Adenylyl cyclase, a coincidence detector for nitric oxide

Adenylyl cyclase isoforms 5 and 6 in the cardiovascular system: complex regulation and divergent roles

Adenylyl cyclases (ACs) are crucial effector enzymes that transduce divergent signals from upstream receptor pathways and are responsible for catalyzing the conversion of ATP to cAMP. The ten AC isoforms are categorized into four main groups; the class III or calcium-inhibited family of ACs comprises AC5 and AC6. These enzymes are very closely related in structure and have a paucity of selective activators or inhibitors, making it difficult to distinguish them experimentally. AC5 and AC6 are highly expressed in the heart and vasculature, as well as the spinal cord and brain; AC6 is also abundant in the lungs, kidney, and liver. However, while AC5 and AC6 have similar expression patterns with some redundant functions, they have distinct physiological roles due to differing regulation and cAMP signaling compartmentation. AC5 is critical in cardiac and vascular function; AC6 is a key effector of vasodilatory pathways in vascular myocytes and is enriched in fetal/neonatal tissues. Expression of both AC5 and AC6 decreases in heart failure; however, AC5 disruption is cardio-protective, while overexpression of AC6 rescues cardiac function in cardiac injury. This is a comprehensive review of the complex regulation of AC5 and AC6 in the cardiovascular system, highlighting overexpression and knockout studies as well as transgenic models illuminating each enzyme and focusing on post-translational modifications that regulate their cellular localization and biological functions. We also describe pharmacological challenges in the design of isoform-selective activators or inhibitors for AC5 and AC6, which may be relevant to developing new therapeutic approaches for several cardiovascular diseases.

Nitric oxide augments signaling for contraction in hypoxic pulmonary arterial smooth muscle—Implications for hypoxic pulmonary hypertension

Introduction: Hypoxic persistent pulmonary hypertension in the newborn (PPHN) is usually treated with oxygen and inhaled nitric oxide (NO), both pulmonary arterial relaxants. But treatment failure with NO occurs in 25% of cases. We previously demonstrated that 72 h exposure to hypoxia, modeling PPHN, sensitized pulmonary artery smooth muscle cells (PASMC) to the contractile agonist thromboxane and inhibited relaxant adenylyl cyclase (AC) activity.

Methods: In this study, we examined the effects of sodium nitroprusside (SNP), as NO donor, on the thromboxane-mediated contraction and NO-independent relaxation pathways and on reactive oxygen species (ROS) accumulation in PASMC. In addition, we examined the effect of the peroxynitrite scavenger 5,10,15,20-Tetrakis (4-sulfonatophenyl)porphyrinato Iron (III) (FeTPPS) on these processes.

Results: Exposure of PASMC to 72 h hypoxia increased total intracellular ROS compared to normoxic control cells and this was mitigated by treatment of cells with either SNP or FeTPPS. Total protein nitrosylation was increased in hypoxic PASMC compared to controls. Both normoxic and hypoxic cells treated with SNP exhibited increased total protein nitrosylation and intracellular nitrite; this was reduced by treatment with FeTPPS. While cell viability and mitochondrial number were unchanged by hypoxia, mitochondrial activity was decreased compared to controls; addition of FeTPPS did not alter this. Basal and maximal mitochondrial metabolism and ATP turnover were reduced in hypoxic PASMC compared to controls. Hypoxic PASMC had higher basal Ca2+, and a heightened peak Ca2+ response to thromboxane challenge compared to controls. Addition of SNP further elevated the peak Ca2+ response, while addition of FeTPPS brought peak Ca2+ response down to control levels. AC mediated relaxation was impaired in hypoxic PASMC compared to controls but was normalized following treatment with FeTPPS. Addition of SNP inhibited adenylyl cyclase activity in both normoxic and hypoxic PASMC. Moreover, addition of the Ca2+ chelator BAPTA improved AC activity, but the effect was minimal.

Discussion: We conclude that NO independently augments contraction and inhibits relaxation pathways in hypoxic PASMC, in part by a mechanism involving nitrogen radical formation and protein nitrosylation. These observations may partially explain impaired effectiveness of NO when treating hypoxic pulmonary hypertension.

Critical cysteines in the functional interaction of adenylyl cyclase isoform 6 with Gαs

Activation of adenylyl cyclases (ACs) by G‐protein Gαs catalyzes the production of cyclic adenosine monophosphate (cAMP), a key second messenger that regulates diverse physiological responses. There are 10 AC isoforms present in humans, with AC5 and AC6 proposed to play vital roles in cardiac function. We have previously shown that under hypoxic conditions, AC6 is amenable to post‐translational modification by nitrosylation, resulting in decreased AC catalytic activity. Using a computational model of the AC6–Gαs complex, we predicted key nitrosylation‐amenable cysteine residues involved in the interaction of AC6 with Gαs and pursued a structure–function analysis of these cysteine residues in both AC6 and Gαs. Our results based on site‐directed mutagenesis of AC6 and Gαs, a constitutively active Gαs, AC activity, and live cell intracellular cAMP assays suggest that Cys1004 in AC6 (subunit C2) and Cys237 in Gαs are present at the AC–Gαs interface and are important for the activation of AC6 by Gαs. We further provide mechanistic evidence to show that mutating Cys 1004 in the second catalytic domain of AC6 makes it amenable to inhibition by Gαi, which may account for decreased functional activity of AC6 when this residue is unavailable.

Nitric oxide-targeted protein phosphorylation during human sperm capacitation

Among many other molecules, nitric oxide insures the correct progress of sperm capacitation by mediating phosphorylation events. For a more comprehensive understanding of how this happens, we capacitated human spermatozoa from healthy men in the presence/absence of S-Nitrosoglutathione, a nitric oxide donor, two nitric oxide synthase inhibitors, N^G-Nitro-l-arginine Methyl Ester Hydrochloride and Aminoguanidine Hemisulfate salt and, finally, with/without l-Arginine, the substrate for nitric oxide synthesis, and/or human follicular fluid. When analyzing the phosphorylation of protein kinase A substrates and tyrosine residues, we particularly observed how the inhibition of nitric oxide synthesis affects certain protein bands (~ 110, ~ 87, ~ 75 and ~ 62 kD) by lowering their phosphorylation degree, even when spermatozoa were incubated with l-Arginine and/or follicular fluid. Mass spectrometry analysis identified 29 proteins in these species, related to: spermatogenesis, binding to the zona pellucida, energy and metabolism, stress response, motility and structural organization, signaling and protein turnover. Significant changes in the phosphorylation degree of specific proteins could impair their biological activity and result in severe fertility-related phenotypes. These findings provide a deeper understanding of nitric oxide’s role in the capacitation process, and consequently, future studies in infertile patients should determine how nitric oxide mediates phosphorylation events in the species here described.

Gαi1 inhibition mechanism of ATP-bound adenylyl cyclase type 5

Conversion of adenosine triphosphate (ATP) to the second messenger cyclic adenosine monophosphate (cAMP) is an essential reaction mechanism that takes place in eukaryotes, triggering a variety of signal transduction pathways. ATP conversion is catalyzed by the enzyme adenylyl cyclase (AC), which can be regulated by binding inhibitory, Gα_i, and stimulatory, Gα_s subunits. In the past twenty years, several crystal structures of AC in isolated form and complexed to Gα_s subunits have been resolved. Nevertheless, the molecular basis of the inhibition mechanism of AC, induced by Gα_i, is still far from being fully understood. Here, classical molecular dynamics simulations of the isolated holo AC protein type 5 and the holo binary complex AC5:Gα_i have been analyzed to investigate the conformational impact of Gα_i association on ATP-bound AC5. The results show that Gα_i appears to inhibit the activity of AC5 by preventing the formation of a reactive ATP conformation.

Involvement of nitric oxide during in vitro oocyte maturation, sperm capacitation and in vitro fertilization in pig

The importance of porcine species for meat production is undeniable. Due to the genetic, anatomical, and physiological similarities with humans, from a biomedical point of view, pig is considered an ideal animal model for the study and development of new therapies for human diseases. The in vitro production (IVP) of porcine embryos has become widespread as a result of these qualities and there is significant demand for these embryos for research purposes. However, the efficiency of porcine embryo IVP remains very low, which hinders its use as a model for research. The high degree of polyspermic fertilization is the main problem that affects in vitro fertilization (IVF) in porcine species. Furthermore, oocyte in vitro maturation (IVM) is another important step that could be related to polyspermic fertilization and low embryo production. The presence of nitric oxide synthase (NOS), the enzyme that produces nitric oxide (NO), has been detected in the oviduct, the ovary, the oocyte and the sperm cell of porcine species. Its functions include regulating oviductal activity, ovulation, acquisition of meiotic competence, oocyte activation, sperm capacitation, and gamete interaction. Therefore, in this review, we summarize the current knowledge on the role of NO/NOS system in each of the steps that lead to the production of porcine embryos in an in vitro environment, i.e. IVM, sperm capacitation, IVF, and embryo culture. We also discuss the possible ways in which the NO/NOS system could be used to enhance IVP of porcine embryos.

The Roles of NO and H2S in Sperm Biology: Recent Advances and New Perspectives

After being historically considered as noxious agents, nitric oxide (NO) and hydrogen sulfide (H2S) are now listed as gasotransmitters, gaseous molecules that play a key role in a variety of cellular functions. Both NO and H2S are endogenously produced, enzymatically or non-enzymatically, and interact with each other in a range of cells and tissues. In spite of the great advances achieved in recent decades in other biological systems, knowledge about H2S function and interactions with NO in sperm biology is in its infancy. Here, we aim to provide an update on the importance of these molecules in the physiology of the male gamete. Special emphasis is given to the most recent advances in the metabolism, mechanisms of action, and effects (both physiological and pathophysiological) of these gasotransmitters. This manuscript also illustrates the physiological implications of NO and H2S observed in other cell types, which might be important for sperm function. The relevance of these gasotransmitters to several signaling pathways within sperm cells highlights their potential use for the improvement and successful application of assisted reproductive technologies.

Regulation of adenylyl cyclase 5 in striatal neurons confers the ability to detect coincident neuromodulatory signals

Long-term potentiation and depression of synaptic activity in response to stimuli is a key factor in reinforcement learning. Strengthening of the corticostriatal synapses depends on the second messenger cAMP, whose synthesis is catalysed by the enzyme adenylyl cyclase 5 (AC5), which is itself regulated by the stimulatory Gα_olf and inhibitory Gα_i proteins. AC isoforms have been suggested to act as coincidence detectors, promoting cellular responses only when convergent regulatory signals occur close in time. However, the mechanism for this is currently unclear, and seems to lie in their diverse regulation patterns. Despite attempts to isolate the ternary complex, it is not known if Gα_olf and Gα_i can bind to AC5 simultaneously, nor what activity the complex would have. Using protein structure-based molecular dynamics simulations, we show that this complex is stable and inactive. These simulations, along with Brownian dynamics simulations to estimate protein association rates constants, constrain a kinetic model that shows that the presence of this ternary inactive complex is crucial for AC5’s ability to detect coincident signals, producing a synergistic increase in cAMP. These results reveal some of the prerequisites for corticostriatal synaptic plasticity, and explain recent experimental data on cAMP concentrations following receptor activation. Moreover, they provide insights into the regulatory mechanisms that control signal processing by different AC isoforms.

Adenylyl cyclases (ACs) are enzymes that can translate extracellular signals into the intracellular molecule cAMP, which is thus a 2nd messenger of extracellular events. The brain expresses nine membrane-bound AC variants, and AC5 is the dominant form in the striatum. The striatum is the input stage of the basal ganglia, a brain structure involved in reward learning, i.e. the learning of behaviors that lead to rewarding stimuli (such as food, water, sugar, etc). During reward learning, cAMP production is crucial for strengthening the synapses from cortical neurons onto the striatal principal neurons, and its formation is dependent on several neuromodulatory systems such as dopamine and acetylcholine. It is, however, not understood how AC5 is activated by transient (subsecond) changes in the neuromodulatory signals. Here we combine several computational tools, from molecular dynamics and Brownian dynamics simulations to bioinformatics approaches, to inform and constrain a kinetic model of the AC5-dependent signaling system. We use this model to show how the specific molecular properties of AC5 can detect particular combinations of co-occuring transient changes in the neuromodulatory signals which thus result in a supralinear/synergistic cAMP production. Our results also provide insights into the computational capabilities of the different AC isoforms.

Association of Both Inhibitory and Stimulatory Gα Subunits Implies Adenylyl Cyclase 5 Deactivation

Adenylyl cyclase (AC) generates cyclic AMP required for a variety of cellular functions, and its regulation plays a major role in cellular signal transduction in eukaryotes and prokaryotes. All membrane-bound AC isoforms in eukaryotes can be activated by stimulatory G-proteins, but only AC1, AC5, and AC6 can be both stimulated and inhibited by active Gα subunits, Gα_s and Gα_i, respectively. In principle, these Gα_i-sensitive AC isoforms could form both binary and ternary complexes with Gα subunits due to the noncompetitive association of inhibitory and stimulatory Gα. However, the formation and possible catalytic activity of a putative ternary complex have not yet been experimentally confirmed due to its proposed short-lived nature. Here, the catalytic activity of such a ternary complex consisting of apo AC5, stimulatory Gα_olf, and inhibitory Gα_i1 is investigated via classical molecular dynamics simulations. Trajectories of inhibited and stimulated binary complexes, AC5:Gα_i1 and AC5:Gα_olf, respectively, as well as Gα-free AC5 were also obtained to compare the sampled AC5 conformation in the ternary complex to those sampled under different Gα conditions. This comparison suggests that association of both Gα subunits results in an AC5 conformation similar to that sampled by the AC5:Gα_i1 complex, indicating that the ternary complex mainly samples an inactive conformation.

Nitric oxide modulates μ-opioid receptor function in vitro

Painful diabetic neuropathy (PDN) is a type of peripheral neuropathic pain that develops as a consequence of prolonged hyperglycaemia-induced injury to the long nerves. Apart from pain, PDN is also characterized by morphine hyposensitivity. Intriguingly, in streptozotocin (STZ)-induced diabetic rats exhibiting marked morphine hyposensitivity, dietary administration of the nitric oxide (NO) precursor, L-arginine at 1 g/d, progressively rescued morphine efficacy and potency over an 8-week treatment period. In earlier work, single bolus doses of the furoxan nitric oxide (NO) donor, PRG150 (3-methylfuroxan-4-carbaldehyde), evoked dose-dependent pain relief in STZ-diabetic rats but the efficacious doses were 3-4 orders of magnitude higher in advanced diabetes than that required in early STZ diabetes. Together, these findings suggested a role for NO in the modulation of μ-opioid (MOP) receptor signalling. Therefore, the present study was designed to assess a role for NO released from PRG150, in modulating MOP receptor function in vitro. Here, we show an absolute requirement for the MOP receptor, but not the δ-opioid (DOP) or the κ-opioid (KOP) receptor, to transduce the cellular effects of PRG150 on forskolin-stimulated cAMP responses in vitro. PRG150 did not interact with the classical naloxone-sensitive binding site of the MOP receptor, and its effects on cAMP responses in HEK-MOP cells were also naloxone-insensitive. Nevertheless, the inhibitory effects of PRG150 on forskolin-stimulated cAMP responses in HEK-MOP cells were dependent upon pertussis toxin (PTX)-sensitive G_i/o proteins as well as membrane lipid rafts and src kinase. Together, our findings implicate a role for NO in modulating MOP receptor function in vivo.

CB1 cannabinoid receptor-phosphorylated fourth intracellular loop structure-function relationships

A peptide comprising the juxtamembrane C-terminal intracellular loop 4 (IL4) of the CB₁ cannabinoid receptor possesses three Serine residues (Ser402, Ser411 and Ser415). Here we report the effect of Ser phosphorylation on the CB₁ IL4 peptide conformation and cellular signaling functions using nuclear magnetic resonance spectroscopy, circular dichroism, G protein activation and cAMP production. Circular dichroism studies indicated that phosphorylation at various Ser residues induced helical structure in different environments. NMR data indicates that helical content varies in the order of IL4pSer411 > IL4pSer415 > IL4 > IL4pSer402. The efficacy of phosphorylated IL4 peptides in activating Go and Gi3 ([³⁵S]GTPγS binding) and inhibiting cAMP accumulation in N18TG2 cells were correlated with helicity changes. Treatment of cells with bradykinin, which activates PKC, augmented CB₁-mediated inhibition of cAMP accumulation, and this was reversed by a PKC inhibitor, suggesting that phosphorylation of serine might be a physiologically relevant modification in vivo. We conclude that phosphorylation-dependent alterations of helicity of CB₁ IL4 peptides can increase efficacy of G protein signaling.

Hypoxia inhibits adenylyl cyclase catalytic activity in a porcine model of persistent pulmonary hypertension of the newborn

Persistent pulmonary hypertension of the newborn (PPHN) features hypoxemia, pulmonary vasoconstriction, and impaired cardiac inotropy. We previously reported low basal and stimulated cAMP in hypoxic pulmonary artery smooth muscle cells (PASMCs). We now examine pulmonary arterial adenylyl cyclase (AC) activity and regulation in hypoxic PPHN. PPHN was induced in newborn swine by normobaric hypoxia (fraction of inspired oxygen 0.10) for 72 h and compared with age-matched normoxic controls. We studied relaxation of pulmonary arterial (PA) rings to AC activator forskolin and cGMP activator sodium nitroprusside (SNP) by isometric myography, ATP content, phosphodiesterase activity, AC content, isoform expression, and catalytic activity in presence or absence of Gαs-coupled receptor agonists, forskolin, or transnitrosylating agents in human and neonatal porcine PASMCs and HEK293T stably expressing AC isoform 6, after 72 h hypoxia (10% O₂) or normoxia (21% O₂). Relaxation to forskolin and SNP were equally impaired in PPHN PA. AC-specific activity decreased in hypoxia. PASMC from PPHN swine had reduced AC activity despite exposure to normoxia in culture; transient hypoxia in vitro further decreased AC activity. Prostacyclin receptor ligand affinity decreased, but its association with Gαs increased in hypoxia. Total AC content was unchanged by hypoxia, but AC6 increased in hypoxic cells and PPHN pulmonary arteries. Impairment of AC6 activity in hypoxia was associated with nitrosylation. PPHN PA relaxation is impaired because of loss of AC activity. Hypoxic AC is inhibited because of S-nitrosylation; inhibition persists after removal from hypoxia. Downregulation of AC-mediated relaxation in hypoxic PA has implications for utility of Gαs-coupled receptor agonists in PPHN treatment.

International Union of Basic and Clinical Pharmacology. CI. Structures and Small Molecule Modulators of Mammalian Adenylyl Cyclases

Adenylyl cyclases (ACs) generate the second messenger cAMP from ATP. Mammalian cells express nine transmembrane AC (mAC) isoforms (AC1-9) and a soluble AC (sAC, also referred to as AC10). This review will largely focus on mACs. mACs are activated by the G-protein Gαs and regulated by multiple mechanisms. mACs are differentially expressed in tissues and regulate numerous and diverse cell functions. mACs localize in distinct membrane compartments and form signaling complexes. sAC is activated by bicarbonate with physiologic roles first described in testis. Crystal structures of the catalytic core of a hybrid mAC and sAC are available. These structures provide detailed insights into the catalytic mechanism and constitute the basis for the development of isoform-selective activators and inhibitors. Although potent competitive and noncompetitive mAC inhibitors are available, it is challenging to obtain compounds with high isoform selectivity due to the conservation of the catalytic core. Accordingly, caution must be exerted with the interpretation of intact-cell studies. The development of isoform-selective activators, the plant diterpene forskolin being the starting compound, has been equally challenging. There is no known endogenous ligand for the forskolin binding site. Recently, development of selective sAC inhibitors was reported. An emerging field is the association of AC gene polymorphisms with human diseases. For example, mutations in the AC5 gene (ADCY5) cause hyperkinetic extrapyramidal motor disorders. Overall, in contrast to the guanylyl cyclase field, our understanding of the (patho)physiology of AC isoforms and the development of clinically useful drugs targeting ACs is still in its infancy.

Cannabinoid receptor interacting protein (CRIP1a) attenuates CB1R signaling in neuronal cells

CB1 cannabinoid receptors (CB1R) are one of the most abundantly expressed G protein coupled receptors (GPCR) in the CNS and regulate diverse neuronal functions. The identification of GPCR interacting proteins has provided additional insight into the fine-tuning and regulation of numerous GPCRs. The cannabinoid receptor interacting protein 1a (CRIP1a) binds to the distal carboxy terminus of CB1R, and has been shown to alter CB1R-mediated neuronal function [1]. The mechanisms by which CRIP1a regulates CB1R activity have not yet been identified; therefore the focus of this investigation is to examine the cellular effects of CRIP1a on CB1R signaling using neuronal N18TG2 cells stably transfected with CRIP1a over-expressing and CRIP1a knockdown constructs. Modulation of endogenous CRIP1a expression did not alter total levels of CB1R, ERK, or forskolin-activated adenylyl cyclase activity. When compared to WT cells, CRIP1a over-expression reduced basal phosphoERK levels, whereas depletion of CRIP1a augmented basal phosphoERK levels. Stimulation of phosphoERK by the CB1R agonists WIN55212-2, CP55940 or methanandamide was unaltered in CRIP1a over-expressing clones compared with WT. However, CRIP1a knockdown clones exhibited enhanced ERK phosphorylation efficacy in response to CP55940. In addition, CRIP1a knockdown clones displayed a leftward shift in CP55940-mediated inhibition of forskolin-stimulated cAMP accumulation. CB1R-mediated Gi3 and Go activation by CP99540 was attenuated by CRIP1a over-expression, but robustly enhanced in cells depleted of CRIP1a. Conversely, CP55940-mediated Gi1 and Gi2 activation was significant enhanced in cells over-expressing CRIP1a, but not in cells deficient of CRIP1a. These studies suggest a mechanism by which endogenous levels of CRIP1a modulate CB1R-mediated signal transduction by facilitating a Gi/o protein subtype preference for Gi1 and Gi2, accompanied by an overall suppression of G-protein-mediated signaling in neuronal cells.

Adenylyl cyclase VI mediates vasopressin-stimulated ENaC activity

Vasopressin modulates sodium reabsorption in the collecting duct through adenylyl cyclase-stimulated cyclic AMP, which exists as multiple isoforms; the specific isoform involved in vasopressin-stimulated sodium transport is unknown. To assess this, we studied mice deficient in adenylyl cyclase type VI specifically in the principal cells of the collecting duct. Knockout mice had increased urine volume and reduced urine sodium concentration, but regardless of the level of sodium intake, they did not exhibit significant alterations in urinary sodium excretion, arterial pressure, or pulse rate. Plasma renin concentration was elevated in knockout mice, however, suggesting a compensatory response. Valsartan significantly reduced arterial pressure in knockout mice but not in controls. Knockout mice had decreased renal cortical mRNA content of all three epithelial sodium channel (ENaC) isoforms, and total cell sodium channel isoforms α and γ were reduced in these animals. Patch-clamp analysis of split-open cortical collecting ducts revealed no difference in baseline activity of sodium channels, but knockout mice had abolished vasopressin-stimulated ENaC open probability and apical membrane channel number. In summary, these data suggest that adenylyl cyclase VI mediates vasopressin-stimulated ENaC activity in the kidney.

Cannabinoid CB1 receptors transactivate multiple receptor tyrosine kinases and regulate serine/threonine kinases to activate ERK in neuronal cells

BACKGROUND AND PURPOSE

Signalling networks that regulate the progression of cannabinoid CB(1) receptor-mediated extracellular signal-regulated kinase (ERK) activation in neurons are poorly understood. We investigated the cellular mechanisms involved in CB(1) receptor-stimulated ERK phosphorylation in a neuronal cell model.

EXPERIMENTAL APPROACH

Murine N18TG2 neuronal cells were used to analyse the effect of specific protein kinase and phosphatase inhibitors on CB(1) receptor-stimulated ERK phosphorylation. The LI-COR In Cell Western assay and immunoblotting were used to measure ERK phosphorylation.

KEY RESULTS

The time-course of CB(1) receptor-stimulated ERK activation occurs in three phases that are regulated by distinct cellular mechanisms in N18TG2 cells. Phase I (0-5 min) maximal ERK phosphorylation is mediated by CB(1) receptor-stimulated ligand-independent transactivation of multiple receptor tyrosine kinases (RTKs). Phase I requires G(i/o) βγ subunit-stimulated phosphatidylinositol 3-kinase activation and Src kinase activation and is modulated by inhibition of cAMP-activated protein kinase A (PKA) levels. Src kinase activation is regulated by the protein tyrosine phosphatases 1B and Shp1. The Phase II (5-10 min) rapid decline in ERK phosphorylation involves PKA inhibition and serine/threonine phosphatase PP1/PP2A activation. The Phase III (>10 min) plateau in ERK phosphorylation is mediated by CB(1) receptor-stimulated, ligand-independent, transactivation of multiple RTKs.

CONCLUSIONS AND IMPLICATIONS

The complex expression of CB(1) receptor-stimulated ERK activation provides cellular selectivity, modulation of sensitivity to agonists, and coincidence detection with RTK signalling. RTK and PKA pathways may provide routes to novel CB(1) -based therapeutic interventions in the treatment of addictive disorders or neurodegenerative diseases. LINKED ARTICLES This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Pendrin protein abundance in the kidney is regulated by nitric oxide and cAMP

Pendrin is a Cl(-)/HCO(3)(-) exchanger, expressed in the apical regions of some intercalated cell subtypes, and is critical in the pressor response to angiotensin II. Since angiotensin type 1 receptor inhibitors reduce renal pendrin protein abundance in mice in vivo through a mechanism that is dependent on nitric oxide (NO), we asked if NO modulates renal pendrin expression in vitro and explored the mechanism by which it occurs. Thus we quantified pendrin protein abundance by confocal fluorescent microscopy in cultured mouse cortical collecting ducts (CCDs) and connecting tubules (CNTs). After overnight culture, CCDs maintain their tubular structure and maintain a solute gradient when perfused in vitro. Pendrin protein abundance increased 67% in CNT and 53% in CCD when NO synthase was inhibited (N(G)-nitro-L-arginine methyl ester, 100 μM), while NO donor (DETA NONOate, 200 μM) application reduced pendrin protein by ∼33% in the CCD and CNT. When CNTs were cultured in the presence of the guanylyl cyclase inhibitor 1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one (10 μM), NO donors did not alter pendrin abundance. Conversely, pendrin protein abundance rose when cAMP content was increased by the application of an adenylyl cyclase agonist (forskolin, 10 μM), a cAMP analog (8-bromo-cAMP, 1 mM), or a phosphodiesterase inhibitor (BAY60-7550, 50 μM). Since NO reduces cellular cAMP in the CNT, we asked if NO reduces pendrin abundance by reducing cAMP. With blockade of cGMP-stimulated phosphodiesterase II, NO did not alter pendrin protein abundance. We conclude that NO acts through cAMP to reduce pendrin total protein abundance by enhancing cAMP degradation.

Adenylyl cyclase 2 selectively couples to E prostanoid type 2 receptors, whereas adenylyl cyclase 3 is not receptor-regulated in airway smooth muscle

Adenylyl cyclases (ACs) are important regulators of airway smooth muscle function, because β-adrenergic receptor (βAR) agonists stimulate AC activity and cAMP production. We have previously shown in a number of cell types that AC6 selectively couples to βAR and these proteins are coexpressed in lipid rafts. We overexpressed AC2, AC3, and AC6 in mouse bronchial smooth muscle cells (mBSMCs) and human embryonic kidney (HEK)-293 cells by using recombinant adenoviruses and assessed their localization and regulation by various G protein-coupled receptors (GPCRs). AC3 and AC6 were expressed primarily in caveolin-rich fractions, whereas AC2 expression was excluded from these domains. AC6 expression enhanced cAMP production in response to isoproterenol but did not increase responses to butaprost, reflecting the colocalization of AC6 with β(2)AR but not E prostanoid type 2 receptor (EP(2)R) in lipid raft fractions. AC2 expression enhanced butaprost-stimulated cAMP production but had no effect on the β(2)AR-mediated response. AC3 did not couple to any GPCR tested. Forskolin-induced arborization of mBSMCs was assessed as a functional readout of cAMP signaling. Arborization was enhanced by overexpression of AC6 and AC3, but AC2 had no effect. GPCR-stimulated arborization mirrored the selective coupling observed for cAMP production. With the addition of the phosphodiesterase 4 (PDE4) inhibitor rolipram AC2 accelerated forskolin-stimulated arborization. Thus, AC2 selectively couples to EP(2)R, but signals from this complex are limited by PDE4 activity. AC3 does not seem to couple to GPCR in either mBSMCs or HEK-293 cells, so it probably exists in a distinct signaling domain in these cells.

Human bronchial smooth muscle cells express adenylyl cyclase isoforms 2, 4, and 6 in distinct membrane microdomains

Adenylyl cyclases (AC) are important regulators of airway smooth muscle function, because β-adrenergic receptor (AR) agonists stimulate AC activity and increase airway diameter. We assessed expression of AC isoforms in human bronchial smooth muscle cells (hBSMC). Reverse transcriptase-polymerase chain reaction and immunoblot analyses detected expression of AC2, AC4, and AC6. Forskolin-stimulated AC activity in membranes from hBSMC displayed Ca(2+)-inhibited and G(βγ)-stimulated AC activity, consistent with expression of AC6, AC2, and AC4. Isoproterenol-stimulated AC activity was inhibited by Ca(2+) but unaltered by G(βγ), whereas butaprost-stimulated AC activity was stimulated by G(βγ) but unaffected by Ca(2+) addition. Using sucrose density centrifugation to isolate lipid raft fractions, we found that only AC6 localized in lipid raft fractions, whereas AC2 and AC4 localized in nonraft fractions. Immunoisolation of caveolae using caveolin-1 antibodies yielded Ca(2+)-inhibited AC activity (consistent with AC6 expression), whereas the nonprecipitated material displayed G(βγ)-stimulated AC activity (consistent with expression of AC2 and/or AC4). Overexpression of AC6 enhanced cAMP production in response to isoproterenol and beraprost but did not increase responses to prostaglandin E(2) or butaprost. β(2)AR, but not prostanoid EP(2) or EP(4) receptors, colocalized with AC5/6 in lipid raft fractions. Thus, particular G protein-coupled receptors couple to discreet AC isoforms based, in part, on their colocalization in membrane microdomains. These different cAMP signaling compartments in airway smooth muscle cells are responsive to different hormones and neurotransmitters and can be regulated by different coincident signals such as Ca(2+) and G(βγ).

Characterization of vasopressin-responsive collecting duct adenylyl cyclases in the mouse

Little is known about collecting duct adenylyl cyclase (AC) isoforms or regulation in the mouse. We performed RT-PCR for AC isoforms 1-9 in microdissected cortical (CCD) and outer medullary (OMCD) and acutely isolated inner medullary (IMCD) collecting duct. All collecting duct regions contained AC3, AC4, and AC6 mRNA, while CCD and OMCD, but not IMCD, also contained AC5 mRNA. Acutely isolated IMCD expressed AC3, AC4, and AC6 proteins by Western blot analysis. The mIMCD3 cell line expressed AC2, AC3, AC4, AC5, and AC6 mRNA; M-1 CCD cells expressed AC2, 3, 4, and 6, while mpkCCD cell lines contained AC3, AC4, and AC6 mRNA. AVP stimulated cAMP accumulation in acutely isolated mouse IMCD; this was reduced by chelation of extracellular calcium (EGTA) and almost completely abolished by blockade of calmodulin (W-7). Blockade of calmodulin kinase with KN-93 or endoplasmic reticulum calcium ATPase (thapsigargin) also reduced the AVP response. A similar inhibitory effect of W-7, KN-93, and thapsigargin was seen on forskolin-stimulated cAMP content in acutely isolated mouse IMCD. These three agents had the same pattern of blockade of AVP- or forskolin-stimulated AC activity in acutely isolated rat IMCD. AVP responsiveness in primary cultures of mouse IMCD was also reduced by W-7, KN-93, and thapsigargin. Small interfering RNA (siRNA) designed to knock down AC3 or AC6 in primary cultured mouse IMCD significantly reduced AVP-stimulated cAMP accumulation. Together, these data are consistent with a role of AC3 and AC6 in the activation of mouse collecting duct by AVP.

Influence of intramolecular electron transfer mechanism in biological nitration, nitrosation, and oxidation of redox-sensitive amino acids

Using both high-performance liquid chromatography (HPLC) and electron spin resonance (ESR) spin-trappng techniques, we developed an analytical methodology for investigating intramolecular electron transfer-mediated tyrosyl nitration and cysteine nitrosation in model peptides. Peptides N-acetyl-TyrCys-amide (YC), N-acetyl-TyrAlaCys-amide, N-acetyl-TyrAlaAlaCys-amide, and N-acetyl-TyrAlaAlaAlaAlaCys-amide were used as models. Product analysis showed that nitration and oxidation products derived from YC and related peptides in the presence of myeloperoxidase (MPO)/H(2)O(2)/NO(2)(-) were not detectable. The major product was determined to be the corresponding disulfide (e.g., YCysCysY), suggestive of a rapid electron transfer from the tyrosyl radical to the cysteinyl residue. ESR spin-trapping experiments with 5,5'-dimethyl-1-pyrroline N-oxide (DMPO) demonstrated that thiyl radical intermediates were formed from peptides (e.g., YC) treated with MPO/H(2)O(2) and MPO/H(2)O(2)/NO(2)(-). Blocking the thiol group in YC totally abrogated thiyl radical formation. Under similar conditions, we were, however, able to trap the tyrosyl radical using the spin trap dibromonitrosobenzene sulfonic acid (DBNBS). Competition spin-trapping experiments revealed that intramolecular electron transfer is the dominant mechanism for thiyl radical formation in YC peptides. We conclude that a rapid intramolecular electron transfer mechanism between redox-sensitive amino acids could influence both protein nitration and nitrosation reactions. This mechanism brings together nitrative, nitrosative, and oxidative mechanisms in free radical biology.

Production of nitric oxide by carp (Cyprinus carpio L.) kidney leukocytes is regulated by cyclic 3',5'-adenosine monophosphate

The inducible nitric oxide synthase (iNOS) plays a central role in the inflammatory reactions that follow infection or tissue damage. Induction of nitric oxide (NO) synthesis by bacterial lipopolysaccharide (LPS) depends on activation of G protein-coupled receptors in mammals. Thus, it was our intention to evaluate whether similar mechanisms are involved in iNOS activation in fish leukocytes. Therefore, the participation of membrane-bound receptors which activate effectors via G proteins has been confirmed using the G protein inhibitor suramin. Furthermore, the NO produced by iNOS performs both beneficial and detrimental actions. It is thus conceivable that regulatory mechanisms exist which control the timing and intensity of NO production by iNOS in order to outweigh protective effects against detrimental ones. The second messenger cAMP produced by adenylyl cyclases (ACs) plays a key role in the regulation of many cellular functions. Since cAMP signaling inhibits numerous immunological reactions, studies have been carried out to determine whether cAMP-dependent pathways could inhibit NO production by carp leukocytes as well. To measure cellular responses such as NO production by carp leukocytes derived from head and trunk kidneys treatments were performed with the cAMP elevating agents forskolin and dibutyryl-cAMP (db-cAMP) prior to stimulation with Aeromonas hydrophila. Pharmacological studies in stimulated kidney leukocytes showed that increased intracellular cAMP levels lead to reduced NO formation. This reduction of NO production was not due to decreased cell numbers, since a tetrazolium dye-based assay revealed no reduction of cell viability by cyclic nucleotide elevating agents. Thus, our data provide evidence that the AC/cAMP signaling pathway is well established in carp leukocytes. Cyclic AMP leads to type II immune response. We provide evidence that the predominant AC in fish leukocytes is a particulate enzyme due to its sensitivity to forskolin. Treatment of leukocytes with agents increasing intracellular cAMP gave clear evidence for participation of this cyclic nucleotide in immune signaling.

Decreased beta-adrenoceptor chronotropic response in selenium-deficient mice: negative crosstalk between iNOS activity and cAMP accumulation

With the aim to study if selenium (Se) deficiency affects the basal frequency and cardiac response to isoproterenol (ISO), mice were fed a Se-deficient diet (Se-) or the same diet supplemented with 0.2 ppm Se as sodium selenite (Se+) for 4 wk. Atria frequency, cyclic AMP (cAMP) accumulation, nitric oxide synthase (NOS) activity, and beta-adrenoceptor-binding assay were then examined. Results showed that Se-mice have both a reduction in atria frequency as well as in cAMP content but higher NOS activity levels either at basal or after ISO stimulation. These differences were suppressed by feeding Se-mice with a Se-supplemented diet for 1 wk or by inhibition of inducible nitric oxide synthase (iNOS). Alterations observed after ISO stimulation in atria of Se-mice were not related to a beta-adrenoceptor expression modification because specific radioligand-binding parameters in cardiac membranes from Se-mice and Se+ mice were similar. The reduced response on rate and cAMP in atria from Se-mice to direct adenylate cyclase (AC) stimulation by forskolin and the shifted upward levels present in 2-amino-4-methylpyridine-treated Se-mice is in agreement with a negative crosstalk between iNOS activity and AC activity in Se-mice.

Nitric oxide potentiates cAMP-gated cation current in feeding neurons of Pleurobranchaea californica independent of cAMP and cGMP signaling pathways

Critical roles for nitric oxide (NO) in regulating cell and tissue physiology are broadly appreciated, but aspects remain to be explored. In the mollusk Pleurobranchaea, NO synthase activity is high in CNS ganglia containing motor networks for feeding and locomotion, where a cAMP-gated cation current (I(Na,cAMP)) is also prominent in many neurons. We examined effects of NO on I(Na,cAMP) using voltage-clamp methods developed to analyze cAMP signaling in the live neuron, focusing on the identified metacerebral giant neuron of the feeding network. NO donors enhanced the I(Na,cAMP) response to injected cAMP by an averaged 85%. In dose-response measures, NO increased the current stimulated by cAMP injection without altering either apparent cAMP binding affinity or cooperativity of current activation. NO did not detectably alter levels of native cAMP or synthesis or degradation rates as observable in both current saturation and decay rate of I(Na,cAMP) responses to cAMP injection. NO actions were not exerted by cGMP signaling, as they were not mimicked by cGMP analogue nor blocked by inhibitors of guanylate cyclase and protein kinase G. NO potentiation of I(Na,cAMP) was broadly distributed among many other neurons of the feeding motor network in the buccal ganglion. However, NO did not affect a second type of I(Na,cAMP) found in locomotor neurons of the pedal ganglia. These results suggest that NO acts through a novel mechanism to regulate the gain of cAMP-dependent neuromodulatory pathways that activate I(Na,cAMP) and may thereby affect the set points of feeding network excitability and reactivity to exogenous input.

Regulatory properties of adenylate cyclases type 5 and 6: A progress report

Adenylate cyclases (AC) type 5 and 6 comprise the calcium-inhibited family of adenylate cyclase isoforms. Here we review recent discoveries in the regulation of AC5 and AC6 with a focus on posttranslational modifications including glycosylation, nitrosylation, and phosphorylation by the cyclic AMP-dependent protein kinase (PKA), protein kinase C (PKC), and Raf1. We also describe novel signaling interactions such as Galpha(q)-mediated potentiation of AC6 activation. Novel regulators of AC5 and AC6, including small molecules and proteins that physically interact with AC5 and AC6 such as snapin, regulator of G protein signaling 2 (RGS2), protein associated with myc (PAM), and caveolin peptides are discussed. We also describe several recent studies that demonstrate the usefulness of transgenic or adenoviral overexpression of AC5 and AC6 in models for disease states such as cardiovascular hypertrophy. The discovery of novel regulatory mechanisms for AC5 and AC6 and their potential role in crucial physiological processes provide new avenues for research into therapeutic interventions targeting the cyclic AMP pathway.

Intramolecular Electron Transfer between Tyrosyl Radical and Cysteine Residue Inhibits Tyrosine Nitration and Induces Thiyl Radical Formation in Model Peptides Treated with Myeloperoxidase, H2O2, and NO2-

We investigated the effects of a cysteine residue on tyrosine nitration in several model peptides treated with myeloperoxidase (MPO), H(2)O(2), and nitrite anion (NO(2)(-)) and with horseradish peroxidase and H(2)O(2). Sequences of model peptides were acetyl-Tyr-Cys-amide (YC), acetyl-Tyr-Ala-Cys-amide (YAC), acetyl-Tyr-Ala-Ala-Cys-amide (YAAC), and acetyl-Tyr-Ala-Ala-Ala-Ala-Cys-amide (YAAAAC). Results indicate that nitration and oxidation products of tyrosyl residue in YC and other model peptides were barely detectable. A major product detected was the corresponding disulfide (e.g. YCysCysY). Spin trapping experiments with 5,5'-dimethyl-1-pyrroline N-oxide (DMPO) revealed thiyl adduct (e.g. DMPO-SCys-Tyr) formation from peptides (e.g. YC) treated with MPO/H(2)O(2) and MPO/H(2)O(2)/NO(2)(-). The steady-state concentrations of DMPO-thiyl adducts decreased with increasing chain length of model peptides. Blocking the sulfydryl group in YC with methylmethanethiosulfonate (that formed YCSSCH(3)) totally inhibited thiyl radical formation as did substitution of Tyr with Phe (i.e. FC) in the presence of MPO/H(2)O(2)/NO(2)(-). However, increased tyrosine nitration, tyrosine dimerization, and tyrosyl radical formation were detected in the MPO/H(2)O(2)/NO(2)(-)/YCSSCH(3) system. Increased formation of S-nitrosated YC (YCysNO) was detected in the MPO/H(2)O(2)/(*)NO system. We conclude that a rapid intramolecular electron transfer reaction between the tyrosyl radical and the Cys residue impedes tyrosine nitration and induces corresponding thiyl radical and nitrosocysteine product. Implications of this novel intramolecular electron transfer mechanism in protein nitration and nitrosation are discussed.

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.

Dibutyryl cyclic adenosine monophosphate restores the ability of aged Leydig cells to produce testosterone at the high levels characteristic of young cells

The wealth of knowledge about the function and regulation of adult Leydig cells, the cells within the mammalian testis that produce testosterone, make these cells ideal for studying principles and mechanisms of aging. A hallmark of mammalian aging is decreased serum testosterone concentration. In the Brown Norway rat, this has been shown to be associated with the reduced ability of aged Leydig cells to produce testosterone in response to LH. Herein, we demonstrate that culturing the aged cells with dibutyryl cAMP, a membrane-permeable cAMP agonist that bypasses the LH receptor-adenlyly cyclase cascade, restores testosterone production to levels comparable to those of young cells and also restores steroidogenic acute regulatory protein and P450scc, the proteins involved in the rate-limiting steps of steroidogenesis. These results strongly suggest that signal transduction deficits are responsible for reduced steroidogenesis by aged Leydig cells and that bypassing signal transduction reverses the steroidogenic decline by the aged cells.

Inducible nitric oxide synthase modulates lipolysis in adipocytes

The role of inducible nitric oxide synthase (iNOS) in the modulation of adipocyte lipolysis was investigated. Treatment of white and brown adipose cell lines and mouse adipose explants with a mixture of tumor necrosis factor-alpha, interferon-gamma, and lipopolysaccharide (LPS) doubled the lipolytic rate, and this was associated with marked induction of iNOS expression and nitric oxide (NO) production. iNOS inhibition by 1400W, aminoguanidine, or L-NIL pretreatment further increased the cytokine/LPS-mediated lipolysis by 30% (P < 0.05) in cultured adipocytes and in adipose explants. However, this potentiating effect of iNOS inhibition was abolished in adipose explants isolated from iNOS knockout mice. Pharmacological inhibitors of adenylyl cyclase or protein kinase A reduced cytokine/LPS-induced lipolysis and also blunted the potentiating effect of iNOS inhibition on the lipolytic rate. Furthermore, addition of the antioxidants l-cystine and l-glutathione to cytokine/LPS-stimulated adipocytes mimicked the lipolytic effect of iNOS inhibition. In conclusion, inhibition of iNOS activity in adipocytes potentiates cytokine/LPS-induced lipolysis. This effect was fully reversed by adenylyl cyclase and protein kinase A inhibitors but was mimicked by cellular antioxidants. These data suggest that iNOS-mediated NO production counteracts cytokine/LPS-mediated lipolysis in adipocytes and that this feedback mechanism involves an oxidative process upstream of cAMP production in the signaling pathway.

The evolving role of lipid rafts and caveolae in G protein-coupled receptor signaling: implications for molecular pharmacology

The many components of G-protein-coupled receptor (GPCR) signal transduction provide cells with numerous combinations with which to customize their responses to hormones, neurotransmitters, and pharmacologic agonists. GPCRs function as guanine nucleotide exchange factors for heterotrimeric (alpha, beta, gamma) G proteins, thereby promoting exchange of GTP for GDP and, in turn, the activation of 'downstream' signaling components. Recent data indicate that individual cells express mRNA for perhaps over 100 different GPCRs (out of a total of nearly a thousand GPCR genes), several different combinations of G-protein subunits, multiple regulators of G-protein signaling proteins (which function as GTPase activating proteins), and various isoforms of downstream effector molecules. The differential expression of such protein combinations allows for modulation of signals that are customized for a specific cell type, perhaps at different states of maturation or differentiation. In addition, in the linear arrangement of molecular interactions involved in a given GPCR-G-protein-effector pathway, one needs to consider the localization of receptors and post-receptor components in subcellular compartments, microdomains, and molecular complexes, and to understand the movement of proteins between these compartments. Co-localization of signaling components, many of which are expressed at low overall concentrations, allows cells to tailor their responses by arranging, or spatially organizing in unique and kinetically favorable ways, the molecules involved in GPCR signal transduction. This review focuses on the role of lipid rafts and a subpopulation of such rafts, caveolae, as a key spatial compartment enriched in components of GPCR signal transduction. Recent data suggest cell-specific patterns for expression of those components in lipid rafts and caveolae. Such domains likely define functionally important, cell-specific regions of signaling by GPCRs and drugs active at those GPCRs.

Effect of the relation between neural cholinergic action and nitric oxide on ovarian steroidogenesis in prepubertal rats

The coeliac ganglion and the ovary are related by the superior ovarian nerve, which penetrates into the ovary by the hilium and innervates mainly the ovarian stroma. On the other hand, it is known that the gaseous neurotransmitter nitric oxide (NO) and the two isoforms of its synthesis enzyme, the nitric oxide synthetase (NOS), are present in the ovary. Both innervation and NO participate in ovarian steroidogenesis. Therefore, the purposes of this work were (a) to standardize an in vitro coeliac ganglion-superior ovarian nerve-ovary integrated system in prepubertal rats; (b) to determine the presence of NO in the ovary and analyze the ganglionic cholinergic effect on the ovarian release of androstenedione, progesterone and NO; and (c) to assess the steroids/NO relationship. The system was incubated in buffer solution for 120 min, with the ganglion and ovary located in different compartments and linked by the superior ovarian nerve. From the results obtained, it is concluded that the system is viable and functional. The presence of basal NO is stimulated by the cholinergic action, while the release of the steroids is inhibited, which might indicate that the ganglionic cholinergic effect is probably mediated by NO. To our knowledge, this work constitutes the first study of the relationship between the neural cholinergic action and NO on the ovarian steroidogenesis of prepubertal rats.

NO inhibits signal transduction pathway for ATP release from erythrocytes via its action on heterotrimeric G protein Gi

The release of ATP from erythrocytes involves a signal transduction pathway of which cystic fibrosis transmembrane conductance regulator, PKA, adenylyl cyclase, and the heterotrimeric G proteins G(s) and G(i) are components. In the pulmonary circulation, ATP released from the erythrocyte stimulates nitric oxide (NO) synthesis, thereby regulating vascular resistance. We reported that NO liberated from an NO donor inhibited ATP release from erythrocytes in response to decreased Po(2) or mechanical deformation. Here, we investigated the hypothesis that NO inhibits ATP release from erythrocytes via inactivation of G(i). Washed rabbit erythrocytes were incubated in the presence or absence of the NO donor N-(2-aminoethyl)-N-(2-hydroxy-2-nitrosohydrazino)-1,2-ethylenediamine (spermine NONOate; 100 nM, 20 min), followed by treatment with agents that activate specific components of the signal transduction pathway promoting ATP release. Neither ATP release nor cAMP accumulation induced by either forskolin (100 microM, n = 7) or iloprost (100 nM, n = 6) was inhibited by spermine NONOate. These experiments suggest that the inhibitory action of NO is not the result of inactivation of adenylyl cyclase or G(s), respectively. However, spermine NONOate completely inhibited ATP release in response to mastoparan (10 microm, P < 0.05, n = 5), a specific activator of G(i). Spermine (100 nM, 20 min), the polyamine remaining after liberation of NO from spermine NONOate, had no affect on mastoparan-induced ATP release (n = 4). These results support the hypothesis that NO inhibits ATP release from erythrocytes via inactivation of the heterotrimeric G protein G(i).

Nitric oxide inhibition of adenylyl cyclase type 6 activity is dependent upon lipid rafts and caveolin signaling complexes

Several cell types, including cardiac myocytes and vascular endothelial cells, produce nitric oxide (NO) via both constitutive and inducible isoforms of NO synthase. NO attenuates cardiac contractility and contributes to contractile dysfunction in heart failure, although the precise molecular mechanisms for these effects are poorly defined. Adenylyl cyclase (AC) isoforms type 5 and 6, which are preferentially expressed in cardiac myocytes, may be inhibited via a direct nitrosylation by NO. Because endothelial NO synthase (eNOS and NOS3), beta-adrenergic (betaAR) receptors, and AC6 all can localize in lipid raft/caveolin-rich microdomains, we sought to understand the role of lipid rafts in organizing components of betaAR-G(s)-AC signal transduction together with eNOS. Using neonatal rat cardiac myocytes, we found that disruption of lipid rafts with beta-cyclodextrin inhibited forskolin-stimulated AC activity and cAMP production, eliminated caveolin-3-eNOS interaction, and increased NO production. betaAR- and G(s)-mediated activation of AC activity were inhibited by beta-cyclodextrin treatment, but prostanoid receptor-stimulated AC activity, which appears to occur outside caveolin-rich microdomains, was unaffected unless eNOS was overexpressed and lipid rafts were disrupted. An NO donor, SNAP, inhibited basal and forskolin-stimulated cAMP production in both native cardiac myocytes and cardiac myocytes and pulmonary artery endothelial cells engineered to overexpress AC6. These effects of SNAP were independent of guanylyl cyclase activity and were mimicked by overexpression of eNOS. The juxtaposition of eNOS with betaAR and AC types 5 and 6 results in selective regulation of betaAR by eNOS activity in lipid raft domains over other G(s)-coupled receptors localized in nonraft domains. Thus co-localization of multiple signaling components in lipid rafts provides key spatial regulation of AC activity.

Nitric oxide inhibits ATP release from erythrocytes

Erythrocytes have been reported to release ATP from intracellular stores into the surrounding environment in response to decreased oxygen tension and mechanical deformation. This erythrocyte-derived ATP can then act on purinergic receptors present on vascular endothelial cells, resulting in the synthesis and bidirectional release of nitric oxide (NO). NO released abluminally produces relaxation of vascular smooth muscle, thereby increasing vascular caliber, leading to a decrease in deformation-induced ATP release from erythrocytes. In contrast, NO released into the vascular lumen could interact directly with formed elements in the blood, including the erythrocyte. Here, we investigate the hypothesis that NO functions in a negative-feedback manner to inhibit ATP release from the erythrocyte. The NO donor N-(2-aminoethyl)- N-(2-hydroxy-2-nitrosohydrazino)-1,2-ethylenediamine (spermine NONOate) decreased total pulmonary resistance in a dose-dependent manner when administered to isolated perfused rabbit lungs. ATP release from rabbit erythrocytes in response to decreased oxygen tension or mechanical deformation was inhibited by preincubation with spermine NONOate (100 nM, 20 min). Importantly, incubating rabbit erythrocytes with spermine (100 nM, 20 min), the polyamine remaining after the liberation of NO from spermine NONOate, did not affect decreased oxygen tension-induced ATP release. Mechanical deformation-induced ATP release was also inhibited when erythrocytes were preincubated with spermine NONOate. However, NO-depleted spermine NONOate had no effect on mechanical deformation-induced ATP release from rabbit erythrocytes. These data provide support for the hypothesis that NO inhibits ATP release from erythrocytes, thereby identifying an additional role of NO in the regulation of vascular resistance.

Regulation and organization of adenylyl cyclases and cAMP

Adenylyl cyclases are a critically important family of multiply regulated signalling molecules. Their susceptibility to many modes of regulation allows them to integrate the activities of a variety of signalling pathways. However, this property brings with it the problem of imparting specificity and discrimination. Recent studies are revealing the range of strategies utilized by the cyclases to solve this problem. Microdomains are a consequence of these solutions, in which cAMP dynamics may differ from the broad cytosol. Currently evolving methodologies are beginning to reveal cAMP fluctuations in these various compartments.

Effect of radiation on cAMP, cGMP and Ca(2+)(i) pathways and their interactions in rat distal colon

The secretory response implicated in the intestinal response to luminal attack is altered by radiation. The cAMP, cGMP and Ca(2+)(i) pathways leading to secretion as well as the interactions between the cAMP pathway and the cGMP or Ca(2+)(i) pathway were studied in the rat distal colon 4 days after a 9-Gy abdominal X irradiation, when modifications mainly occurred. The secretory response in Ussing chambers and cAMP and cGMP accumulation in single isolated crypts were measured. The muscarinic receptor characteristics were determined in mucosal membrane preparations. The secretory response by the cAMP pathway (stimulated by vasoactive intestinal peptide or forskolin) and the cAMP accumulation in crypts were decreased (P < 0.05) after irradiation. The weak secretory response induced by the cGMP pathway (stimulated by nitric oxide or guanylin) was unaltered by radiation, and the small amount of cGMP determined in isolated crypts from the control group became undetectable in the irradiated group. Inducible NOS was not involved in the hyporesponsiveness to VIP after irradiation (there was no effect of an iNOS inhibitor). The secretory response by the Ca(2+)(i) pathway (stimulated by carbachol) was unaffected despite a decreased number and increased affinity of muscarinic receptors. The non-additivity of VIP and carbachol co-stimulated responses was unmodified. In contrast, VIP and SNP co-stimulation showed that NO enhanced the radiation-induced hyporesponsiveness to VIP through a reduced accumulation of cAMP in crypts. This study provides further understanding of the effect of ionizing radiation on the intracellular signaling pathways.

Angiotensin II enhances adenylyl cyclase signaling via Ca2+/calmodulin. Gq-Gs cross-talk regulates collagen production in cardiac fibroblasts

Cardiac fibroblasts regulate formation of extracellular matrix in the heart, playing key roles in cardiac remodeling and hypertrophy. In this study, we sought to characterize cross-talk between Gq and Gs signaling pathways and its impact on modulating collagen synthesis by cardiac fibroblasts. Angiotensin II (ANG II) activates cell proliferation and collagen synthesis but also potentiates cyclic AMP (cAMP) production stimulated by beta-adrenergic receptors (beta-AR). The potentiation of beta-AR-stimulated cAMP production by ANG II is reduced by phospholipase C inhibition and enhanced by overexpression of Gq. Ionomycin and thapsigargin increased intracellular Ca2+ levels and potentiated isoproterenol- and forskolin-stimulated cAMP production, whereas chelation of Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid/AM inhibited such potentiation. Inhibitors of tyrosine kinases, protein kinase C, or Gbetagamma did not alter this cross-talk. Immunoblot analyses showed prominent expression of adenylyl cyclase 3 (AC3), a Ca2+-activated isoform, along with AC2, AC4, AC5, AC6, and AC7. Of those isoforms, only AC3 and AC5/6 proteins were detected in caveolin-rich fractions. Overexpression of AC6 increased betaAR-stimulated cAMP accumulation but did not alter the size of the ANG II potentiation, suggesting that the cross-talk is AC isoform-specific. Isoproterenol-mediated inhibition of serum-stimulated collagen synthesis increased from 31 to 48% in the presence of ANG II, indicating that betaAR-regulated collagen synthesis increased in the presence of ANG II. These data indicate that ANG II potentiates cAMP formation via Ca2+-dependent activation of AC activity, which in turn attenuates collagen synthesis and demonstrates one functional consequence of cross-talk between Gq and Gs signaling pathways in cardiac fibroblasts.

Effects of nitric oxide and cGMP on inhibin A and inhibin subunit mRNA levels from cultured rat granulosa cells

OBJECTIVE

To determine the effects of nitric oxide (NO) and cGMP on inhibin A and inhibin subunit mRNA levels from cultured rat granulosa cells.

DESIGN

Basic research study.

SETTING

University research laboratory.

ANIMAL(S)

Primary cell culture of granulosa cells obtained from estrogen-treated, immature Sprague-Dawley female rats.

INTERVENTION(S)

Functionally immature rat granulosa cells were incubated for 48 hours with media alone; FSH; forskolin; the NO generator DETA/NO; an inhibitor of soluble guanylyl cyclase (ODQ); and/or a cell-permeable cGMP analog.

MAIN OUTCOME MEASURE(S)

Media concentrations of inhibin A were measured by solid-phase immunosorbent assay. Inhibin alpha and betaA subunit mRNA levels were determined by Northern and slot blot analyses.

RESULT(S)

Whereas FSH caused a 20-fold increase in inhibin A levels compared with untreated granulosa cells, the NO generator DETA/NO significantly inhibited FSH-stimulated inhibin A concentrations. Similarly, cotreatment with FSH plus dibutyryl cGMP significantly attenuated inhibin A concentrations, compared with those in cells treated with FSH alone. Incubation with forskolin (FSK) stimulated inhibin A levels sevenfold, whereas cotreatment with FSK plus DETA/NO or FSK plus dibutyryl cGMP effectively decreased inhibin A concentrations. The effects of NO on inhibin A levels were not prevented by cotreatment with an inhibitor of soluble guanylyl cyclase. In addition, there was no influence of DETA/NO or dibutyryl cGMP on inhibin subunit mRNA levels.

CONCLUSION(S)

These findings indicate that NO and cGMP can attenuate inhibin A concentrations through actions at one or more post-FSH receptor sites. These influences may reflect inhibition of inhibin A secretion, rather than gene expression and protein synthesis. In addition, NO decreases inhibin A concentrations through both cGMP-dependent and -independent pathways. These results suggest local roles for NO and cGMP in the regulation of granulosa cell function.

Roles of cyclic GMP in modulating ovarian functions

The production of a viable oocyte is dependent upon the critical influences of gonadotrophins on follicular development, granulosa cell maturation, ovulation, and luteinization. While the effects of LH and FSH are due in large part to cyclic AMP-dependent signalling mechanisms, it is clear that a number of other factors modulate the actions of gonadotrophins on the ovary via activation of alternative signalling pathways. In this regard, recent studies indicate that the second messenger guanosine 3',5'-cyclic monophosphate (cGMP) mediates a wide range of influences on the ovary. Nitric oxide (NO) is a major regulator of cGMP production via its action on soluble guanylyl cyclase, while natriuretic peptides activate receptors with intrinsic guanylyl cyclase activities. In addition, other factors known to influence ovarian functions are now recognized to act via NO/cGMP pathways. This report will review these previous findings and present new data demonstrating the inhibitory influence of cGMP on cAMP-stimulated LH receptor expression in cultured granulosa cells.

CB(1) cannabinoid receptor-G protein association: a possible mechanism for differential signaling

Effects of cannabinoid compounds on neurons are predominantly mediated by the CB(1) cannabinoid receptor. Onset of signaling cascades in response to cannabimimetic drugs is triggered by the interaction of the cannabinoid receptor with G(i/o) proteins. Much work has been done to delineate the cannabinoid agonist-induced downstream signaling events; however, it remains to define the molecular basis of cannabinoid receptor-G protein interactions that stimulate these signaling pathways. In this review, we discuss several signal transduction pathways, focusing on studies that demonstrate the efficacy of CB(1) receptor agonists through G protein mediated pathways.

Nitric oxide signalling by selective beta(2)-adrenoceptor stimulation prevents ACh-induced inhibition of beta(2)-stimulated Ca(2+) current in cat atrial myocytes

The present study determined the effects of acetylcholine (ACh) on the L-type Ca(2+) current (I(Ca,L)) stimulated by beta(1)- or beta(2)-adrenergic receptor (AR) agonists in cat atrial myocytes. When isoproterenol (ISO; 0.1 microM) plus the beta(2)-AR antagonist ICI 118,551 (ISO-beta(1)-AR stimulation) or 0.1 microM fenoterol, a beta(2)-AR agonist (FEN-beta(2)-AR stimulation) increased I(Ca,L), ACh (1 microM) inhibited I(Ca,L) by -60 +/- 4 and -63 +/- 6 %, respectively. When ISO plus the beta(1)-AR antagonist atenolol (ISO-beta(2)-AR stimulation) or 1 microM zinterol (ZIN-beta(2)-AR stimulation) increased I(Ca,L), ACh-induced inhibition of I(Ca,L) was significantly smaller, at -21 +/- 3 and -24 +/- 3 %, respectively. L-N(5)-(1-iminoethyl)ornithine (L-NIO, 10 microM), an inhibitor of nitric oxide (NO) synthase, enhanced ACh-induced inhibition of I(Ca,L) when stimulated by ZIN-beta(2)-ARs, but not when stimulated by ISO-beta(1)-ARs or FEN-beta(2)-ARs. Haemoglobin (50 microM), a NO scavenger, also enhanced ACh-induced inhibition when I(Ca,L) was stimulated by ZIN-beta(2)-ARs, but not when stimulated by FEN-beta(2)-ARs. ACh-induced inhibition of I(Ca,L) stimulated by ZIN-beta(2)-ARs was not affected by 10 microM 1H-[1,2,4] oxadiazolo[4,3-a] quinoxaline-1-one (ODQ) a guanylate cyclase inhibitor, but was significantly enhanced by 500 microM reduced glutathione or 100 microM dithiothreitol, agents that act as sinks for S-nitrosylation. ACh-induced inhibition was smaller when I(Ca,L) was stimulated by spermine/NO, a NO donor, than by milrinone, a phosphodiesterase type III inhibitor. ISO (ISO-beta(1)/beta(2)-AR stimulation) increased I(Ca,L) and even though ISO releases NO, ACh prominently inhibited I(Ca,L). This inhibitory effect of ACh was enhanced by L-NIO. Stimulation of ZIN-beta(2)-ARs increased intracellular NO, whereas ISO-beta(1)-ARs or FEN-beta(2)-ARs failed to increase intracellular NO. These results indicate that in atrial myocytes, NO released by selective beta(2)-AR stimulation prevents ACh-induced inhibition of I(Ca,L) stimulated by beta(2)-ARs. NO acts via a cGMP-independent, S-nitrosylation mechanism. Although FEN acts via beta(2)-ARs, it fails to stimulate G(i)-/NO signalling and preferentially stimulates G(s)-/adenylate cyclase signalling, similar to beta(1)-ARs. These findings indicate that NO signalling modulates muscarinic receptor inhibition of atrial function stimulated by beta(2)-ARs.

Attenuation of cAMP accumulation in adult rat cardiac fibroblasts by IL-1beta and NO: role of cGMP-stimulated PDE2

Treatment of cultured adult rat cardiac fibroblasts with interleukin-1beta (IL-1beta) induces the inducible nitric oxide synthase (iNOS) expression, increases nitric oxide (NO) and cGMP production, and attenuates cAMP accumulation in response to isoproterenol by ~50%. Reduced cAMP accumulation is due to NO production: the effect is mimicked by NO donors and prevented by N(G)-monomethyl-L-arginine, an NOS inhibitor. Effects of NO are not restricted to the beta-adrenergic response; the response to forskolin is similarly diminished. NO donors only slightly (12%) decrease forskolin-stimulated adenylyl cyclase (AC) activity in cardiac fibroblast plasma membranes, suggesting that the main effect of NO is not a direct one on AC. An inhibitor of soluble guanylyl cyclase inhibits the effects of IL-1beta and NO donors; inhibition of cGMP-dependent protein kinase is without effect. 3-Isobutyl-1-methylxanthine, a nonspecific phosphodiesterase (PDE) inhibitor, and erythro-9-(2-hydroxy-3-nonyl)adenine, a specific inhibitor of the cGMP-stimulated PDE (PDE2), completely restore cAMP accumulation in sodium nitroprusside-treated fibroblasts and largely reverse the attenuated response in IL-1beta-treated fibroblasts. Although NO reportedly acts by reducing AC activity in some cells, in cardiac fibroblasts NO production decreases cAMP accumulation largely by the cGMP-mediated activation of PDE2.

Nitric oxide and the other cyclic nucleotide

Nitric oxide (NO) participates in the regulation of the daily activities of cells as well as in cytotoxic events. Elucidating the mechanism(s) by which NO carries out its diverse functions has been the goal of numerous laboratories. In the cardiovascular system, evidence indicates that NO mediates its effects via an activation of soluble guanylyl cyclase (sGC). In other tissues, it is not clear if sGC is an exclusive target for NO or what the functions of cGMP might be. It is also unlikely that the diversity of NO actions is explained solely by changes in cGMP. This review focuses on the evidence that NO modulates cAMP signalling, with specific attention to the effects of NO on adenylyl cyclase (AC) as the target of NO regulation.

Alterations of the VIP-stimulated cAMP pathway in rat distal colon after abdominal irradiation

Ionizing radiation induces hyporesponsiveness of rat colonic mucosa to vasoactive intestinal peptide (VIP). Possible mechanisms responsible for this hyporesponsiveness of the cAMP communication pathway in rat colon were investigated. VIP- and forskolin-stimulated short-circuit current (I(sc)) responses were studied after a 10-Gy abdominal irradiation in Ussing chambers as well as in single, isolated crypts. Adenylyl cyclase (AC) activity and VIP receptor characteristics were determined in mucosal membrane preparations. In addition, alterations in crypt morphology were studied. Impaired secretory responses to VIP and forskolin were observed 4 days after irradiation (decrease of 80%). cAMP analog-stimulated I(sc) responses were unchanged. In isolated crypts, VIP- and forskolin-stimulated cAMP accumulation was markedly reduced by 80 and 50%, respectively. VIP-stimulated AC activity and VIP receptor number were decreased in membrane preparations. No major change of cellularity was associated with these functional alterations. In conclusion, the decreased secretory responses to VIP of rat colon are associated with reduced cAMP accumulation, decreased AC activity, and diminution of VIP receptor numbers without a marked decrease of crypt cell number.

NO donors potentiate the beta-adrenergic stimulation of I(Ca,L) and the muscarinic activation of I(K,ACh) in rat cardiac myocytes

The effects of nitric oxide (NO) donors on the L-type Ca(2+) current (I(Ca,L)) and the muscarinic activated K(+) current (I(K,ACh)) were studied in isolated rat cardiac myocytes. The nitrosothiol S-nitroso-N-acetyl-D,L-penicillamine (SNAP, 1 pM-1 microM) strongly potentiated the stimulation of the I(Ca,L) elicited by subthreshold concentrations of isoprenaline (Iso, 0.1-0.5 nM) in ventricular myocytes. The effect of SNAP was mimicked by 2-(N,N-diethylamino)-diazenolate-2-oxide (DEANO, 1 pM-1 nM), a NONOate that spontaneously releases NO in a pH-controlled manner, and was blunted by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (100 microM), a NO trap. 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxaline-1-one (10 microM), a guanylyl cyclase inhibitor, did not alter the effect of SNAP. SNAP (1 pM-1 microM) did not modify the effect of L858051 (0.1-0.3 microM), a forskolin analogue that activates adenylyl cyclase, on I(Ca,L) and did not enhance the basal I(Ca,L) in the presence of rolipram (1 microM), a phosphodiesterase type 4 inhibitor. Superfusion with Rp-CPT-cAMPS (500 microM), or internal dialysis with cAMP-dependent protein kinase (cA-PK) inhibitory peptide (PKI; 20 microM), inhibitors of the cA-PK, blunted the effect of SNAP (1 nM and 1 microM) on the Iso-stimulated (1-100 pM) I(Ca,L). SNAP (1 nM and 1 microM) potentiated the threshold stimulation of I(Ca,L) elicited by internal GTP-gammaS (10 microM), a non-hydrolysable analogue of GTP. SNAP (1 pM-1 microM) and DEANO (1 microM) potentiated the stimulation of I(K,ACh) elicited by low concentrations of ACh (1-2 nM) in rat atrial myocytes. The threshold stimulation of I(K,ACh) elicited by internal 5'-guanylylimidodiphosphate (10 microM) was also potentiated by NO donors. SNAP (1 microM) did not modify I(K,ACh) reconstituted in human embryonic kidney 293 cells, in the absence or in the presence of ACh (1 or 10 nM). Taken together, these data suggest that NO is a cGMP-independent modulator of G-protein-coupled muscarinic and beta-adrenergic receptor actions on cardiac ion channels. Although this action of NO seemed to occur at the level of G proteins, it appeared to require a component distinct from receptors, G proteins or their effectors.

Zinc inhibition of cAMP signaling

Zn(2+) is required as either a catalytic or structural component for a large number of enzymes and thus contributes to a variety of important biological processes. We report here that low micromolar concentrations of Zn(2+) inhibited hormone- or forskolin-stimulated cAMP production in N18TG2 neuroblastoma cells. Similarly, low concentrations inhibited hormone- and forskolin-stimulated adenylyl cyclase (AC) activity in membrane preparations and did so primarily by altering the V(max) of the enzyme. Zn(2+) also inhibited recombinant isoforms, indicating that this reflects a direct interaction with the enzyme. The IC(50) for Zn(2+) inhibition was approximately 1-2 microm with a Hill coefficient of 1.33. The dose-response curve for Zn(2+) inhibition was identical for AC1, AC5, and AC6 as well as for the C441R mutant of AC5 whose defect appears to be in one of the catalytic metal binding sites. However, AC2 displayed a distinct dose-response curve. These data in combination with the findings that Zn(2+) inhibition was not competitive with Mg(2+) or Mg(2+)/ATP suggest that the inhibitory Zn(2+) binding site is distinct from the metal binding sites involved in catalysis. The prestimulated enzyme was found to be less susceptible to Zn(2+) inhibition, suggesting that the ability of Zn(2+) to inhibit AC could be significantly influenced by the coincidence timing of the input signals to the enzyme.

Adenylyl cyclase types I and VI but not II and V are selectively inhibited by nitric oxide

Adenylyl cyclase (AC) isoforms catalyze the synthesis of 3',5'-cyclic AMP from ATP. These isoforms are critically involved in the regulation of gene transcription, metabolism, and ion channel activity among others. Nitric oxide (NO) is a gaseous product whose synthesis from L-arginine is catalyzed by the enzyme NO synthase. It has been well established that NO activates the enzyme guanylyl cyclase, but little has been reported on the effects of NO on other important second messengers, such as AC. In the present study, the effects of sodium nitroprusside (SNP), a nitric oxide-releasing compound, on COS-7 cells transfected with plasmids containing AC types I, II, V and VI were evaluated. Total inhibition (approximately 98.5%) of cAMP production was observed in COS-7 cells transfected with the AC I isoform and previously treated with SNP (10 mM) for 30 min, when stimulated with ionomycin. A high inhibition (approximately 76%) of cAMP production was also observed in COS-7 cells transfected with the AC VI isoform and previously treated with SNP (10 mM) for 30 min, when stimulated with forskolin. No effect on cAMP production was observed in cells transfected with AC isoforms II and V.

Nitric oxide inhibition of cAMP synthesis in parotid acini: regulation of type 5/6 adenylyl cyclase

The nitric oxide (NO) donor, GEA 3162, inhibited isoproterenol-induced cyclic AMP (cAMP) accumulation in a concentration- and time-dependent manner in mouse parotid acini; SIN-1 mimicked these effects. Inhibition of stimulated cAMP accumulation was independent of phosphodiesterase activity. GEA 3162 also inhibited forskolin-induced cAMP accumulation. Removal of extracellular Ca(2+), addition of La(3+), or the calmodulin (CaM) inhibitor, calmidazolium, did not prevent the NO-mediated response, and addition of the soluble guanylyl inhibitor, ODQ, did not reverse GEA 3162-induced inhibition of cAMP accumulation. GEA 3162 also inhibited adenylyl cyclase in vitro independently of Ca(2+)/CaM. Further studies revealed that the NO synthase (NOS) inhibitor, 7-nitroindazole (7-NI), reduced significantly thapsigargin-induced Ca(2+) release and capacitative Ca(2+) entry and reversed thapsigargin inhibition of the AC Type 5/6 isoform (AC5/6). Data suggest that NO produced endogenously has dual effects on cAMP accumulation in mouse parotid acini, an inhibitory effect on AC activity and a modulatory effect on capacitative Ca(2+) entry resulting in AC5/6 inhibition.