Tradeoff-in-the-Nephron: A Theory to Explain the Primacy of Phosphate in the Pathogenesis of Secondary Hyperparathyroidism

Chemical evidence for the tradeoff-in-the-nephron hypothesis to explain secondary hyperparathyroidism

Background

Secondary hyperparathyroidism (SHPT) complicates advanced chronic kidney disease (CKD) and causes skeletal and other morbidity. In animal models of CKD, SHPT was prevented and reversed by reduction of dietary phosphate in proportion to GFR, but the phenomena underlying these observations are not understood. The tradeoff-in-the-nephron hypothesis states that as GFR falls, the phosphate concentration in the distal convoluted tubule ([P]_DCT]) rises, reduces the ionized calcium concentration in that segment ([Ca⁺⁺]_DCT), and thereby induces increased secretion of parathyroid hormone (PTH) to maintain normal calcium reabsorption. In patients with CKD, we previously documented correlations between [PTH] and phosphate excreted per volume of filtrate (E_P/C_cr), a surrogate for [P]_DCT. In the present investigation, we estimated [P]_DCT from physiologic considerations and measurements of phosphaturia, and sought evidence for a specific chemical phenomenon by which increased [P]_DCT could lower [Ca⁺⁺]_DCT and raise [PTH].

Methods and findings

We studied 28 patients (“CKD”) with eGFR of 14–49 mL/min/1.73m² (mean 29.9 ± 9.5) and 27 controls (“CTRL”) with eGFR > 60 mL/min/1.73m² (mean 86.2 ± 10.2). In each subject, total [Ca]_DCT and [P]_DCT were deduced from relevant laboratory data. The Joint Expert Speciation System (JESS) was used to calculate [Ca⁺⁺]_DCT and concentrations of related chemical species under the assumption that a solid phase of amorphous calcium phosphate (Ca₃(PO₄)₂ (am., s.)) could precipitate. Regressions of [PTH] on eGFR, [P]_DCT, and [Ca⁺⁺]_DCT were then examined. At filtrate pH of 6.8 and 7.0, [P]_DCT was found to be the sole determinant of [Ca⁺⁺]_DCT, and precipitation of Ca₃(PO₄)₂ (am., s.) appeared to mediate this result. At pH 6.6, total [Ca]_DCT was the principal determinant of [Ca⁺⁺]_DCT, [P]_DCT was a minor determinant, and precipitation of Ca₃(PO₄)₂ (am., s.) was predicted in no CKD and five CTRL. In CKD, at all three pH values, [PTH] varied directly with [P]_DCT and inversely with [Ca⁺⁺]_DCT, and a reduced [Ca⁺⁺]_DCT was identified at which [PTH] rose unequivocally. Relationships of [PTH] to [Ca⁺⁺]_DCT and to eGFR resembled each other closely.

Conclusions

As [P]_DCT increases, chemical speciation calculations predict reduction of [Ca⁺⁺]_DCT through precipitation of Ca₃(PO₄)₂ (am., s.). [PTH] appears to rise unequivocally if [Ca⁺⁺]_DCT falls sufficiently. These results support the tradeoff-in-the-nephron hypothesis, and they explain why proportional phosphate restriction prevented and reversed SHPT in experimental CKD. Whether equally stringent treatment can be as efficacious in humans warrants investigation.

Mediation of the relationship between proteinuria and serum phosphate: Insight from the KNOW-CKD study

Proteinuria and hyperphosphatemia are risk factors for cardiovascular disease in patients with chronic kidney disease (CKD). Although the interaction between proteinuria and the serum phosphate level is well established, the mechanistic link between the two, particularly the extent to which this interaction is mediated by phosphate-regulating factors, remains poorly understood. In this study, we examined the association between proteinuria and the serum phosphate level, as well as potential mediators, including circulating fibroblast growth factor (FGF23)/klotho, the 24-h urinary phosphate excretion rate to glomerular filtration rate ratio (EP/GFR), and the 24-h tubular phosphate reabsorption rate to GFR ratio (TRP/GFR). The analyses were performed with data from 1793 patients in whom 24-h urine protein and phosphate, serum phosphate, FGF23, and klotho levels were measured simultaneously, obtained from the KoreaN cohort study for Outcome in patients With Chronic Kidney Disease (KNOW-CKD). Multivariable linear regression and mediation analyses were performed. Total, direct, and indirect effects were also estimated. Patients with high serum phosphate levels were found to be more likely to exhibit greater proteinuria, higher FGF23 levels, and lower klotho levels. The 24-h EP/GFR increased and the 24-h TRP/GFR decreased with increasing proteinuria and CKD progression. Simple mediation analyses showed that 15.4% and 67.9% of the relationship between proteinuria and the serum phosphate level were mediated by the FGF23/klotho ratio and 24-h EP/GFR, respectively. Together, these two factors accounted for 73.1% of the relationship between serum markers. These findings suggest that proteinuria increases the 24-h EP/GFR via the FGF23/klotho axis as a compensatory mechanism for the increased phosphate burden well before the reduction in renal function is first seen.

Parathyroid Hormone, Fibroblast Growth Factor 23, and Parameters of Phosphate Reabsorption

BACKGROUND

The serum phosphorus concentration ([P]s) is the sum of EP/Ccr and TRP/Ccr, where Ccr is creatinine clearance and EP and TRP are rates of excretion and reabsorption of phosphate. In chronic kidney disease (CKD), parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) mediate reduction of TRP/Ccr, and [PTH] and [FGF23] are linear functions of EP/Ccr. If controls and patients with CKD are considered together, TRP/Ccr is a hyperbolic function of EP/Ccr. Given these observations, we hypothesized that hyperbolas would describe relationships of phosphate reabsorption to [PTH] and [FGF23].

METHODS

We studied 30 patients and 28 controls with mean eGFR of 29.5 and 86.0 mL/min/1.73 m2, respectively. All analyses combined both subsets. We measured fasting [PTH] 1-84 and intact [FGF23], and determined contemporaneous EP/Ccr, TRP/Ccr, fractional excretion of phosphorus (FEP), and phosphate tubular maximum per volume of filtrate (TmP/GFR). We examined linear regressions of TRP/Ccr and TmP/GFR on 100/[PTH] and 100/[FGF23]; from linear equations we derived hyperbolic equations relating reabsorptive parameters to hormone concentrations.

RESULTS

TRP/Ccr and TmP/GFR were linear functions of 100/[PTH] and 100/[FGF23] and hyperbolic functions of [PTH] and [FGF23]. TRP/Ccr and TmP/GFR fell minimally over the ranges of EP/Ccr, [PTH], and [FGF23] seen in CKD. FEP rose with EP/Ccr despite stable phosphate reabsorption.

CONCLUSIONS

Hyperbolas describe relationships of TRP/Ccr and TmP/GFR to [PTH] and [FGF23] if subjects with normal and reduced GFR are analyzed together. Although FEP rises with [PTH] and [FGF23] as GFR falls, the simultaneous increments do not signify hormonally mediated reductions in phosphate reabsorption.