Chemical and hormonal determinants of vascular calcification in vitro

Chronic kidney disease and vascular remodelling: molecular mechanisms and clinical implications

CKD (chronic kidney disease) is a severe and complex disease with a very high prevalence of CV (cardiovascular) complications. CKD patients are exposed to haemodynamic disturbances in addition to severe metabolic abnormalities that lead to a specific form of arterial remodelling, which contributes to the development of CV disease. Arterial calcification is a major event in the arterial remodelling process and is strongly linked to mineral metabolism abnormalities associated with CKD. Arterial remodelling is not limited to arterial calcification and modifications in arterial wall composition are also observed. Activation of the RAS (renin-angiotensin system), ET-1 (endothelin-1), endothelial dysfunction, oxidative stress and ADMA (asymmetric ω-NG,NG-dimethylarginine), as well as the anti-aging molecule Klotho, are implicated in this process. The present review details the mechanisms involved in arterial calcification and arterial remodelling associated with CKD, and provides the clinical consequences of large and small artery stiffness and remodelling in CKD patients.

Avoiding harm and achieving optimal dialysis outcomes--the dialysate component

Appropriate dialysate composition is critical for effective and safe hemodialysis. Unfortunately, there are few randomized trials to guide practice, and although solute clearance is well understood, there is a limited understanding of balance in dialysis patients. The current practice of simply trying to normalize serum electrolyte and mineral concentrations measured predialysis may not provide optimal care. More thought should be given to normalizing balance with respect to sodium, bicarbonate, magnesium, and potassium and minimizing wide swings in serum concentrations that may have adverse effects. In practice, this would require longer or more frequent dialysis with less steep chemical gradients. With respect to calcium, the goal should be to optimize bone and vascular health. Clinicians should also be mindful that the dialysis procedure itself exposes patients to potential toxins, and efforts to minimize these risks should be stressed.

Bone-specific alkaline phosphatase concentrations are less variable than those of parathyroid hormone in stable hemodialysis patients

Abnormalities of bone mineral metabolism and vascular calcification are prevalent in patients with kidney failure. Clinical management is based on biochemical targets, in particular parathyroid hormone (PTH) concentrations, but this has many limitations including high biological variation. A possible alternative is bone-specific alkaline phosphatase (ALP); therefore, we evaluated the biological variation of this marker in patients undergoing hemodialysis. Bone ALP was measured in non-fasting serum samples taken twice a week over a 6-week period in 22 stable hemodialysis patients and 12 healthy volunteers. The within-individual coefficients of variance were calculated and used to derive the critical difference required to be certain that an observed change was significant. The coefficient of variance for bone ALP was significantly higher in hemodialysis patients compared to healthy individuals. Seven samples were required to estimate the homeostatic set point of bone ALP, within 10%, in a hemodialysis patient. The concentration of serial bone ALP measurements would need to change by 36% between any two measurements before it can be considered a significant change. Since the biological variation of bone ALP is less than half that reported for PTH, our study provides further support for the use of bone ALP as an alternative marker of bone mineral metabolism in the setting of chronic kidney disease–mineral and bone disorder.

The risk for medial arterial calcification in CKD

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.

Arterial calcification in chronic kidney disease: key roles for calcium and phosphate

Vascular calcification contributes to the high risk of cardiovascular mortality in chronic kidney disease (CKD) patients. Dysregulation of calcium (Ca) and phosphate (P) metabolism is common in CKD patients and drives vascular calcification. In this article, we review the physiological regulatory mechanisms for Ca and P homeostasis and the basis for their dysregulation in CKD. In addition, we highlight recent findings indicating that elevated Ca and P have direct effects on vascular smooth muscle cells (VSMCs) that promote vascular calcification, including stimulation of osteogenic/chondrogenic differentiation, vesicle release, apoptosis, loss of inhibitors, and extracellular matrix degradation. These studies suggest a major role for elevated P in promoting osteogenic/chondrogenic differentiation of VSMC, whereas elevated Ca has a predominant role in promoting VSMC apoptosis and vesicle release. Furthermore, the effects of elevated Ca and P are synergistic, providing a major stimulus for vascular calcification in CKD. Unraveling the complex regulatory pathways that mediate the effects of both Ca and P on VSMCs will ultimately provide novel targets and therapies to limit the destructive effects of vascular calcification in CKD patients.

Vascular and valvular calcification in chronic peritoneal dialysis patients

Cardiovascular disease accounts over half of the total mortality in peritoneal dialysis (PD) patients. In addition, there is an increasing recognition of a high prevalence of vascular and valvular calcification that may contribute to the increased all-cause and cardiovascular mortality in the PD patients. Disturbed mineral metabolism in association with chronic kidney disease has been suggested as one of the major contributing factors to the increased vascular/valvular calcification in this population. In this paper, we provide an overview of the prevalence and importance of this complication in the PD patients. In addition, we review the contributing factors and some emerging mechanisms for this complication. Furthermore, we discuss some therapeutic strategies that may be useful in limiting the progression of vascular/valvular calcification in the PD population.

The chemistry of thiosulfate and vascular calcification

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.

Association of Serum Phosphate and Related Factors in ESRD-Related Vascular Calcification

Vascular calcification is common in ESRD patients and is important in increasing mortality from cardiovascular complications in these patients. Hyperphosphatemia related to chronic kidney disease is increasingly known as major stimulus for vascular calcification. Hyperphosphatemia and vascular calcification become popular discussion among nephrologist environment more than five decades, and many researches have been evolved. Risk factors for calcification are nowadays focused for the therapeutic prevention of vascular calcification with the hope of reducing cardiovascular complications.

Cell biological and physicochemical aspects of arterial calcification

Processes similar to endochondral or intramembranous bone formation occur in the vascular wall. Bone and cartilage tissue as well as osteoblast- and chondrocyte-like cells are present in calcified arteries. As in bone formation, apoptosis and matrix vesicles play an important role in the initiation of vascular calcification. Recent evidence indicates that nanocrystals initially formed in the vessel wall may actively be involved in the progression of the calcification process. This review focuses on the cellular and structural similarities between bone formation and vascular calcification and discusses the initial events in this pathological mineralization process.

Direct effects of phosphate on vascular cell function

Elevated serum phosphate has clinically been associated with vascular stiffness and cardiovascular mortality. Mechanistic studies over the past decade regarding local effects of phosphate on the vessel wall have provided insight into various pathways that culminate in vascular calcification. Smooth muscle cell phenotype change and apoptosis play prominent roles. The sodium-phosphate cotransporter PiT-1 is required for the osteochondrogenic differentiation of smooth muscle cells in vitro. Less is known about phosphate-driven valve interstitial cell calcification and elastin degradation. In this article, we review the current knowledge about phosphate-induced changes in the vascular wall.

Recent progress in the treatment of 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.

Serum alkaline phosphatase and phosphate and risk of mortality and hospitalization

BACKGROUND AND OBJECTIVES

Elevated alkaline phosphatase (AlkPhos) and phosphate levels are associated with cardiovascular morbidity and mortality in patients receiving dialysis. A retrospective cohort study was conducted to test these associations in outpatients with an estimated GFR > or =60 ml/min/1.73 m(2).

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Patients with serum AlkPhos and phosphate levels measured between 2000 and 2002 (n = 10,743) at Montefiore Medical Center (MMC) clinics were followed through September 11, 2008 (median 6.8 years). Mortality data were obtained via Social Security Administration records (n = 949 deaths). Hospitalization data were obtained from MMC records.

RESULTS

The mean age was 51 years, 64% were women, 22% were white, 26% were non-Hispanic black, 16% were Hispanic, 13% had a diagnosis of hypertension, 9% had diabetes mellitus, and 8% had cardiovascular disease at baseline. AlkPhos and phosphate were independently associated with mortality and cardiovascular-related hospitalization after multivariable adjustment. Comparing patients in the highest (> or =104 U/L) versus lowest quartile of AlkPhos (< or =66 U/L), the adjusted hazard ratio (HR) for mortality was 1.65 (P trend across quartiles <0.001). For the highest compared with the lowest quartile of serum phosphate (> or =3.8 mg/dl versus < or =3.0 mg/dl), the adjusted HR for mortality was 1.29 (P trend across quartiles = 0.008). High AlkPhos but not phosphate levels were also associated with all-cause, infection-related, and fracture-related hospitalization.

CONCLUSIONS

Higher levels of serum AlkPhos and phosphate were associated with increased mortality and cardiovascular-related hospitalization in an inner-city clinic population. Further studies are needed to elucidate mechanisms underlying these associations.

Association of serum alkaline phosphatase and bone mineral density in maintenance hemodialysis patients

Recent studies indicate that serum alkaline phosphatase (AlkPhos), a surrogate of high turnover bone disease, is associated with coronary artery calcification and death risk in maintenance hemodialysis (MHD) patients. The association between AlkPhos and bone mineral density (BMD) is not well studied. We studied the association between AlkPhos and dual-energy X-ray absorptiometry-assessed BMD in a group of MHD patients in Southern California. In 154 MHD patients, aged 55.3 +/- 13.6 years, including 42% women, 38% Hispanics, 42% African Americans, and 55% diabetics, the mean serum AlkPhos was 121 +/- 63 U/L (median: 101, Q(25-75): 81-141); 36% had AlkPhos>/=120 U/L and 50% had a total T-score< or =-1. Whereas the total BMD did not correlate with age (r=0.01, P=0.99) or body mass index (r=0.10, P=0.22), it correlated negatively with AlkPhos (r=-0.25, P=0.002), including after multivariate adjustment (r=-0.24, P=0.003). The proportion of patients with a high coronary artery calcification score>400 was incrementally higher across worsening BMD tertiles (P trend=0.04). The BMD was significantly worse in MHD patients with serum AlkPhos> or =120 U/L compared with <120 U/L (1.01 +/- 0.016 vs. 1.08 +/- 0.013 g/cm(2), respectively, P<0.001). The multivariate adjusted odds ratio of AlkPhos> or =120 U/L for having a total T-score<-1.0 was 2.3 (1.1-4.8, P=0.037). Among routine clinical and biochemical markers, serum AlkPhos> or =120 U/L was a better predictor of total T-score< or =-1 in MHD patients. An association exists between higher serum AlkPhos and worse dual-energy X-ray absorptiometry-assessed BMD in MHD patients. Given these findings, studies are indicated to examine whether interventions that lower serum AlkPhos improve BMD in MHD patients.

Vascular smooth muscle cell calcification and SLC20 inorganic phosphate transporters: effects of PDGF, TNF-alpha, and Pi

Pi transport by vascular smooth muscle cells (VSMC) has been proposed to play an important role in the pathogenesis of vascular calcification. In this study, we have determined the correlation between calcification induced by Pi, platelet-derived growth factor (PDGF)-BB, and tumor necrosis factor-alpha and Pi transport activity in primary cultures of rat aortic VSMC. These agents induced calcification and increased the expression of Cbfa1, Msx2, and Bmp2 osteogene messenger RNA in rat aortic VSMC, while Pi transport rate was not modified per milligram of protein. Only PDGF increased Pi transport when it was expressed per unit of DNA, as PDGF also increased total cell protein by 100%, while DNA content and number of cells were not modified. PDGF increased the expression of the Pi transporter, Pit-1, but membrane protein biotinylation showed that Pit-1 abundance was not modified in the cell surface. Immunofluorescence revealed that, under basal conditions, Pit-1 is only slightly expressed at the cell membrane, but strongly expressed inside the cell. The intracellular signal colocalizes with endoplasmic reticulum (ER) markers, and PDGF increases Pit-1 expression in the ER but not the cell membrane. In conclusion, Pi transport across the plasma membrane does not correlate directly with calcification, but the expression of Pit-1 in the ER opens new possibilities for the study of the pathogenesis of vascular calcification.

Vascular calcification: the killer of patients with chronic kidney disease

Cardiovascular complications are the leading cause of death in patients with chronic kidney disease (CKD). Vascular calcification is a common complication in CKD, and investigators have demonstrated that the extent and histoanatomic type of vascular calcification are predictors of subsequent vascular mortality. Although research efforts in the past decade have greatly improved our knowledge of the multiple factors and mechanisms involved in vascular calcification in patients with kidney disease, many questions remain unanswered. No longer can we accept the concept that vascular calcification in CKD is a passive process resulting from an elevated calcium-phosphate product. Rather, as a result of the metabolic insults of diabetes, dyslipidemia, oxidative stress, uremia, and hyperphosphatemia, "osteoblast-like" cells form in the vessel wall. These mineralizing cells as well as the recruitment of undifferentiated progenitors to the osteochondrocyte lineage play a critical role in the calcification process. Important transcription factors such as Msx 2, osterix, and RUNX2 are crucial in the programming of osteogenesis. Thus, the simultaneous increase in arterial osteochondrocytic programs and reduction in active cellular defense mechanisms creates the "perfect storm" of vascular calcification seen in ESRD. Innovative clinical studies addressing the combined use of inhibitors that work on vascular calcification through distinct molecular mechanisms, such as fetuin-A, osteopontin, and bone morphogenic protein 7, among others, will be necessary to reduce significantly the accrual of vascular calcifications and cardiovascular mortality in kidney disease. In addition, the roles of oxidative stress and inflammation on the fate of smooth muscle vascular cells and their function deserve further translational investigation.

Association of serum alkaline phosphatase with coronary artery calcification in maintenance hemodialysis patients

BACKGROUND AND OBJECTIVES

Recent in vitro studies have shown a link between alkaline phosphatase and vascular calcification in patients with chronic kidney disease (CKD). High serum levels of alkaline phosphatase are associated with increased death risk in epidemiologic studies of maintenance hemodialysis (MHD) patients. We hypothesized that coronary artery calcification is independently associated with increased serum alkaline phosphatase levels in MHD patients.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We examined the association of coronary artery calcification score (CACS) and alkaline phosphatase in 137 randomly selected MHD patients for whom markers of malnutrition, inflammation, and bone and mineral disorders were also measured.

RESULTS

Serum alkaline phosphatase was the only measure with significant and robust association with CACS (P < 0.003), whereas either other biochemical markers had no association with CACS or their association was eliminated after controlling for case-mix variables. Serum alkaline phosphatase >120 IU/L was a robust predictor of higher CACS and was particularly associated with the likelihood of CACS >400 (multivariate odds ratio 5.0 95% confidence interval 1.6 to 16.3; P = 0.007). Serum alkaline phosphatase of approximately 85 IU/L seemed to be associated with the lowest likelihood of severe coronary artery calcification, but in the lowest tertile of alkaline phosphatase, the CACS predictability was not statistically significant.

CONCLUSIONS

An association between serum alkaline phosphatase level and CACS exists in MHD patients. Given the high burden of vascular calcification in patients with CKD, examining potential therapeutic interventions to modulate the alkaline phosphatase pathway may be warranted.

Progression of coronary artery calcification in renal transplantation and the role of secondary hyperparathyroidism and inflammation

BACKGROUND AND OBJECTIVES

Transplantation should favorably affect coronary calcification (CAC) progression in dialysis; however, changes in CAC score in the individual patient are not reliably evaluated.

DESIGN, SETTING, PARTICIPANTS & MEASUREMENTS

The authors used special tables of reproducibility limits for each score level to study, by multislice computed tomography and biochemistries, the 2-year changes in CAC in 41 transplant patients (age 48 +/- 13 yr, 25 men, dialysis vintage 4.8 +/- 4.3 yr, underwent transplant 6.2 +/- 5.5 yr prior). Thirty balanced dialysis patients served as controls.

RESULTS

In the study group, Agatston score was stable, and C-reactive protein decreased, whereas fetuin and osteoprotegerin increased. In the control group, Agatston score increased, parathyroid hormone and phosphate decreased, and inflammation markers were persistently twice as high as in the study group. With regard to individual changes, 12.2% transplant patients worsened, compared with 56.6% of patients in dialysis (P < 0.0001). Patients without calcification at entry showed slower progression in transplantation (8.3%) than in dialysis (44.4%; P < 0.034), and the difference was similar to that observed in cases with CAC (17.6% versus 61.9%; P < 0.007). Discriminant analysis indicated parathyroid hormone, the modality of therapy (dialysis or transplantation), and erythrocyte sedimentation rate as the variables most associated with worsening.

CONCLUSIONS

Renal transplantation lowers but does not halt CAC progression. Inflammation and hyperparathyroidism are associated with progression in the populations studied.

Systemic correlates of angiographic coronary artery disease

Coronary angiography allows a direct evaluation of coronary anatomy. The aim of the present investigation was to search for correlations between the magnitude of coronary artery disease, as assessed by angiography, and a number of systemic parameters. A group of 116 patients (80 male, 36 female) with coronary heart disease diagnosed by angiography, aged 62.0±10.5 years, was the subject of an observational study. Correlation and linear regression analysis using coronary artery disease burden (CADB - sum of the percentage of the luminal stenosis encountered in all the lesions of the coronary arterial trees) as dependent variable, and age, sex, plasma calcium, phosphorus, magnesium, glucose, HDL cholesterol, LDL cholesterol, triglycerides, uric acid, estimated glomerular filtration rate and body mass index as independent variables, were carried out. Significant correlation values versus CADB were seen with age (r 0.19, p 0.04), uric acid (r 0.18, p 0.048) and fasting plasma glucose (r 0.33, p<0.001). Linear regression analysis, yielding a global significance level of 0.002, showed a significant value for glucose (p 0.018) and for sex (0.008). In conclusion, among several systemic parameters studied, plasma glucose was found to be correlated to coronary artery atherosclerosis lesions.

Treatment of vascular calcification

A variety of potential therapies for vascular calcification, based either on the underlying biology or physical chemistry or solely on empiric observations in patients, may be effective but lack rigorous testing. Pasch et al. provide convincing evidence that sodium thiosulfate prevents medial vascular calcification in uremic rats. Although this provides some scientific basis for the clinical use of thiosulfate, uncertainty about mechanism of action and safety still remains.

The emerging role of phosphate in vascular calcification

Vascular calcification is recognized as a major contributor to cardiovascular disease (CVD) in end stage renal disease (ESRD) patients. Susceptibility to vascular calcification is genetically determined and actively regulated by diverse inducers and inhibitors. One of these inducers, hyperphosphatemia, promotes vascular calcification and is a nontraditional risk factor for CVD mortality in ESRD patients. Vascular smooth muscle cells (SMCs) respond to elevated phosphate levels by undergoing an osteochondrogenic phenotype change and mineralizing their extracellular matrix through a mechanism requiring sodium-dependent phosphate cotransporters. Disease states and cytokines can increase expression of sodium-dependent phosphate cotransporters in SMCs, thereby increasing susceptibility to calcification even at phosphate concentrations that are in the normal range.

Adequate phosphate binding with lanthanum carbonate attenuates arterial calcification in chronic renal failure rats

BACKGROUND

Hyperphosphataemia is a risk factor for arterial calcification contributing to the high cardiovascular mortality in patients with chronic kidney disease. Calcium-based phosphate binders can induce hypercalcaemia and are associated with progression of vascular calcification. Therefore, the effect of lanthanum carbonate, a non-calcium phosphate binder, on the development of vascular calcification was investigated in uraemic rats.

METHODS

Chronic renal failure (CRF) was induced by feeding rats an adenine-enriched diet for 4 weeks. After 2 weeks, 1% or 2% lanthanum carbonate was added to the diet for 6 weeks. Calcification in the aorta, carotid and femoral arteries was evaluated histomorphometrically, biochemically and by ex vivo micro-CT. Chondro-/osteogenic conversion of vascular smooth muscle cells was also analysed in the rat aorta.

RESULTS

Treatment with 1% lanthanum carbonate (1% La) did not reduce vascular calcification, but in the 2% lanthanum carbonate (2% La) group vascular calcium content and area% Von Kossa positivity were decreased compared with control CRF rats. The aortic calcified volume measured with ex vivo micro-CT was significantly reduced in rats treated with 2% La. Although calcification was inhibited by treatment with 2% La, the chondrocyte transcription factor sox-9 was abundantly expressed in the aorta.

CONCLUSION

Treatment of CRF rats with 2% La reduces the development of vascular calcification by adequate phosphate binding resulting in a decreased supply of phosphate as a substrate for vascular calcification.

Differential effects of intermittent PTH(1-34) and PTH(7-34) on bone microarchitecture and aortic calcification in experimental renal failure

PTH(1-84) and PTH(7-84) are elevated in chronic kidney disease (CKD). These peptides, as their shorter analogs PTH(1-34) and PTH(7-34) both promote PTH receptor (PTH1R) internalization but only PTH(1-34) and PTH(1-84) activate the receptor. Here, we examined the effects of intermittent administration of PTH(1-34) and PTH(7-34) on mineral ion metabolism, bone architecture, and vascular calcification in rats with experimental CKD. CKD with or without parathyroidectomy (PTX) was established by 5/6 nephrectomy (NPX) in rats. Animals were divided into 4 groups: Sham PTX+ sham NPX (Sham); PTX+ sham NPX (PTX); Sham PTX+NPX (NPX); PTX+NPX (PTX/NPX). Rats were treated with single daily doses of 40 microg/kg PTH(1-34), PTH(7-34), or vehicle. Creatinine was higher in NPX and Ca lower in PTX and PTX/NPX groups than in Sham or NPX rats. Plasma phosphate was higher in PTX, NPX and PTX/NPX than in Sham rats. PTH(1-34) was more hypercalcemic than PTH(7-34) in PTX rats. Fractional bone volume in rats treated with PTH(1-34) increased significantly in all groups compared to that of vehicle treatment. In addition, trabecular number, thickness and volumetric bone density increased in rats treated with PTH(1-34). In contrast, PTH(1-34) diminished vascular calcification. Bone and renal PTH1R mRNA expression was reduced as much or more in PTX/NPX rats as in NPX alone, whereas PTH(7-34) had no effect on PTH1R expression. Renal but not bone PTH1R mRNA increased in response to PTH(1-34). These findings suggest that PTH(1-34) exerts greater hypercalcemic and anabolic effects in parathyroidectomized and/or nephrectomized rats than does PTH(7-34). There was no evidence for significant bone or vascular actions of PTH(7-34). We conclude that PTH(1-34) protects against vascular calcification and bone demineralization in experimental renal failure.

Vitamin D and osteogenic differentiation in the artery wall

Vascular calcification is widespread, particularly in patients with chronic kidney disease, who receive, among other treatments, active vitamin D supplements. Emerging evidence indicates that vascular calcification is a regulated process that resembles embryonic endochondral osteogenesis, involving osteoblastic differentiation of vascular smooth muscle cells. In experimental animal models, high dosages of vitamin D consistently promote vascular calcification. In particular, the vitamin D-fed rat is frequently used as a model to assess putative regulators of calcific vasculopathy. The artery wall calcification in these animals most likely results from multiple mechanisms involving systems physiology of the complex, bone-vascular-renal-endocrine axis. Genetically engineered mice with upregulated vitamin D signaling pathways have also shed light on the molecular intermediaries, including fibroblast growth factor-23 and transcriptional intermediary factor 1-alpha. In contrast to the studies of animals, studies of humans show that vitamin D has an inverse relationship or little effect. This difference between in vitro and in vivo findings is most likely, again, due to the complex, systemic feedback regulatory mechanisms that control calcium-phosphate metabolism. Recent epidemiologic evidence suggests that there is a narrow range of vitamin D levels in which vascular function is optimized. Levels above or below this range seem to confer a significant increase in risk for cardiovascular disease. There is some evidence to suggest that dietary vitamin D may be carried by lipoprotein particles into cells of the artery wall and atherosclerotic plaque, where it may be converted to active form by monocyte-macrophages. These findings raise interesting questions regarding the effects of vitamin D intake on atherosclerotic calcification and cardiovascular risk.

Upregulation of alkaline phosphatase and pyrophosphate hydrolysis: potential mechanism for uremic vascular calcification

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.

Metabolic acidosis inhibits soft tissue calcification in uremic rats

Metabolic acidosis is common in patients with chronic kidney disease, which is known to affect bone metabolism. We examined the effect of metabolic acidosis on the development of vascular and other soft-tissue calcifications in uremic rats treated with calcitriol. Extraskeletal calcification was measured in vivo, in control rats and rats with a remnant kidney model of uremia with or without ammonium chloride-induced acidosis. Soft-tissue calcification was assessed histologically, by measurement of the expression of the sodium-dependent phosphate cotransporter Pit-1 and by quantification of tissue calcium and phosphorus. Calcitriol administration to uremic rats resulted in significant deposition of material positive for von Kossa stain in the aorta, stomach, and kidney, elevated aortic calcium and phosphorus, increased aortic Pit-1 expression, and high mortality. Calcitriol-treated uremic rats with acidosis did not develop aortic or soft-tissue calcification, did not increase aortic Pit-1 expression, and had significantly lower mortality. Additionally, an acidotic environment prevented calcification of vascular smooth muscle cells in vitro. Our study shows that metabolic acidosis inhibits extraskeletal calcification.

The fallacy of the calcium-phosphorus product

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 x 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 x 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.

Sodium-dependent phosphate cotransporters and vascular calcification

PURPOSE OF REVIEW

Vascular calcification is associated with cardiovascular events in patients with end-stage renal disease and diabetes. Hyperphosphatemia is a risk factor for vascular calcification in these patients. Sodium-dependent phosphate cotransporters are required for cellular phosphate uptake. This review focuses on the potential role of phosphate transport and type III sodium-dependent phosphate cotransporters in the process of vascular calcification.

RECENT FINDINGS

Consistent with clinical and animal studies, elevated phosphate induces mineralization of cultured smooth muscle cells in vitro. Calcification is concomitant with osteochondrogenic phenotype change in smooth muscle cells characterized by induction of osteochondrogenic differentiation marker, Runx2, and inhibition of smooth muscle cell lineage marker, SM22. Inhibition of the type III sodium-dependent phosphate cotransporter, Pit-1, blocks phosphate-induced smooth muscle cell calcification. Moreover, the phosphate-induced osteochondrogenic phenotype modulation is also abrogated by Pit-1 inhibition. Pit-1 is upregulated by several calcification-promoting factors, including tumor necrosis factor-alpha, bone morphogenetic protein 2, platelet-derived growth factor and elevated calcium.

SUMMARY

Phosphate uptake via Pit-1 is required for osteochondrogenic phenotypic change and calcification of vascular smooth muscle cells in vitro. Modulation of Pit-1 expression or its transport activity may provide a novel therapeutic target for intervention of vascular calcification.

Vascular calcification: contribution of parathyroid hormone in renal failure

Hyperphosphatemia is a driving force in the pathogenesis of vascular calcification (VC) and secondary hyperparathyroidism associated with renal failure. To test for the possible contribution of parathyroid hormone (PTH) to cardiovascular calcification, we removed the parathyroid glands from rats but infused synthetic hormone at a supraphysiologic rate. All rats were pair-fed low, normal, or high phosphorus diets and subjected to a sham or 5/6 nephrectomy (remnant kidney). Control rats were given a normal diet and underwent both sham parathyroidectomy and 5/6 nephrectomy. Heart weight/body weight ratios and serum creatinine levels were higher in remnant kidney rats than in the sham-operated rats. Remnant kidney rats on the high phosphorus diet and PTH replacement developed hyperphosphatemia and hypocalcemia along with low bone trabecular volume. Remnant kidney rats on the low phosphorus diet or intact kidney rats on a normal phosphorus diet, each with hormone replacement, developed hypercalcemia. All rats on PTH replacement developed intense aortic medial calcification, and some animals presented coronary calcification. We suggest that high PTH levels induce high bone turnover and medial calcification resembling Mömckeberg's sclerosis independent of uremia. This model may be useful in defining mechanisms underlying VC.

Vascular calcification: not so crystal clear

Patients with renal failure are predisposed to calcification of the medial layer of arteries. This calcification is far more complex than simple precipitation of calcium and phosphate and involves multiple forms of calcium phosphate. Like bone, calcification in the vessels also involves biologic events. The two are necessarily linked and unraveling the pathophysiology will require an understanding of both.

Characterization of phosphate transport in rat vascular smooth muscle cells: implications for vascular calcification

OBJECTIVE

Hyperphosphatemia and inorganic phosphate (Pi) transport by vascular smooth muscle cells (VSMCs) have been implicated in the pathogenesis of vascular calcification. The aim of this work has been to characterize Pi transport in VSMCs.

METHODS AND RESULTS

Primary cultures of VSMCs express both high affinity Na-dependent and Na-independent components of Pi transport. Under physiological conditions both transport systems are saturated, show similar activity, and are inhibited by increasing pH. The Na-dependent transport is also weakly inhibited by phosphonoformic acid (PFA) (3.9 mmol/L IC50 at 0.05 mmol/L Pi). Real-time polymerase chain reaction shows that Pit1 and Pit2 are expressed to the same degree, and no other Pi transporters are significantly expressed. When expressed in Xenopus oocytes they are strictly Na-dependent, with high affinities for Pi, and are inhibited by increasing pH, but only weakly inhibited by PFA. We have used RNA interference to demonstrate that Pit1 and Pit2 are the transporters responsible for Na-dependent Pi transport in VSMCs.

CONCLUSIONS

Taken together these novel findings suggest new roles of Pi transport in the pathogenesis of VC and have implications as potential future clinical targets.

Vitamin D and vascular calcification

PURPOSE OF REVIEW

Vascular calcification is frequently found in patients with osteoporosis, atherosclerosis and chronic kidney disease, leading to high morbidity and mortality rates. The effects of vitamin D excess and deficiency on vascular calcification are reviewed in this article.

RECENT FINDINGS

There is evidence from experimental studies that mediacalcinosis induced by vitamin D excess is an active and reversible process. Vitamin D excess, however, is rarely seen in the general human population. Experimental data also demonstrate that physiologic vitamin D actions include the inhibition of processes that are important for intimal and medial artery calcification such as pro-inflammatory cytokine release, adhesion molecule release, and proliferation and migration of vascular smooth muscle cells. In uremic rats, low levels of the vitamin D hormone calcitriol are associated with massive vascular and soft tissue calcifications. Whereas retrospective studies already indicate a beneficial effect of active vitamin D on mortality rates in chronic kidney disease, little is yet known about the effect of vitamin D deficiency on cardiovascular morbidity and mortality in the general population.

SUMMARY

Available data indicate that vitamin D exerts a biphasic 'dose response' curve on vascular calcification with deleterious consequences not only of vitamin D excess but also of vitamin D deficiency.

Mechanisms of vascular calcification

Vascular calcification is highly prevalent and correlated with high rates of cardiovascular mortality in chronic kidney disease patients. Recent evidence suggests that mineral, hormonal, and metabolic imbalances that promote phenotype change in vascular cells as well as deficiencies in specific mineralization inhibitory pathways may be important contributory factors for vascular calcification in these patients. This article reviews current mechanisms proposed for the regulation of vascular calcification and data supporting their potential contribution to this process in chronic kidney disease.