exhibit many features of human IH including normal SCa (15), increased intestinal Ca absorption (45) and bone resorption (43), decreased renal tubule Ca reabsorption (57), and normal S1,25D levels in addition to decreased bone mineral density (24, 32) and have a polygenic mode of inheritance (35). GHS rats have elevated levels of vitamin D receptor (VDR) protein in Ca-transporting organs including the kidney, intestine, and bone (30,43,45,57).In humans, the changes in intestine, kidney, and ...