Background and objectives: Hepcidin is a key regulator of iron homeostasis, but its study in the setting of chronic kidney disease (CKD) has been hampered by the lack of validated serum assays.Design, setting, participants, & measurements: This study reports the first measurements of bioactive serum hepcidin using a novel competitive ELISA in 48 pediatric (PCKD2-4) and 32 adult (ACKD2-4) patients with stages 2 to 4 CKD along with 26 pediatric patients with stage 5 CKD (PCKD5D) on peritoneal dialysis.Results: When compared with their respective controls (pediatric median ‫؍‬ 25.3 ng/ml, adult ‫؍‬ 72.9 ng/ml), hepcidin was significantly increased in PCKD2-4 (127.3 ng/ml), ACKD2-4 (269.9 ng/ml), and PCKD5D (652.4 ng/ml). Multivariate regression analysis was used to assess the relationship between hepcidin and indicators of anemia, iron status, inflammation, and renal function. In PCKD2-4 (R 2 ‫؍‬ 0.57), only ferritin correlated with hepcidin. In ACKD2-4 (R 2 ‫؍‬ 0.78), ferritin and soluble transferrin receptor were associated with hepcidin, whereas GFR was inversely correlated. In PCKD5D (R 2 ‫؍‬ 0.52), percent iron saturation and ferritin were predictors of hepcidin. In a multivariate analysis that incorporated all three groups (R 2 ‫؍‬ 0.6), hepcidin was predicted by ferritin, C-reactive protein, and whether the patient had stage 5D versus stages 2 to 4 CKD.Conclusions: These findings suggest that increased hepcidin across the spectrum of CKD may contribute to abnormal iron regulation and erythropoiesis and may be a novel biomarker of iron status and erythropoietin resistance.

The antibacterial protein hepcidin regulates the absorption, tissue distribution, and extracellular concentration of iron by suppressing ferroportin-mediated export of cellular iron. In CKD, elevated hepcidin and vitamin D deficiency are associated with anemia. Therefore, we explored a possible role for vitamin D in iron homeostasis. Treatment of cultured hepatocytes or monocytes with prohormone 25-hydroxyvitamin D or active 1,25-dihydroxyvitamin D decreased expression of hepcidin mRNA by 0.5-fold, contrasting the stimulatory effect of 25-hydroxyvitamin D or 1,25-dihydroxyvitamin D on related antibacterial proteins such as cathelicidin. Promoterreporter and chromatin immunoprecipitation analyses indicated that direct transcriptional suppression of hepcidin gene (HAMP) expression mediated by 1,25-dihydroxyvitamin D binding to the vitamin D receptor caused the decrease in hepcidin mRNA levels. Suppression of HAMP expression was associated with a concomitant increase in expression of the cellular target for hepcidin, ferroportin protein, and decreased expression of the intracellular iron marker ferritin. In a pilot study with healthy volunteers, supplementation with a single oral dose of vitamin D (100,000 IU vitamin D 2 ) increased serum levels of 25D-hydroxyvitamin D from 2762 ng/ml before supplementation to 4463 ng/ml after supplementation (P,0.001). This response was associated with a 34% decrease in circulating levels of hepcidin within 24 hours of vitamin D supplementation (P,0.05). These data show that vitamin D is a potent regulator of the hepcidin-ferroportin axis in humans and highlight a potential new strategy for the management of anemia in patients with low vitamin D and/or CKD.

The molecular bases of inwardly rectifying K(+) (Kir) currents and K(+)-induced dilations were examined in cerebral arteries of mice that lack the Kir2.1 and Kir2.2 genes. The complete absence of the open reading frame in animals homozygous for the targeted allele was confirmed. Kir2.1(-/-) animals die 8 to 12 hours after birth, apparently due to a complete cleft of the secondary palate. In contrast, Kir2.2(-/-) animals are viable and fertile. Kir currents were observed in cerebral artery myocytes isolated from control neonatal animals but were absent in myocytes from Kir2.1(-/-) animals. Voltage-dependent K(+) currents were similar in cells from neonatal control and Kir2.1(-/-) animals. An increase in the extracellular K(+) concentration from 6 to 15 mmol/L caused Ba(2+)-sensitive dilations in pressurized cerebral arteries from control and Kir2.2 mice. In contrast, arteries from Kir2.1(-/-) animals did not dilate when the extracellular K(+) concentration was increased to 15 mmol/L. In summary, Kir2.1 gene expression in arterial smooth muscle is required for Kir currents and K(+)-induced dilations in cerebral arteries.

Ventricular myocytes demonstrate a steeply inwardly rectifying K+ current termed IK1. We investigated the molecular basis for murine IK1 by removing the genes encoding Kir2.1 and Kir2.2. The physiological consequences of the loss of these genes were studied in newborn animals because mice lacking Kir2.1 have a cleft palate and die shortly after birth. Kir2.1 ‐/‐ ventricular myocytes lack detectable IK1 in whole‐cell recordings in 4 mM external K+. In 60 mM external K+ a small, slower, residual current is observed. Thus Kir2.1 is the major determinant of IK1. Sustained outward K+ currents and Ba2+ currents through L‐ and T‐type channels were not significantly altered by the mutation. A 50 % reduction in IK1 was observed in Kir2.2‐/‐ mice, raising the possibility that Kir2.2 can also contribute to the native IK1. Kir2.1 ‐/‐ myocytes showed significantly broader action potentials and more frequent spontaneous action potentials than wild‐type myocytes. In electrocardiograms of Kir2.1‐/‐ neonates, neither ectopic beats nor re‐entry arrhythmias were observed. Thus the increased automaticity and prolonged action potential of the mutant ventricular myocytes were not sufficiently severe to disrupt the sinus pacing of the heart. The Kir2.1‐/‐ mice, however, had consistently slower heart rates and this phenotype is likely to arise indirectly from the influence of Kir2.1 outside the heart. Thus Kir2.1 is the major component of murine IK1 and the Kir2.1‐/‐ mouse provides a model in which the functional consequences of removing IK1 can be studied at both cellular and organismal levels.