Increased acute vascular deposition of radioactive phosphate is an index of calcification

Abstract

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.