The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review.

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Nitric Oxide and Peroxynitrite in Health and Disease

Nitric oxide contrasts with most intercellular messengers because it diffuses rapidly and isotropically through most tissues with little reaction but cannot be transported through the vasculature due to rapid destruction by oxyhemoglobin. The rapid diffusion of nitric oxide between cells allows it to locally integrate the responses of blood vessels to turbulence, modulate synaptic plasticity in neurons, and control the oscillatory behavior of neuronal networks. Nitric oxide is not necessarily short lived and is intrinsically no more reactive than oxygen. The reactivity of nitric oxide per se has been greatly overestimated in vitro because no drain is provided to remove nitric oxide. Nitric oxide persists in solution for several minutes in micromolar concentrations before it reacts with oxygen to form much stronger oxidants like nitrogen dioxide. Nitric oxide is removed within seconds in vivo by diffusion over 100 microns through tissues to enter red blood cells and react with oxyhemoglobin. The direct toxicity of nitric oxide is modest but is greatly enhanced by reacting with superoxide to form peroxynitrite (ONOO-). Nitric oxide is the only biological molecule produced in high enough concentrations to out-compete superoxide dismutase for superoxide. Peroxynitrite reacts relatively slowly with most biological molecules, making peroxynitrite a selective oxidant. Peroxynitrite modifies tyrosine in proteins to create nitrotyrosines, leaving a footprint detectable in vivo. Nitration of structural proteins, including neurofilaments and actin, can disrupt filament assembly with major pathological consequences. Antibodies to nitrotyrosine have revealed nitration in human atherosclerosis, myocardial ischemia, septic and distressed lung, inflammatory bowel disease, and amyotrophic lateral sclerosis.

Nitric oxide, superoxide, and peroxynitrite: the good, the bad, and ugly.

Background. Endothelium regulates vascular tone by influencing the contractile activity of vascular smooth muscle. This regulatory effect of the endothelium on blood vessels has been shown to be impaired in atherosclerotic arteries in humans and animals and in animal models of hypertension. Methods. To determine whether patients with essential hypertension have an endothelium-dependent abnormality in vascular relaxation, we studied the response of the forearm vasculature to acetylcholine (an endotheliumdependent vasodilator) and sodium nitroprusside (a direct dilator of smooth muscle) in 18 hypertensive patients (mean age[±SD], 50.7± 10 years; 10 men and 8 women) two weeks after the withdrawal of antihypertensive medications and in 18 normal controls (mean age, 49.9±9; 9 men and 9 women). The drugs were infused at increasing concentrations into the brachial artery, and the response in forearm blood flow was measured by strain-gauge plethysmography. Results. The basal forearm blood flow was simila...

https://www.nejm.org/doi/pdf/10.1056/NEJM199007053230105

Abnormal endothelium-dependent vascular relaxation in patients with essential hypertension.

Effects of endothelium-derived nitric oxide on peripheral arteriolar tone in man.

Fk 506 for liver, kidney, and pancreas transplantation

Constriction of vascular smooth muscle in response to the stimulus of raised intravascular pressure—the myogenic response1,2— represents a positive feedback mechanism which, if unopposed, could theoretically lead to instability in the intact circulation3,4. Dilation in response to increased intraluminal flow would provide an opposing feedback mechanism which could confer overall stability4. Flow-dependent dilation in conduit vessels5–7 is mediated by endothelium-derived relaxing factor (EDRF)8–14, but the relationship between flow and EDRF activity has not been studied in resistance vessels in situ. We here demonstrate that EDRF can coordinate the aggregate hydrodynamic properties of an intact network. Under control conditions, EDRF maintains a fourth-power relationship between diameter and flow so that the pressure gradient in each vessel asymptotically approaches a constant value at high flow rates. Basal EDRF release may also maintain a similar spatial distribution of flow at different flow rates, even under conditions of moderate pharmacological constriction.

EDRF coordinates the behaviour of vascular resistance vessels.

The protective role of eicosapentaenoic acid [EPA] in the pathogenesis of nephrolithiasis

1
Microradiographic techniques have been used to show that endothelium-derived relaxing factor (EDRF), which is believed to be nitric oxide, influences vasomotor responses in small arteries and arterioles down to 25 μm in diameter in an isolated, intact, buffer-perfused ear preparation of the rabbit. Arteries down to 75 μm in diameter, i.e. the central ear artery (G0) and its first three generations of branch vessels (G1, G2 and G3) were studied quantitatively.

2
Relative constrictor responses to 1 μm 5-hydroxytryptamine (5-HT) and the combination of 1 μm 5-HT and 1 μm histamine diminished progressively from G0 to G3. Constrictor responses to 5-HT were doubled in all generations by 1 μm haemoglobin which abolishes EDRF activity.

3
Relative dilator responses to acetylcholine or to substance P in preconstricted arteries were, in contrast, equal in the different generations. Mean —log (IC50) values calculated from diameter measurements were 7.63 ± 0.10 m and 9.80 ± 0.11 m, respectively. These dilator responses were abolished by 1 μm haemoglobin, implying that they were EDRF-mediated. Spatial homogeneity of relative dilator responses was found also with glyceryl trinitrate (10 or 50 μm) whose activity is thought to depend on biotransformation to nitric oxide, in both the presence and the absence of haemoglobin.

4
This finding of spatial homogeneity of the diameter response to changes in EDRF activity (or to glyceryl trinitrate) implies that EDRF influences hydrodynamic resistance more in vessels where constrictor tone is high.

Endothelium-derived relaxing factor (EDRF) and resistance vessels in an intact vascular bed: a microangiographic study of the rabbit isolated ear

The role of EDRF in flow distribution: a microangiographic study of the rabbit isolated ear.

SUMMARY

Contact X-ray microscopy (microradiography) is a method of studying the microstructure of biological tissue. These techniques have been used to study the historadiological details of human breast tissue and sections of human ear ossicles. X-ray microscopy can also be used to demonstrate variations in structural densities seen in histological specimens including the detection of microcalcification. A modification of existing apparatus is described which has resulted in improved image-contrast and detail.



The ability of X-rays to penetrate relatively thick sections of tissue makes it an ideal method for studying the morphology of biological structures, particularly in calcified tissue. The tissues may be further examined by conventional histology, elemental analysis, etc. The technique has a complementary role to alternative methods of tissue microscopy.

R. Ll. Davies

Papers

EDRF coordinates the behaviour of vascular resistance vessels.

The protective role of eicosapentaenoic acid [EPA] in the pathogenesis of nephrolithiasis

Endothelium-derived relaxing factor (EDRF) and resistance vessels in an intact vascular bed: a microangiographic study of the rabbit isolated ear

The role of EDRF in flow distribution: a microangiographic study of the rabbit isolated ear.

Developments in contact X-ray microscopy in biomedical research