Medial arterial calcification is accelerated in patients with CKD and strongly associated with increased arterial rigidity and cardiovascular mortality. Recently, a novel in vitro blood test that provides an overall measure of calcification propensity by monitoring the maturation time (T50) of calciprotein particles in serum was described. We used this test to measure serum T50 in a prospective cohort of 184 patients with stages 3 and 4 CKD, with a median of 5.3 years of follow-up. At baseline, the major determinants of serum calcification propensity included higher serum phosphate, ionized calcium, increased bone osteoclastic activity, and lower free fetuin-A, plasma pyrophosphate, and albumin concentrations, which accounted for 49% of the variation in this parameter. Increased serum calcification propensity at baseline independently associated with aortic pulse wave velocity in the complete cohort and progressive aortic stiffening over 30 months in a subgroup of 93 patients. After adjustment for demographic, renal, cardiovascular, and biochemical covariates, including serum phosphate, risk of death among patients in the lowest T50 tertile was more than two times the risk among patients in the highest T50 tertile (adjusted hazard ratio, 2.2; 95% confidence interval, 1.1 to 5.4; P=0.04). This effect was lost, however, after additional adjustment for aortic stiffness, suggesting a shared causal pathway. Longitudinally, serum calcification propensity measurements remained temporally stable (intraclass correlation=0.81). These results suggest that serum T50 may be helpful as a biomarker in designing methods to improve defenses against vascular calcification.

Muscle injury (rhabdomyolysis) and subsequent deposition of myoglobin in the kidney causes renal vasoconstriction and renal failure. We tested the hypothesis that myoglobin induces oxidant injury to the kidney and the formation of F₂-isoprostanes, potent renal vasoconstrictors formed during lipid peroxidation. In low density lipoprotein (LDL), myoglobin induced a 30-fold increase in the formation of F₂-isoprostanes by a mechanism involving redox cycling between ferric and ferryl forms of myoglobin. In an animal model of rhabdomyolysis, urinary excretion of F₂-isoprostanes increased by 7.3-fold compared with controls. Administration of alkali, a treatment for rhabdomyolysis, improved renal function and significantly reduced the urinary excretion of F₂-isoprostanes by ∼80%. EPR and UV spectroscopy demonstrated that myoglobin was deposited in the kidneys as the redox competent ferric myoglobin and that it’s concentration was not decreased by alkalinization. Kinetic studies demonstrated that the reactivity of ferryl myoglobin, which is responsible for inducing lipid peroxidation, is markedly attenuated at alkaline pH. This was further supported by demonstrating that myoglobin-induced oxidation of LDL was inhibited at alkaline pH. These data strongly support a causative role for oxidative injury in the renal failure of rhabdomyolysis and suggest that the protective effect of alkalinization may be attributed to inhibition of myoglobin-induced lipid peroxidation.

Aortic stiffness and chronic kidney disease are closely linked by shared risk factors and associated increased cardiovascular mortality. At lower levels of renal function, aortic stiffness is independently associated with glomerular filtration rate. However, the longitudinal impact of aortic stiffness on renal function has not been reported previously. A cohort of 133 patients with chronic kidney disease stages 3 and 4 (estimated glomerular filtration rate: 15 to 59 mL/min per 1.73 m ² ) underwent prospective measurement of arterial stiffness parameters and monitoring of renal function. Aortic pulse wave velocity measurement was performed in 120 patients. The mean age was 69±12 years (mean±SD; 103 men, 30 women, and 23.3% diabetic). Mean systolic blood pressure was 155±21 mm Hg, and mean diastolic blood pressure was 83±11 mm Hg. The mean Modification of Diet in Renal Disease estimated glomerular filtration rate was 32±11 mL/min per 1.73 m ² . Change in renal function was measured using reciprocal creatinine plots and the dichotomous combined end point of ≥25% decline in renal function or start of renal replacement therapy. After stepwise multivariate analysis, aortic pulse wave velocity was independently associated with gradient of reciprocal creatinine plot ( r =0.46; P =0.014). In multivariate analysis of the end point of ≥25% decline in renal function or start of renal replacement therapy, independent predictors were aortic pulse wave velocity ( r =0.48; P =0.002), systolic blood pressure ( r =0.17; P =0.039), and urine protein:creatinine ratio ( r =0.20; P =0.021). We, therefore, conclude that aortic stiffening is independently associated with rate of change in renal function in patients with chronic kidney disease stages 3 and 4.

FGF23 measurement may have a role in the management of patients with chronic kidney disease (CKD). Our objective was to study the biological variability of plasma intact FGF23 (iFGF23) and C-terminal FGF23 (cFGF23) concentrations. Plasma samples were taken from 12 healthy adults at multiple time points on 2 consecutive days to assess diurnal variability of FGF23 concentrations. Early-morning fasting and nonfasting samples were also taken over a 6-wk period to estimate components of biological variation. Samples from 170 volunteers were used to define reference intervals. FGF23 concentrations were measured by commercial ELISA. Western blotting was used to analyze FGF23 species from the plasma of healthy adults and patients with predialysis CKD and those undergoing dialysis. A total of 180 healthy adults and 18 adults with stage 3-5D CKD participated in this study at a hospital research unit. Estimates were made of the biological variability of plasma FGF23 concentrations. iFGF23, but not cFGF23, showed significant diurnal variation. cFGF23 had a significantly lower intra-individual variation than iFGF23 (8.3 vs. 18.3%) but higher inter-individual variation than iFGF23 (28.9 vs. 19.2%). Fourteen samples would be needed to estimate an individual's homeostatic set point (within 10%) for iFGF23 compared with only three samples for cFGF23. Using Western blotting, C-terminal FGF23 fragments were detected in the plasma of individuals with normal renal function and at all stages of renal disease. The percent cFGF23 was significantly higher in those without renal impairment (P < 0.001) and was positively correlated with plasma phosphate concentration in those with normal renal function. The high intra-individual biological variability of iFGF23 may limit its clinical use as a diagnostic or management tool. Risk-related thresholds may be more appropriate for clinical decision making based on cFGF23 measurements than conventional reference intervals. FGF23 cleavage pathways may be an important natural regulatory mechanism for phosphate control.

Vascular stiffening occurs in normal ageing and is accelerated in chronic kidney disease (CKD). Vascular calcification contributes to this stiffening and to the high incidence of vascular morbidity and mortality in this population. A network of inhibitors work in concert to reduce mineralization risk in extra-osseous tissue. Fetuin-A is an important systemic inhibitor of ectopic calcification. A fraction of the total circulating fetuin-A interacts with mineral ions to form stable colloidal complexes, calciprotein particles (CPP), preventing deposition. We sought to assess whether CPP fetuin-A levels were associated with procalcific factors and aortic stiffness in a cohort of patients with Stages 3 and 4 CKD. We measured fetuin-A CPP levels, serum inflammatory markers [C-reactive protein (CRP), interleukin-6, tumour necrosis factor-α], oxidized low-density lipoprotein (oxLDL), bone morphogenetic protein-2 (BMP-2) and -7 (BMP-7) and aortic pulse wave velocity (APWV) in a cohort of 200 CKD patients. Serum measurements were also made in 78 healthy controls. CPP fetuin-A phosphorylation was characterized by phosphate-affinity gel chromatography. Fetuin-A-containing CPPs were only detectable in the serum of CKD patients. Inflammatory markers, oxLDL and BMP-2 levels were all significantly higher in the CKD than control subjects. CPP fetuin-A levels were independently associated with serum phosphate, high-sensitivity C-reactive protein, oxLDL, BMP-2/7 ratio and inversely with estimated glomerular filtration rate (model R² = 0.51). After adjusting for confounders, CPP fetuin-A levels were independently associated with APWV. Only phosphorylated fetuin-A was present in serum CPP. Increased CPP fetuin-A levels reflect an increasingly procalcific milieu and are associated with increased aortic stiffness in patients with pre-dialysis CKD.