Myoglobin may serve a variety of functions in muscular oxygen supply, such as O 2 storage, facilitated O 2 diffusion, and myoglobin-mediated oxidative phosphorylation. We studied the functional consequences of a myoglobin deficiency on cardiac function by producing myoglobinknockout (myo ؊͞؊ ) mice. )]. These data demonstrate that disruption of myoglobin results in the activation of multiple compensatory mechanisms that steepen the pO 2 gradient and reduce the diffusion path length for O 2 between capillary and the mitochondria; this suggests that myoglobin normally is important for the delivery of oxygen.

Abstract-To investigate the role of adenosine formed extracellularly in vascular homeostasis, mice with a targeted deletion of the cd73/ecto-5Ј-nucleotidase were generated. Southern blot, RT-PCR, and Western blot analysis confirmed the constitutive knockout. In vivo analysis of hemodynamic parameters revealed no significant differences in systolic blood pressure, ejection fraction, or cardiac output between strains. However, basal coronary flow measured in the isolated perfused heart was significantly lower (Ϫ14%; PϽ0.05) in the mutant. Immunohistochemistry revealed strong CD73 expression on the endothelium of conduit vessels in wild-type (WT) mice. Time to carotid artery occlusion after ferric chloride (FeCl 3 ) was significantly reduced by 20% in cd73 Ϫ/Ϫ mice (PϽ0.05). Bleeding time after tail tip resection tended to be shorter in cd73mice (Ϫ35%). In vivo platelet cAMP levels were 0.96Ϯ0.46 in WT versus 0.68Ϯ0.27 pmol/10 6 cells in cd73 Ϫ/Ϫ mice (PϽ0.05). Under in vitro conditions, platelet aggregation in response to ADP (0.05 to 10 mol/L) was undistinguishable between the two strains. In the cremaster model of ischemia-reperfusion, the increase in leukocyte attachment to endothelium was significantly higher in cd73 Ϫ/Ϫ compared with WT littermates (WT 98% versus cd73 Ϫ/Ϫ 245%; PϽ0.005). The constitutive adhesion of monocytes in ex vivo-perfused carotid arteries of WT mice was negligible but significantly increased in arteries of cd73 Ϫ/Ϫ mice (PϽ0.05). Thus, our data provide the first evidence that adenosine, extracellularly formed by CD73, can modulate coronary vascular tone, inhibit platelet activation, and play an important role in leukocyte adhesion to the vascular endothelium in vivo. Key Words: transgenic mice Ⅲ adenosine Ⅲ ecto-5Ј-nucleotidase Ⅲ vascular inflammation Ⅲ thrombosis C D73/ecto-5Ј-nucleotidase, a 70-kDa glycosylphosphatidylinositol (GPI)-anchored cell surface molecule, is expressed on the vascular endothelium and catalyzes the extracellular conversion of 5Ј-AMP to adenosine. 1,2 CD73 is the final step of the extracellular nucleotide breakdown cascade that also involves membrane-associated CD39/ATPdiphosphohydrolase. 3 The product of CD73 is adenosine, a purine nucleoside that has been implicated in many physiological and pathophysiological events. 4 There are four known G-coupled adenosine receptors: A 1 , A 2A , A 2B , and A 3 , each of which operates via different intracellular signaling mechanisms and exhibits distinct patterns of tissue distribution. 5 In human neutrophils, adenosine A 1 and A 2 receptor occupancy mediate opposing roles for adenosine in inflammation: A 1 activation is proinflammatory, whereas the A 2 receptor plays an anti-inflammatory role. 6 A 2 receptor activation inhibits the neutrophil oxidative burst, whereas the A 3 receptor inhibits neutrophil degranulation 7 and may play an important role in inflammation by inhibiting eosinophil migration. 8 Recently, deletion of the A 2A receptor in transgenic mice revealed that this receptor is critical for the limitation a...

The present study explored the role of myoglobin (Mb) in cardiac NO homeostasis and its functional relevance by employing isolated hearts of wild-type (WT) and myoglobin knockout mice. 1 H NMR spectroscopy was used to measure directly the conversion of oxygenated Mb (MbO2) to metmyoglobin (metMb) by reaction with NO. NO was applied intracoronarily (5 nM to 25 M), or its endogenous production was stimulated with bradykinin (Bk; 10 nM to 2 M). We found that infusion of authentic NO solutions dose-dependently (> 2.5 M NO) increased metMb formation in WT hearts that was rapidly reversible on cessation of NO infusion. Likewise, Bk-induced release of NO was associated with significant metMb formation in the WT (>1 M Bk and IIa skeletal and cardiac muscle tissue (1). As a major breakthrough in understanding globular protein structure, its tertiary structure was derived from x-ray diffraction studies by John Kendrew and his colleagues as early as the 1950s (2). Mb is a relatively small (M r 16,700) and densely packed protein consisting of a single polypeptide chain of 153 amino acid residues. It contains an iron-porphyrin heme group identical to that of hemoglobin (Hb), and like Hb is capable of reversible oxygenation and deoxygenation. In mammals, half O 2 saturation of Mb is achieved at an intracellular O 2 partial pressure as low as 2.4 mmHg (1 mmHg ϭ 133 Pa; ref.3), suggesting a predominance of oxygenated Mb (MbO 2 ) under basal conditions. Mb's function as an oxygen store is well accepted. Mb serves as a short-term O 2 reservoir in exercising skeletal muscle and in the beating heart, tiding the muscle over from one contraction to the next (4). In diving mammals, the concentrations of Mb exceed those of terrestrial mammals up to 10-fold, and Mb most likely serves for the extension of diving time when pulmonary ventilation ceases (5). Similarly, in mammals and humans adapted to high altitudes, Mb is expressed in high concentrations in skeletal muscle (6).It has been proposed that Mb facilitates intracellular delivery of O 2 , in that Mb adjacent to the cell membrane picks up oxygen, traverses the cytosol by translational diffusion to unload O 2 in the vicinity of mitochondria, and finally diffuses back to the cell membrane in the deoxygenated state (7). This circuit, termed ''facilitated O 2 diffusion,'' may be a critical link between capillary O 2 supply and O 2 -consuming cytochromes within mitochondria in the steady state. Facilitated O 2 diffusion has been unambiguously demonstrated in concentrated Mb solutions (8), but experiments carried out in isolated cells, papillary muscle, and at the whole organ level have yielded conflicting results (9-11). Likewise, model calculations have both refuted and supported the contribution of Mb-bound O 2 to total O 2 flux (11, 12).The recent generation of transgenic mice lacking Mb has shed new light on the role of Mb in the intracellular delivery of O 2 (13,14). Loss of Mb led to a surprisingly benign phenotype, with exercise and reproductive capacity, as well as cardiac and ...

Abstract-For the specific analysis of endothelial NO synthase (eNOS) function in the coronary vasculature, we generated a mouse homozygous for a defective eNOS gene (eNOSϪ/Ϫ). Western blot as well as immunohistochemical staining revealed the absence of eNOS protein in eNOSϪ/Ϫ mice. Aortic endothelial cells derived from eNOSϪ/Ϫ mice displayed only background levels of NO x formation compared with wild-type (WT) cells (88 versus 1990 pmol). eNOSϪ/Ϫ mice were hypertensive (mean arterial pressure, 135Ϯ15 versus 107Ϯ8 mm Hg in WT) without the development of cardiac hypertrophy. Coronary hemodynamics, analyzed in Langendorff-perfused hearts, showed no differences either in basal coronary flow or in maximal and repayment flow of reactive hyperemia. Acute NOS inhibition with N -nitro-L-arginine methyl ester (L-NAME) in WT hearts substantially reduced basal flow and reactive hyperemia. The coronary response to acetylcholine (ACh) (500 nmol/L) was biphasic: An initial vasoconstriction (flow, Ϫ35%) in WT hearts was followed by sustained vasodilation (ϩ190%). L-NAME significantly reduced vasodilation in WT hearts (ϩ125%) but did not alter the initial vasoconstriction. In eNOSϪ/Ϫ hearts, the initial vasoconstriction was augmented (Ϫ70%), whereas the ACh-induced vasodilation was not affected. Inhibition of cyclooxygenase with diclofenac converted the ACh-induced vasodilation into vasoconstriction (Ϫ49% decrease of basal flow). This effect was even more pronounced in eNOSϪ/Ϫ hearts (Ϫ71%). Our results demonstrate that (1) acute inhibition of eNOS reveals a role for NO in setting the basal coronary vascular tone as well as participation in reactive hyperemia and the response to ACh; (2) chronic inhibition of NO formation in eNOSϪ/Ϫ mutant mice induces no changes in basal coronary flow and reactive hyperemia, suggesting the activation of important compensatory mechanisms; and (3) Key Words: heart Ⅲ gene targeting Ⅲ reactive hyperemia Ⅲ coronary flow Ⅲ blood pressure E ndothelial NO synthase, also called type III NO synthase, is the major NOS isoenzyme that is widely expressed in endothelial cells throughout the vascular bed. It is generally accepted that endothelium-derived NO is an important factor in the control of basal vascular tone.1 NO is also involved in receptor-mediated vasodilation in response to various agonists such as ACh, ATP, thrombin, bradykinin, and others. Through experiments using NOS inhibitors 2,3 but also by use of genetically modified animals, 4,5 it has been demonstrated that functional inactivation of eNOS activity results in hypertension. In addition to the control of vascular tone, NO inhibits platelet aggregation and leukocyte adhesion to the vessel wall as well as proliferation and migration of smooth muscle cells. Thus, eNOS is considered to play an important role in maintaining the antiatherogenic surface of the vessel wall. 6In the heart, eNOS is expressed primarily in the coronary and endocardial endothelia. In addition, eNOS has been localized to cardiac myocytes and the specialized cells of s...

To elucidate the physiological role of the AMP-adenosine metabolic cycle and to investigate the relation between AMP and adenosine formation, the O2 supply of isolated guinea pig hearts was varied (95% to 10% O2). The net adenosine formation rate (AMP-->adenosine) and coronary venous effluent adenosine release rate were measured; free cytosolic AMP was determined by 31P-nuclear magnetic resonance. Switching from 95% to 40% O2 increased free AMP and adenosine formation 4-fold, whereas free cytosolic adenosine and venous adenosine release rose 15- to 20-fold. In the AMP range from 200 to 3000 nmol/L, there was a linear correlation between free AMP and adenosine formation (R2 = .71); however, adenosine release increased several-fold more than formation. At 95% O2, only 6% of the adenosine formed was released; however, this fraction increased to 22% at 40% O2, demonstrating reduced adenosine salvage. Selective blockade of adenosine deaminase and adenosine kinase indicated that flux through adenosine kinase decreased from 85% to 35% of adenosine formation in hypoxia. Mathematical model analysis indicated that this apparent decrease in enzyme activity was not due to saturation but to the inhibition of adenosine kinase activity to 6% of the basal levels. The data show (1) that adenosine formation is proportional to the AMP substrate concentration and (2) that hypoxia decreases adenosine kinase activity, thereby shunting myocardial adenosine from the salvage pathway to venous release. In conclusion, because of the normal high turnover of the AMP-adenosine metabolic cycle, hypoxia-induced inhibition of adenosine kinase causes the amplification of small changes in free AMP into a major rise in adenosine. This mechanism plays an important role in the high sensitivity of the cardiac adenosine system to impaired oxygenation.