Prevention of phosphate-induced progression of uremia in rats by 3-phosphocitric acid

Contribution of phosphate and FGF23 to CKD progression

PURPOSE OF REVIEW

Progressive forms of chronic kidney disease (CKD) exhibit kidney inflammation and fibrosis that drive continued nephron loss; however, factors responsible for the development of these common pathologic features remain poorly defined. Recent investigations suggest pathways involved in maintaining urinary phosphate excretion in CKD may be contributing to kidney function decline. This review provides an update on recent evidence linking altered phosphate homeostasis to CKD progression.

RECENT FINDINGS

High dietary phosphate intake and increased serum concentrations of fibroblast growth factor 23 (FGF23) both increase urinary phosphate excretion and are associated with increased risk of kidney function decline. Recent investigations have discovered high concentrations of tubular phosphate promote phosphate-based nanocrystal formation that drives tubular injury, cyst formation, and fibrosis.

SUMMARY

Studies presented in this review highlight important scientific discoveries that have molded our current understanding of the contribution of altered phosphate homeostasis to CKD progression. The collective observations from these investigations implicate phosphaturia, and the resulting formation of phosphate-based crystals in tubular fluid, as unique risk factors for kidney function decline. Developing a better understanding of the relationship between tubular phosphate handling and kidney pathology could result in innovative strategies for improving kidney outcomes in patients with CKD.

Dietary phosphate restriction attenuates polycystic kidney disease in mice

Studies in rodents with reduced nephron mass have suggested a strong positive correlation between dietary phosphate consumption and CKD progression. Prior work by our group demonstrated that dietary phosphate restriction can prevent tubular injury and microcyst formation in rodents with glomerulonephritis. Tubular injury and cystic dilation of tubules are key contributors to kidney function decline in polycystic kidney disease (PKD). Here, we determined whether dietary phosphate restriction slows renal cyst growth and fibrosis in a mouse model of PKD. Pcy/pcy mice received a normal phosphate (0.54%) or a phosphate-restricted (0.02%) diet ( n = 10/group) from 7 to 20 wk of age. All of the other major dietary constituents, including protein source and content, were comparable between the two diets. At 20 wk, body weight, kidney weight-to-body weight ratio (KW/BW), cystic area, cyst number, and kidney fibrosis were quantified. Pcy/pcy mice fed a phosphate-restricted diet had lower serum phosphate, fibroblast growth factor 23, and parathyroid hormone levels, along with elevated serum calcium levels and increased kidney Klotho gene expression compared with mice that consumed the control diet. Dietary phosphate restriction resulted in a 25% lower KW/BW ratio and reduced the cyst number, cystic index, and gene expression for the tubular injury markers neutrophil gelatinase-associated lipocalin and interleukin-18. Mice fed the phosphate-restricted diet exhibited lower kidney expression for pathways involved in collagen deposition and myofibroblast activation (collagen type I-α1, phosphorylated SMAD3, and α-smooth muscle actin); however, histological differences in kidney fibrosis were not appreciated. Dietary phosphate restriction slows cystogenesis and inhibits the activation of key pathways in the generation of kidney fibrosis in PKD mice.

Dietary Phosphorus Intake and the Kidney

Although phosphorus is an essential nutrient required for multiple physiological functions, recent research raises concerns that high phosphorus intake could have detrimental effects on health. Phosphorus is abundant in the food supply of developed countries, occurring naturally in protein-rich foods and as an additive in processed foods. High phosphorus intake can cause vascular and renal calcification, renal tubular injury, and premature death in multiple animal models. Small studies in human suggest that high phosphorus intake may result in positive phosphorus balance and correlate with renal calcification and albuminuria. Although serum phosphorus is strongly associated with cardiovascular disease, progression of kidney disease, and death, limited data exist linking high phosphorus intake directly to adverse clinical outcomes. Further prospective studies are needed to determine whether phosphorus intake is a modifiable risk factor for kidney disease.

Effect of ammonium chloride and dietary phosphorus in the azotaemic rat. Part II--Kidney hypertrophy and calcium deposition

BACKGROUND

Kidney hypertrophy is stimulated by both partial nephrectomy and NH(4)Cl administration. Also, parathyroidectomy (PTX) has been reported to prevent kidney hypertrophy induced by a high protein diet. Our goal was to determine in the azotaemic rat: (i) the combined effects of NH(4)Cl administration and dietary phosphorus on the development of kidney hypertrophy and calcium deposition in the kidney and (ii) whether the absence of parathyroid hormone (PTH) affected the development of kidney hypertrophy and calcium deposition.

METHODS

High (HPD, 1.2%), normal (NPD, 0.6%) or low (LPD, <0.05%) phosphorus diets were given to 5/6 nephrectomized rats for 30 days. In each dietary group, one-half of the rats were given NH(4)Cl in the drinking water. The six groups of rats were: (i) HPD + NH(4)Cl; (ii) HPD; (iii) NPD + NH(4)Cl; (iv) NPD; (v) LPD + NH(4)Cl and (vi) LPD. In a separate study, PTX was performed to determine whether PTH affected renal hypertrophy in 5/6 nephrectomized rats given NH(4)Cl.

RESULTS

Both with and without NH(4)Cl (+/-NH(4)Cl), kidney weight was greatest (P<0.05) in the HPD groups. In each dietary phosphorus group, kidney weight was greater (P<0.05) in the NH(4)Cl group. In both the +/-NH(4)Cl groups, kidney calcium content was greatest (P<0.05) in the HPD group, but was less (P<0.05) in the NPD and HPD groups given NH(4)Cl. An inverse correlation was present between creatinine clearance and kidney calcium content (r = -0.51, P<0.001). When factored for kidney weight, creatinine clearance was less (P<0.05) in the HPD group in both the +/-NH(4)Cl groups, but was greater in the HPD + NH(4)Cl than in the HPD group. In PTX rats, kidney weight was greater (P<0.05) and kidney calcium deposition was less (P<0.05) in rats given NH(4)Cl.

CONCLUSIONS

In azotaemic rats studied for 30 days, NH(4)Cl administration induced kidney hypertrophy. A HPD also induced kidney hypertrophy. The effects on kidney calcium deposition were divergent for which NH(4)Cl administration decreased and a HPD increased calcium deposition. The inverse correlation between kidney calcium content and creatinine clearance suggests that kidney calcium deposition is harmful to renal function. When factored for kidney weight, the lower creatinine clearance in the high phosphorus group suggests that kidney hypertrophy does not completely compensate for the harmful effects of a HPD. This result also suggests that a longer study would probably result in more rapid deterioration in the high phosphorus group. In PTX rats, the absence of PTH did not prevent NH(4)Cl from inducing kidney hypertrophy and reducing kidney calcium deposition. In conclusion, NH(4)Cl and dietary phosphorus each independently affect kidney growth and calcium deposition in the growing rat with renal failure.

An open-label, crossover study of a new phosphate-binding agent in haemodialysis patients: ferric citrate

BACKGROUND

Hyperphosphataemia contributes to secondary hyperparathyroidism and renal osteodystrophy in patients with end-stage renal disease (ESRD). Calcium salts are widely employed to bind dietary phosphate (P) but they may promote positive net calcium balance and metastatic calcification. We recently reported that ferric compounds bind intestinal phosphate in studies of normal and azotemic rats.

METHODS

To extend this observation, we performed an open-label, random order, crossover comparison study of ferric citrate and calcium carbonate in haemodialysis patients from two teaching hospitals. The study sample consisted of 23 women and 22 men with an average age of 52.5 +/- 11.8 (SD) years and an average weight of 54.5 +/- 10.7 kg. All forms of iron therapy were discontinued. Two weeks before the study, patients were instructed to discontinue all P-binding agents. The patients were randomly assigned to receive either calcium carbonate (3 g/day) or ferric citrate (3 g/day) for 4 weeks followed by a 2 week washout period, and then crossed over to the other P-binding agent for 4 weeks.

RESULTS

From a baseline concentration of 5.6 +/- 1.5 mg/dl, the serum P increased during the washout period to 7.2 +/- 1.9 mg/dl prior to calcium carbonate treatment, and to 6.7 +/- 1.9 mg/dl prior to ferric citrate treatment. The serum P concentration fell significantly during treatment with both calcium carbonate (7.2 +/- 1.9 to 5.2 +/- 1.5 mg/dl, P<0.0001) and ferric citrate (6.7 +/- 1.9 to 5.7 +/- 1.6 mg/dl, P<0.0001). The results were not influenced by order of treatment. Under the conditions of the study protocol, ferric citrate was less effective than calcium carbonate at lowering the serum phosphate concentration. The serum Ca concentration increased during treatment with calcium carbonate but not ferric citrate. Ferric citrate treatment did not affect the serum concentration of aluminium. Ferric citrate treatment was associated with mild and generally tolerable gastrointestinal symptoms.

CONCLUSION

Ferric citrate shows promise as a means of lowering the serum phosphate concentration in haemodialysis patients. Further studies are needed to find the optimal dose.

Are we mismanaging calcium and phosphate metabolism in renal failure?

Secondary hyperparathyroidism and renal osteodystrophy are the consequences of abnormal calcium, phosphate, and calcitriol metabolism ensuing from renal failure. Evidence suggests that calcium balance tends to become negative as we grow older than 35 years of age; however, the current dialysis modalities provide patients regardless of age with excessive calcium during dialysis. Administration of calcitriol in the management of hyperparathyroidism further increases the calcium and phosphate absorption. Furthermore, the current thrice-weekly renal replacement therapies fail to remove the daily absorbed phosphate, and we have to use calcium carbonate as a primary phosphate-binding agent to reduce intestinal phosphate absorption. The large calcium mass transfer and phosphate retention could lead to soft tissue calcification, especially in older end-stage renal disease (ESRD) patients. Consequently, only by maintaining a negative calcium balance during renal replacement therapy can we safely use calcitriol and calcium carbonate for the management of secondary hyperparathyroidism. Recent studies have indicated that phosphate restriction alone independent of plasma calcitriol or calcium can lower plasma parathyroid hormone (PTH) in renal failure and prevent hyperplasia of parathyroid glands. Therefore, phosphate control perhaps is the most important means to prevent secondary hyperparathyroidism. Previous studies have shown that ferric compounds are potent phosphate-binding agents; hence, these compounds warrant further trial in the management of phosphate metabolism in renal failure.

Use of phosphonocarboxylic acids as inhibitors of sodium-phosphate cotransport

Phosphonocarboxylic acids, initially developed as antiviral agents, are found to be specific inhibitors of phosphate (P(i)) transport across cell membranes. Foscarnet (PFA), the most potent and the most widely used compound, can induce phosphaturia both after parenteral and oral administration. Furthermore, it can inhibit intestinal phosphate absorption when administered orally. PFA absorption and bioavailability are increased in animals on phosphate-restricted diets. PFA also blunts the adaptive increase in intestinal and renal Na(+)-P(i) cotransport which accompanies dietary phosphorus restriction. Finally, PFA is shown to inhibit hydroxyapatite crystal formation and calcium-phosphate precipitation when tested in in vitro systems. These properties, and the low toxicity of PFA, point to potential new applications for PFA and some of its analogs in clinical conditions such as chronic renal insufficiency, where phosphate retention may lead to progression of renal failure and to other serious complications.

High-performance liquid chromatographic and mass spectrometric identification of phosphocitrate synthesized by human kidney homogenate

Two high-performance liquid chromatographic techniques for the analysis of phosphocitrate, a metabolite synthesized by the cell that is thought to play a role in a number of physiological as well as pathological events, have been developed. The mass spectra of the isolated compound obtained under fast-atom bombardment and collisionally activated decomposition mass-analysed ion kinetic energy conditions are also reported.

Hypertension-related renal injury: a major contributor to end-stage renal disease

The US Renal Data System Annual Report reveals that 57% of new cases and of end-stage renal disease are attributed to hypertensive nephropathy and diabetic nephropathy. Analyses of the data on serum creatinine from the Multiple Risk Factor Intervention Trial confirms that one in 20 of the hypertensive men exhibits a rate of decline in renal function that equals or exceeds 3% per year, the rate of loss being greater in older men, black men, and men with higher baseline blood pressure. Effective blood pressure treatment with maintenance of diastolic blood pressure below 95 mm Hg protected renal function in non-blacks but not in blacks, despite comparable blood pressure reduction in blacks. A longitudinal study of a diabetic cohort (n = 131) revealed that hypertension, plasma angiotensin II, and aldosterone are independent predictors of accelerated loss of renal function in diabetic nephropathy, possibly aggravated by diuretic usage as part of the antihypertensive regimen in diabetic nephropathy. No significant loss of renal function could be documented in those individuals whose blood pressure was adequately controlled (systolic blood pressure, < 140 mm Hg). These findings provide emphasis for the importance of adequate blood pressure control in both essential hypertension and hypertension associated with diabetes mellitus. They also provide support for the proposal that careful blood pressure control offers promise for reducing the incidence of end-stage renal disease in both hypertensive nephropathy and diabetic nephropathy.

Effects of dietary protein and phosphorus restriction on the progression of chronic renal failure

In recent years, evidence has accumulated suggesting that early dietary intervention in the form of protein restriction can dramatically alter the natural history of chronic renal insufficiency. This article reviews the literature in this area and summarizes the questions that remain for future investigators to answer. Interest in the role of protein intake in renal disease dates back to the early 1900s, when several investigators found that the progression of renal failure was accelerated in rats and rabbits fed high-protein diets. Subsequent work in animal models has demonstrated that nephron loss, resulting from a variety of disease states, is associated with elevated intraglomerular pressures and flows. Several investigators now have postulated that these abnormal hemodynamics, resulting from intrarenal vasodilation and hyperperfusion, damage the glomerulus and may produce further nephron loss independent of the initial renal insult. Dietary protein restriction appears to reduce these pressures and flows towards normal in animals, although the operative mechanism has yet to be identified. Limited studies of dietary protein restriction in human renal disease have suggested a beneficial effect, but carefully controlled prospective studies will be necessary to establish clear therapeutic efficacy.

Parenteral calcitriol for treatment of severe renal osteodystrophy in children with chronic renal insufficiency

Renal osteodystrophy is a common and incapacitating complication of chronic renal failure in children. Standard therapy with oral calcium supplements, phosphate binders, and vitamin D preparations is often inadequate to control progressive bone disease. We report the use of parenteral calcitriol therapy in two children, aged 2 and 15 years, respectively, with chronic renal failure. This treatment effectively suppressed secondary hyperparathyroidism in both patients, causing a nearly 50% reduction in circulating parathyroid hormone level and a parallel decline in serum alkaline phosphatase activity. In the younger patient, therapy was associated with healing of subperiosteal bone resorption and accelerated growth velocity. These findings indicate that parenteral administration of calcitriol may be an effective treatment option in some patients with refractory renal osteodystrophy and secondary hyperparathyroidism.

Relation between renal calcium content and renal impairment in 246 human renal biopsies

Tissue calcium content from 246 diagnostic human renal biopsies was measured to assess whether elevated tissue calcium concentration could be demonstrated to exist early during the course of human renal disease or was only a manifestation of advanced renal impairment. Renal calcium content correlated significantly with serum creatinine (r = +0.23, P less than 0.001, N = 246); serum phosphate P less than 0.001, N = 169) but not with serum calcium (r = -0.10, P greater than 0.1, N = 193). Fivefold greater calcium content was measured in biopsied patients with normal renal function than in normal postmortem renal tissue (35.7 +/- 5.2 vs. 7.6 +/- 0.7 mgCa/100 g wet renal tissue, P less than 0.001). Those biopsied patients with significant functional impairment (SCr greater than 1.5 mg/dl) had a higher mean level of serum phosphorus and serum [Ca] X [P] product than patients with normal renal function (5.19 +/- 0.22 vs. 3.92 +/- 0.11 mg P/dl and 44.8 +/- 1.8 vs. 35.7 +/- 1.2 mg2/dl2, respectively), and slightly higher renal calcium content (85.3 +/- 32.2 vs. 35.7 +/- 5.2 Ca/100 g wet renal tissue, P = 0.06), which correlated with histologic calcium deposition (r = +0.52, P less than 0.02, N = 20). These findings are consistent with the hypothesis that renal calcium deposition begins early in the course of a variety of renal diseases and hence may play a secondary pathogenetic role that accelerates progression to chronic renal failure. Severity of renal calcium deposition is equally closely related to hyperphosphatemia and to the level of renal impairment.