Aims Non-renal extravasation of phosphate from the circulation and transient accumulation into tissues and extracellular fluid is a regulated process of acute phosphate homeostasis that is not well understood. This process is especially relevant in the setting of chronic kidney disease (CKD), where exposure to increased phosphate is prolonged due to inefficient kidney excretion. Furthermore, CKD-associated mineral dysregulation induces pathological accumulation of phosphate causing vascular calcification (VC). Our objective was to determine whether the systemic response to acute phosphate challenges is altered by VC. Methods/Results After bolus phosphate administration, circulating and tissue deposition of this challenge was assessed in two rat models of VC using a radiolabelled phosphate tracer. In an adenine-induced model of CKD (N = 70), animals with VC had a blunted elevation of circulating 33PO4 following oral phosphate administration (p < 0.01), and the discordant deposition could be traced to the calcified arteries (11.4[7.5,13.1]vs.43.0[35.5, 53.7] pmol/ng tissue, p < 0.001). In a non-CKD model of VC, calcification was induced with 0.5ug/kg calcitriol and then withdrawn (N = 24). New phosphate uptake by the calcified vasculature correlated to the pre-existing burden of calcification (r = 38, p < 0.001) and was substantially attenuated in the absence of calcification stimulus (p < 0.01). Phosphate accrual was stimulated by the phosphate challenge, and not present to the same degree during passive disposition of circulating phosphate. Further, the form of phosphate that deposited to the vasculature was predominately amorphous inorganic phosphate, and not that which was bound in matured calciprotein particles. Conclusions In the process of calcification, arteries acutely deposit substantial amorphous phosphate while blunting the elevation in the circulation, thereby altering the systemic disposition of phosphate, and identifying VC as a participatory mineral homeostatic organ. This study demonstrates the negative vascular consequence of acute fluctuations in circulating phosphate, and supports the importance of phosphate bioavailability and diet management in CKD patients as a mediator of cardiovascular risk.

<b><i>Background:</i></b> The Wnt/β-catenin pathway has been implicated in the development of adynamic bone disease in early-stage chronic kidney disease (CKD). Dickkopf-related protein 1 (DKK1) and sclerostin are antagonists of the Wnt/β-catenin pathway yet have not been widely used as clinical indicators of bone disease. This study characterized levels of DKK1, sclerostin, and other biomarkers of mineral metabolism in participants across a spectrum of inulin-measured glomerular filtration rate (GFR). <b><i>Methods:</i></b> GFR was measured by urinary inulin clearance (mGFR) in 90 participants. Blood samples were obtained for measurement of circulating DKK1, sclerostin, fibroblast growth factor 23 (FGF-23), parathyroid hormone (PTH), calcium, phosphate, α-klotho, and vitamin D metabolites including 25-hydroxyvitamin D₃ and 1,25-dihydroxyvitamin D₃. Spearman correlations and linear regressions were used where appropriate to examine the associations between measured values. <b><i>Results:</i></b> The median [IQR] age was 64 years [53.0–71.0], and the median [IQR] mGFR was 32.6 [21.7–60.6] mL/min. DKK1 decreased (<i>r</i> = 0.6, <i>p</i> &#x3c; 0.001) and sclerostin increased (<i>r</i> = −0.4, <i>p</i> &#x3c; 0.001) as kidney function declined, and both were associated with phosphate, PTH, FGF-23, and 1,25-dihydroxyvitamin D₃ in the unadjusted analysis. After adjustment for age and mGFR, DKK1 remained significantly associated with PTH. <b><i>Conclusion:</i></b> The results of this study demonstrate opposing trends in Wnt/β-catenin pathway inhibitors, DKK1 and sclerostin, as mGFR declines. Unlike sclerostin, DKK1 levels decreased significantly as mGFR declined and was independently associated with PTH. Future studies should determine whether measurement of Wnt signaling inhibitors may be useful in predicting bone histomorphometric findings and important clinical outcomes in patients with CKD.

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Mineral bone disorder and vascular calcification (VC) are substantial contributors to the elevated cardiovascular disease (CVD) burden in chronic kidney disease patients. The degree to which the uremic milieu and mineral dysregulation individually contribute to this CVD is poorly understood. Calcitriol, the active form of vitamin D, is a key regulator of mineral metabolism. In this study, we present a model of rapid calcitriol‐induced VC in rats with normal kidney function, with findings that point to active and non‐active processes of calcification being fundamentally different phenotypes.Male Sprague Dawley rats (n=17, 15 weeks) were injected with 0.5 μg/kg/day calcitriol SQ for 8 days. On the 9th day, half of the animals were sacrificed (n=8, CxE), and the remainder were sacrificed 14 days after the cessation of calcitriol (n=9, CxL). Control animals (n=6) followed the same protocol. Circulating calcium, phosphate, and hormones regulating mineral metabolism (parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF‐23)) were measured every two days. At sacrifice, animals underwent an IV infusion of 300μmol PO4 spiked with radioactive 33PO4. Forty body tissues, including 14 across the vascular tree, were harvested and analyzed for mineral content/VC (calcium and phosphate) as well as acute phosphate deposition (33PO4 accrual).Daily administration of high‐dose calcitriol significantly elevated FGF‐23 and calcium, and suppressed PTH after only 2 doses, however, 14 days after calcitriol withdrawal, the analytes had fully normalized (Figure 1). Substantial VC was generated over the 8 days of treatment (7.8±4.2 v. 166±142 nmol Ca/mg tissue, p<0.001) and persisted 14 days after stopping treatment (540±492 nmol Ca/mg tissue). Following IV infusion of phosphate labelled with 33PO4, animals sacrificed directly following calcitriol treatment (CxE) had greater accrual of phosphate acutely in the vascular bed compared to those sacrificed 14 days after the cessation of treatment (CxL; −50.4%, p<0.001), despite having sustained levels of resident VC (Figure 2).Despite the similar magnitude of VC, acute deposition of phosphate following the IV load, a potential measure of calcification activity, was significantly greater during calcitriol treatment (CxE) than after stopping it (CxL). This suggests that the VC itself does not mediate the increased accrual of phosphate, but rather the cellular phenotype of the tissue. Also, it was this acute activity, not the resident VC, that aligned with dysregulated mineral metabolism markers. The transcriptional, cellular, and physiological differences of these two states, as well as their differential associations with circulating analytes are not currently differentiated in VC research. These differences likely have an important role in VC present in humans.Support or Funding InformationThis research was funded by Canadian Institutes of Health Research and Vanier Canada Graduate Scholarships.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.