Nitrite anions comprise the largest vascular storage pool of nitric oxide (NO), provided that physiological mechanisms exist to reduce nitrite to NO. We evaluated the vasodilator properties and mechanisms for bioactivation of nitrite in the human forearm. Nitrite infusions of 36 and 0.36 micromol/min into the forearm brachial artery resulted in supra- and near-physiologic intravascular nitrite concentrations, respectively, and increased forearm blood flow before and during exercise, with or without NO synthase inhibition. Nitrite infusions were associated with rapid formation of erythrocyte iron-nitrosylated hemoglobin and, to a lesser extent, S-nitroso-hemoglobin. NO-modified hemoglobin formation was inversely proportional to oxyhemoglobin saturation. Vasodilation of rat aortic rings and formation of both NO gas and NO-modified hemoglobin resulted from the nitrite reductase activity of deoxyhemoglobin and deoxygenated erythrocytes. This finding links tissue hypoxia, hemoglobin allostery and nitrite bioactivation. These results suggest that nitrite represents a major bioavailable pool of NO, and describe a new physiological function for hemoglobin as a nitrite reductase, potentially contributing to hypoxic vasodilation.

The therapeutic goal in peripheral arterial disease (PAD) patients is to restore blood flow to ischemic tissue. Stem cell transplantation offers a new avenue to enhance arteriogenesis and angiogenesis. Two major problems with cell therapies are poor cell survival and the lack of visualization of cell delivery and distribution. To address these therapeutic barriers, allogeneic bone marrow-derived mesenchymal stem cells (MSCs) were encapsulated in alginate impregnated with a radiopaque contrast agent (MSC-Xcaps.) In vitro MSC-Xcap viability by a fluorometric assay was high (96.9% ± 2.7% at 30 days postencapsulation) and as few as 10 Xcaps were visible on clinical x-ray fluoroscopic systems. Using an endovascular PAD model, rabbits (n = 21) were randomized to receive MSC-Xcaps (n = 6), empty Xcaps (n = 5), unencapsulated MSCs (n = 5), or sham intramuscular injections (n = 5) in the ischemic thigh 24 hours postocclusion. Immediately after MSC transplantation and 14 days later, digital radiographs acquired on a clinical angiographic system demonstrated persistent visualization of the Xcap injection sites with retained contrast-to-noise. Using a modified TIMI frame count, quantitative angiography demonstrated a 65% improvement in hind limb perfusion or arteriogenesis in MSC-Xcap-treated animals versus empty Xcaps. Post-mortem immunohistopathology of vessel density by anti-CD31 staining demonstrated an 87% enhancement in angiogenesis in Xcap-MSC-treated animals versus empty Xcaps. MSC-Xcaps represent the first x-ray-visible cellular therapeutic with enhanced efficacy for PAD treatment.

Purpose To investigate the utility of inversion recovery with ON‐resonant water suppression (IRON) to create positive signal in normal lymph nodes after injection of superparamagnetic nanoparticles. Materials and Methods Experiments were conducted on six rabbits, which received a single bolus injection of 80 μmol Fe/kg monocrystalline iron oxide nanoparticle (MION‐47). Magnetic resonance imaging (MRI) was performed at baseline, 1 day, and 3 days after MION‐47 injection using conventional T1‐ and T2*‐weighted sequences and IRON. Contrast‐to‐noise ratios (CNR) were measured in blood and in paraaortic lymph nodes. Results On T2*‐weighted images, as expected, signal attenuation was observed in areas of paraaortic lymph nodes after MION‐47 injection. However, using IRON the paraaortic lymph nodes exhibited very high contrast enhancement, which remained 3 days after injection. CNR with IRON was 2.2 ± 0.8 at baseline, increased markedly 1 day after injection (23.5 ± 5.4, P < 0.01 vs. baseline), and remained high after 3 days (21.8 ± 5.7, *P < 0.01 vs. baseline). CNR was also high in blood 1 day after injection (42.7 ± 7.2 vs. 1.8 ± 0.7 at baseline, P < 0.01) but approached baseline after 3 days (1.9 ± 1.4, P = NS vs. baseline). Conclusion IRON in conjunction with superparamagnetic nanoparticles can be used to perform ‘positive contrast’ MR‐lymphography, particularly 3 days after injection of the contrast agent, when signal is no longer visible within blood vessels. The proposed method may have potential as an adjunct for nodal staging in cancer screening. J. Magn. Reson. Imaging 2008;27:1175–1180. © 2008 Wiley‐Liss, Inc.