Modulation of bradykinin receptor ligand binding affinity and its coupled G-proteins by nitric oxide

Targeting hydrogen sulfide and nitric oxide to repair cardiovascular injury after trauma

The systemic cardiovascular effects of major trauma, especially neurotrauma, contribute to death and permanent disability in trauma patients and treatments are needed to improve outcomes. In some trauma patients, dysfunction of the autonomic nervous system produces a state of adrenergic overstimulation, causing either a sustained elevation in catecholamines (sympathetic storm) or oscillating bursts of paroxysmal sympathetic hyperactivity. Trauma can also activate innate immune responses that release cytokines and damage-associated molecular patterns into the circulation. This combination of altered autonomic nervous system function and widespread systemic inflammation produces secondary cardiovascular injury, including hypertension, damage to cardiac tissue, vascular endothelial dysfunction, coagulopathy and multiorgan failure. The gasotransmitters nitric oxide (NO) and hydrogen sulfide (H2S) are small gaseous molecules with potent effects on vascular tone regulation. Exogenous NO (inhaled) has potential therapeutic benefit in cardio-cerebrovascular diseases, but limited data suggests potential efficacy in traumatic brain injury (TBI). H2S is a modulator of NO signaling and autonomic nervous system function that has also been used as a drug for cardio-cerebrovascular diseases. The inhaled gases NO and H2S are potential treatments to restore cardio-cerebrovascular function in the post-trauma period.

Tracing the path of inhaled nitric oxide: Biological consequences of protein nitrosylation

Nitric oxide (NO) is a comprehensive regulator of vascular and airway tone. Endogenous NO produced by nitric oxide synthases regulates multiple signaling cascades, including activation of soluble guanylate cyclase to generate cGMP, relaxing smooth muscle cells. Inhaled NO is an established therapy for pulmonary hypertension in neonates, and has been recently proposed for the treatment of hypoxic respiratory failure and acute respiratory distress syndrome due to COVID-19. In this review, we summarize the effects of endogenous and exogenous NO on protein S-nitrosylation, which is the selective and reversible covalent attachment of a nitrogen monoxide group to the thiol side chain of cysteine. This posttranslational modification targets specific cysteines based on the acid/base sequence of surrounding residues, with significant impacts on protein interactions and function. S-nitrosothiol (SNO) formation is tightly compartmentalized and enzymatically controlled, but also propagated by nonenzymatic transnitrosylation of downstream protein targets. Redox-based nitrosylation and denitrosylation pathways dynamically regulate the equilibrium of SNO-proteins. We review the physiological roles of SNO proteins, including nitrosohemoglobin and autoregulation of blood flow through hypoxic vasodilation, and pathological effects of nitrosylation including inhibition of critical vasodilator enzymes; and discuss the intersection of NO source and dose with redox environment, in determining the effects of protein nitrosylation.

Nitric Oxide and S-Nitrosylation in Cardiac Regulation: G Protein-Coupled Receptor Kinase-2 and β-Arrestins as Targets

Cardiac diseases including heart failure (HF), are the leading cause of morbidity and mortality globally. Among the prominent characteristics of HF is the loss of β-adrenoceptor (AR)-mediated inotropic reserve. This is primarily due to the derangements in myocardial regulatory signaling proteins, G protein-coupled receptor (GPCR) kinases (GRKs) and β-arrestins (β-Arr) that modulate β-AR signal termination via receptor desensitization and downregulation. GRK2 and β-Arr2 activities are elevated in the heart after injury/stress and participate in HF through receptor inactivation. These GPCR regulators are modulated profoundly by nitric oxide (NO) produced by NO synthase (NOS) enzymes through S-nitrosylation due to receptor-coupled NO generation. S-nitrosylation, which is NO-mediated modification of protein cysteine residues to generate an S-nitrosothiol (SNO), mediates many effects of NO independently from its canonical guanylyl cyclase/cGMP/protein kinase G signaling. Herein, we review the knowledge on the NO system in the heart and S-nitrosylation-dependent modifications of myocardial GPCR signaling components GRKs and β-Arrs.

The vital role for nitric oxide in intraocular pressure homeostasis

Catalyzed by endothelial nitric oxide (NO) synthase (eNOS) activity, NO is a gaseous signaling molecule maintaining endothelial and cardiovascular homeostasis. Principally, NO regulates the contractility of vascular smooth muscle cells and permeability of endothelial cells in response to either biochemical or biomechanical cues. In the conventional outflow pathway of the eye, the smooth muscle-like trabecular meshwork (TM) cells and Schlemm's canal (SC) endothelium control aqueous humor outflow resistance, and therefore intraocular pressure (IOP). The mechanisms by which outflow resistance is regulated are complicated, but NO appears to be a key player as enhancement or inhibition of NO signaling dramatically affects outflow function; and polymorphisms in NOS3, the gene that encodes eNOS modifies the relation between various environmental exposures and glaucoma. Based upon a comprehensive review of past foundational studies, we present a model whereby NO controls a feedback signaling loop in the conventional outflow pathway that is sensitive to changes in IOP and its oscillations. Thus, upon IOP elevation, the outflow pathway tissues distend, and the SC lumen narrows resulting in increased SC endothelial shear stress and stretch. In response, SC cells upregulate the production of NO, relaxing neighboring TM cells and increasing permeability of SC's inner wall. These IOP-dependent changes in the outflow pathway tissues reduce the resistance to aqueous humor drainage and lower IOP, which, in turn, diminishes the biomechanical signaling on SC. Similar to cardiovascular pathogenesis, dysregulation of the eNOS/NO system leads to dysfunctional outflow regulation and ocular hypertension, eventually resulting in primary open-angle glaucoma.

Vasomotor effects of 5-hydroxytryptamine, histamine, angiotensin II, acetylcholine, noradrenaline, and bradykinin on the cerebral artery of bottlenose dolphin (Tursiops truncatus)

From an evolutionary aspect, dolphins share a very close phylogenetic relationship with pigs. Previously, we characterized porcine cerebral artery responsiveness to intrinsic vasoactive substances. Therefore, here, we investigated dolphin (Tursiops truncatus) cerebral artery responsiveness to 5-hydroxytryptamine (5-HT), histamine (His), angiotensin (Ang) II, acetylcholine (ACh), noradrenaline (NA), and bradykinin (BK) to characterize their related receptor subtypes. We also compared dolphin cerebral artery responsiveness with porcine cerebral artery responsiveness. We found that 5-HT and His induced concentration-dependent contraction of the dolphin cerebral artery. Ketanserin (a 5-HT₂ antagonist) and methiothepin (a 5-HT₁ and 5-HT₂ antagonist) shifted the concentration-response curve for 5-HT to the right. Although diphenhydramine (an H₁ antagonist) shifted the concentration-response curve for His to the right, cimetidine (an H₂ antagonist) had no such effect. Ang II and ACh did not produce any vasomotor actions. NA induced concentration-dependent relaxation. Propranolol (a β antagonist) shifted the concentration-response curve for NA to the right, whereas phentolamine (an α antagonist) had no significant effect. BK induced relaxation followed by contraction in pre-contracted arteries with intact endothelium. HOE140 (a B₂ antagonist) shifted the concentration-response curve for BK to the right, whereas des-Arg⁹-[Leu⁸]-BK (a B₁ antagonist) had no significant effect. These results suggest that 5-HT₁, 5-HT₂, and H₁ receptor subtypes are important in arterial contraction and that β and B₂ receptor subtypes modify these contractions to relaxations. The responsiveness of the dolphin cerebral artery is very similar to that of porcine cerebral artery, supporting their evolutionary linkage.

Endogenous and Exogenous Vanilloids Evoke Disparate TRPV1 Activation to Produce Distinct Neuronal Responses

Neuronal signals are processed along the nociceptive pathway to convey discriminative information, which would manifest in the produced pain sensation. The transient receptor potential vanilloid 1 (TRPV1), an important signaling complex in nociceptors termini, is activated by different noxious stimuli that underlie distinct pain sensations. For example, while endovanilloids are associated with inflammatory pain and hypersensitivity through TRPV1 activation, the exovanilloid toxin, capsaicin, evokes an acute pain by activating this channel. Differences in the TRPV1 activation profile evoked by exogenous and endogenous vanilloids were suggested to underlie this disparity in pain sensations. However, the cellular processes that lead to these differences in pain sensation mediated by the same channel are not fully understood. Here, we sought to describe the neuronal response of TRPV1-expressing nociceptors to exo-and endovanilloids. To this end, we performed current-clamp recordings in rat trigeminal neurons exposed to either capsaicin or intracellular endovanilloids produced downstream of the bradykinin receptor BK2. Our results show that lipoxygenase metabolites generate persistent TRPV1-dependent action potential firing while capsaicin evokes robust depolarization and high-frequency firing that is quickly terminated by depolarization block. Additionally, we found that a weak TRPV1 activation prolongs action potential firing. Overall, our results indicate different firing patterns evoked by inflammatory mediators and capsaicin via TRPV1 that correlate with the respective subsequent pain sensation. These findings also suggest that differences in neuronal activation stem from the variable degree of TRPV1 activation they produce.

Effects of Post-translational Modifications on Membrane Localization and Signaling of Prostanoid GPCR-G Protein Complexes and the Role of Hypoxia

G protein-coupled receptors (GPCRs) play a pivotal role in the adaptive responses to cellular stresses such as hypoxia. In addition to influencing cellular gene expression profiles, hypoxic microenvironments can perturb membrane protein localization, altering GPCR effector scaffolding and altering downstream signaling. Studies using proteomics approaches have revealed significant regulation of GPCR and G proteins by their state of post-translational modification. The aim of this review is to examine the effects of post-translational modifications on membrane localization and signaling of GPCR-G protein complexes, with an emphasis on vascular prostanoid receptors, and to highlight what is known about the effect of cellular hypoxia on these mechanisms. Understanding post-translational modifications of protein targets will help to define GPCR targets in treatment of disease, and to inform research into mechanisms of hypoxic cellular responses.

S-nitrosoglutathione inhibits cerebrovascular angiotensin II-dependent and -independent AT1 receptor responses: A possible role of S-nitrosation

BACKGROUND AND PURPOSE

Angiotensin II (AngII) and NO regulate the cerebral circulation. AngII AT₁ receptors exert ligand-dependent and ligand-independent (myogenic tone [MT]) vasoconstriction of cerebral vessels. NO induces post-translational modifications of proteins such as S-nitrosation (redox modification of cysteine residues). In cultured cells, S-nitrosation decreases AngII's affinity for the AT₁ receptor. The present work evaluated the functional consequences of S-nitrosation on both AngII-dependent and AngII-independent cerebrovascular responses.

EXPERIMENTAL APPROACH

S-Nitrosation was induced in rat isolated middle cerebral arteries by pretreatment with the NO donors, S-nitrosoglutathione (GSNO) or sodium nitroprusside (SNP). Agonist-dependent activation of AT₁ receptors was evaluated by obtaining concentration-response curves to AngII. Ligand-independent activation of AT₁ receptors was evaluated by calculating MT (active vs. passive diameter) at pressures ranging from 20 to 200 mmHg in the presence or not of a selective AT₁ receptor inverse agonist.

KEY RESULTS

GSNO or SNP completely abolished the AngII-dependent AT₁ receptor-mediated vasoconstriction of cerebral arteries. GSNO had no impact on responses to other vasoconstrictors sharing (phenylephrine, U46619) or not (5-HT) the same signalling pathway. MT was reduced by GSNO, and the addition of losartan did not further decrease MT, suggesting that GSNO blocks AT₁ receptor-dependent MT. Ascorbate (which reduces S-nitrosated compounds) restored the response to AngII but not the soluble GC inhibitor ODQ, suggesting that these effects are mediated by S-nitrosation rather than by S-nitrosylation.

CONCLUSIONS AND IMPLICATIONS

In rat middle cerebral arteries, GSNO pretreatment specifically affects the AT₁ receptor and reduces both AngII-dependent and AngII-independent activation, most likely through AT₁ receptor S-nitrosation.

Regulation of vascular tone homeostasis by NO and H2S: Implications in hypertension

Nitric oxide (NO) and hydrogen sulfide (H2S) are two gasotransmitters that are produced in the vasculature and contribute to the regulation of vascular tone. NO and H2S are synthesized in both vascular smooth muscle and endothelial cells; NO functions primarily through the sGC/cGMP pathway, and H2S mainly through activation of the ATP-dependent potassium channels; both leading to relaxation of vascular smooth muscle cells. A deficit in the NO/H2S homeostasis is involved in the pathogenesis of various cardiovascular diseases, especially hypertension. It is now becoming increasingly clear that there are important interactions between NO and H2S and that have a profound impact on vascular tone and this may provide insights into the new therapeutic interventions. The aim of this review is to provide a better understanding of individual and interactive roles of NO and H2S in vascular biology. Overall, available data indicate that both NO and H2S contribute to vascular (patho)physiology and in regulating blood pressure. In addition, boosting NO and H2S using various dietary sources or donors could be a hopeful therapeutic strategy in the management of hypertension.

Vascular nitric oxide: Beyond eNOS

As the first discovered gaseous signaling molecule, nitric oxide (NO) affects a number of cellular processes, including those involving vascular cells. This brief review summarizes the contribution of NO to the regulation of vascular tone and its sources in the blood vessel wall. NO regulates the degree of contraction of vascular smooth muscle cells mainly by stimulating soluble guanylyl cyclase (sGC) to produce cyclic guanosine monophosphate (cGMP), although cGMP-independent signaling [S-nitrosylation of target proteins, activation of sarco/endoplasmic reticulum calcium ATPase (SERCA) or production of cyclic inosine monophosphate (cIMP)] also can be involved. In the blood vessel wall, NO is produced mainly from l-arginine by the enzyme endothelial nitric oxide synthase (eNOS) but it can also be released non-enzymatically from S-nitrosothiols or from nitrate/nitrite. Dysfunction in the production and/or the bioavailability of NO characterizes endothelial dysfunction, which is associated with cardiovascular diseases such as hypertension and atherosclerosis.

Nitric oxide-dependent hypotensive effects of wax gourd juice

ETHNOPHARMACOLOGICAL RELEVANCE

The wax gourd (Benincasa hispida (Thunb) Cong.) is a long-season vegetable that has been used in traditional Chinese medicine to treat high blood pressure. However, precise details of its effect and the mechanism of action involved are still lacking.

MATERIALS AND METHODS

Ten-fold-condensed wax gourd juice was used for the experiments. We measured (1) blood pressure of anesthetized normal Wistar rats in vivo, (2) isolated rat aortic contraction and relaxation, and (3) nitric oxide production from cultured porcine endothelial cells. The rats mentioned had not been treated with the investigational medicine.

RESULTS

Intravenous injection of the juice produced a dose-dependent decrease in blood pressure. Treatment with the juice induced concentration-dependent relaxation of isolated rat aortic rings that had been precontracted with noradrenaline. The relaxation induced by the juice was strongly inhibited by treatment with the nitric oxide (NO) synthase inhibitor N(G)-nitro-l-arginine methyl ester hydrochloride (l-NAME) or endothelial denudation. Treatment with the juice produced NO from cultured porcine aortic endothelial cells. This NO production was significantly inhibited by l-NAME.

CONCLUSIONS

The present findings suggest that wax gourd juice exerts a hypotensive effect via endothelium-dependent vasodilation. The main endothelium-derived relaxing factor involved might be NO.

S-nitrosylation-regulated GPCR signaling

G protein-coupled receptors (GPCRs) are the most numerous and diverse type of cell surface receptors, accounting for about 1% of the entire human genome and relaying signals from a variety of extracellular stimuli that range from lipid and peptide growth factors to ions and sensory inputs. Activated GPCRs regulate a multitude of target cell functions, including intermediary metabolism, growth and differentiation, and migration and invasion. The GPCRs contain a characteristic 7-transmembrane domain topology and their activation promotes complex formation with a variety of intracellular partner proteins, which form basis for initiation of distinct signaling networks as well as dictate fate of the receptor itself. Both termination of active GPCR signaling and removal from the plasma membrane are controlled by protein post-translational modifications of the receptor itself and its interacting partners. Phosphorylation, acylation and ubiquitination are the most studied post-translational modifications involved in GPCR signal transduction, subcellular trafficking and overall expression. Emerging evidence demonstrates that protein S-nitrosylation, the covalent attachment of a nitric oxide moiety to specified cysteine thiol groups, of GPCRs and/or their associated effectors also participates in the fine-tuning of receptor signaling and expression. This newly appreciated mode of GPCR system modification adds another set of controls to more precisely regulate the many cellular functions elicited by this large group of receptors. This article is part of a Special Issue entitled: Regulation of cellular processes by S-nitrosylation.

Cyclic GMP modulates the expression of Gi protein and adenylyl cyclase signaling in vascular smooth muscle cells

We have recently shown that the nitric oxide (NO) donor, SNAP, decreased the expression of Gialpha proteins and associated functions in vascular smooth muscle cells. Because NO stimulates soluble guanylyl cyclase and increases the levels of guanosine 3\',5\'-cyclic monophosphate (cGMP), the present studies were undertaken to investigate whether cGMP can also modulate the expression of Gi proteins and associated adenylyl cyclase signaling. A10 vascular smooth muscle cells (VSMCs) and primary cultured cells from aorta of Sprague Dawley rats were used for these studies. The cells were treated with 8-bromoguanosine 3\',5\'-cyclic monophosphate (8BrcGMP) for 24 h and the expression of Gialpha proteins was determined by immunobloting techniques. Adenylyl cyclase activity was determined by measuring [32P]cAMP formation for [alpha-32P]ATP. Treatment of cells with 8-BrcGMP (0.5 mM) decreased the expression of Gialpha-2 and Gialpha-3 by about 30-45%, which was restored towards control levels by KT5823, an inhibitor of protein kinase G. On the other hand, the levels of Gsalpha protein were not altered by this treatment. The decreased expression of Gialpha proteins by 8Br-cGMP treatment was reflected in decreased Gi functions. For example, the inhibition of forskolin (FSK)-stimulated adenylyl cyclase activity by low concentrations of GTPgammaS (receptor-independent Gi functions) was significantly decreased by 8Br-cGMP treatment. In addition, exposure of the cells to 8Br-cGMP also resulted in the attenuation of angiotensin (Ang) II- and C-ANP4-23 (a ring-deleted analog of atrial natriuretic peptide [ANP])-mediated inhibition of adenylyl cyclase activity (receptor-dependant functions of Gi). On the other hand, Gsalpha-mediated stimulations of adenylyl cyclase by GTPgammaS, isoproterenol and FSK were significantly augmented in 8Br-cGMP-treated cells. These results indicate that 8Br-cGMP decreased the expression of Gialpha proteins and associated functions in VSMCs. From these studies, it can be suggested that 8Br-cGMP-induced decreased levels of Gi proteins and resultant increased levels of cAMP may be an additional mechanism through which cGMP regulates vascular tone and thereby blood pressure.

Red not dead: signaling in and from erythrocytes

The oxygen required to meet metabolic needs of all tissues is delivered by the erythrocyte, a small, flexible cell which, in mammals, is devoid of a nucleus and mitochondria. Despite its simple appearance, this 'bag of hemoglobin' has an important role in its own distribution, enabling the delivery of oxygen to precisely meet localized metabolic need. When an erythrocyte enters an area in which tissue oxygen demand exceeds supply, a signaling pathway is activated resulting in the release of adenosine 5'-triphosphate (ATP). This ATP acts in a paracrine fashion to increase vascular caliber resulting in increased oxygen delivery. Defects in this pathway are found in erythrocytes of humans with type 2 diabetes (DM2) and could contribute to the perfusion abnormalities in skeletal muscle associated with this disease.

Flavin adenine dinucleotide may release preformed stores of nitrosyl factors from the vascular endothelium of conscious rats

This study determined whether flavin adenine dinucleotide (FAD) may elicit vasodilation in conscious rats via release of preformed endothelium-derived nitrosyl factors. Injections 1-6 (inj(1-6)) of FAD (2.5 micromol/kg, IV) elicited pronounced and equivalent vasodilator responses in saline-treated rats. Inj(1) of FAD elicited pronounced vasodilation in L-NAME-treated rats pretreated with the nitric oxide (NO) synthesis inhibitor, NG-nitro-L-arginine (L-NAME; 50 micromol/kg, IV), whereas Inj(2-6) elicited progressively smaller responses such that inj(6) elicited minor responses. The vasodilator responses elicited by the endothelium-dependent agonist, acetylcholine, were markedly attenuated in L-NAME-treated rats that had received inj(1-6) of FAD but not in saline-treated rats that had received inj(1-6) of FAD. The vasodilator actions of L-S-nitrosocysteine and the NO donor, sodium nitroprusside, were not diminished after the injections of FAD in saline- or in L-NAME-treated rats. Binding studies demonstrated that the densities of muscarinic M3 receptors were increased in thoracic aorta endothelium of rats treated with L-NAME + inj(1-6) of saline or L-NAME + inj(1-6) of FAD as compared to rats treated with saline + inj(1-6) of saline or saline + inj(1-6) of FAD. The progressive loss of response to injections of FAD in L-NAME-treated rats coupled with the loss of response to acetylcholine suggests that FAD elicits the use-dependent depletion of vesicular pools of nitrosyl factors in endothelial cells that cannot be replenished in the absence of NO synthesis.

Human recombinant chromogranin A-derived vasostatin-1 mimics preconditioning via an adenosine/nitric oxide signaling mechanism

The acidic protein chromogranin A (CgA) is the precursor of several regulatory peptides generated by specific proteolytic processes. Human recombinant CgA NH(2)-terminal fragment STA-CgA(1-78) (hrSTA-CgA(1-78)), containing vasostatin-1 (CgA(1-76)) domain, exerts a negative inotropic effect and counteracts the beta-adrenergic positive inotropic effect on the rat heart. We hypothesized an involvement of nitric oxide (NO)-dependent pathway in both cardiodepression and cardioprotection by hrSTA-CgA(1-78). We also hypothesized an involvement of adenosine A(1) receptor and protein kinase C (PKC) in cardioprotection by hrSTA-CgA(1-78). Therefore, we evaluated whether 1) the cardioinhibition mediated by hrSTA-CgA(1-78) involves the G(i/o) proteins/NO-dependent signal transduction cascade, 2) hrSTA-CgA(1-78) induces ischemic preconditioning-like protective effects on the myocardium, and 3) inhibition of NO synthase (NOS), adenosine A(1) receptor, or PKC affects hrSTA-CgA(1-78) protection. Using the isolated rat heart, we found that the reduction of left ventricular pressure (LVP), rate-pressure product, and maximal values of the first derivative of LVP elicited by hrSTA-CgA(1-78) at 33 nM is abolished by blocking G(i/o) proteins with pertussis toxin, scavenging NO with hemoglobin, and blocking NOS activity with N(G)-monomethyl-l-arginine or N(5)-(iminoethyl)-l-ornithine, soluble guanylate cyclase with 1H-[1,2,4]oxadiazole-[4,4-a]quinoxalin-1-one, and protein kinase (PKG) with KT5823. Data suggest the involvement of the G(i/o) proteins/NO-cGMP-PKG pathway in the hrSTA-CgA(1-78)-dependent cardioinhibition. When given before 30 min of ischemia, hrSTA-CgA(1-78) significantly reduced the size of the infarct from 64 +/- 4 to 32 +/- 3% of the left ventricular mass. This protective effect was abolished by either NOS inhibition or PKC blockade and was attenuated, but not suppressed, by the blockade of A(1) receptors. These results suggest that hrSTA-CgA(1-78) activity triggers two different pathways: one of these pathways is mediated by A(1) receptors, and the other is mediated by NO release. As with repeated brief preconditioning ischemia, hrSTA-CgA(1-78) may be considered a stimulus strong enough to trigger both pathways, which may converge on PKC.

S-nitrosylation of cysteine 289 of the AT1 receptor decreases its binding affinity for angiotensin II

1. Nitric oxide (NO) is known to affect the properties of various proteins via the S-nitrosylation of cysteine residues. This study evaluated the direct effects of the NO donor sodium nitroprusside (SNP) on the pharmacological properties of the AT1 receptor for angiotensin II expressed in HEK-293 cells. 2. SNP dose-dependently decreased the binding affinity of the AT1 receptor without affecting its total binding capacity. This modulatory effect was reversed within 5 min of removing SNP. 3. The effect of SNP was not modified in the presence of the G protein uncoupling agent GTPgammaS or the soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. 4. The binding properties of a mutant AT1 receptor in which all five cysteine residues within the transmembrane domains had been replaced by serine was not affected by SNP. Systematic analysis of mutant AT1 receptors revealed that cysteine 289 conferred the sensitivity to SNP. 5. These results suggest that NO decreased the binding affinity of the AT1 receptor by S-nitrosylation of cysteine 289. This modulatory mechanism may be particularly relevant in pathophysiological situations where the beneficial effects of NO oppose the deleterious effects of angiotensin II.

Nitric oxide modulates Gi-protein expression and adenylyl cyclase signaling in vascular smooth muscle cells

We have previously shown that treatment of rats with the nitric oxide (NO) synthase inhibitor N6-nitro-L-arginine methyl ester for 4 weeks resulted in the augmentation of blood pressure and enhanced levels of Gialpha proteins. The present studies were undertaken to investigate if NO can modulate the expression of Gi proteins and associated adenylyl cyclase signaling. A10 vascular smooth muscle cells (VSMC) and primary cultured cells from aorta of Sprague-Dawley rats were used for these studies. The cells were treated with S-nitroso-N-acetylpenicillamine (SNAP) or sodium nitroprusside (SNP) for 24 h and the expression of Gialpha proteins was determined by immunobloting techniques. Adenylyl cyclase activity was determined by measuring [32P]cAMP formation for [alpha-32P]ATP. Treatment of cells with SNAP (100 microM) or SNP (0.5 mM) decreased the expression of Gialpha-2 and Gialpha-3 by about 25-40% without affecting the levels of Gsalpha proteins. The decreased expression of Gialpha proteins was reflected in decreased Gi functions (receptor-independent and -dependent) as demonstrated by decreased or attenuated forskolin-stimulated adenylyl cyclase activity by GTPgammaS and inhibition of adenylyl cyclase activity by angiotensin II and C-ANP4-23, a ring-deleted analog of atrial natriuretic peptide (ANP) that specifically interacts with natriuretic peptide receptor-C (NPR-C) in SNAP-treated cells. The SNAP-induced decreased expression of Gialpha-2 and Gialpha-3 proteins was not blocked by 1H[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one, an inhibitor of soluble guanylyl cyclase, or KT5823, an inhibitor of protein kinase G, but was restored toward control levels by uric acid, a scavenger of peroxynitrite and Mn(111)tetralis (benzoic acid porphyrin) MnTBAP, a peroxynitrite scavenger and a superoxide dismutase mimetic agent that inhibits the production of peroxynitrite, suggesting that NO-mediated decreased expression of Gialpha protein was cGMP-independent and may be attributed to increased levels of peroxynitrite. In addition, Gsalpha-mediated stimulation of adenylyl cyclase by GTPgammaS, isoproterenol, and forskolin was significantly augmented in SNAP-treated cells. These results indicate that NO decreased the expression of Gialpha protein and associated functions in VSMC by cGMP-independent mechanisms. From these studies, it can be suggested that NO-induced decreased levels of Gi proteins and resultant increased levels of cAMP may be an additional mechanism through which NO regulates blood pressure.

S-nitrosoglutathione inhibits alpha1-adrenergic receptor-mediated vasoconstriction and ligand binding in pulmonary artery

Endogenous nitric oxide donor compounds (S-nitrosothiols) contribute to low vascular tone by both cGMP-dependent and -independent pathways. We have reported that S-nitrosoglutathione (GSNO) inhibits 5-hydroxytryptamine (5-HT)-mediated pulmonary vasoconstriction via a cGMP-independent mechanism likely involving S-nitrosylation of its G protein-coupled receptor (GPCR) system. Because catecholamines, like 5-HT, constrict lung vessels via a GPCR coupled to G(q), we hypothesized that S-nitrosothiols modify the alpha1-adrenergic GPCR system to inhibit pulmonary vasoconstriction by receptor agonists, e.g., phenylephrine (PE). Rat pulmonary artery rings were pretreated for 30 min with and without an S-nitrosothiol, either GSNO or S-nitrosocysteine (CSNO), and constricted with sequential concentrations of PE (10(-8)-10(-6) M). Effective cGMP-dependence was tested in rings pretreated with soluble guanylate cyclase inhibitors {either 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or LY-83583} or G kinase inhibitor (KT-5823), and a thiol reductant [dithiothreitol (DTT)] was used to test reversibility of S-nitrosylation. Both S-nitrosothiols attenuated the PE dose response. The GSNO effect was not prevented by LY-83583, ODQ, or KT-5823, indicating cGMP independence. GSNO inhibition was reversed by DTT, consistent with S-nitrosylation or other GSNO-mediated cysteine modifications. In CSNO-treated lung protein, the alpha1-adrenergic receptor was shown to undergo S-nitrosylation in vitro using a biotin switch assay. Studies of alpha1-adrenergic receptor subtype expression and receptor density by saturation binding with 125I-HEAT showed that GSNO decreased alpha1-adrenergic receptor density but did not alter affinity for antagonist or agonist. These data demonstrate a novel cGMP-independent mechanism of reversible alpha1-adrenergic receptor inhibition by S-nitrosothiols.

International union of pharmacology. XLV. Classification of the kinin receptor family: from molecular mechanisms to pathophysiological consequences

Kinins are proinflammatory peptides that mediate numerous vascular and pain responses to tissue injury. Two pharmacologically distinct kinin receptor subtypes have been identified and characterized for these peptides, which are named B1 and B2 and belong to the rhodopsin family of G protein-coupled receptors. The B2 receptor mediates the action of bradykinin (BK) and lysyl-bradykinin (Lys-BK), the first set of bioactive kinins formed in response to injury from kininogen precursors through the actions of plasma and tissue kallikreins, whereas the B(1) receptor mediates the action of des-Arg9-BK and Lys-des-Arg9-BK, the second set of bioactive kinins formed through the actions of carboxypeptidases on BK and Lys-BK, respectively. The B2 receptor is ubiquitous and constitutively expressed, whereas the B1 receptor is expressed at a very low level in healthy tissues but induced following injury by various proinflammatory cytokines such as interleukin-1beta. Both receptors act through G alpha(q) to stimulate phospholipase C beta followed by phosphoinositide hydrolysis and intracellular free Ca2+ mobilization and through G alpha(i) to inhibit adenylate cyclase and stimulate the mitogen-activated protein kinase pathways. The use of mice lacking each receptor gene and various specific peptidic and nonpeptidic antagonists have implicated both B1 and B2 receptors as potential therapeutic targets in several pathophysiological events related to inflammation such as pain, sepsis, allergic asthma, rhinitis, and edema, as well as diabetes and cancer. This review is a comprehensive presentation of our current understanding of these receptors in terms of molecular and cell biology, physiology, pharmacology, and involvement in human disease and drug development.

S-nitrosothiols modulate G protein-coupled receptor signaling in a reversible and highly receptor-specific manner

Background

Recent studies indicate that the G protein-coupled receptor (GPCR) signaling machinery can serve as a direct target of reactive oxygen species, including nitric oxide (NO) and S-nitrosothiols (RSNOs). To gain a broader view into the way that receptor-dependent G protein activation – an early step in signal transduction – might be affected by RSNOs, we have studied several receptors coupling to the G_i family of G proteins in their native cellular environment using the powerful functional approach of [³⁵S]GTPγS autoradiography with brain cryostat sections in combination with classical G protein activation assays.

Results

We demonstrate that RSNOs, like S-nitrosoglutathione (GSNO) and S-nitrosocysteine (CysNO), can modulate GPCR signaling via reversible, thiol-sensitive mechanisms probably involving S-nitrosylation. RSNOs are capable of very targeted regulation, as they potentiate the signaling of some receptors (exemplified by the M2/M4 muscarinic cholinergic receptors), inhibit others (P2Y₁₂ purinergic, LPA₁lysophosphatidic acid, and cannabinoid CB₁ receptors), but may only marginally affect signaling of others, such as adenosine A₁, μ-opioid, and opiate related receptors. Amplification of M2/M4 muscarinic responses is explained by an accelerated rate of guanine nucleotide exchange, as well as an increased number of high-affinity [³⁵S]GTPγS binding sites available for the agonist-activated receptor. GSNO amplified human M4 receptor signaling also under heterologous expression in CHO cells, but the effect diminished with increasing constitutive receptor activity. RSNOs markedly inhibited P2Y₁₂ receptor signaling in native tissues (rat brain and human platelets), but failed to affect human P2Y₁₂ receptor signaling under heterologous expression in CHO cells, indicating that the native cellular signaling partners, rather than the P2Y₁₂ receptor protein, act as a molecular target for this action.

Conclusion

These in vitro studies show for the first time in a broader general context that RSNOs are capable of modulating GPCR signaling in a reversible and highly receptor-specific manner. Given that the enzymatic machinery responsible for endogenous NO production is located in close proximity with the GPCR signaling complex, especially with that for several receptors whose signaling is shown here to be modulated by exogenous RSNOs, our data suggest that GPCR signaling in vivo is likely to be subject to substantial, and highly receptor-specific modulation by NO-derived RSNOs.

Chronic administration of nitric oxide reduces angiotensin II receptor type 1 expression and aldosterone synthesis in zona glomerulosa cells

Acute nitric oxide (NO) inhibits angiotensin II (ANG II)-stimulated aldosterone synthesis in zona glomerulosa (ZG) cells. In this study, we investigated the effects of chronic administration of NO on the ANG II receptor type 1 (AT1) expression and aldosterone synthesis. ZG cells were treated daily with DETA NONOate (10(-4) M), an NO donor, for 0, 12, 24, 48, 72, and 96 h. Chinese hamster ovary (CHO) cells, stably transfected with the AT1B receptor, were used as a positive control. Western blot analysis indicated that AT1 receptor expression was decreased as a function of time of NO administration in both CHO and ZG cells. ANG II binding to its receptors was determined by radioligand binding. NO treatment of ZG cells for 96 h resulted in a decrease in ANG II binding compared with control. The receptor density was decreased to 1,864 +/- 129 fmol/mg protein from 3,157 +/- 220 fmol/mg protein (P < 0.005), but the affinity was not changed (1.95 +/- 0.22 vs. 1.88 +/- 0.21 nM). Confocal Raman microspectroscopy and immunocytochemistry both confirmed that the expression of AT1 receptors in ZG cells decreased with chronic NO administration. In addition, chronic NO administration also decreased the expression of cholesterol side-chain cleavage enzyme in ZG cells and inhibited ANG II- and 25-hydroxycholesterol-stimulated aldosterone synthesis in ZG cells. This study demonstrates that chronic administration of NO inhibits aldosterone synthesis in ZG cells by downregulation of the expression of both AT1 receptors and cholesterol side-chain cleavage enzyme.

Modulation of A1 adenosine receptor signaling by peroxynitrite

Nitric oxide (NO) is a gaseous free radical involved in many pathophysiological processes. During oxidative stress, NO, its derivatives and adenosine are released. Considering adenosine neuroprotective role in the central nervous system (CNS) and toxicity of NO, we investigated the effect of a NO/peroxynitrite (ONOO(-)) donor, 3-morpholinosydnonimine (SIN-1), on A(1) adenosine receptor (A(1)AR) signaling pathway in rat cortical membranes. Membrane treatment with 0.5mM SIN-1 for various periods of time (0-240min) decreased specific binding of the radiolabeled A(1)AR agonist, [3H]N(6)-cyclohexyladenosine ([3H]CHA), in a time-dependent manner, reaching the steady state after 120min. The inhibitory effect of SIN-1 was concentration-dependent, with an EC(50) value of 0.60+/-0.30mM (N=3). Membrane pre-incubation with the superoxide anion (O(2)z.rad;(-)) scavenger superoxide dismutase (SOD) followed by SIN-1 addition, abolished SIN-1 inhibition of [3H]CHA binding. Membrane treatment with 0.5mM SIN-1 for 120min caused a significant 2-fold increase of the K(D) value for [3H]CHA without changing the B(max) value. Moreover, pre-incubation of membranes with A(1)AR agonists, CHA or N(6)-(2-phenylisopropyl)-adenosine (R-PIA) before SIN-1 addition increased the inhibitory effect while the selective A(1)AR antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) had no activity. Membrane treatment with SIN-1 decreased receptor-stimulated guanosine 5'-O-(gamma[35S]thio)triphosphate ([35S]GTPgammaS) binding in a concentration-dependent manner. This treatment influenced [35S]GTPgammaS binding affinity for A(1)AR activated G(i) proteins in cortical membranes. These findings suggest that ONOO(-) modulates A(1)AR signaling pathways by affecting receptor G(i) protein coupling.

The [35S]GTPgammaS binding assay: approaches and applications in pharmacology

Receptors of the of seven transmembrane spanning, heterotrimeric G protein coupled family (GPCR) play crucial roles in regulating physiological functions and consequently are targets for the action of many classes of drugs. Activation of receptor by agonist leads to the dissociation of GDP from Galpha of the Galphabetagamma heterotrimer, followed by the binding of GTP to Galpha and subsequent modulation of downstream effectors. The G protein heterotrimer is reformed by GTPase activity of the Galpha subunit, forming Galpha-GDP and so allowing Galpha and Gbetagamma to recombine. The [35S]GTPgammaS assay measures the level of G protein activation following agonist occupation of a GPCR, by determining the binding of the non-hydrolyzable analog [35S]GTPgammaS to Galpha subunits. Thus, the assay measures a functional consequence of receptor occupancy at one of the earliest receptor-mediated events. The assay allows for traditional pharmacological parameters of potency, efficacy and antagonist affinity, with the advantage that agonist measures are not subjected to amplification or other modulation that may occur when analyzing parameters further downstream of the receptor. In general the assay is experimentally more feasible for receptors coupled to the abundant G(i/o) proteins. Nevertheless, [35S]GTPgammaS binding assays are used with GPCRs that couple to the G(s) and G(q) families of G proteins, especially in artificial expression systems, or using receptor-Galpha constructs or immunoprecipitation of [35S]GTPgammaS-labeled Galpha. The relative simplicity of the assay has made it very popular and its use is providing insights into contemporary pharmacological topics including the roles of accessory proteins in signaling, constitutive activity of receptors and agonist specific signaling.

Effect of dietary nitric oxide on gastric mucosal mast cells in absence or presence of an experimental gastritis in rats

Synthetic nitric oxide donors are known to protect the gastric mucosa from damage and dietary nitrate is known to release NO in the stomach. Mast cells have been found to be involved in gastric mucosal damage in humans or in rodents, and recent studies have pointed out the possibility of nitric oxide from endogenous or exogenous origin to modulate mast cell reactivity. This study aimed to determine whether the protective effect afforded by dietary nitrate against gastric mucosal damage was linked to mast cell stabilization. Mast cell involvement in iodoacetamide-induced gastritis was investigated in rats receiving oral administration of iodoacetamide together with the mast cell stabilizer doxantrazole (ip) or its solvent. The effects of dietary nitrate on mast cells during gastritis were investigated in rats receiving iodoacetamide orally, associated or not with KNO3. Control groups were given water instead of iodoacetamide either with or without KNO3, doxantrazole or its solvent. After sacrifice, blood samples were taken to determine RMCP II serum level and the stomach was resected in order to determine myeloperoxidase (MPO) activity and mucosal mast cell (MMC) number. Iodoacetamide significantly increased gastric MPO activity but did not modify RMCP II serum level or MMC number. Doxantrazole and KNO3 significantly reduced iodoacetamide-induced increase in gastric MPO activity, increased MMC number, and decreased RMCP II serum level in basal conditions. Only doxantrazole was able to modify all parameters under inflammatory conditions. These results suggest that nitric oxide released by dietary nitrate in the stomach stabilizes mast cells in basal conditions but exerts its protective effect against experimental gastritis through other pathways.

L-arginine protects the mesenteric vascular circulation against cardiopulmonary bypass-induced vascular dysfunction

BACKGROUND

The aim of our study was to determine whether addition of the nitric oxide donor l-arginine at reperfusion may prevent the cardiopulmonary bypass (CPB)-induced vascular alterations in the intestine.

METHODS

Twelve dogs underwent 90-minute hypothermic CPB. After 60 minutes, the cardiac arrest-treated group (n = 6) received 40 mg/kg intravenous bolus l-arginine, followed by 3 mg/kg/min infusion for 20 minutes. Hemodynamic parameters, blood gases, lactate, and glucose were monitored. Reactive hyperemia (RH) in response to superior mesenteric artery ischemia and vasorelaxation to systemically administered vasoactive drugs (acetylcholine [ACH] and sodium nitroprusside) were assessed before and after CPB and defined as percent change of vascular resistance.

RESULTS

In the control group, CPB reduced reactive hyperemia (RH) (-26 +/- 15% vs -53 +/- 5%), and the response to ACH (-30 +/- 3% vs -42 +/- 7%). In the treated group, the post-CPB endothelial dysfunction was reversed (-37 +/- 1%, P <.05 vs control group) and RH partially recovered (-34 +/- 4%, P <.05). Administration of l-arginine resulted in a higher mesenteric oxygen delivery, increased nitrite/nitrate production, and lower lactate release from the mesenteric vascular circulation after reperfusion.

CONCLUSIONS

CPB disrupts some of the regulatory functions of the endothelial cell in the mesenterium and these are mostly related to nitric oxide unavailability. Systemic supplementation of l-arginine at reperfusion prevents the CPB-induced mesenteric endothelial dysfunction in association with an increased blood distribution and a reduced metabolic impairment.

Relaxation to bradykinin in bovine pulmonary supernumerary arteries can be mediated by both a nitric oxide-dependent and -independent mechanism

1. The aim of the present study was to determine the relative contribution of prostanoids, nitric oxide and K(+) channels in the bradykinin-induced relaxation of bovine pulmonary supernumerary arteries. 2. In endothelium-intact, but not denuded rings, bradykinin produced a concentration-dependent relaxation (pEC(50), 9.6+/-0.1), which was unaffected by the cyclo-oxygenase inhibitor indomethacin. The nitric oxide scavenger hydroxocobalamin (200 micro M, pEC(50), 8.5+/-0.2) and the nitric oxide synthase inhibitor L-NAME (100 micro M, pEC(50), 8.9+/-0.1) and the combination of L-NAME and hydroxocobalamin (pEC(50), 8.1+/-0.2) produced rightward shifts in the bradykinin concentration response curve. 3. The guanylyl cyclase inhibitor ODQ (10 micro M, pEC(50), 9.6+/-0.4) did not affect the response to bradykinin. 4. Elevating the extracellular [K(+)] to 30 mM did not affect the response to bradykinin but abolished the response when ODQ or L-NAME was present. 5. The K(+) channel blocker apamin (100 nM), combined with charybdotoxin (100 nM), produced a small reduction in the maximum response to bradykinin but they abolished the response to bradykinin when ODQ, L-NAME or hydroxocobalamin were present. Apamin (100 nM) combined with iberiotoxin (100 nM) also reduced the response to bradykinin in the presence of hydroxocobalamin or L-NAME. 6. The concentration response curve for sodium nitroprusside-induced relaxation was abolished by ODQ (10 micro M) and shifted to the right by apamin and charybdotoxin. 7. These studies suggest that in bovine pulmonary supernumerary arteries bradykinin can stimulate the formation of nitric oxide and activate an EDHF-like mechanism and that either of these pathways alone can mediate the bradykinin-induced relaxation. In addition nitric oxide, acting through guanylyl cyclase, can activate an apamin/charbydotoxin-sensitive K(+) channel in this tissue.

Heterogeneity of bronchial endothelial cell permeability

In vivo models of airway inflammation suggest that most protein transudation occurs from bronchial microcirculation. However, due to technical limitations in the isolation and culture of bronchial endothelial cells, most studies of lung vascular permeability have focused on pulmonary endothelium. Thus conditions for culture of sheep bronchial artery endothelial cells (BAEC) and bronchial microvascular endothelial cells (BMVEC) were established. The bronchial artery and the mainstem bronchi, stripped of epithelium, were dissected, and endothelial cells were isolated by enzymatic treatment. BAEC and BMVEC demonstrated positive staining for factor VIII-related antigen, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate-labeled low-density lipoprotein, and PECAM-1. Radioligand binding studies confirmed equivalent numbers of bradykinin B(2) receptors on BAEC and BMVEC. Permeability of BAEC and BMVEC was determined after treatment with bradykinin and thrombin by comparing the translocation of FITC-dextran (mol wt 9,500) across confluent monolayers (n = 10-12). Bradykinin caused a maximal increase in permeability in BAEC (165% increase) and BMVEC (144% increase) by 15 min compared with vehicle controls. Thrombin treatment altered BMVEC permeability only, reaching a maximal response at 60 min (109% increase). These results demonstrate bronchial endothelial cell heterogeneity and establish methods to determine intracellular mechanisms contributing to airway disease in relevant cell systems.

Dehydroepiandrosterone activates endothelial cell nitric-oxide synthase by a specific plasma membrane receptor coupled to Galpha(i2,3)

The adrenal steroid dehydroepiandrosterone (DHEA) has no known cellular receptor or unifying mechanism of action, despite evidence suggesting beneficial vascular effects in humans. Based on previous data from our laboratory, we hypothesized that DHEA binds to specific cell-surface receptors to activate intracellular G-proteins and endothelial nitric-oxide synthase (eNOS). We now pharmacologically characterize a putative plasma membrane DHEA receptor and define its associated G-proteins. The [3H]DHEA binding to isolated plasma membranes from bovine aortic endothelial cells was of high affinity (K(d) = 48.7 pm) and saturable (B(max) = 500 fmol/mg protein). Structurally related steroids failed to compete with DHEA for binding. The putative DHEA receptor was functionally coupled to G-proteins, because guanosine 5'-O-(3-thio)triphosphate (GTPgammaS) inhibited [3H]DHEA binding to plasma membranes by 69%, and DHEA increased [35S]GTPgammaS binding by 157%. DHEA stimulated [35S]GTPgammaS binding to Galpha(i2) and Galpha(i3), but not to Galpha(i1) or Galpha(o). Pretreatment of plasma membranes with antibody to Galpha(i2) or Galpha(i3), but not to Galpha(i1), inhibited the DHEA activation of eNOS. Thus, DHEA receptors are expressed on endothelial cell plasma membranes and are coupled to eNOS activity through Galpha(i2) and Galpha(i3). These novel findings should allow us to isolate the putative receptor and reevaluate the physiological role of DHEA activity.

Pulmonary vasoconstriction by serotonin is inhibited by S-nitrosoglutathione

Nitric oxide (NO) functions as an endothelium-derived relaxing factor by activating guanylate cyclase to increase cGMP levels. However, NO and related species may also regulate vascular tone by cGMP-independent mechanisms. We hypothesized that naturally occurring NO donors could decrease the pulmonary vascular response to serotonin (5-HT) in the intact lung through chemical interactions with 5-HT(2) receptors. In isolated rabbit lung preparations and isolated pulmonary artery (PA) rings, 50-250 microM S-nitrosoglutathione (GSNO) inhibited the response to 0.01-10 microM 5-HT. The vasoconstrictor response to 5-HT was mediated by 5-HT(2) receptors in the lung, since it could be blocked completely by the selective inhibitor ketanserin (10 microM). GSNO inhibited the response to 5-HT by 77% in intact lung and 82% in PA rings. In PA rings, inhibition by GSNO could be reversed by treatment with the thiol reductant dithiothreitol (10 mM). 3-Morpholinosydnonimine (100-500 microM), which releases NO and O simultaneously, also blocked the response to 5-HT. Its chemical effects, however, were distinct from those of GSNO, because 5-HT-mediated vasoconstriction was not restored in isolated rings by dithiothreitol. In the intact lung, neither NO donor altered the vascular response to endothelin, which activates the same second-messenger vasoconstrictor system as 5-HT. These findings, which did not depend on guanylate cyclase, are consistent with chemical modification by NO of the 5-HT(2) G protein-coupled receptor system to inhibit vasoconstriction, possibly by S-nitrosylation of the receptor or a related protein. This study demonstrates that GSNO can regulate vascular tone in the intact lung by a reversible mechanism involving inhibition of the response to 5-HT.

Kinetic analysis of M2 muscarinic receptor activation of Gi in Sf9 insect cell membranes

A steady-state kinetic mechanism describing the interaction of M(2) muscarinic acetylcholine receptors and the guanine nucleotide-binding protein G(i)alpha(2)beta(1)gamma(3) are presented. Data are consistent with two parallel pathways of agonist-promoted GTPase activity arising from receptor coupled to a single or multiple guanine nucleotide-binding proteins. An aspartate 103 to asparagine receptor mutation resulted in a receptor lacking the ability to catalyze the binding of guanosine-5'-O-(3-thiotriphosphate) or guanosine triphosphate hydrolysis by the G protein. An aspartate 69 to asparagine receptor mutant was able to catalyze agonist-specific guanine nucleotide exchange and GTPase activity. A threonine 187 to alanine receptor mutation resulted in a receptor that catalyzed guanine nucleotide exchange comparable with wild-type receptors but had reduced ability to stimulate GTP hydrolysis. A tyrosine 403 to phenylalanine receptor mutation resulted in an increase in agonist-promoted GTPAse activity compared with wild type. The observation that the threonine 187 and tyrosine 403 mutants promote guanine nucleotide exchange similarly to wild type but alter GTPase activity compared with wild type suggests that the effects of the mutations arise downstream from guanine nucleotide exchange and may result from changes in receptor-G protein dissociation.

Nitric oxide modulates cardiac Na(+) channel via protein kinase A and protein kinase G

We directly tested the effects of nitric oxide (NO) on Na(+) channels in guinea pig and mouse ventricular myocytes using patch-clamp recordings. We have previously shown that NO donors have no observed effects on expressed Na(+) channels. In contrast, NO (half-blocking concentration of 523 nmol/L) significantly reduces peak whole-cell Na(+) current (I(Na)) in isolated ventricular myocytes. The inhibitory effect of NO on I(Na) was not associated with changes in activation, inactivation, or reactivation kinetics. At the single-channel level, the reduction in macroscopic current was mediated by a decrease in open probability and/or a decrease in the number of functional channels with no change in single-channel conductance. Application of cell permeable analogs of cGMP or cAMP mimics the inhibitory effects of NO. Furthermore, the effects of NO on I(Na) can only be blocked by inhibition of both cGMP and cAMP pathways. Sulfhydryl-reducing agent does not reverse the effect of NO. In summary, although NO exerts its action via the known guanylyl cyclase (GC)/cGMP pathway, our findings provide evidence that NO can mediate its function via a GC/cGMP-independent mechanism involving the activation of adynylyl cyclase (AC) and cAMP-dependent protein kinase.

Sulfhydryl G proteins and phospholipase A(2)-associated G proteins are involved in adrenergic signal transduction in the rat pineal gland

The rat pineal gland with its circadian noradrenaline-regulated melatonin rhythm is an excellent model for studying adrenergic signal transduction with respect to cAMP and cGMP formation. The stimulatory G(s) proteins play a well-established role in this process. In contrast, the potential roles of the inhibitory G(i) proteins, the functionally unclear other G(o) proteins, and a number of G protein subtypes are not known. The present study examines the effects on beta(1)- and beta(1)-plus-alpha(1)-stimulated cAMP and cGMP formation of a number of G protein modulators in rat pinealocyte suspension cultures. The effects of the nitric oxide donor sodium nitroprusside on cGMP were also examined. The results showed that drugs that activate G proteins of the G(i)/G(o) family, i.e., pertussis toxin, mastoparan, and compound 48/80, had no effect on unstimulated, isoproterenol (beta(1))-stimulated, or combined isoproterenol/phenylephrine (beta(1)-plus()-alpha(1))-stimulated cAMP and cGMP accumulation. However, in this experimental paradigm, the inhibitors of sulfhydryl G proteins (N-ethylmaleimide) and those of phospholipase A2-related G proteins (isotetrandrine) exerted a clear inhibitory effect. Sodium-nitroprusside-stimulated cGMP accumulation was also inhibited. These results confirm a previous report that members of the G(i)/G(o) family, which are present in the rat pineal gland, do not play a major role in adrenergic signal transduction. The new finding that sulfhydryl G proteins and phospholipase A2-associated G proteins exert a clear stimulatory effect on adrenergic signal transduction suggests that they are subtypes of G(s) proteins.

G protein-mediated inhibitory effect of a nitric oxide donor on the L-type Ca2+ current in rat ventricular myocytes

1. The role of the cGMP pathway in the modulation of the cardiac L-type Ca2+ current (ICa,L) by nitric oxide (NO) was examined in rat ventricular myocytes. 2. The NO donors DEANO, SIN-1, SNP, SNAP and GSNO had no significant effects on basal ICa,L. However, DEANO (100 microM) inhibited ICa,L after the current had been previously stimulated by either isoprenaline (Iso, 1-10 nM), a beta-adrenergic agonist, or isobutylmethyl-xanthine (IBMX, 10-80 microM), a wide spectrum phosphodiesterase (PDE) inhibitor. 3. The anti-adrenergic effect of DEANO on ICa,L was not mimicked by other NO donors (SIN-1, SNAP and SPNO). 4. The NO-sensitive guanylyl cyclase inhibitor ODQ (10 microM), antagonized the inhibitory effect of DEANO on ICa,L. Likewise, inhibitors of the cGMP-dependent protein kinase (cG-PK), Rp-8-chloro-phenylthio-cGMP (10 microM) and KT5823 (0.1 and 0.3 microM), also abolished the inhibitory effect of DEANO on Iso (1-10 nM)-stimulated ICa,L. 5. Intracellular dialysis with exogenous cAMP (10-100 microM) blunted the inhibitory effect of DEANO (10 and 100 microM) on ICa,L. SNAP and SNP also had no effect on the cAMP-stimulated ICa,L. 6. Pre-treatment of the myocytes with pertussis toxin (0.5 microg ml-1, 4-6 h at 37 degrees C) eliminated the inhibitory effect of DEANO (100 microM) on ICa,L, in the presence of either Iso (0.01 and 1 nM) or IBMX (10-80 microM). 7. These results demonstrate that DEANO produces anti-adrenergic effects in rat ventricular myocytes. This effect of DEANO occurs in a cGMP-dependent manner, and involves activation of cG-PK and regulation of a pertussis toxin-sensitive G protein.

Nitric oxide attenuates signal transduction: possible role in dissociating caveolin-1 scaffold

Caveolae harbor different serpentine receptors, intracellular components of signaling cascades, and certain enzymes, including endothelial nitric oxide synthase (eNOS). The regulation of eNOS activity by Ca(2+)/calmodulin and caveolin has been described. We have previously demonstrated that nitric oxide (NO) can modulate signaling initiated via receptors localized to caveolae. In the present study, we show that NO donors induced an increase in the monomeric form of this scaffolding protein in cultured endothelial cells, the effect mimicked by 8-bromo cGMP. Proximity imaging of endothelial cells transfected with the thermotolerant green fluorescent protein-caveolin-1 construct demonstrated that sodium nitroprusside resulted in the increased fluorescence ratio of 410:470 nm, consistent with the distancing of fluorescently tagged caveolin-1. Pulse labeling of endothelial cells with cholera toxin B subunit indicated that sodium nitroprusside reversibly decreased its binding. Signaling via G protein-coupled receptors resident to caveolae was inhibited by pretreatment with NO donor. The data demonstrate that NO modulation of cell signaling is accomplished in part by regulating the state of caveolin-1 oligomerization. NO-induced attenuation of signaling involves reversible dissociation of caveolin scaffold, thus providing both spatial and temporal modulation of signal transduction.

Relationship between flow rate and NO production in postnatal mesenteric arteries

We studied mesenteric arterial arcades from 3- and 35-day-old swine to determine the relationship between perfusate flow rate and release of nitric oxide (NO) into mesenteric effluent. Mesenteric arterial arcades were perfused under controlled-flow conditions with a peristaltic pump using warm oxygenated Krebs buffer. Basal rates of NO production were 43.6 +/- 4.2 vs. 12.1 +/- 2.5 nmol/min in 3- vs. 35-day-old mesentery during perfusion at in vivo flow rates (9 vs. 20 ml/min, respectively). Rate of NO production was directly related to flow rate over a wide range of flows (5-40 ml/min) in 3- but not 35-day-old mesentery. Both age groups demonstrated a brisk, albeit brief, increase in NO production in response to infusion of NO-dependent vasodilator substance P (10(-8) M/min). Tyrosine kinase inhibitor herbimycin A and L-arginine analog L-NMMA significantly attenuated flow-induced increase in NO production, and phosphatase inhibitor phenylarsine oxide increased magnitude of flow-induced increase in NO production in 3-day-olds. Removal of extracellular Ca(2+) and depletion of intracellular Ca(2+) stores (Ca(2+)-free Krebs with EGTA plus thapsigargin) had no effect on NO production in either group. Thus, basal rate of NO production is greater in mesenteric arterial arcades from 3- than from 35-day old swine, a direct relationship between flow rate and NO production rate is present in mesentery from 3- but not 35-day-olds, and phosphorylation events are necessary for this interaction to occur.

Suppression of endothelial nitric oxide production after withdrawal of statin treatment is mediated by negative feedback regulation of rho GTPase gene transcription

BACKGROUND

Statins improve endothelial function by upregulating endothelial nitric oxide (NO) production that is mediated by inhibiting the isoprenylation of rho GTPase. Withdrawal of statin treatment could suppress endothelial NO production and may impair vascular function.

METHODS AND RESULTS

To test this hypothesis, mice were treated for 14 days with 10 mg/kg atorvastatin per day; this led to the upregulation of endothelial NO synthase expression and activity by 2.3- and 3-fold, respectively. Withdrawal of statins resulted in a dramatic, 90% decrease of NO production after 2 days. In mouse aortas and cultured endothelial cells, statins upregulated the expression of rho GTPase in the cytosol, but statins blocked isoprenoid-dependent rho membrane translocation and GTP-binding activity. Inhibiting the downstream targets of rho showed that rho expression is controlled by a negative feedback mechanism mediated by the actin cytoskeleton. Measuring rho mRNA half-life and nuclear run-on assays demonstrated that statins or disruption of actin stress fibers increased rho gene transcription but not rho mRNA stability. Therefore, treatment with statins leads to the accumulation of nonisoprenylated rho in the cytosol. Withdrawing statin treatment restored the availability of isoprenoids and resulted in a massive membrane translocation and activation of rho, causing downregulation of endothelial NO production.

CONCLUSIONS

Withdrawal of statin therapy in normocholesterolemic mice results in a transient increase of rho activity, causing a suppression of endothelial NO production. The underlying molecular mechanism is a negative feedback regulation of rho gene transcription mediated by the actin cytoskeleton.

Blockade of voltage-sensitive Ca(2+)-channels markedly diminishes nitric oxide- but not L-S-nitrosocysteine- or endothelium-dependent vasodilation in vivo

The aim of this study was to determine the hemodynamic responses elicited by systemic injections of (i) the nitric oxide (NO)-donors, sodium nitroprusside (10 nmol/kg, i.v.) and (Z)-1-(N-methyl-N-(6(N-methylammoniohexyl)amino))diazen-1-ium-1, 2-diolate (MAHMA NONOate, 25 nmol/kg, i.v.), (ii) the endothelium-derived S-nitrosothiol, L-S-nitrosocysteine (100 nmol/kg, i.v.), and (iii) the endothelium-dependent agonist, acetylcholine (1.0 microg/kg, i.v.), in anesthetized rats, before and after injection of the voltage-sensitive Ca(2+)-channel (Ca(VS)(2+)-channel) blocker, nifedipine (500 nmol/kg, i.v.). Before injection of nifedipine, the agents produced similar falls in mean arterial blood pressure, and in hindquarter and mesenteric vascular resistances. The depressor and vasodilator responses elicited by sodium nitroprusside and MAHMA NONOate were markedly attenuated by nifedipine. The falls in mean arterial blood pressure and mesenteric resistance elicited by L-S-nitrosocysteine and acetylcholine were not attenuated but the falls in hindquarter resistance were slightly attenuated by nifedipine. The cyclooxygenase inhibitor, indomethacin (10 mg/kg, i.v.), did not affect the actions of sodium nitroprusside, MAHMA NONOate, L-S-nitrosocysteine or acetylcholine or the effects of nifedipine on the hemodynamic actions of these compounds. The decomposition of sodium nitroprusside (0.2 nmol/ml), MAHMA NONOate (0.5 nmol/ml) and L-S-nitrosocysteine (2 nmol/ml) to NO upon addition to rat blood was not affected by nifedipine (10 microM). These findings suggest that (i) exogenously applied NO relaxes resistance arteries in vivo by inhibition of Ca(VS)(2+)-channels whereas L-S-nitrosocysteine and the non-prostanoid endothelium-derived relaxing factor (EDRF) released by acetylcholine acts by additional mechanisms, and (ii) this EDRF may be an S-nitrosothiol which acts independently of its decomposition to NO.

Endothelin-mediated vasoconstriction in postischemic newborn intestine

We previously suggested that the profound, sustained vasoconstriction noted in 3-day-old swine intestine after a moderate episode of ischemia-reperfusion (I/R) reflects the unmasking of underlying constrictor tone consequent to a loss of endothelium-derived nitric oxide (NO). In this study, we sought to determine whether endothelin-1 (ET-1) was the unmasked constrictor and whether selective loss of endothelial ET(B) receptors, which mediate NO-based vasodilation, participated in the hemodynamic consequences of I/R in newborn intestine. Studies were performed in innervated, autoperfused intestinal loops in 3- and 35-day-old swine. Selective blockade of ET(A) receptors with BQ-610 had no effect on hemodynamics under control conditions; however, when administered before and during I/R, BQ-610 significantly attenuated the post-I/R vasoconstriction and reduction in arteriovenous O(2) difference in the younger group. In 3-day-old intestine, reduction of intestinal O(2) uptake to a level similar to that noted after I/R by lowering tissue temperature had no effect on the response to BQ-610 or ET-1, indicating that the change in response to BQ-610 noted after I/R was not simply consequent to the reduction in tissue O(2) demand. In studies in mesenteric artery rings suspended in myographs, we observed a leftward shift in the dose-response curve for ET-1 after selective blockade of ET(B) receptors with BQ-788 in 3- but not 35-day-old swine. Rings exposed to I/R in vivo behaved in a manner similar to control rings treated with BQ-788 or endothelium-denuded non-I/R rings.

Beta-adrenoceptor dysfunction after inhibition of NO synthesis

G(s) protein-coupled beta-adrenoceptors rapidly desensitize on exposure to agonists in reconstituted membrane preparations, whereas rapid tachyphylaxis to beta-adrenoceptor-mediated vasodilation does not readily occur in vivo. This study examined the possibility that endothelium-derived nitrosyl factors prevent the rapid desensitization of beta-adrenoceptors in the vascular smooth muscle of resistance arteries in pentobarbital-anesthetized rats. The fall in mean arterial blood pressure and in hindquarter vascular resistance produced by the beta-adrenoceptor agonist isoproterenol (ISO, 0.1 to 10 microg/kg IV) was slightly but significantly smaller in rats treated with the NO synthase inhibitor N:(G)-nitro-L-arginine methyl ester (L-NAME, 100 micromol/kg IV) than in saline-treated rats. The ISO-induced fall in mesenteric resistance was similar in L-NAME-treated and in saline-treated rats. The fall in hindquarter vascular resistance and in mesenteric resistance produced by ISO (8 x 10 microg/kg IV) was subject to tachyphylaxis on repeated injection in rats treated with L-NAME (100 micromol/kg IV) but not in rats treated with saline. Injections of L-S:-nitrosocysteine (1200 nmol/kg IV), a lipophobic S:-nitrosothiol, before each injection of ISO (10 microg/kg IV) prevented tachyphylaxis to ISO in L-NAME-treated rats. The vasodilator effects of ISO (0.1 to 10 microg/kg IV) in L-NAME-treated rats that received 8 injections of ISO (10 microg/kg IV) were markedly smaller than in L-NAME-treated rats that received 8 injections of saline. These results indicate that (1) the vasodilator actions of ISO in pentobarbital-anesthetized rats only minimally involve the release of endothelium-derived nitrosyl factors, (2) the effects of ISO are subject to development of tachyphylaxis in L-NAME-treated rats, and (3) tachyphylaxis to ISO is prevented by L-S:-nitrosocysteine. These findings suggest that endothelium-derived nitrosyl factors may prevent desensitization of beta-adrenoceptors in vivo.

Nitric oxide produced via neuronal NOS may impair vasodilatation in septic rat skeletal muscle

Impaired vascular responsiveness in sepsis may lead to maldistribution of blood flow in organs. We hypothesized that increased production of nitric oxide (NO) via inducible nitric oxide synthase (iNOS) mediates the impaired dilation to ACh in sepsis. Using a 24-h cecal ligation and perforation (CLP) model of sepsis, we measured changes in arteriolar diameter and in red blood cell velocity (V(RBC)) in a capillary fed by the arteriole, following application of ACh to terminal arterioles of rat hindlimb muscle. Sepsis attenuated both ACh-stimulated dilation and V(RBC) increase. In control rats, arteriolar pretreatment with the NO donors S-nitroso-N-acetylpenicillamine or sodium nitroprusside reduced diameter and V(RBC) responses to a level that mimicked sepsis. In septic rats, arteriolar pretreatment with the "selective" iNOS blockers aminoguanidine (AG) or S-methylisothiourea sulfate (SMT) restored the responses to the control level. The putative neuronal NOS (nNOS) inhibitor 7-nitroindazole also restored the response toward control. At 24-h post-CLP, muscles showed no reduction of endothelial NOS (eNOS), elevation of nNOS, and, surprisingly, no induction of iNOS protein; calcium-dependent constitutive NOS (eNOS+nNOS) enzyme activity was increased whereas calcium-independent iNOS activity was negligible. We conclude that 1) AG and SMT inhibit nNOS activity in septic skeletal muscle, 2) NO could impair vasodilative responses in control and septic rats, and 3) the source of increased endogenous NO in septic muscle is likely upregulated nNOS rather than iNOS. Thus agents released from the blood vessel milieu (e.g., NO produced by skeletal muscle nNOS) could affect vascular responsiveness.

Biology of nitric oxide signaling

The free radical nitric oxide (NO) has emerged in recent years as a fundamental signaling molecule for the maintenance of homeostasis, as well as a potent cytotoxic effector involved in the pathogenesis of a wide range of human diseases. Although this paradoxical fate has generated confusion, separating the biological actions of NO on the basis of its physiologic chemistry provides a conceptual framework which helps to distinguish between the beneficial and toxic consequences of NO, and to envision potential therapeutic strategies for the future. Under normal conditions, NO produced in low concentration acts as a messenger and cytoprotective (antioxidant) factor, via direct interactions with transition metals and other free radicals. Alternatively, when the circumstances allow the formation of substantial amounts of NO and modify the cellular microenvironment (formation of the superoxide radical), the chemistry of NO will turn into indirect effects consecutive to the formation of dinitrogen trioxide and peroxynitrite. These "reactive nitrogen species" will, in turn, mediate both oxidative and nitrosative stresses, which form the basis of the cytotoxicity generally attributed to NO, relevant to the pathophysiology of inflammation, circulatory shock, and ischemia-reperfusion injury.

Role of endothelin-1 in regulation of the postnatal intestinal circulation

Newborn intestine is uniquely prone to vasoconstriction in response to a wide variety of perturbations. To test the hypothesis that endothelin (ET)-1 is an important factor in this process, we determined the effects of exogenous ET-1 administration and blockade of endogenous ET-1 in vivo and in vitro in 3- and 35-day-old swine. Intramesenteric artery administration of exogenous ET-1 to vascularly isolated in vivo gut loops (10(-9) M/kg bolus) caused vasoconstriction and reduced gut O(2) uptake similarly in these age groups. Selective blockade of ET(A) or ET(B) receptors with BQ-610 or BQ-788, respectively, in vascularly isolated in vivo gut loops had no effect on gut vascular resistance or O(2) uptake in either age group; within in vitro gut loops, BQ-610 significantly increased vasoconstriction when perfusion pressure was reduced below baseline, but only in 3-day-old animals; i.e., it impaired the autoregulatory response to perfusion pressure reduction. Exogenous ET-1 significantly decreased capillary perfusion within in vitro gut loops, as evidenced by a decrease in capillary filtration coefficient, but only in 3-day-old animals; furthermore, blockade of endogenous ET-1 activity with BQ-610 significantly enhanced capillary filtration coefficient in 3-day-old animals and increased O(2) extraction ratio. ET-1 did not depress intestinal metabolic rate, as evidenced by its effect on the O(2) uptake-blood flow relationship; it did compromise tissue oxygenation because of its effects on intestinal O(2) transport. ET-1 concentration in mesenteric venous effluent exceeded arterial concentration, but only in 3-day-old intestine, suggesting production of ET-1 by newborn intestine. We conclude that ET-1 exerts an age-dependent effect on intestinal hemodynamics in postnatal intestine, having a greater impact in 3- than in 35-day-old intestine.

Tachyphylaxis to PACAP-27 after inhibition of NO synthesis: a loss of adenylate cyclase activation

The vasodilator effects of pituitary adenylate cyclase activating polypeptide (PACAP-27) are subject to tachyphylaxis in rats treated with the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). This study examined whether this tachyphylaxis is due to the loss of vasodilator potency of cAMP generated by activation of the G(s) protein-coupled PACAP receptors. Five successive treatments with PACAP-27 (2 nmol/kg iv) produced pronounced vasodilator responses in saline-treated rats that were not subject to tachyphylaxis. The first injection of PACAP-27 (2 nmol/kg iv) in L-NAME (50 micromol/kg iv)-treated rats produced vasodilator responses of similar magnitude to those in saline-treated rats, whereas four subsequent injections produced progressively and markedly smaller responses. The hemodynamic effects of the membrane-permeable cAMP analog 8-(4-chlorophenylthiol)-cAMP (8-CPT-cAMP; 5-15 micromol/kg iv) were similar in L-NAME-treated rats and in L-NAME-treated rats that had received the five injections of PACAP-27. In addition, five injections of 8-CPT-cAMP (10 micromol/kg iv) produced pronounced vasodilator responses in saline- and L-NAME-treated rats that were not subject to the development of tachyphylaxis. These results suggest that a loss of biological potency of cAMP is not responsible for tachyphylaxis to PACAP-27 in L-NAME-treated rats. This tachyphylaxis may be due to the inability of the G(s) protein-coupled PACAP receptor to activate adenylate cyclase.

Nitric oxide modulates beta(2)-adrenergic receptor palmitoylation and signaling

To determine whether nitric oxide (NO) modulates the beta-adrenergic signaling pathway, we treated cells expressing beta(2)-adrenergic receptors (beta(2)AR) with the NO donors, 3-morpholinosydnonimine (SIN-1) and 1,2,3,4-oxatriazolium, 5-amino-3-(3-chloro-2-methylphenyl)chloride and determined the intracellular production of cAMP after exposure to beta-adrenergic receptor agonists, cholera toxin and forskolin. NO significantly decreased the potency of the beta-adrenergic agonist, isoproterenol, to stimulate cAMP production without affecting the stimulatory action of forskolin and cholera toxin, which directly activate adenylyl cyclase and G(s), respectively. Treatment with the NO donor increased the guanyl nucleotide-sensitive high affinity constant for the agonist, isoproterenol, thus suggesting that it reduced functional coupling between the receptor and G(s). Stimulation of endogenous NO production by lipopolysaccharide in RAW 264.7 macrophages also caused a significant increase in the EC(50) for isoproterenol-stimulated cAMP production. SIN-1 treatment also led to a reduction in both basal and isoproterenol-stimulated incorporation of [(3)H]palmitate into the beta(2)AR. Signaling through the nonpalmitoylated, Gly(341)beta(2)AR mutant was unchanged by SIN-1 treatment. Given the link between beta(2)AR palmitoylation and its responsiveness to agonist, these results suggest that the primary action of NO was depalmitoylation of the beta(2)AR resulting in decreased signaling through the beta(2)AR.

Stimulation of bradykinin B2-receptors on endothelial cells induces relaxation and contraction in porcine basilar artery in vitro

1. The aim of the present study was to characterize the subtypes of bradykinin (BK) receptors that evoke the relaxation and contraction induced by BK and to identify the main contracting and relaxing factors in isolated porcine basilar artery by measuring changes in isometric tension and a thromboxane (TX) metabolite. 2. Endothelial denudation completely abolished both responses. [Thi5,8, D-Phe7]-BK (a B2-receptor antagonist) inhibited the BK-induced relaxation and contraction, whereas des-Arg9, [Leu8]-BK (a B1-receptor antagonist) had no effect. 3. L-nitro-arginine (L-NA, a nitric oxide synthase inhibitor) completely inhibited BK-induced relaxation. Indomethacin (a cyclo-oxygenase inhibitor) completely and ONO-3708 (a TXA2/prostaglandin H2 receptor antagonist) partially inhibited BK-induced contraction, whereas OKY-046 (a TXA2 synthase inhibitor) and nordihydroguaiaretic acid (a lipoxygenase inhibitor) did not. 4. In the presence of L-NA, the contractile response to BK was inhibited by indomethacin or ONO-3708 and was competitively antagonized by [Thi5,8, D-Phe7]-BK (pA2=7.50). In the presence of indomethacin, the relaxant response to BK was inhibited by L-NA and was competitively antagonized by [Thi5,8, D-Phe7]-BK (pA2=7.59). 5. TXA2 release was not induced by BK-stimulation. 6. These results suggest that the endothelium-dependent relaxation and contraction to BK in the porcine basilar artery is mediated via activation of endothelial B2-receptors. The main relaxing factor may be NO and the main contracting factor may be prostaglandin H2.

Rapid tachyphylaxis to hemodynamic effects of PACAP-27 after inhibition of nitric oxide synthesis

The vasodilator effects of pituitary adenylate cyclase-activating polypeptide (PACAP)-27 are subject to tachyphylaxis in rats treated with the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME). We examined whether this tachyphylaxis could be prevented by administration of the putative endothelium-derived nitrosyl factor S-nitroso-L-cysteine (L-SNC) and whether L-SNC may exert its effects via increases in cGMP levels in vascular smooth muscle. Five doses of PACAP-27 (2 nmol/kg iv) produced pronounced vasodilator responses in saline-treated rats. These responses were not subject to tachyphylaxis. The first injection of PACAP-27 (2 nmol/kg iv) in L-NAME-treated (50 micromol/kg iv) rats produced vasodilator responses similar to those in saline-treated rats, whereas subsequent injections produced progressively smaller responses. The injection of L-SNC (1,200 nmol/kg iv) before each injection of PACAP-27 prevented tachyphylaxis to the Gs protein-coupled receptor agonist in L-NAME-treated rats, whereas equihypotensive doses of the NO donor sodium nitroprusside (100 micrograms/kg iv) did not. The injection of the membrane-permeant cGMP analog 8-(4-chlorophenylthio)guanosine 3',5'-cyclic monophosphate (8-CPT-cGMP; 30 micromol/kg iv) to L-NAME-treated rats restored resting hemodynamic values to pre-L-NAME levels but did not prevent the development of tachyphylaxis to PACAP-27. These results suggest that nitrosyl factors prevent the development of tachyphylaxis to the hemodynamic actions of PACAP-27. These nitrosyl factors may act independently of their ability to generate cGMP in vascular smooth muscle.

Effects of sustained low-flow perfusion on the response to vasoconstrictor agents in postnatal intestine

This laboratory has previously reported that sustained reduction of blood flow in newborn intestine causes a triphasic increase in vascular resistance that occurs over 3-4 h and that these changes are mediated, in part, by loss of endothelial nitric oxide (NO) production. This study examines the effects of exposure to sustained low-flow perfusion on the subsequent response to three contractile agonists: ANG II, norepinephrine (NE), and endothelin-1 (ET-1). Gut loops from 3- and 35-day-old swine were exposed to low-flow conditions in vivo (i.e., reduction of flow to approximately 50% of baseline) for 30 min or 5 h. Thereafter, they were removed to an extracorporeal perfusion circuit for in vitro hemodynamic assessment; alternatively, the mesenteric artery perfusing the gut loop was removed and cut into rings for assessment of isometric tension development. Gut loops from 3-day-old subjects exposed to low-flow conditions demonstrated significantly increased contractile responses to ANG II, NE, and ET-1; also, mesenteric artery rings from these gut loops demonstrated a significant reduction of the ED50 for all three agonists. Similar changes were not observed in intestine or mesenteric artery rings from older subjects. Sustained blockade of endogenous NO synthesis with NG-monomethyl- L-arginine duplicated the effects of exposure to sustained low-flow perfusion. It appears that sustained reduction of blood flow in newborn intestine decreases constitutive NO production, which in turn causes a generalized enhancement of the contractile efficacy of ANG II, NE, and ET-1.