Background Chronic iron-overload (CIO) is associated with blood disorders such as thalassemias and hemochromatosis. A major prognostic indicator of survival in patients with CIO is iron-mediated cardiomyopathy, characterized by contractile dysfunction and electrical disturbances including slow heart rate (HR; bradycardia) and heart block. Methods and Results We have used a mouse model of CIO to investigate the effects of iron on sinoatrial node (SAN) function. As in humans, CIO reduced HR (~20%) in conscious mice as well as in anesthetized mice with autonomic nervous system blockade and in isolated Langendorff-perfused mouse hearts, suggesting bradycardia originates from altered intrinsic SAN pacemaker function. Indeed, spontaneous action potential frequencies in SAN myocytes with CIO were reduced in association with decreased L-type Ca2+ current (ICa,L) densities and positive (rightward) voltage shifts in ICa,L activation. Pacemaker current (If) current was not affected by CIO. Since ICa,L in SAN myocytes (as well as in atrial and conducting system myocytes) activates at relatively negative potentials due to the presence of CaV1.3 channels (in addition to CaV1.2 channels), our data suggest that elevated iron preferentially suppresses CaV1.3 channel function. Consistent with this suggestion, CIO reduced CaV1.3 mRNA levels by ~40% in atrial tissue (containing SAN) and did not lower HR in CaV1.3 knockout mice. CIO also induced PR interval prolongation, heart block, and atrial fibrillation, conditions also seen in CaV1.3 knockout mice. Conclusion Our results demonstrate that CIO selectively reduces CaV1.3-mediated ICa,L leading to bradycardia, slowing of electrical conduction and atrial fibrillation as seen in iron-overload patients.

The adipocyte enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) amplifies local glucocorticoid action by generating active glucocorticoids from inactive metabolites and has emerged as a key player in the pathogenesis of central obesity and metabolic syndrome. However, the regulation of adipocyte 11beta-HSD1 is incompletely understood. Therefore, the present study was designed to investigate the effects of insulin and glucocorticoid as well as their underlying molecular mechanisms on 11beta-HSD1 activity and expression in 3T3-L1 adipocytes and determine whether the in vitro findings could be confirmed in vivo. Our main in vitro findings are 1) insulin stimulated whereas dexamethasone inhibited 11beta-HSD1 activity and expression in a time- and concentration-dependent manner; 2) the effect of dexamethasone was mimicked by both cortisol and corticosterone but blocked by the glucocorticoid receptor antagonist RU486; 3) the p38 MAPK inhibitor SB220025, but not the ERK inhibitor U0126 or the phosphatidylinositol 3-kinase inhibitor LY294002, prevented insulin stimulation of 11beta-HSD1 activity; and 4) although dexamethasone did not alter the half-life of 11beta-HSD1 mRNA, insulin doubled it. Taken together, these in vitro results demonstrate that insulin stimulates adipocyte 11beta-HSD1 through a posttranscriptional mechanism that involves activation of the p38 MAPK signaling pathway, whereas dexamethasone exerts an opposite effect by a glucocorticoid receptor-mediated transcriptional mechanism. In contrast, both insulin and dexamethasone augmented 11beta-HSD1 activity and expression in rat white adipose tissue in vivo, thus confirming the role of insulin but revealing a fundamental difference regarding the role of dexamethasone in regulating adipocyte 11beta-HSD1 between the two model systems.