NO has a significant effect on vascular smooth muscle tone and blood pressure. Numerous endothelium-dependent vasodilators, such as acetylcholine and bradykinin, act by increasing intracellular calcium levels in endothelial cells, leading to the synthesis of NO. NO diffuses to vascular smooth muscle, leading to vasorelaxation (Figure 19–2). Mice with a knockout mutation in the eNOS gene display increased vascular tone and elevated mean arterial pressure, indicating that eNOS is a fundamental regulator of blood pressure.
Regulation of vasorelaxation by endothelial-derived nitric oxide (NO). Endogenous vasodilators, eg, acetylcholine and bradykinin, cause calcium (Ca2+) efflux from the endoplasmic reticulum in the luminal endothelial cells into the cytoplasm. Calcium binds to calmodulin (CaM), which activates endothelial NO synthase (eNOS), resulting in NO synthesis from L-arginine. NO diffuses into smooth muscle cells, where it activates soluble guanylyl cyclase and cyclic guanosine monophosphate (cGMP) synthesis from guanosine triphosphate (GTP). cGMP binds and activates protein kinase G (PKG), resulting in an overall reduction in calcium influx, and inhibition of calcium-dependent muscle contraction. PKG can also block other pathways that lead to muscle contraction. cGMP signaling is terminated by phosphodiesterases, which convert cGMP to GMP.
Apart from being a vasodilator and regulating blood pressure, NO also has antithrombotic effects. Both endothelial cells and platelets contain eNOS, which acts via an NO-cGMP pathway to inhibit platelet activation, an initiator of thrombus formation. In diseases such as diabetes, endothelial cells are dysfunctional and produce reduced levels of NO, resulting in an increased propensity for abnormal platelet function and thrombosis. NO may have an additional inhibitory effect on blood coagulation by enhancing fibrinolysis via an effect on plasminogen.
NO also protects against atherogenesis. A major antiatherogenic mechanism of NO involves the inhibition of proliferation and migration of vascular smooth muscle cells. In animal models, myointimal proliferation following angioplasty can be blocked by NO donors, by NOS gene transfer, and by NO inhalation. NO reduces the ability of monocytes and leukocytes to adhere to endothelial cells, which is an early step in the development of atheromatous plaques. This effect is due to the inhibitory effect of NO on the expression of adhesion molecules on the endothelial cell surface. The antiatherogenic effect of NO may also involve an antioxidant effect, blocking the oxidation of low-density lipoproteins and thus preventing or reducing the formation of foam cells in the vascular wall. Plaque formation is also reduced by NO-dependent reduction of endothelial cell permeability to lipoproteins. The importance of eNOS in cardiovascular disease is supported by experiments showing increased atherosclerosis in animals treated with eNOS inhibitors. Atherosclerosis risk factors, such as smoking, hyperlipidemia, diabetes, and hypertension, are associated with decreased endothelial NO production, and thus with a loss of the diverse antiatherogenic effects of NO.
Sepsis is a systemic inflammatory response caused by infection. Endotoxin components from the bacterial wall along with endogenously generated tumor necrosis factor-α and other cytokines induce synthesis of iNOS in macrophages, neutrophils, and T cells, as well as hepatocytes, smooth muscle cells, endothelial cells, and fibroblasts. This widespread generation of NO results in exaggerated hypotension, shock, and, in some cases, death. This hypotension is reduced or reversed by NOS inhibitors in humans as well as in animal models (Table 19–3). A similar reversal of hypotension is produced by compounds that prevent the action of NO, such as the sGC inhibitor methylene blue. Furthermore, knockout mice lacking a functional iNOS gene are more resistant to endotoxin than wild-type mice. However, despite the ability of NOS inhibitors to ameliorate hypotension in sepsis, there is no overall improvement in survival in patients with gram-negative sepsis treated with NOS inhibitors. The absence of benefit may reflect the inability of the NOS inhibitors used in these trials to differentiate between NOS isoforms, or may reflect concurrent inhibition of beneficial aspects of iNOS signaling.
TABLE 19–3Some inhibitors of nitric oxide synthesis or action. ||Download (.pdf) TABLE 19–3 Some inhibitors of nitric oxide synthesis or action.
|Inhibitor ||Mechanism ||Comment |
|Nω-Monomethyl-L-arginine (L-NMMA) ||Competitive inhibitor, binds arginine-binding site in NOS ||Nonselective NOS inhibitor |
|Nω-Nitro-L-arginine methyl ester (L-NAME) ||Competitive inhibitor, binds arginine-binding site in NOS ||Nonselective NOS inhibitor |
|7-Nitroindazole ||Competitive inhibitor, binds both tetrahydrobiopterin and arginine-binding sites in NOS ||Partially selective for NOS-1 in vivo |
|BBS-2 ||Inhibits iNOS dimerization ||Also weakly inhibits nNOS and eNOS |
|Hemoglobin ||NO scavenger || |
The generation of NO has both beneficial and detrimental roles in the host immune response and in inflammation. The host response to infection or injury involves the recruitment of leukocytes and the release of inflammatory mediators, such as tumor necrosis factor and interleukin-1. This leads to a marked increase in iNOS levels and activity in leukocytes, fibroblasts, and other cell types. The NO that is produced, along with peroxynitrite that forms from its interaction with superoxide, is an important microbicide. NO also appears to play an important protective role in the body via immune cell function. When challenged with foreign antigens, TH1 cells (see Chapter 55) respond by synthesizing NO, which has roles in TH1 cells. The importance of NO in TH1 cell function is demonstrated by the impaired protective response to injected parasites in animal models after inhibition of iNOS. NO also stimulates the synthesis of inflammatory prostaglandins by activating cyclooxygenase isoenzyme 2 (COX-2). Through its effects on COX-2, its direct vasodilatory effects, and other mechanisms, NO generated during inflammation contributes to the erythema, vascular permeability, and subsequent edema associated with acute inflammation.
However, in both acute and chronic inflammatory conditions, prolonged or excessive NO production may exacerbate tissue injury. Indeed, psoriasis lesions, airway epithelium in asthma, and inflammatory bowel lesions in humans all demonstrate elevated levels of NO and iNOS, suggesting that persistent iNOS induction may contribute to disease pathogenesis. Moreover, these tissues also exhibit increased levels of nitrotyrosine, indicating excessive formation of peroxynitrite. In several animal models of arthritis, increasing NO production by dietary L-arginine supplementation exacerbates arthritis, whereas protection is seen with iNOS inhibitors. Thus, inhibition of the NO pathway may have a beneficial effect on a variety of acute and chronic inflammatory diseases.
THE CENTRAL NERVOUS SYSTEM
NO has an important role in the central nervous system as a neurotransmitter (see Chapter 21). Unlike classic transmitters such as glutamate or dopamine, which are stored in synaptic vesicles and released in the synaptic cleft upon vesicle fusion, NO is not stored, but rather is synthesized on demand and immediately diffuses to neighboring cells. NO synthesis is induced at postsynaptic sites in neurons, most commonly upon activation of the NMDA subtype of glutamate receptor, which results in calcium influx and activation of nNOS. In several neuronal subtypes, eNOS is also present and activated by neurotransmitter pathways that lead to calcium influx. NO synthesized postsynaptically may function as a retrograde messenger and diffuse to the presynaptic terminal to enhance the efficiency of neurotransmitter release, thereby regulating synaptic plasticity, the process of synapse strengthening that underlies learning and memory. Because aberrant NMDA receptor activation and excessive NO synthesis is linked to excitotoxic neuronal death in several neurologic diseases, including stroke, amyotrophic lateral sclerosis, and Parkinson’s disease, therapy with NOS inhibitors may reduce neuronal damage in these conditions. However, clinical trials have not clearly supported any benefit of NOS inhibition, which may reflect nonselectivity of the inhibitors, resulting in inhibition of the beneficial effects of eNOS.
THE PERIPHERAL NERVOUS SYSTEM
Nonadrenergic, noncholinergic (NANC) neurons are widely distributed in peripheral tissues, especially the gastrointestinal and reproductive tracts (see Chapter 6). Considerable evidence implicates NO as a mediator of certain NANC actions, and some NANC neurons appear to release NO. Penile erection is thought to be caused by the release of NO from NANC neurons; NO promotes relaxation of the smooth muscle in the corpora cavernosa—the initiating factor in penile erection—and inhibitors of NOS have been shown to prevent erection caused by pelvic nerve stimulation in the rat. An established approach in treating erectile dysfunction is to enhance the effect of NO signaling by inhibiting the breakdown of cGMP by the phosphodiesterase (PDE isoform 5) present in the smooth muscle of the corpora cavernosa with drugs such as sildenafil, tadalafil, and vardenafil (see Chapter 12).
NO is administered by inhalation to newborns with hypoxic respiratory failure associated with pulmonary hypertension. The current treatment for severely defective gas exchange in the newborn is with extracorporeal membrane oxygenation (ECMO), which does not directly affect pulmonary vascular pressures. NO inhalation dilates pulmonary vessels, resulting in decreased pulmonary vascular resistance and reduced pulmonary artery pressure. Inhaled NO also improves oxygenation by reducing mismatch of ventilation and perfusion in the lung. Inhalation of NO results in dilation of pulmonary vessels in areas of the lung with better ventilation, thereby redistributing pulmonary blood flow away from poorly ventilated areas. NO inhalation does not typically exert pronounced effects on the systemic circulation. Inhaled NO has also been shown to improve cardiopulmonary function in adult patients with pulmonary artery hypertension.
An additional approach for treating pulmonary hypertension is to potentiate the actions of NO in pulmonary vascular beds. Due to the enrichment of PDE-5 in pulmonary vascular beds, PDE-5 inhibitors such as sildenafil and tadalafil induce vasodilation and marked reductions in pulmonary hypertension (see also Chapters 12 and 17).
SUMMARY Nitric Oxide
|Subclass, Drug ||Mechanism of Action ||Effects ||Clinical Applications ||Pharmacokinetics, Toxicity, Interactions |
|NITRIC OXIDE (NO) ||NO activates soluble guanylyl cyclase to elevate cGMP levels in vascular smooth muscle ||Vasodilator • relaxes other smooth muscle • inhalation of NO leads to increased blood flow to parts of the lung exposed to NO and decreased pulmonary vascular resistance ||Hypoxic respiratory failure and pulmonary hypertension ||Inhaled gas • Toxicity: Methemoglobinemia |