ContextThe efficient sequestration of hemoglobin by the red blood cell membrane
and the presence of multiple hemoglobin clearance mechanisms suggest a critical
need to prevent the buildup of this molecule in the plasma. A growing list
of clinical manifestations attributed to hemoglobin release in a variety of
acquired and iatrogenic hemolytic disorders suggests that hemolysis and hemoglobinemia
should be considered as a novel mechanism of human disease.Evidence AcquisitionPertinent scientific literature databases and references were searched
through October 2004 using terms that encompassed various aspects of hemolysis,
hemoglobin preparations, clinical symptoms associated with plasma hemoglobin,
nitric oxide in hemolysis, anemia, pulmonary hypertension, paroxysmal nocturnal
hemoglobinuria, and sickle-cell disease.Evidence SynthesisHemoglobin is released into the plasma from the erythrocyte during intravascular
hemolysis in hereditary, acquired, and iatrogenic hemolytic conditions. When
the capacity of protective hemoglobin-scavenging mechanisms has been saturated,
levels of cell-free hemoglobin increase in the plasma, resulting in the consumption
of nitric oxide and clinical sequelae. Nitric oxide plays a major role in
vascular homeostasis and has been shown to be a critical regulator of basal
and stress-mediated smooth muscle relaxation and vasomotor tone, endothelial
adhesion molecule expression, and platelet activation and aggregation. Thus,
clinical consequences of excessive cell-free plasma hemoglobin levels during
intravascular hemolysis or the administration of hemoglobin preparations include
dystonias involving the gastrointestinal, cardiovascular, pulmonary, and urogenital
systems, as well as clotting disorders. Many of the clinical sequelae of intravascular
hemolysis in a prototypic hemolytic disease, paroxysmal nocturnal hemoglobinuria,
are readily explained by hemoglobin-mediated nitric oxide scavenging.ConclusionA growing body of evidence supports the existence of a novel mechanism
of human disease, namely, hemolysis-associated smooth muscle dystonia, vasculopathy,
and endothelial dysfunction.

https://jamanetwork.com/journals/jama/articlepdf/200611/JCR50001.pdf

The clinical sequelae of intravascular hemolysis and extracellular plasma hemoglobin: a novel mechanism of human disease.

Regulation of cAMP and cGMP signaling: new phosphodiesterases and new functions.

Modern molecular biology has revealed vast numbers of large and complex proteins and genes that regulate body function. By contrast, discoveries over the past ten years indicate that crucial features of neuronal communication, blood vessel modulation and immune response are mediated by a remarkably simple chemical, nitric oxide (NO). Endogenous NO is generated from arginine by a family of three distinct calmodulin- dependent NO synthase (NOS) enzymes. NOS from endothelial cells (eNOS) and neurons (nNOS) are both constitutively expressed enzymes, whose activities are stimulated by increases in intracellular calcium. Immune functions for NO are mediated by a calcium-independent inducible NOS (iNOS). Expression of iNOS protein requires transcriptional activation, which is mediated by specific combinations of cytokines. All three NOS use NADPH as an electron donor and employ five enzyme cofactors to catalyze a five-electron oxidation of arginine to NO with stoichiometric formation of citrulline. The highest levels of NO throughout the body are found in neurons, where NO functions as a unique messenger molecule. In the autonomic nervous system NO functions NO functions as a major non-adrenergic non-cholinergic (NANC) neurotransmitter. This NANC pathway plays a particularly important role in producing relaxation of smooth muscle in the cerebral circulation and the gastrointestinal, urogenital and respiratory tracts. Dysregulation of NOS activity in autonomic nerves plays a major role in diverse pathophysiological conditions including migraine headache, hypertrophic pyloric stenosis and male impotence. In the brain, NO functions as a neuromodulator and appears to mediate aspects of learning and memory. Although endogenous NO was originally appreciated as a mediator of smooth muscle relaxation, NO also plays a major role in skeletal muscle. Physiologically muscle-derived NO regulates skeletal muscle contractility and exercise-induced glucose uptake. nNOS occurs at the plasma membrane of skeletal muscle which facilitates diffusion of NO to the vasculature to regulate muscle perfusion. nNOS protein occurs in the dystrophin complex in skeletal muscle and NO may therefore participate in the pathophysiology of muscular dystrophy. NO signalling in excitable tissues requires rapid and controlled delivery of NO to specific cellular targets. This tight control of NO signalling is largely regulated at the level of NO biosynthesis. Acute control of nNOS activity is mediated by allosteric enzyme regulation, by posttranslational modification and by subcellular targeting of the enzyme. nNOS protein levels are also dynamically regulated by changes in gene transcription, and this affords long-lasting changes in tissue NO levels. While NO normally functions as a physiological neuronal mediator, excess production of NO mediates brain injury. Overactivation of glutamate receptors associated with cerebral ischemia and other excitotoxic processes results in massive release of NO. As a free radical, NO is inherently reactive and mediates cellular toxicity by damaging critical metabolic enzymes and by reacting with superoxide to form an even more potent oxidant, peroxynitrite. Through these mechanisms, NO appears to play a major role in the pathophysiology of stroke, Parkinson's disease, Huntington's disease and amyotrophic lateral sclerosis.

Endogenous nitric oxide synthesis: Biological functions and pathophysiology

Cyclic nucleotide phosphodiesterases (PDEs) comprise a superfamily of metallophosphohydrolases that specifically cleave the 3',5'-cyclic phosphate moiety of cAMP and/or cGMP to produce the corresponding 5'-nucleotide. PDEs are critical determinants for modulation of cellular levels of cAMP and/or cGMP by many stimuli. Eleven families of PDEs with varying selectivities for cAMP or cGMP have been identified in mammalian tissues. Within these families, multiple isoforms are expressed either as products of different genes or as products of the same gene through alternative splicing. Regulation of PDEs is important for controlling myriad physiological functions, including the visual response, smooth muscle relaxation, platelet aggregation, fluid homeostasis, immune responses, and cardiac contractility. PDEs are critically involved in feedback control of cellular cAMP and cGMP levels. Activities of the various PDEs are highly regulated by a panoply of processes, including phosphorylation events, interaction with small molecules such as cGMP or phosphatidic acid, subcellular localization, and association with specific protein partners. The PDE superfamily continues to be a major target for pharmacological intervention in a number of medically important maladies.

Cyclic nucleotide phosphodiesterases: relating structure and function.

Erection is basically a spinal reflex that can be initiated by recruitment of penile afferents, but also by visual, olfactory, and imaginary stimuli. The reflex involves both autonomic and somatic efferents and is modulated by supraspinal influences. Several central transmitters involved in the erectile control have been identified. Dopamine, acetylcholine, nitric oxide (NO), and peptides, such as oxytocin and adrenocorticotropic/alpha-melanocyte-stimulating hormone, seem to have a facilitatory role, whereas serotonin may be either facilitatory or inhibitory, and enkephalins are inhibitory. Peripherally, the balance between contractant and relaxant factors controls the degree of contraction of the smooth muscle of the corpora cavernosa and determines the functional state of the penis. Noradrenaline contracts both corpus cavernosum and penile vessels via stimulation of alpha(1)-adrenoceptors. Neurogenic NO is considered the most important factor for relaxation of penile vessels and corpus cavernosum. The role of other mediators released from nerves or endothelium has not been definitely established. Erectile dysfunction (ED) may be due to inability of penile smooth muscles to relax. This inability can have multiple causes. However, patients with ED respond well to the pharmacological treatments that are currently available. The drugs used are able to substitute, partially or completely, the malfunctioning endogenous mechanisms that control penile erection. Most drugs have a direct action on penile tissue facilitating penile smooth muscle relaxation, including prostaglandin E(1), NO donors, phosphodiesterase inhibitors, and alpha-adrenoceptor antagonists. Dopamine receptors in central nervous centers participating in the initiation of erection have been targeted for the treatment of ED. Apomorphine, administered sublingually, is the first of such drugs.

Pharmacology of Penile Erection

Effects of sildenafil on the relaxation of human corpus cavernosum tissue in vitro and on the activities of cyclic nucleotide phosphodiesterase isozymes

Mary E. Price

Papers

Effects of sildenafil on the relaxation of human corpus cavernosum tissue in vitro and on the activities of cyclic nucleotide phosphodiesterase isozymes