cGMP Degradation Research Articles

Lowered cAMP and cGMP signalling in the brain during levodopa‐induced dyskinesias in hemiparkinsonian rats: new aspects in the pathogenetic mechanisms

Dysregulation of dopamine receptors is thought to underlie levodopa-induced dyskinesias in experimental models of Parkinson's disease. It is unknown whether an imbalance of the second messengers, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), is involved in the alterations of levodopa/dopamine signal transduction. We examined cAMP and cGMP signalling in the interconnected cortico-striatal-pallidal loop at the peak of levodopa-induced dyskinesias in rats with 6-hydroxydopamine lesions in the substantia nigra. In addition, we examined the role of phosphodiesterase (PDE) and the rate of cAMP and cGMP degradation on the severity of levodopa-induced dyskinesias in animals pretreated with PDE inhibitor, zaprinast. Unilateral lesion of substantia nigra led to an increase in cAMP but a decrease in cGMP levels in the ipsilateral basal ganglia. After chronic levodopa treatment, cAMP and cGMP were differentially regulated in eukinetic animals: the cAMP level increased in the cortex and striatum but decreased in the globus pallidus of both hemispheres, whereas the cGMP decreased below baseline levels in the contralateral cortico-striatal-pallidal regions. In dyskinetic animals chronic levodopa treatment led to an absolute decrease in cAMP and cGMP levels in cortico-striatal-pallidal regions of both hemispheres. Pretreatment with zaprinast reduced the severity of levodopa-induced dyskinesias, and partly prevented the decrease in cyclic nucleotides compared with pretreatment with saline-levodopa. In conclusion, using a rat model of hemiparkinsonism, we observed a significant reduction in the levels of cyclic nucleotides in both hemispheres at the peak of levodopa-induced dyskinesias. We propose that such a decrease in cyclic nucleotides may partly result from increased catabolism through PDE overactivity.

Ca2+‐dependent Regulation of Phototransduction†

Photon absorption by rhodopsin triggers the phototransduction signaling pathway that culminates in degradation of cGMP, closure of cGMP-gated ion channels and hyperpolarization of the photoreceptor membrane. This process is accompanied by a decrease in free Ca(2+) concentration in the photoreceptor cytosol sensed by Ca(2+)-binding proteins that modulate phototransduction and activate the recovery phase to reestablish the photoreceptor dark potential. Guanylate cyclase-activating proteins (GCAPs) belong to the neuronal calcium sensor (NCS) family and are responsible for activating retinal guanylate cyclases (retGCs) at low Ca(2+) concentrations triggering synthesis of cGMP and recovery of the dark potential. Here we review recent structural insight into the role of the N-terminal myristoylation in GCAPs and compare it to other NCS family members. We discuss previous studies identifying regions of GCAPs important for retGC1 regulation in the context of the new structural data available for myristoylated GCAP1. In addition, we present a hypothetical model for the Ca(2+)-triggered conformational change in GCAPs and retGC1 regulation. Finally, we briefly discuss the involvement of mutant GCAP1 proteins in the etiology of retinal degeneration as well as the importance of other Ca(2+) sensors in the modulation of phototransduction.

Negative-Feedback Loop Attenuates Hydrostatic Lung Edema via a cGMP-Dependent Regulation of Transient Receptor Potential Vanilloid 4

Although the formation of hydrostatic lung edema is generally attributed to imbalanced Starling forces, recent data show that lung endothelial cells respond to increased vascular pressure and may thus regulate vascular permeability and edema formation. In combining real-time optical imaging of the endothelial Ca(2+) concentration ([Ca(2+)](i)) and NO production with filtration coefficient (K(f)) measurements in the isolated perfused lung, we identified a series of endothelial responses that constitute a negative-feedback loop to protect the microvascular barrier. Elevation of lung microvascular pressure was shown to increase endothelial [Ca(2+)](i) via activation of transient receptor potential vanilloid 4 (TRPV4) channels. The endothelial [Ca(2+)](i) transient increased K(f) via activation of myosin light-chain kinase and simultaneously stimulated NO synthesis. In TRPV4 deficient mice, pressure-induced increases in endothelial [Ca(2+)](i), NO synthesis, and lung wet/dry weight ratio were largely blocked. Endothelial NO formation limited the permeability increase by a cGMP-dependent attenuation of the pressure-induced [Ca(2+)](i) response. Inactivation of TRPV4 channels by cGMP was confirmed by whole-cell patch-clamp of pulmonary microvascular endothelial cells and intravital imaging of endothelial [Ca(2+)](i). Hence, pressure-induced endothelial Ca(2+) influx via TRPV4 channels increases lung vascular permeability yet concomitantly activates an NO-mediated negative-feedback loop that protects the vascular barrier by a cGMP-dependent attenuation of the endothelial [Ca(2+)](i) response. The identification of this novel regulatory pathway gives rise to new treatment strategies, as demonstrated in vivo in rats with acute myocardial infarction in which inhibition of cGMP degradation by the phosphodiesterase 5 inhibitor sildenafil reduced hydrostatic lung edema.

Effects of l‐arginine and phosphodiesterase‐5 inhibitor, sildenafil, on inflammation and airway responsiveness of sensitized BP2 mice

Nitric oxide (NO) levels are elevated in the exhaled breath of asthmatic patients and NO is considered as a biomarker of airway inflammation. However, the functions of NO in the airways are not completely understood. L-arginine, as the substrate of NO synthases, is the precursor of NO which stimulates guanylate cyclase and leads to the formation of cyclic GMP (cGMP). Sildenafil, a phosphodiestérase-5 (PDE-5) inhibitor, prevents the degradation of cGMP. In this study the effects of L-arginine and sildenafil treatment, alone or in combination, were evaluated in ovalbumin-sensitized BP2 mice. These effects concerning the airway responsiveness to inhaled methacholine (MCh) were evaluated by whole-body plethysmography (WBP), the inflammatory response evaluated by bronchoalveolar lavage fluid (BALF) analyses and lung tissue biopsies (eosinophilic inflammation associated with lung remodelling), and NO metabolite measurements (by Griess reaction) in BALF. Ovalbumin sensitization induced: (a) an inflammatory reaction with eosinophil and neutrophil influx in BALF and lung; and (b) an increased bronchial responsiveness to MCh. L-arginine treatment [50 mg/kg intraperitoneally (i.p.), for 7 days] increased the relative amount of eosinophils and neutrophils in BALF, had a tendency to increase the airway responsiveness to inhaled MCh and increased the NO metabolite level in BAL. Sildenafil treatment (20 mg/kg i.p. for 7 days) did not affect the airway responsiveness to MCh and had a lower effect compared with L-arginine on inflammatory reactions. The combination of the two treatments resulted in a dramatic enhancement of the airway responsiveness to inhaled MCh. The relative amount of eosinophils was increased and lung histology showed obvious worsened tissular lesions such as epithelial shedding and hypertrophy, hyperplasia of smooth muscle cells, and fibrosis. These findings are consistent with the notion that NO production plays a role in the development of airway inflammation and hyperresponsiveness of sensitized mice and highlighted the potential risk of the L-arginine dietary complement or PDE5 treatment in asthmatic patients.

Conversion of Phosphodiesterase-5 (PDE5) Catalytic Site to Higher Affinity by PDE5 Inhibitors

Phosphodiesterase-5 (PDE5) specifically hydrolyzes cGMP, thereby contributing to modulation of intracellular levels of this nucleotide. In the present study, preincubation with cGMP increased PDE5 catalytic activity for cGMP degradation, and it converted the PDE5 catalytic site to a form that was more potently inhibited by each of the three PDE5 catalytic site-specific inhibitors: sildenafil, vardenafil, and tadalafil. These results implied that elevated cGMP initiates a physiological negative feedback on the cGMP pathway by increasing the affinity of the PDE5 catalytic site for cGMP. This increase in catalytic site activity or affinity for inhibitors could be caused by binding of cGMP to either the PDE5 allosteric sites, catalytic site, or both. Whether occupation of the catalytic site alone could mediate the effect was examined using radiolabeled PDE5 inhibitors in the absence of cGMP. Exchange-dissociation of [(3)H]sildenafil (Viagra), [(3)H]vardenafil (Levitra), or [(3)H]tadalafil (Cialis) from full-length PDE5 or isolated catalytic domain revealed two kinetic components (slow and fast). Extended preincubation of full-length PDE5, but not isolated catalytic domain, with (3)H inhibitors converted the biphasic pattern to a single slow (high-affinity) component. Studies of amino-terminally truncated PDE5 established that full-length mammalian GAF-B (cGMP-binding phosphodiesterase, Anabaena adenylyl cyclases, Escherichia coli FhlA) subdomain conjoined with the catalytic domain was sufficient for this conversion. In conclusion, binding of substrate or substrate analogs such as PDE5 inhibitors to the catalytic site converts a fast (low-affinity) inhibitor dissociation component of the PDE5 catalytic site to a slow (high-affinity) inhibitor dissociation component. This effect is predicted to improve the substrate affinity or inhibitory potencies of these compounds in intact cells.

The Involvement of the Nitric Oxide in the Effects and Expression of Opioid Receptors During Peripheral Inflammation

Peripheral inflammation enhances the antinociceptive effects of opioid receptor agonists through the activation of peripheral opioid receptors whose expression also increases during inflammatory pain. Similarly, intestinal inflammation also increases the antitransit and antiexudative effects of opioids as well as the expression of neuronal and extra-neuronal opioid receptors located in the gut. Nitric oxide has been described either as pro- or antiinflammatory and could produce both pro- and antinociceptive effects. In addition, numerous studies have shown that the L-arginine-nitric oxide-cGMP system participates in the antinociceptive and in the intestinal effects produced by opioids during peripheral inflammation by enhancing their effects. Thus, substances capable of inhibiting cyclic guanosine-3',5'-monophosphate (cGMP) degradation or nitric oxide donors increase the analgesic effects of opioid receptor agonists during peripheral inflammation. At the same time, the administration of nitric oxide synthase (NOS) or guanylate cyclase inhibitors decreases those effects. In accordance with these results, different clinical trials have also demonstrated that the co-administration of nitric oxide donors with opioids is highly beneficial in the treatment of pain in patients. In the gut, nitric oxide has a further pro- and antiinflammatory action. It is also involved in the enhanced antitransit and antiexudative effects produced by opioids and in the up-regulation of the mu-opioid receptor gene transcription observed in the inflamed intestine. To sum up, a better knowledge of the involvement of the L-arginine-nitric oxide-cGMP pathway in the opioid mechanisms of action and a better understanding of the pathways that regulate the expression of opioid receptors during peripheral inflammation are essential to developing improved analgesic/antiinflammatory therapies.

Endothelin-1 reduces mesenteric microvascular hydraulic permeability via cyclic AMP and protein kinase A signal transduction

We have previously shown that endothelin-1 (ET-1) decreases microvascular hydraulic permeability. In this study, we tested the hypothesis that ET-1 exerts its permeability-decreasing effect through cAMP, cGMP, and protein kinase A (PKA) by determining the effect of ET-1 on venular fluid leak during inhibition of cAMP synthesis, inhibition of cGMP degredation, and inhibition of PKA. Rat mesenteric venules were cannulated to measure hydraulic permeability, L p (units × 10 −7 cm/(s cmH 2O)). L p was measured during continuous perfusion of 80 pM ET-1 and either (1) an inhibitor of cAMP synthesis (10 μM 2′,5′ddA), (2) an inhibitor of cGMP degradation (100 μM Zaprinast), or (3) an inhibitor of PKA (10 μM H-89). Inhibition of cAMP synthesis blocked the permeability decreasing effects of ET-1. The peak L p of the cAMP inhibitor alone and with ET-1 was 4.11 ± 0.53 and 3.86 ± 0.19, respectively ( p = 0.36, n = 6). Inhibition of cGMP degradation did not block the permeability decreasing effects of ET-1. The peak L p during inhibition of cGMP degradation alone and with ET-1 was 2.26 ± 0.15 and 1.44 ± 0.09, respectively ( p < 0.001, n = 6). Inhibition of PKA activation blocked the permeability decreasing effects of ET-1. The peak L p of the PKA inhibitor alone and with ET-1 was 2.70 ± 0.15 and 2.59 ± 0.15, respectively ( p = 0.38, n = 6). The data support the notion that the signal transduction mechanism of ET-1 with regard to decreasing microvascular fluid leak involves cAMP production and PKA activation, but not cGMP degradation. Further understanding of intracellular mechanisms that control microvascular fluid leak could lead to the development of a pharmacologic therapy to control third space fluid loss in severely injured or septic patients.

Effect of new sildenafil analogues in the rabbit isolated aorta

Background Nitric oxide (NO) diffuses into vascular smooth muscle cells to cause stimulation of guanylyl cyclase to elevate cGMP. The accumulated cGMP is degraded by phosphodiesterase-5 (PDE5) in cavernosal and vascular smooth muscle (Boollel et al., 1996). PDE5 inhibitors, such as sildenafil (SILD), block the degradation of cGMP, and lead vasorelaxation through increases in cGMP levels. The major side-effect of PDE5 inhibitors is associated with vasodilatation, producing a significant hypotension in individuals who take organic nitrates.

KH-305 투여가 흰쥐 음경조직의 Nitric Oxide Synthase활성 및 Erectile Dysfunction에 미치는 영향

This study was designed to investigate the effects of KH-305 on erectile dysfunction in young rats, via nitric oxide (NO)-cGMP pathways. After oral administration of the KH-305 mixture (50, 100, 200, 300 mg/kg) to young rats for 10 days, NOS and SOD protein expressions in penile tissue and testosterone in plasma were measured. cGMP degradation was also investigated using bovine vascular smooth muscle cells pretreated with an NO donor, S-nitroso-N-Acetylpenicillamine (SNAP). The penile expression levels of nNOS and eNOS-dependent NOS activities as well as SOD preventing oxidative stress by overproduction of NO were increased significantly. Also, the concentration of testosterone in the plasma was increased. In vitro, cGMP concen-trations were decreased dose dependently in the KH-305. These results suggest that KH-305 may be useful in erectile dysfunction.

Overview of PDEs and Their Regulation

Contraction and relaxation of vascular smooth muscle and cardiac myocytes are key physiological events in the cardiovascular system. These events are regulated by second messengers, cAMP and cGMP, in response to extracellular stimulants. The strength of signal transduction is controlled by intracellular cyclic nucleotide concentrations, which are determined by a balance in production and degradation of cAMP and cGMP. Degradation of cyclic nucleotides is catalyzed by 3',5'-cyclic nucleotide phosphodiesterases (PDEs), and therefore regulation of PDEs hydrolytic activity is important for modulation of cellular functions. Mammalian PDEs are composed of 21 genes and are categorized into 11 families based on sequence homology, enzymatic properties, and sensitivity to inhibitors. PDE families contain many splice variants that mostly are unique in tissue-expression patterns, gene regulation, enzymatic regulation by phosphorylation and regulatory proteins, subcellular localization, and interaction with association proteins. Each unique variant is closely related to the regulation of a specific cellular signaling. Thus, multiple PDEs function as a particular modulator of each cardiovascular function and regulate physiological homeostasis.

TcrPDEA1, a cAMP-specific phosphodiesterase with atypical pharmacological properties from Trypanosoma cruzi

Cyclic nucleotide phosphodiesterases (PDEs) catalyze the degradation of cAMP and cGMP, and regulate a variety of cellular processes by controlling the levels of these second messengers. We have previously described the presence of both a calcium-stimulated adenylyl cyclase and two membrane-bound cAMP-specific PDEs (one of them strongly associated to the flagellum and the other one with a possible vesicular localization) in Trypanosoma cruzi. Here we report the identification and characterization of TcrPDEA1, a singular phosphodiesterase of T. cruzi which is resistant to the typical phosphodiesterase inhibitors, such as IBMX, papaverine and theofylline. TcrPDEA1 is a single copy gene that encodes a 620-amino acid protein, which is grouped with PDE1 family members, mainly with its kinetoplastid orthologs. TcrPDEA1 was able to complement a mutant yeast strain deficient in PDE genes, demonstrating that this enzyme is a functional phosphodiesterase. TcrPDEA1 is specific for cAMP with a high Km value (191.1±6.5μM). Cyclic GMP neither activates the enzyme nor competes as a substrate. In addition, calcium-calmodulin did not affect the kinetic parameters and, as its counterpart in T. brucei, magnesium showed to be crucial for its activity and stability. Although TcrPDEA1 function remains unclear, its presence points out the high complexity of the cAMP signaling in trypanosomatids and the possible compartmentalization of the enzymes involved in the cAMP pathway.

The Role of Nitric Oxide in Erectile Dysfunction: Implications for Medical Therapy

Erectile dysfunction is a common, multifactorial disorder that is associated with aging and a range of organic and psychogenic conditions, including hypertension, hypercholesterolemia, diabetes mellitus, cardiovascular disease, and depression. Penile erection is a complex process involving psychogenic and hormonal input, and a neurovascular nonadrenergic, noncholinergic mechanism. Nitric oxide (NO) is believed to be the main vasoactive nonadrenergic, noncholinergic neurotransmitter and chemical mediator of penile erection. Released by nerve and endothelial cells in the corpora cavernosa of the penis, NO activates soluble guanylyl cyclase, which increases 3',5'-cyclic guanosine monophosphate (cGMP) levels. Acting as a second messenger molecule, cGMP regulates the activity of calcium channels as well as intracellular contractile proteins that affect the relaxation of corpus cavernosum smooth muscle. Impaired NO bioactivity is a major pathogenic mechanism of erectile dysfunction. Treatment of erectile dysfunction often requires combinations of psychogenic and medical therapies, many of which have been only moderately successful in the past. The advent of oral phosphodiesterase type 5 (PDE-5) inhibitors, however, has greatly enhanced erectile dysfunction treatment; patients have demonstrated high tolerability and success rates for improved erectile function. The efficacy of the PDE-5 inhibitors also serves to illustrate the importance of the NO-cGMP pathway in erectile function since these agents counteract the degradation of NO-generated cGMP. Because not all patients respond to PDE-5 inhibitors, additional therapies are being investigated, such as soluble guanylyl cyclase activators and NO donors, which act on NO-independent and NO-dependent pathways, respectively.

Phosphodiesterase 5 Inhibitors - Drug Design and Differentiation Based on Selectivity, Pharmacokinetic and Efficacy Profiles

The discovery that inhibition of phosphodiesterase-5 (PDE5) reduces the degradation of cGMP, allowing erectile function to occur by relaxation of penile smooth muscle, represents a revolutionary approach or the treatment of erectile dysfunction (ED). Three PDE5 inhibitors (sildenafil, tadalafil, and vardenafil) are clinically available at this time, and extensive drug design efforts are registered for finding agents with a better activity, enhanced selectivity and reduced side effects. Many classes of such compounds have been reported, belonging to diverse chemical entities. The drug design has been very much facilitated after the report of the X-ray crystal structure of the PDE5 catalytic domain in complex with the three clinically used derivatives. PDE5 inhibitor therapy, has been found to be effective in special clinical populations, such as those with prostate cancer, diabetes, and cardiovascular disease. The duration of action of sildenafil and vardenafil is of about 4 hours, whereas that of tadalafil is of about 36 hours, and the overall safety of the treatments is good. There is a risk of hypotension if nitrates are given concurrently with the PDE5 inhibitors. Common side-effects include headache, facial flushing, nasal congestion, dyspepsia and transient visual impairment. There are pharmacological interactions between these drugs and other medications metabolized by the cytochrome P450 (P3A4 isoform), such as the azole antifungals, erythromycin and the HIV protease inhibitors.

Minimal Determinants for Binding Activated Gα from the Structure of a Gαi1−Peptide Dimer,

G-proteins cycle between an inactive GDP-bound state and an active GTP-bound state, serving as molecular switches that coordinate cellular signaling. We recently used phage display to identify a series of peptides that bind G alpha subunits in a nucleotide-dependent manner [Johnston, C. A., Willard, F. S., Jezyk, M. R., Fredericks, Z., Bodor, E. T., Jones, M. B., Blaesius, R., Watts, V. J., Harden, T. K., Sondek, J., Ramer, J. K., and Siderovski, D. P. (2005) Structure 13, 1069-1080]. Here we describe the structural features and functions of KB-1753, a peptide that binds selectively to GDP x AlF4(-)- and GTPgammaS-bound states of G alpha(i) subunits. KB-1753 blocks interaction of G alpha(transducin) with its effector, cGMP phosphodiesterase, and inhibits transducin-mediated activation of cGMP degradation. Additionally, KB-1753 interferes with RGS protein binding and resultant GAP activity. A fluorescent KB-1753 variant was found to act as a sensor for activated G alpha in vitro. The crystal structure of KB-1753 bound to G alpha(i1) x GDP x AlF4(-) reveals binding to a conserved hydrophobic groove between switch II and alpha3 helices and, along with supporting biochemical data and previous structural analyses, supports the notion that this is the site of effector interactions for G alpha(i) subunits.

Physiology of Erectile Function: An Update on Intracellular Molecular Processes

Objectives To provide a comprehensive update of current knowledge concerning the molecular mechanisms underlying the erectile physiology. Methods Results from numerous investigations, including both basic and clinical studies, have been considered. In particular, we pointed out the advances concerning the peripheral control of erection that ultimately influence the functional state of the penis. Results Numerous neurotransmitters and endothelial factors modulate the penile vasculature and smooth muscle tone of corpora cavernosa in the penis. The regulation of adequate intracellular calcium levels represents the key determinant of the smooth muscle tone. Following the sexual stimulation, the activation of smooth muscle relaxation in the penis is mainly mediated by nitric oxide (NO), which acts via cyclic guanosine monophosphate (cGMP)-mediated intracellular signalling. The pro-erectile NO/cGMP pathway is coupled to the anti-erectile RhoA/Rho-kinase calcium-sensitizing pathway, which was recently highlighted as another important regulator of the erectile function. In the last decade, the enzyme phosphodiesterase 5 (PDE5), critically involved in the degradation of cGMP and thereby in the maintenance of penile detumescence, has gained attention as target enzyme of the most used drugs (PDE5 inhibitors) for the treatment of erectile dysfunction. Moreover, androgens play an important role in peripheral regulation of erection, acting positively either on the enzyme which synthesizes NO (NOS), and on that involved in the degradation of cGMP (PDE5). Conclusion The recent advances added to new insights into our knowledge of erectile physiology and leaded to the improvement in the clinical management of men affected by erectile dysfunction.

The cGMP/Protein Kinase G Pathway Contributes to Dihydropyridine-sensitive Calcium Response and Cytokine Production in TH2 Lymphocytes

Th2 lymphocytes differ from other CD4+ T lymphocytes not only by their effector tasks but also by their T cell receptor (TCR)-dependent signaling pathways. We previously showed that dihydropyridine receptors (DHPR) involved in TCR-induced calcium inflow were selectively expressed in Th2 cells. In this report, we studied whether cGMP-dependent protein kinase G (PKG) activation was implicated in the regulation of DHPR-dependent calcium response and cytokine production in Th2 lymphocytes. The contribution of cGMP in Th2 signaling was supported by the following results: 1) TCR activation elicited cGMP production, which triggered calcium increase responsible for nuclear factor of activated T cell translocation and Il4 gene expression; 2) guanylate cyclase activation by nitric oxide donors increased intracellular cGMP concentration and induced calcium inflow and IL-4 production; 3) reciprocally, guanylate cyclase inhibition reduced calcium response and Th2 cytokine production associated with TCR activation. In addition, DHPR blockade abolished cGMP-induced [Ca2+]i increase, indicating that TCR-induced DHP-sensitive calcium inflow is dependent on cGMP in Th2 cells. Th2 lymphocytes from PKG1-deficient mice displayed impaired calcium signaling and IL-4 production, as did wild-type Th2 cells treated with PKG inhibitors. Altogether, our data indicate that, in Th2 cells, cGMP is produced upon TCR engagement and activates PKG, which controls DHP-sensitive calcium inflow and Th2 cytokine production.

Nuclear Signaling in Smooth Muscle Cells

See related article, pages 777–784 The second messenger cyclic GMP (cGMP) mediates signaling in the nervous system, genitourinary system, and the gastrointestinal tract. In the cardiovascular system, cGMP regulates vasorelaxation, vascular remodeling, platelet activation, and cardiac contractility. The ability of cGMP to regulate such a large number of temporally and spatially disparate processes is attributable to the many different isozymes that exist for synthesis and degradation of cGMP. These enzymes exhibit different subcellular localization and mechanisms of activation. Thus, cGMP is synthesized from GTP by both soluble and particulate forms of guanylyl cyclase. Soluble guanylyl cyclases are heterodimeric cytosolic enzymes that consist of α and β subunits. There are several isoforms of the α and β subunit, α1β1 being the most abundant combination in mammalian tissues.1–3 Soluble guanylyl cyclase is activated by nitric oxide (NO) and NO-releasing drugs. Particulate forms of guanylyl cyclase, on the other hand, are transmembrane proteins that are activated by the natriuretic peptides ANP, BNP, and CNP.4 Similarly, cGMP is degraded by a large number of cyclic nucleotide phosphodiesterases (PDEs). To date, 21 different PDE genes divided among 11 gene families have been identified in mammals. Most PDE families contain more than one gene, and most genes code for more than one mRNA. Although some PDE gene families hydrolyze cAMP exclusively (PDE4, PDE7, PDE8), many of these enzymes hydrolyze cGMP, or both cAMP and cGMP.5 PDE5 is the major cGMP-specific PDE gene family in some cell types. Other cGMP-hydrolyzing PDE gene families include PDE1, PDE2, PDE6, PDE9, PDE10, PDE11, and possibly PDE3.6 Any particular cell type typically expresses several different guanylyl cyclases and cGMP-hydrolyzing PDEs, which provide the cell with tools to precisely regulate cGMP synthesis and hydrolysis in different …

Desensitization of NO/cGMP Signaling in Smooth Muscle: Blood Vessels Versus Airways

The NO/cGMP signaling pathway plays a major role in the cardiovascular system, in which it is involved in the regulation of smooth muscle tone and inhibition of platelet aggregation. Under pathophysiological conditions such as endothelial dysfunction, coronary artery disease, and airway hyperreactivity, smooth muscle containing arteries and bronchi are of great pharmacological interest. In these tissues, NO mediates its effects by stimulating guanylyl cyclase (GC) to form cGMP; the subsequent increase in cGMP is counteracted by the cGMP-specific phosphodiesterase (PDE5), which hydrolyzes cGMP. In platelets, allosteric activation of PDE5 by cGMP paralleled by phosphorylation has been shown to govern the sensitivity of NO/cGMP signaling. Here, we demonstrate that the functional responsiveness to NO correlates with the relative abundance of GC and PDE5 in aortic and bronchial tissue, respectively. We show a sustained desensitization of the NO-induced relaxation of aortic and bronchial rings caused by a short-term exposure to NO. The NO treatment caused heterologous desensitization of atrial natriuretic peptide-induced relaxation, whereas relaxation by the cGMP analog 8-pCPT-cGMP was unperturbed. Impaired relaxation was shown to be paralleled by PDE5 phosphorylation; this indicates enhanced cGMP degradation as a mechanism of desensitization. In summary, our results demonstrate the physiological impact of PDE5 activation on the control of smooth muscle tone and provide an explanation for the apparent impairment of NO-induced vasorelaxation.

Phosphodiesterase 5 Inhibitor Ameliorates Renal Resistance to Atrial Natriuretic Peptide Associated with Obesity and Hyperleptinemia

Abnormal neurohormonal regulation of renal sodium handling plays an important role in obesity-associated hypertension. We investigated the effect of experimental obesity on renal response to atrial natriuretic peptide (ANP). The effect of ANP was studied in three groups of rats: (1) lean controls, (2) animals made obese by a highly palatable diet, (3) rats treated with adipose tissue hormone, leptin, for 7 days to reproduce hyperleptinemia observed in obesity. ANP administered at a dose of 50 pmol/kg min(-1) induced about a 3-fold lower increase in Na+ and cGMP excretion in obese and leptin-treated rats than in the control group. ANP decreased Na+,K+-ATPase activity in the renal medulla only in the control group. Natriuretic effect of exogenous cGMP was also impaired in obese and leptin-treated rats. In contrast, hydrolysis-resistant cGMP derivative, 8-bromo-cGMP exerted comparable natriuretic effects in all groups. Neutral endopeptidase inhibitor, phosphoramidon, and ANP clearance receptor antagonist, C-ANP, increased urinary ANP excretion in all groups to a similar level, but their natriuretic effect was impaired in obese and leptin-treated groups. A specific inhibitor of cGMP-degrading phosphodiesterase, zaprinast, had comparable natriuretic and Na+,K+-ATPase-lowering effects in all groups and restored normal sensitivity to ANP. (1) Dietary-induced obesity is accompanied by impaired natriuretic effect of ANP, (2) ANP resistance in obesity may be accounted for by increased leptin level, (3) accelerated degradation of cGMP may contribute to ANP resistance associated with obesity and hyperleptinemia, suggesting that inhibiting cGMP-specific phosphodiesterases may be useful in the treatment of obesity-associated hypertension.

Nasal Obstruction as a Common Side-Effect of Sildenafil Citrate

Sildenafil citrate is an effective oral drug for erectile dysfunction. The main action of sildenafil is the enhancement of the effect of nitric oxide (NO) by inhibiting the cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase-5 (PDE-5), an enzyme responsible for degradation of cGMP. NO is also present in the nasal mucosa and is responsible for vasodilation causing congestion and nasal obstruction. The aim of this study was to detect the effect of sildenafil on nasal mucosa in terms of nasal obstruction. A total of 16 patients presented to urology clinic suffering from impotence and medicated with sildenafil were included in the study. Before and after oral administration of 50 mg sildenafil, in all of the patients the nasal patency was examined by active anterior rhinomanometry (a method of assessing nasal resistance) using air pressure of 150 Pascal. In addition, all patients were asked about their sensation of nasal patency to detect the symptomatic nasal obstruction. There was a significant decrease in nasal air flow values (cm(3)/s) (p < 0.05). Except for three cases, all patients indicated that they had the sensation of nasal obstruction after the use of sildenafil (p < 0.05). Nasal obstruction is a common complaint for the patients using Sildenafil.