Vascular calcification maladaptively participates in acute phosphate homeostasis.

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 AND 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 In a non-CKD model of VC, calcification was induced with 0.5 ug/kg calcitriol and then withdrawn (N = 24).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.

The 1,24,25(OH)3D3 metabolite in clinical and experimental CKD: Impact of calcitriol treatment.

Calcitriol, and other vitamin D receptor activators, remain a primary treatment for elevated parathyroid hormone levels in patients with end stage kidney disease. The objective of this study was to assess the 24-hydroxylation-mediated metabolism of 25(OH)D3 and 1,25(OH)2D3 in rats with experimental kidney disease treated with calcitriol and in a cross-sectional analysis of patients requiring hemodialysis. Methods: Animals were stratified by creatinine into a time control group or calcitriol (20 ng/kg/day) for 3 weeks following CKD induction using a dietary adenine model (0.25% adenine). Hemodialysis patients were recruited and demographic data including calcitriol prescription was obtained by chart review and participant interview. Vitamin D metabolites were assessed using LC-MS/MS. In the rat model, 1,25(OH)2D3 levels increased substantially in calcitriol-treated rats yet there was no increase in its primary metabolite: 1,24,25(OH)2D3. A lower ratio of 1,24,25(OH)2D3:1,25(OH)2D3 (1,25-VMR) was associated with increased calcium levels in calcitriol treated rats. In hemodialysis patients (N = 86), the level of 1,25(OH)2D3 was substantially higher in calcitriol-treated patients yet there was no difference between groups in 1,24,25(OH)3D3, resulting in a marked decrease in the 1,25-VMR in calcitriol treated patients. In hemodialysis patients treated with calcitriol, 1,25(OH)2D3 and a lower ratio between 1,24,25(OH)3D3 and 1,25(OH)2D3 were associated with higher serum calcium levels. Impaired metabolism of exogenous calcitriol may contribute to the adverse effects associated with this treatment. A better understanding of the uniquely dysfunctional catabolic vitamin D profile in CKD may guide more effective treatment strategies.

The metabolism of 1,25(OH)2D3 in clinical and experimental kidney disease.

Chronic kidney disease (CKD) results in calcitriol deficiency and altered vitamin D metabolism. The objective of this study was to assess the 24-hydroxylation-mediated metabolism of 25(OH)D3 and 1,25(OH)2D3 in a cross-sectional analysis of participants with a range of kidney function assessed by precise measured GFR (mGFR) (N = 143) and in rats with the induction and progression of experimental kidney disease. Vitamin D metabolites were assessed with LC-MS/MS. Circulating measures of 24-hydroxylation of 25(OH)D3 (24,25(OH)2D3:25(OH)D3) precisely decreased according to mGFR in humans and progressively in rats with developing CKD. In contrast, the 1,24,25(OH)3D3: 1,25(OH)2D3 vitamin D metabolite ratio increased in humans as the mGFR decreased and in rats with the induction and progression of CKD. Human participants taking cholecalciferol had higher circulating 1,24,25(OH)3D3, despite no increase of 1,25(OH)2D3. This first report of circulating 1,24,25(OH)3D3 in the setting of CKD provides novel insight into the uniquely altered vitamin D metabolism in this setting. A better understanding of the uniquely dysfunctional catabolic vitamin D profile in CKD may guide more effective treatment strategies. The potential that 24-hydroxylated products have biological activity of is an important area of future research.