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Objective: Warfarin is associated with medial arterial calcification in humans, but the magnitude and specificity of this effect and the role of other risk factors are unknown. Using serial mammograms, progression of arterial calcification was compared in women receiving no anticoagulants, warfarin, or other anticoagulants, and before, during, and after warfarin use. Approach and Results: Warfarin users with mammograms were identified by computerized searches of medical records that included renal function and diabetes mellitus. Lengths of calcified arterial segments were measured, with progression expressed as millimeters per breast per year and presented as medians and interquartile range (IQR). In women with normal renal function (estimated glomerular filtration rate >60 mL/minute per 1.73 m 2 ), progression was 3.9-fold greater in warfarin users: 9.9 (3.8-16) versus 2.5 (0.7-6.7) in controls, P =0.0003, but not increased in users of other anticoagulants. In longitudinal analyses, progression increased from 2.1 (IQR, 0.3-3.9) to 13.8 (IQR, 7.8-38.7; P =0.011) after starting warfarin (n=11) and decreased from 8.8 (IQR, 1.1-10) to 1.9 (IQR, -10 to 6.7; P =0.024) after discontinuation of warfarin (n=13). Progression of calcification was similar in warfarin users with chronic kidney disease (7.3 [IQR, 3.6-17], n=29) but markedly accelerated in warfarin users with end-stage renal disease (47 [IQR, 31-183], n=11; P =0.0002). Progression was similar in diabetic and nondiabetic warfarin users (10.1 [IQR, 3.8-24] versus 7.8 [IQR, 3.6-15]) and did not correlate with age ( r =0.09) or duration of warfarin therapy ( r =0.12)., Conclusions: Warfarin significantly accelerates medial arterial calcification in humans. This effect is markedly augmented in end-stage renal disease.

Pathologic cardiovascular calcification is associated with a number of conditions and is a common complication of chronic kidney disease. Because ambient calcium and phosphate levels together with properties of the vascular matrix favor calcification even under normal conditions, endogenous inhibitors such as pyrophosphate play a key role in prevention. Genetic diseases and animal models have elucidated the metabolism of extracellular pyrophosphate and demonstrated the importance of pyrophosphate deficiency in vascular calcification. Therapies based on pyrophosphate metabolism have been effective in animal models, including renal failure, and hold promise as future therapies to prevent vascular calcification., (Copyright © 2018 International Society of Nephrology. Published by Elsevier Inc. All rights reserved.)

A number of histologic changes are associated with the medial arterial calcification that occurs in chronic kidney disease, leading to several different hypotheses concerning the underlying mechanism. Careful timing of these changes in relation to the onset of the calcification, as reported in this issue of the journal, can shed light on which changes are pathogenic as opposed to secondary in reaction to the calcification., (Copyright © 2017 International Society of Nephrology. Published by Elsevier Inc. All rights reserved.)

Chronic decline in renal function is accompanied by deterioration of bone structure and function and progressive calcification of the vascular system. Both disease states have been linked with increased morbidity and mortality in chronic kidney disease. The severe alterations of mineral metabolism inherent with loss of renal function have an impact on vascular calcification development and progression, and several investigators have focused on ways to reduce their impact on vascular health. Imaging has contributed an important role in the assessment of vascular calcification, and the impact of various interventions aimed at curbing their progression., (© 2017 Wiley Periodicals, Inc.)

The extent to which vascular calcification is reversible and the possible mechanisms are unclear. To address this, calcified aortas from uremic mice were transplanted orthotopically into normal mice, and the calcium content, histology, and minerals of the allografts were compared with the nontransplanted donor aorta. Calcium content decreased immediately after transplantation but remained constant thereafter, with 68% ± 12% remaining after 34 weeks. X-ray diffraction showed the presence of apatite in both donor aortas and allografts. Osteoclasts were absent in the allografts and there was no expression of the macrophage marker CD11b, the osteoclast marker tartrate-resistant acid phosphatase, or carbonic anhydrase II. The initial loss of calcium was less in heavily calcified aortas and was associated with an increase in the Ca/P ratio from 1.49 to 1.63, consistent with a loss of nonapatitic calcium. The results indicate that vascular calcification persists after reversal of uremia, because of a lack of active resorption of apatite. This failure to resorb established calcifications may contribute to the severity of vascular calcification and suggests that therapy should be aimed at prevention., (Copyright © 2017 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.)

Summary: Calcium supplements have been associated with increased cardiovascular risk, but the mechanism is unknown. We investigated the effects of calcium supplements on the propensity of serum to calcify, based on the transition time of primary to secondary calciprotein particles (T50). Changes in serum calcium were related to changes in T50., Introduction: Calcium supplements have been associated with increased cardiovascular risk; however, it is unknown whether this is related to an increase in vascular calcification., Methods: We investigated the acute and 3-month effects of calcium supplements on the propensity of serum to calcify, based on the transition time of primary to secondary calciprotein particles (T50), and on three possible regulators of calcification: fetuin-A, pyrophosphate and fibroblast growth factor-23 (FGF23). We randomized 41 postmenopausal women to 1 g/day of calcium as carbonate, or to a placebo containing no calcium. Measurements were performed at baseline and then 4 and 8 h after their first dose, and after 3 months of supplementation. Fetuin-A, pyrophosphate and FGF23 were measured in the first 10 participants allocated to calcium carbonate and placebo who completed the study., Results: T50 declined in both groups, the changes tending to be greater in the calcium group. Pyrophosphate declined from baseline in the placebo group at 4 h and was different from the calcium group at this time point (p = 0.04). There were no other significant between-groups differences. The changes in serum total calcium from baseline were significantly related to changes in T50 at 4 h (r = -0.32, p = 0.05) and 8 h (r = -0.39, p = 0.01), to fetuin-A at 3 months (r = 0.57, p = 0.01) and to pyrophosphate at 4 h (r = 0.61, p = 0.02)., Conclusions: These correlative findings suggest that serum calcium concentrations modulate the propensity of serum to calcify (T50), and possibly produce counter-regulatory changes in pyrophosphate and fetuin-A. This provides a possible mechanism by which calcium supplements might influence vascular calcification.

Background: Matrix Gla protein is a vitamin K-dependent inhibitor of vascular calcification. Warfarin use is associated with increased breast arterial calcification, but whether this is reflective of other arteries or occurs in men is unclear. In this study, the prevalence of calcification in peripheral arteries was compared in patients with and without warfarin therapy., Methods and Results: This retrospective matched cohort study assessed 430 patients with radiographs performed during or after warfarin therapy who were identified by a computerized search of medical records. Each patient was matched to a patient without warfarin exposure based on age, sex, and diabetes status. Patients with warfarin exposure <1 month, history of end-stage renal disease, or serum creatinine >2.0 mg/dl were excluded. Radiographs were reviewed visually for arterial calcification. The prevalence of arterial calcification was 44% greater in patients with versus without warfarin use (30.2% versus 20.9%, P=0.0023) but not on radiographs performed before warfarin therapy (26.4% versus 22.4%, n=156) or prior to 5 years of warfarin therapy. The increase was noted only in the ankle and foot, was limited to a medial pattern of calcification, and was similar in men and women., Conclusions: Warfarin use is associated with lower extremity arterial calcification in both men and women independent of age, sex, diabetes status, and other patient characteristics. This may have implications for the choice of therapies for long-term anticoagulation., (© 2016 The Authors. Published on behalf of the American Heart Association, Inc., by Wiley Blackwell.)

The presence of bone tissue in calcified arteries may provide insights into the pathophysiology and potential reversibility of calcification, but the prevalence, distribution, and determinants of bone and osteoclasts in calcified arteries are unknown. Specimens of 386 arteries from lower limb amputations in 108 patients were examined retrospectively. Calcification was present in 282 arteries from 89 patients, which was medial in 64%, intimal in 9%, and both in 27%. Bone was present in 6% of arteries, essentially all of which were heavily calcified. Multiple sampling revealed that the true prevalence of bone in heavily calcified arteries was 25%. Bone was more common in medial rather than intimal calcifications (10% vs 3%, p=0.03) but did not vary with artery location (above vs below the knee). Heavily calcified arteries with bone were more likely to come from patients who were older (p=0.04), had diabetes (p=0.06), or were receiving warfarin (p=0.06), but there was no association with gender or renal failure. Bone was almost always adjacent to calcifications, along the periphery, but never within. Staining for the bone-specific proteins osteocalcin and osterix was noted in 20% and 45% of heavily calcified arteries without visible bone. Osteoclasts were present in 4.9% of arteries, all of which were heavily calcified and most of which contained bone. The frequent absence of bone in heavily calcified vessels and the histologic pattern strongly suggests a secondary rather than primary event. Recruitment of osteoclasts to vascular calcifications can occur but is rare, suggesting a limited capacity to reverse calcifications., (© The Author(s) 2015.)

Medial vascular calcification (MVC) is a pathological phenomenon that causes vascular stiffening and can lead to heart failure; it is common to a variety of conditions, including aging, chronic kidney disease, diabetes, obesity, and a variety of rare genetic diseases. These conditions share the common feature of tissue-nonspecific alkaline phosphatase (TNAP) upregulation in the vasculature. To evaluate the role of TNAP in MVC, we developed a mouse model that overexpresses human TNAP in vascular smooth muscle cells in an X-linked manner. Hemizygous overexpressor male mice (Tagln-Cre(+/-) ; Hprt(ALPL) (/Y) or TNAP-OE) show extensive vascular calcification, high blood pressure, and cardiac hypertrophy, and have a median age of death of 44 days, whereas the cardiovascular phenotype is much less pronounced and life expectancy is longer in heterozygous (Tagln-Cre(+/-) ; Hprt(ALPL) (/-) ) female TNAP-OE mice. Gene expression analysis showed upregulation of osteoblast and chondrocyte markers and decreased expression of vascular smooth muscle markers in the aortas of TNAP-OE mice. Through medicinal chemistry efforts, we developed inhibitors of TNAP with drug-like pharmacokinetic characteristics. TNAP-OE mice were treated with the prototypical TNAP inhibitor SBI-425 or vehicle to evaluate the feasibility of TNAP inhibition in vivo. Treatment with this inhibitor significantly reduced aortic calcification and cardiac hypertrophy, and extended lifespan over vehicle-treated controls, in the absence of secondary effects on the skeleton. This study shows that TNAP in the vasculature contributes to the pathology of MVC and that it is a druggable target., (© 2014 American Society for Bone and Mineral Research.)

Objective: The histopathology of peripheral arterial disease and the accompanying calcification are poorly defined, and it is not known whether this varies according to different risk factors., Approach and Results: Sections from 176 upper and lower leg arteries were examined histologically in specimens from amputations of 60 patients with peripheral arterial disease, of whom 58% had diabetes mellitus, 35% had end-stage renal disease, and 48% had a history of smoking. The most common findings were calcification of the media (72% of arteries) and intimal thickening without lipid (68% of arteries), with the presence of atheromas in only 23% of arteries. Intimal calcification occurred in 43% and was generally much less extensive than medial calcification. Nonatheromatous intimal thickening was frequently severe, resulting in complete occlusion in some vessels. The absence of lipid and macrophages was confirmed by staining with oil red O and staining for CD68. Other than a greater prevalence and severity of medial calcification in end-stage renal disease, the findings did not differ between diabetics, patients with end-stage renal disease, or smokers., Conclusions: The results indicate that the majority of arteries in patients with peripheral arterial disease have a vascular lesion that is distinct from atherosclerosis, suggesting a different pathogenesis. This pattern does not differ substantially between patients with different risk factors for peripheral arterial disease. The bulk of vascular calcification in the lower extremities is medial rather than intimal., (© 2014 American Heart Association, Inc.)

Objective: Matrix gla protein is a vitamin K-dependent inhibitor of medial arterial calcification whose synthesis and activity are blocked by warfarin. Warfarin induces arterial calcification in experimental models, but whether this occurs in humans is unclear. This was addressed by examining breast arterial calcification, which is exclusively medial and easily identified on mammograms., Approach and Results: Screening mammograms from women with current, past, or future warfarin use were examined for the presence of arterial calcification and compared with mammograms obtained in untreated women matched for age and diabetes mellitus. Women with a serum creatinine >2.0 mg/dL or a history of end-stage renal disease were excluded. In 451 women with mammograms performed after ≥1 month of warfarin therapy, the prevalence of arterial calcification was 50% greater than in 451 untreated women (39.0% versus 25.9%; P<0.0001). However, in 159 mammograms performed before warfarin therapy, the prevalence of arterial calcification was not increased (26.4% versus 25.8%). The increased prevalence varied with duration of treatment, from 25.0% for <1 year to 74.4% for >5 years. In a multivariable logistic model, only age and duration of warfarin, but not the period of time after stopping warfarin, were significant determinants of arterial calcification in women with current or past warfarin use., Conclusions: The prevalence of breast arterial calcification is increased in women with current or past warfarin use independent of other risk factors and conditions predating warfarin use. This effect appears to be cumulative and may be irreversible., (© 2014 American Heart Association, Inc.)

Plasma levels of pyrophosphate, an endogenous inhibitor of vascular calcification, are reduced in end-stage renal disease and correlate inversely with arterial calcification. However, it is not known whether the low plasma levels are directly pathogenic or are merely a marker of reduced tissue levels. This was tested in an animal model in which aortas were transplanted between normal mice and Enpp1(-/-) mice lacking ectonucleotide pyrophosphatase phosphodiesterase, the enzyme that synthesizes extracellular pyrophosphate. Enpp1(-/-) mice had very low plasma pyrophosphate and developed aortic calcification by 2 months that was greatly accelerated with a high-phosphate diet. Aortas of Enpp1(-/-) mice showed no further calcification after transplantation into wild-type mice fed a high-phosphate diet. Aorta allografts of wild-type mice calcified in Enpp1(-/-) mice but less so than the adjacent recipient Enpp1(-/-) aorta. Donor and recipient aortic calcium contents did not differ in transplants between wild-type and Enpp1(-/-) mice, demonstrating that transplantation per se did not affect calcification. Histology revealed medial calcification with no signs of rejection. Thus, normal levels of extracellular pyrophosphate are sufficient to prevent vascular calcification, and systemic Enpp1 deficiency is sufficient to produce vascular calcification despite normal vascular extracellular pyrophosphate production. This establishes an important role for circulating extracellular pyrophosphate in preventing vascular calcification.

The pathophysiology of medial arterial calcification in chronic kidney disease (CKD) is unclear but has been ascribed to phenotypic changes in vascular smooth muscle, possibly in conjunction with intimal proliferation and atherosclerosis. As the prevalence of calcification in breast arteries is increased in women with CKD and end-stage renal disease (ESRD), this was examined histologically in mastectomy specimens from 19 women with CKD or ESRD. Arterial calcification was present in 18, was exclusively medial, and occurred in vessels as small as arterioles. Intimal thickening was common but unrelated to calcification. There was no evidence of atherosclerosis. The earliest calcification presented as small punctate lesions scattered throughout the media, often with calcification of the internal elastic lamina. Arterial calcification was present in all samples from an age- and diabetes-matched cohort without CKD but was much milder. While smooth muscle cell density was reduced one-third in arteries from patients with ESRD, the cells appeared normal, expressed SM22α, and exhibited no apoptosis. Staining for the bone-specific protein osteocalcin, the osteoblastic transcription factors Runx2 or osterix, or the chondrocytic transcription factor SOX9 was absent in regions of early calcification. Thus, medial calcification in breast arteries of patients with CKD can occur in the absence of smooth muscle cell apoptosis and/or osteogenic transdifferentiation. This suggests that the pathologic mineralization process may differ from one arterial type to the other.

Objective: Calcitriol and various analogs are commonly used to suppress secondary hyperparathyroidism in chronic kidney disease but may also exacerbate vascular calcification. Although this could be because of increased intestinal calcium and phosphate absorption, direct effects through vitamin D receptors (VDRs) on vascular smooth muscle have also been proposed., Approach and Results: The role of these receptors was investigated by examining gene regulation in rat aortas treated with calcitriol ex vivo and in vivo and by transplanting aortas from VDR-null (VDR(-/-)) mice into wild-type mice before induction of uremia and treatment with calcitriol. In cultured rat aortas, calcitriol increased the expression of mRNA for CYP24A1 but not mRNA for any bone-related or calcification-related genes. Gene expression in aortas in vivo was not altered by doses of calcitriol that promote calcification. Calcitriol markedly increased aortic calcification in uremic mice and this did not differ between VDR(-/-) aortic allografts and VDR(+/+) recipient aortas., Conclusions: Calcitriol promotes vascular calcification through a systemic action rather than through a direct vascular action.

Background: Thiosulfate has been shown to inhibit vascular calcification in uremic rats and may inhibit calcification in humans with end-stage renal disease but whether this is due to a systemic or local action is unknown. The underlying mechanism is also unclear but complexation of calcium ions has been proposed., Methods: In vitro assays were used to determine the effect of thiosulfate on vascular calcification and hydroxyapatite formation., Results: Thiosulfate (EC50: 1-2 mM) prevented calcification of injured or devitalized aortas but not uninjured aortas, and similar results were obtained with sulfate. There was no effect on reversal of calcification. Measurements with an ion-sensitive electrode (corrected for changes in ionic strength) revealed a very weak interaction between thiosulfate and Ca(2+) (K(a) = 10.9 ± 1.0 × 10(-6) M(-1)) that resulted in a 4% decrease in ionized Ca(2+) in culture medium at 5 mM thiosulfate and a corresponding 5% increase in the solubility product for calcium-phosphate. Adjustment of the total Ca(2+) concentration to account for this did not prevent the inhibition of aortic calcification by thiosulfate. Thiosulfate did not inhibit hydroxyapatite formation from seed crystals or the calcification of purified elastin and did not alter medium pH., Conclusions: Thiosulfate inhibits vascular calcification at millimolar concentrations through a direct extracellular effect that does not require intact smooth muscle cells but is related to cellular injury. This effect is not specific for thiosulfate since sulfate has similar properties. Inhibition cannot be explained by effects on ionized calcium, calcium-phosphate solubility, pH, oxidative stress or hydroxyapatite formation.

Background and Objectives: CKD is a risk factor for medial artery calcification, but the CKD stage at which this risk begins is unknown. Because breast arterial calcification (BAC) is a marker of generalized medial arterial calcification, mammography was used to detect medial arterial calcification in women with different CKD stages., Design, Setting, Participants, & Measurements: This was a retrospective, cross-sectional study of women with and without CKD matched for age and diabetes and identified from mammograms obtained in 2006-2011. BAC was scored as present or absent per visual inspection., Results: A total of 146 women with stage 3 CKD and 54 with stage 4/5 CKD were identified. An additional 21 patients with ESRD were identified and added to a previous cohort of 71 patients. Mean age was 64 years for CKD 3, 63 for CKD 4, and 59 for ESRD. Half of each group had diabetes. Compared with controls, the odds ratios for BAC were 1.44 in CKD 3 (95% confidence interval [CI], 0.82-2.53), 2.69 in CKD 4 (95% CI, 1.14-6.33), and 7.19 in ESRD (95% CI, 3.77-13.7) and did not differ with diabetic status or race. In a multivariable logistic model, age (P<0.001) and estimated GFR (P=0.005) were independent predictors of BAC. The odds ratio for BAC increased 4% for each milliliter per minute per 1.73 m(2) decrease in estimated GFR. The prevalence of BAC in CKD was increased in each decade of age over 49 years., Conclusions: CKD is an independent risk factor for medial arterial calcification.

Extracellular inorganic pyrophosphate (ePP(i)) is an important endogenous inhibitor of vascular calcification, but it is not known whether systemic or local vascular PP(i) metabolism controls calcification. To determine the role of ePP(i) in vascular smooth muscle, we identified the pathways responsible for ePP(i) production and hydrolysis in rat and mouse aortas and manipulated them to demonstrate their role in the calcification of isolated aortas in culture. Rat and mouse aortas contained mRNA for ectonucleotide pyrophosphatase/phosphodiesterases (NPP1-3), the putative PP(i) transporter ANK, and tissue-nonspecific alkaline phosphatase (TNAP). Synthesis of PP(i) from ATP in aortas was blocked by β,γ-methylene-ATP, an inhibitor of NPPs. Aortas from mice lacking NPP1 (Enpp1(-/-)) did not synthesize PP(i) from ATP and exhibited increased calcification in culture. Although ANK-mediated transport of PP(i) could not be demonstrated in aortas, aortas from mutant (ank/ank) mice calcified more in culture than did aortas from normal (ANK/ANK) mice. Hydrolysis of PP(i) was reduced 25% by β,γ-methylene-ATP and 50% by inhibition of TNAP. Hydrolysis of PP(i) was increased in cells overexpressing TNAP or NPP3 but not NPP1 and was not reduced in Enpp1(-/-) aortas. Overexpression of TNAP increased calcification of cultured aortas. The results show that smooth muscle NPP1 and TNAP control vascular calcification through effects on synthesis and hydrolysis of ePP(i), indicating an important inhibitory role of locally produced PP(i). Smooth muscle ANK also affects calcification, but this may not be mediated through transport of PP(i). NPP3 is identified as an additional pyrophosphatase that could influence vascular calcification.

Vascular calcification is common in patients with advanced chronic kidney disease and is associated with poorer outcomes. Although the pathophysiology is not completely understood, it is clear that it is a multifactorial process involving altered mineral metabolism, as well as changes in systemic and local factors that can promote or inhibit vascular calcification, and all of these are potential therapeutic targets. Current therapy is closely linked to strategies for preventing disordered bone and mineral metabolism in advanced kidney disease and involves lowering the circulating levels of both phosphate and calcium. The efficacy of compounds that specifically target calcification, such as bisphosphonates and thiosulfate, has been shown in animals but only in small numbers of humans, and safety remains an issue. Additional therapies, such as pyrophosphate, vitamin K, and lowering of pH, are supported by animal studies, but are yet to be investigated clinically. As the mineral composition of vascular calcifications is the same as in bone, potential effects on bone must be addressed with any therapy for vascular calcification. [ABSTRACT FROM AUTHOR]

Extracellular inorganic pyrophosphate (PPi) is a potent suppressor of physiological calcification in bone and pathological calcification in blood vessels. Ectonucleotide pyrophosphatase/phosphodiesterases (eNPP5) generate PP via the hydrolysis of ATP released into extracellular compartments by poorly understood mechanisms. Here we report that cultured vascular smooth muscle cells (VSMC) from rat aorta generate extracellular PPi via an autocrine mechanism that involves ATP release tightly coupled to eNPP activity. The nucleotide analog β-γ-methylene ATP (MeATP or AMPPCP) was used to selectively suppress ATP metabolism by eNPPs but not the CD39-type ecto-ATPases. In the absence of MeATP, VSMC generated extracellular PP to accumulate ⩾600 nM within 2 h while steadily maintaining extracellular ATP at 1 nM. Conversely, the presence of MeATP completely suppressed PPi accumulation while increasing ATP accumulation. Probenecid, which inhibits PPi efflux dependent on ANK, a putative PPi transporter or transport regulator, reduced extracellular PPi accumulation by approximately twofold. This indicates that autocrine ATP release coupled to eNPP activity comprises ⩾0% of the extracellular PP-generating capacity of VSMC. The accumulation of extracellular PPi and ATP was markedly attenuated by reduced temperature but was insensitive to brefeldin A, which suppresses constitutive exocytosis of Golgi-derived secretory vesicles. The magnitude of extracellular PP accumulation in VSMC cultures increased with time postplating, suggesting that ATP release coupled to PPi generation is upregulated as cultured VSMC undergo contact-inhibition of proliferation or deposit extracellular matrix. [ABSTRACT FROM AUTHOR]

The article describes the effect of three novel inhibitors of the physiological pyrophosphatase activity of alkaline phosphatase on vascular smooth muscle cell (VSMC) calcification. According to the authors, the three compounds were shown to reduce the calcification by Enppl-/- VSMCs. They add that, using an ex vivo rat whole-aorta PPi hydrolysis assay, pyrophosphatase activity was inhibited by all three lead compounds.

Scattered through the practice of medicine are dogmas with little or no scientific basis. One of these is the product of the serum calcium and phosphorus concentrations, the so-called calcium-phosphorus product or Ca × P. The assumption that ectopic calcification will occur when the product of the serum calcium and phosphorus concentrations exceeds a particular threshold has become standard practice in nephrology even though there is little scientific basis. Experimental support is lacking, the chemistry underlying the use of the product is oversimplified and the concept that ectopic calcification is simply the result of supersaturation is biologically flawed. The evidence that the Ca × P is an independent risk factor for mortality and morbidity is also questionable. Although ectopic calcification can occur in many sites, this review will focus on vascular calcification, as it is the most common site and the site most likely to affect patient outcomes.Kidney International (2007) 72, 792–796; doi:10.1038/sj.ki.5002412; published online 4 July 2007 [ABSTRACT FROM AUTHOR]

Pyrophosphate is a potent inhibitor of medial vascular calcification where its level is controlled by hydrolysis via a tissue-nonspecific alkaline phosphatase (TNAP). We sought to determine if increased TNAP activity could explain the pyrophosphate deficiency and vascular calcification seen in renal failure. TNAP activity increased twofold in intact aortas and in aortic homogenates from rats made uremic by feeding adenine or by 5/6 nephrectomy. Immunoblotting showed an increase in protein abundance but there was no increase in TNAP mRNA assessed by quantitative polymerase chain reaction. Hydrolysis of pyrophosphate by rat aortic rings was inhibited about half by the nonspecific alkaline phosphatase inhibitor levamisole and was reduced about half in aortas from mice lacking TNAP. Hydrolysis was increased in aortic rings from uremic rats and all of this increase was inhibited by levamisole. An increase in TNAP activity and pyrophosphate hydrolysis also occurred when aortic rings from normal rats were incubated with uremic rat plasma. These results suggest that a circulating factor causes pyrophosphate deficiency by regulating TNAP activity and that vascular calcification in renal failure may result from the action of this factor. If proven by future studies, this mechanism will identify alkaline phosphatase as a potential therapeutic target.Kidney International (2008) 73, 1024–1030; doi:10.1038/ki.2008.26; published online 20 February 2008 [ABSTRACT FROM AUTHOR]