cGMP: the wayward child of the cyclic nucleotide family

3',5'-cIMP as Potential Second Messenger in the Vascular Wall

Traditionally, only the 3',5'-cyclic monophosphates of adenosine and guanosine (produced by adenylyl cyclase and guanylyl cyclase, respectively) are regarded as true "second messengers" in the vascular wall, despite the presence of other cyclic nucleotides in different tissues. Among these noncanonical cyclic nucleotides, inosine 3',5'-cyclic monophosphate (cIMP) is synthesized by soluble guanylyl cyclase in porcine coronary arteries in response to hypoxia, when the enzyme is activated by endothelium-derived nitric oxide. Its production is associated with augmentation of vascular contraction mediated by stimulation of Rho kinase. Based on these findings, cIMP appears to meet most, if not all, of the criteria required for it to be accepted as a "second messenger," at least in the vascular wall.

Methylene blue and its analogues as antidepressant compounds

Methylene Blue (MB) is considered to have diverse medical applications and is a well-described treatment for methemoglobinemias and ifosfamide-induced encephalopathy. In recent years the focus has shifted to MB as an antimalarial agent and as a potential treatment for neurodegenerative disorders such as Alzheimer's disease. Of interest are reports that MB possesses antidepressant and anxiolytic activity in pre-clinical models and has shown promise in clinical trials for schizophrenia and bipolar disorder. MB is a noteworthy inhibitor of monoamine oxidase A (MAO-A), which is a well-established target for antidepressant action. MB is also recognized as a non-selective inhibitor of nitric oxide synthase (NOS) and guanylate cyclase. Dysfunction of the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) cascade is strongly linked to the neurobiology of mood, anxiety and psychosis, while the inhibition of NOS and/or guanylate cyclase has been associated with an antidepressant response. This action of MB may contribute significantly to its psychotropic activity. However, these disorders are also characterised by mitochondrial dysfunction and redox imbalance. By acting as an alternative electron acceptor/donor MB restores mitochondrial function, improves neuronal energy production and inhibits the formation of superoxide, effects that also may contribute to its therapeutic activity. Using MB in depression co-morbid with neurodegenerative disorders, like Alzheimer's and Parkinson's disease, also represents a particularly relevant strategy. By considering their physicochemical and pharmacokinetic properties, analogues of MB may provide therapeutic potential as novel multi-target strategies in the treatment of depression. In addition, low MAO-A active analogues may provide equal or improved response with a lower risk of adverse effects.

Generation of purified nitric oxide from liquid N2O4 for the treatment of pulmonary hypertension in hypoxemic swine

Inhaled nitric oxide (NO) selectively dilates pulmonary blood vessels, reduces pulmonary vascular resistance (PVR), and enhances ventilation-perfusion matching. However, existing modes of delivery for the treatment of chronic pulmonary hypertension are limited due to the bulk and heft of large tanks of compressed gas. We present a novel system for the generation of inhaled NO that is based on the initial heat-induced evaporation of liquid N2O4 into gas phase NO2 followed by the room temperature reduction to NO by an antioxidant, ascorbic acid cartridge just prior to inhalation. The biologic effects of NO generated from liquid N2O4 were compared with the effects of NO gas, on increased mean pulmonary artery pressure (mPAP) and PVR in a hypoxemic (FiO2 15%) swine model of pulmonary hypertension. We showed that NO concentration varied directly with the fixed cross sectional flow of the outflow aperture when studied at temperatures of 45, 47.5 and 50°C and was independent of the rate of heating. Liquid N2O4-sourced NO at 1, 5, and 20 ppm significantly reduced the elevated mPAP and PVR induced by experimental hypoxemia and was biologically indistinguishable from gas source NO in this model. These experiments show that it is feasible to generate highly purified NO gas from small volumes of liquid N2O4 at concentrations sufficient to lower mPAP and PVR in hypoxemic swine, and suggest that a miniaturized ambulatory system designed to generate biologically active NO from liquid N2O4 is achievable.

Localization of hypoxanthine-guanine phosphoribosyltransferase mRNA in the mouse brain by in situ hybridization

Congenital deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) in humans results in a severe neurogenetic disorder known as the Lesch-Nyhan syndrome. Since little information concerning the precise localization of HPRT in the brain is currently available, we have used in situ hybridization to examine the expression of HPRT mRNA in the mouse brain. The results showed that HPRT mRNA is expressed in many regions of the normal mouse brain, with high levels in most, but not all neurons. In contrast, glial cells did not express detectable levels of HPRT mRNA. No HPRT mRNA was detected in the brains of mutant mice carrying a deletion in the HPRT gene.

Phosphodiesterase inhibitor improves renal tubulointerstitial hypoxia of the diabetic rat kidney

BACKGROUND/AIMS

Renal hypoxia is involved in the pathogenesis of diabetic nephropathy. Pentoxifyllin (PTX), a nonselective phosphodiesterase inhibitor, is used to attenuate peripheral vascular diseases. To determine whether PTX can improve renal hypoxia, we investigated its effect in the streptozocin (STZ)-induced diabetic kidney.

METHODS

PTX (40 mg/kg, p.o.) was administered to STZ-induced diabetic rats for 8 weeks. To determine tissue hypoxia, we examined hypoxic inducible factor-1α (HIF-1α), heme oxygenase-1 (HO-1), vascular endothelial growth factor (VEGF), and glucose transporter-1 (GLUT-1) levels. We also tested the effect of PTX on HIF-1α in renal tubule cells.

RESULTS

PTX reduced the increased protein creatinine ratio in diabetic rats at 8 weeks. HIF-1α, VEGF, and GLUT-1 mRNA expression increased significantly, and the expression of HO-1 also tended to increase in diabetic rats. PTX significantly decreased mRNA expression of HIF-1α and VEGF at 4 and 8 weeks, and decreased HO-1 and GLUT-1 at 4 weeks. The expression of HIF-1α protein was significantly increased at 4 and 8 weeks in tubules in the diabetic rat kidney. PTX tended to decrease HIF-1α protein expression at 8 weeks. To examine whether PTX had a direct effect on renal tubules, normal rat kidney cells were stimulated with CoCl(2) (100 µM), which enhanced HIF-1α mRNA and protein levels under low glucose conditions (5.5 mM). Their expressions were similar even after high glucose (30 mM) treatment. PTX had no effect on HIF-1α expression.

CONCLUSIONS

PTX attenuates tubular hypoxia in the diabetic kidney.

Investigation of the regulation of transcriptional changes in Ancylostoma caninum larvae following serum activation, with a focus on the insulin-like signalling pathway

The exit from dauer in the free-living nematode Caenorhabditis elegans is under the control of a single amphidial neuron (ASJ) of the insulin-like signalling pathway. Mutations of this pathway have the ability to suppress entry into the dauer stage. It has been postulated that insulin-like signalling plays a significant role in the response to serum stimulation in vitro of the third-stage larvae (L3s) of the canine hookworm Ancylostoma caninum. To test for the possible involvement of the insulin-like signalling cascade in the response to serum stimulation, the effects of two signalling stimulants (8-bromo cGMP and arecoline) and four inhibitors, namely 4,7-phenanthroline, phosphoinositide-3 kinase (PI3K), Akt inhibitor IV and rapamycin on feeding and on levels of selected activation-associated mRNAs in serum-stimulated L3s were explored. L3s of A. caninum were pre-incubated with or without the appropriate inhibitor/agonist. Following serum-stimulation, the feeding activity was assessed. The transcription levels of a number of activation-associated mRNAs linked to particular expressed sequence tags (ESTs) were investigated by reverse transcription, real-time PCR (rtPCR). The treatment of worms with 4,7-phenanthroline completely suppressed feeding and significantly reduced the differential levels of most activation-associated mRNAs, whereas the treatment with cGMP resulted in the resumption of feeding in almost 85% of the L3s and yielded a specific transcriptional profile consistent with that following serum stimulation. The treatment of L3s with arecoline resulted in the resumption of feeding in approximately 85% of L3s, but did not result in a transcriptomic profile consistent with activation. A complete reduction in feeding was recorded in the presence of the PI3K inhibitor LY294002 (1mM) and resulted in a pronounced dampening of differential transcription in response to serum stimulation for the molecules examined. Akt inhibitor IV resulted in a approximately 70% reduction in feeding but had almost no effect on the level of any of the activation-associated mRNAs studied. Rapamycin was shown to have a weak effect on feeding, and several of the mRNAs studied exhibited greater than expected transcription following treatment. The complexities of activation-associated transcription could not be addressed using the current approach. A larger number of mRNAs needs to be investigated in order to predict or identify regulatory mechanisms proposed to function in the insulin-like signalling pathway in A. caninum.

Nitric oxide: an inhibitory retrograde modulator in the crustacean heart

The nervous system innervates most of the organs in the body, and controls and coordinates their activities. Effective coordination depends on accurate feedback from target organs. Recent studies have identified a target-based feedback mechanism that regulates a simple neural circuit, the cardiac ganglion-a network of nine neurons whose rhythmic bursts of action potentials drive the contractions of the crustacean heart. The feedback agent, nitric oxide (NO), is produced by the target organ (the heart), and acts on the neural circuit (the ganglion), thus serving as a retrograde, trans-synaptic signaling molecule. NO decreases the ganglionic burst rate, which has both negative chronotropic and negative inotropic effects on the heartbeat. This article will review the evidence identifying NO as an inhibitory modulator in the crustacean heart, and will present new data showing that these inhibitory effects are not mediated by cGMP, the canonical downstream agent mobilized by NO in many other systems. Rather, our data suggest that in the crustacean heart cGMP may play a secondary role in the process of adaptation that occurs in during prolonged exposures to NO.

Expression of nitric oxide synthase and nitric oxide-sensitive guanylate cyclase in the crustacean cardiac ganglion

The cardiac ganglion is a simple central pattern-generating network that controls the rhythmic contractions of the crustacean heart. Enzyme assays and Western blots show that whole heart homogenates from the crab Cancer productus contain high levels of nitric oxide synthase (NOS), an enzyme that catalyzes the conversion of arginine to citrulline with concomitant production of the transmitter nitric oxide (NO). Crab heart NOS is calcium-dependent and has an apparent molecular weight of 110 kDa. In the cardiac ganglion, antibodies to NOS and citrulline indicate the presence of a NOS-like protein and NOS enzymatic activity in the four small pacemaker neurons and the five large motor neurons of the cardiac network. In addition, all cardiac neurons label positively with an antibody to cyclic guanosine monophosphate (cGMP). The NO donor sodium nitroprusside (SNP, 10 mM) stimulates additional cGMP production in the isolated ganglion. This increase is blocked by [(1)H](1,2,4)oxadiazole(4,3-a)quinoxalin-1-one (ODQ, 50 microM), an inhibitor of the NO-sensitive soluble guanylate cyclase (sGC). Taken together, our data indicate that NO- and cGMP-mediated signaling pathways are enriched in the cardiac system relative to other crab tissues and that the cardiac network may be a target for extrinsic and intrinsic neuromodulation via NO produced from the heart musculature and individual cardiac neurons, respectively. The crustacean cardiac ganglion is therefore a promising system for studying cellular and synaptic mechanisms of nitrergic neuromodulation in a simple pattern-generating network.

Assay of human platelet guanylate cyclase activity by high-performance liquid chromatography with fluorescence derivatization

A selective and sensitive high-performance liquid chromatography (HPLC) method with fluorescence derivatization for the assay of guanylate cyclase (GC) activity is described. GTP and cGMP, which are the substrate and the product of GC, respectively, and other guanine-containing compounds are selectively converted by the reaction with (3,4-dimethoxyphenyl)glyoxal to the fluorescent derivatives. The derivatives were separated by reversed-phase HPLC. The limit of detection at a signal-to-noise ratio of 3 for cGMP was 10 fmol on the column. The sensitivity of this method was less than that of the conventional radioisotopic method, but this method is simple and convenient. Human platelet GC activity was measured, and the effects of some compounds were investigated.

Phosphodiesterase 5 inhibitors and nitrergic transmission-from zaprinast to sildenafil

Phosphodiesterase 5 terminates the cellular actions of the second messenger molecule cyclic GMP; inhibitors of phosphodiesterase 5 will therefore increase and prolong the actions of endogenous substances that signal via the cyclic GMP pathway, including nitric oxide released as a neurotransmitter from nitrergic nerves. To date, the most widely used phosphodiesterase 5 inhibitors, zaprinast and sildenafil, have proved vital in the elucidation of the widespread role of cyclic GMP in nitrergic transmission and, specifically in the case of sildenafil, have provided a major breakthrough in the treatment of erectile dysfunction in men. Although still a matter of debate, early evidence indicates that sildenafil may also be of benefit in some forms of sexual dysfunction in women. The remarkable clinical success of sildenafil has prompted the search for further novel phosphodiesterase 5 inhibitors which might be used to enhance nitrergic function in other disease states.

Nitric oxide (NO.) stabilizes whereas nitrosonium (NO+) enhances filopodial outgrowth by rat retinal ganglion cells in vitro

Recent observations suggest that nitric oxide (NO(.)) can increase or decrease growth cone motility. Here, these apparently paradoxical results are explained by distinct actions of different NO-related species. Filopodial morphology of 223 rat retinal ganglion cells was monitored under computer-enhanced video microscopy in the presence of NO synthase (NOS) substrates or inhibitors, donors of specific NO-related species, and membrane-permeant cyclic nucleotide analogs. Physiological NOS activity induced filopodial outgrowth, whereas inhibition of NOS stabilized filopodia. Similar to NOS, nitrosonium (NO(+) transfer) and peroxynitrite (ONOO(-)), which can regulate the activity of growth-associated proteins by S-nitrosylation and oxidation, respectively, induced filopodial outgrowth. In contrast, NO(.), which stimulates guanylate cyclase to increase cGMP, stabilized filopodial activity. Thus disparate NO-related species may offer a dynamic process of filopodial growth regulation.

A transmembrane guanylyl cyclase (DAF-11) and Hsp90 (DAF-21) regulate a common set of chemosensory behaviors in caenorhabditis elegans

Caenorhabditis elegans daf-11 and daf-21 mutants share defects in specific chemosensory responses mediated by several classes of sensory neurons, indicating that these two genes have closely related functions in an assortment of chemosensory pathways. We report that daf-11 encodes one of a large family of C. elegans transmembrane guanylyl cyclases (TM-GCs). The cyclic GMP analogue 8-bromo-cGMP rescues a sensory defect in both daf-11 and daf-21 mutants, supporting a role for DAF-11 guanylyl cyclase activity in this process and further suggesting that daf-21 acts at a similar step. daf-11::gfp fusions are expressed in five identified pairs of chemosensory neurons in a pattern consistent with most daf-11 mutant phenotypes. We also show that daf-21 encodes the heat-shock protein 90 (Hsp90), a chaperone with numerous specific protein targets. We show that the viable chemosensory-deficient daf-21 mutation is an unusual allele resulting from a single amino acid substitution and that the daf-21 null phenotype is early larval lethality. These results demonstrate that cGMP is a prominent second messenger in C. elegans chemosensory transduction and suggest a previously unknown role for Hsp90 in regulating cGMP levels.

Critical role of sulfenic acid formation of thiols in the inactivation of glyceraldehyde-3-phosphate dehydrogenase by nitric oxide

The relationship between possible modifications of the thiol groups of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) by nitric oxide (NO) and modified enzyme activity was examined. There are 16 free thiols, including 4 active site thiols, in a tetramer of GAPDH molecule. NO donors, sodium nitroprusside (SNP), and S-nitroso-N-acetyl-DL-penicillamine (SNAP) decreased the number of free thiols with a concomitant inhibition of GAPDH activity in a concentration- and time-dependent manner. After treatment for 30 min, free thiols were maximally decreased to 8-10 per GAPDH tetramer and enzyme activity was also inhibited to 5-10% of control activity. In the presence of 30 mM dithiothreitol (DTT), these effects were completely blocked. Since similar results were obtained in the case of hydrogen peroxide (H2O2) treatment, which is known to oxidize the thiols, these effects of nitric oxide donors were probably due to modification of thiol groups present in a GAPDH molecule. On the other hand, DTT posttreatment after the treatment of GAPDH with SNP, SNAP, or H2O2 did not completely restore the modified thiols and the inhibited enzyme activity. DTT posttreatment after the 30-min-treatment with these agents restored free thiols to 14 in all treatments. In the case of SNAP treatment, all 4 active sites were restored and enzyme activity reached more than 80% of the control activity, but in two other cases one active site remained modified and enzyme activity was restored to about only 20%. Therefore, all 4 free thiols in the active site seem to be very important for full enzyme activity. DTT posttreatment in the presence of sodium arsenite, which is known to reduce sulfenic acid to thiol, almost completely restored both thiol groups and enzyme activity. These findings suggest that nitric oxide inhibits GAPDH activity by modifications of the thiols which are essential for this activity, and that the modification includes formation of sulfenic acid, which is not restored by DTT. S-nitrosylation, which is one type of thiol modification by NO, occurred when GAPDH was treated with SNAP but not SNP. Analysis of thiol modification showed that SNAP preferentially nitrosylated the active site thiols, the nitrosylation of which fully disappeared by DTT posttreatment. It seems that SNAP nitrosylates the active site thiols of GAPDH to prevent these thiols from oxidizing to sulfenic acid.

The parafascicular thalamic nucleus but not the prefrontal cortex facilitates the nitric oxide/cyclic GMP pathway in rat striatum

We investigated whether the parafascicular thalamic nucleus and the prefrontal cortex, the two major excitatory inputs to the striatum, modulate the nitric oxide/cyclic GMP pathway in rat striatum. Electrical stimulation (10 pulses of 0.5 ms, 10 V applied at 10 Hz, 140 microA) delivered bilaterally to the parafascicular thalamic nucleus for a total of 4, 10 and 20 min, time-dependently facilitated cyclic GMP output in the dorsal striatum of freely moving rats, assessed by trans-striatal microdialysis. Electrical stimulation to the prefrontal cortex for a total duration of 20 min did not affect striatal cyclic GMP levels. The facilitatory effect observed after electrical stimulation of the parafascicular thalamic nucleus was blocked by co-perfusion with tetrodotoxin, suggesting that the effect is mediated by neuronal process(es). The non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (30 microM infused into the dorsal striatum), and the competitive one, 3-[(R)-carboxypiperazin-4-yl]-propyl-phosphonic acid (50 microM infused), but not local perfusion of the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid antagonist, 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (15 microM perfused locally), abolished the cyclic GMP response in the striatum. The nitric oxide synthase inhibitor, 7-nitroindazole, applied locally (1 mM), blocked the electrically evoked increase in striatal extracellular cyclic GMP. This increase was also prevented by local application (100 and 300 microM) of 1H-(1,2,4)-oxadiazolo-(4,3a)-quinoxalin-1-one, a selective inhibitor of soluble guanylyl cyclase. The results provide direct functional evidence of selective thalamic facilitation of the nitric oxide/cyclic GMP pathway in the dorsal striatum, through activation of N-methyl-D-aspartate receptors.

Cyclic-3',5'-nucleotide phosphodiesterase isozymes in cell biology and pathophysiology of the kidney

Investigations of recent years revealed that isozymes of cyclic-3', 5'-nucleotide phosphodiesterase (PDE) are a critically important component of the cyclic-3',5'-adenosine monophosphate (cAMP) protein kinase A (PKA) signaling pathway. The superfamily of cyclic-3', 5'-phosphodiesterase (PDE) isozymes consists of at least nine gene families (types): PDE1 to PDE9. Some PDE families are very diverse and consist of several subtypes and numerous PDE isoform-splice variants. PDE isozymes differ in molecular structure, catalytic properties, intracellular regulation and location, and sensitivity to selective inhibitors, as well as differential expression in various cell types. A number of type-specific "second-generation" PDE inhibitors have been developed. Current evidence indicates that PDE isozymes play a role in several pathobiologic processes in kidney cells. In rat mesangial cells, PDE3 and PDE4 compartmentalize cAMP signaling to the PDE3-linked cAMP-PKA pathway that modulates mitogenesis and PDE4-linked cAMP-PKA pathway that modulates generation of reactive oxygen species. Administration of selective PDE isozyme inhibitors in vivo suppresses proteinuria and pathologic changes in experimental anti-Thy-1.1 mesangial proliferative glomerulonephritis in rats. Increased activity of PDE5 (and perhaps also PDE9) in glomeruli and in cells of collecting ducts in sodium-retaining states, such as nephrotic syndrome, accounts for renal resistance to atriopeptin; diminished ability to excrete sodium can be corrected by administration of the selective PDE5 inhibitor zaprinast. Anomalously high PDE4 activity in collecting ducts is a basis of unresponsiveness to vasopressin in mice with hereditary nephrogenic diabetes insipidus. Apparently, PDE isozymes apparently also play an important role in the pathogenesis of acute renal failure of different origins. Administration of PDE isozyme-selective inhibitors suppresses some components of immune responses to allograft transplant and improves preservation and survival of transplanted organ. PDE isozymes are a target for action of numerous novel selective PDE inhibitors, which are key components in the design of novel "signal transduction" pharmacotherapies of kidney diseases.

Light-increased cGMP and K+ conductance in the hyperpolarizing receptor potential of Onchidium extra-ocular photoreceptors

The phototransduction mechanism of the extra-ocular photoreceptor cells Ip-2 and Ip-1 in the mollusc Onchidium ganglion was examined. Previous work showed that the depolarizing receptor potential of another extra-ocular photoreceptor cell, A-P-1 is produced by a decrease of the light-sensitive K+ conductance activated by a second messenger, cGMP and is inactivated by the hydrolysis of cGMP. Here, a hyperpolarizing receptor potential of Ip-2 or Ip-1 was associated with an increase in membrane conductance. When Ip-2 or Ip-1 was voltage-clamped near the resting membrane potential, light induced an outward photocurrent corresponding to the above hyperpolarization. The spectral sensitivity had a peak at 510 nm. The shift of reversal potentials of the photocurrent depended on the Nernst equation of K(+)-selective conductance. The photocurrent was blocked by 4-AP and L-DIL, which are effective blockers of the A-P-1 light-sensitive K+ conductance. These results suggested that the hyperpolarization is mediated by increasing a similar light-sensitive K+ conductance to that of A-P-1. The injection of cGMP or Ca2+ into a cell produced a K+ current that mimicked the photocurrent. 4-AP and L-DIL both abolished the cGMP-activated K+ current, while TEA suppressed only the Ca(2+)-activated K+ current. These results indicated that cGMP is also a second messenger that regulates the light-sensitive K+ conductance. The photocurrent was blocked by LY-83583, a guanylate cyclase (GC) inhibitor, but was unaltered by zaprinast, a phosphodiesterase (PDE) inhibitor. Together, the present results suggest that increasing the internal cGMP in Ip-2 or Ip-1 cells light-activates GC rather than inhibits PDE, thereby leading to an increase of the light-sensitive K+ conductance and the hyperpolarization.

Safety and efficacy of sildenafil citrate in the treatment of male erectile dysfunction

Sildenafil citrate, an oral therapy for erectile dysfunction, is a selective inhibitor of cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase type 5 (PDE5), the predominant isozyme metabolizing cGMP in the corpus cavernosum. Chemically, it is a compound of the pyrazolo-pyrimidinyl-methylpiperazine class. Sildenafil has no direct relaxant effect on human corpus cavernosum but enhances the relaxant effect of nitric oxide (NO) on the corpus cavernosum by inhibiting PDE5, which is responsible for degradation of cGMP in this tissue. When sexual stimulation causes local release of NO, inhibition of PDE5 by sildenafil increases concentrations of cGMP in the corpus cavernosum, causing smooth muscle relaxation and blood flow into the penis, resulting in an erection. Sildenafil at recommended doses has no effect in the absence of sexual stimulation. The drug is rapidly absorbed after oral administration, with absolute bioavailability of 40%. Its pharmacokinetics are dose proportional over the recommended dosage range. Maximum plasma concentrations are reached within 30 to 120 minutes after oral dosing in the fasting state. Sildenafil is cleared predominantly by the hepatic microsomal isoenzymes CYP3A4 (major route) and CYP2C9 (minor route). Clinical studies assessed the effect of sildenafil on the ability of men with erectile dysfunction to engage in sexual activity and, specifically, to achieve and maintain an erection sufficient for satisfactory sexual intercourse. Sildenafil was evaluated at doses of 25, 50, and 100 mg in randomized, double-masked, placebo-controlled clinical trials of up to 6 months' duration. The drug was administered to hundreds of patients aged 19 to 87 years having erectile dysfunction of various etiologies for a mean duration of 5 years. Sildenafil was associated with statistically significant improvement in erectile function compared with placebo. Adverse effects reported at a rate of >2% were headache, flushing, dyspepsia, nasal congestion, urinary tract infection, abnormal vision, diarrhea, dizziness, and rash. No cases of priapism were reported. The use of sildenafil is contraindicated in men who are taking organic nitrates, because of the potential for a precipitous decrease in blood pressure. Postmarketing reports and surveillance have revealed at least 39 deaths with sildenafil use in men having a history of heart disease, men taking nitrate medications, and men in poor physical health due to lack of exercise. Many of the men who experienced serious adverse effects or death had a variety of concomitant diseases and were taking multiple medications.

Real-time patch-cram detection of intracellular cGMP reveals long-term suppression of responses to NO and muscarinic agonists

Cyclic GMP (cGMP) is a crucial intracellular messenger in neuronal, muscle, and endocrine cells. The intracellular concentration of cGMP is regulated by various neurotransmitters, including acetylcholine (ACh) and nitric oxide (NO). While much is known about the biochemical steps leading to cGMP synthesis, little is known about cGMP kinetics in intact cells. Here, we use "patch-cramming," in which an excised, inside-out membrane patch containing cyclic nucleotide-gated ion channels is used as a biosensor, to obtain the first real-time measurements of cGMP in intact cells. Patch-cramming experiments on neuroblastoma cells show that both muscarinic agonists and NO rapidly elevate cGMP. NO elicits cGMP responses repeatedly without decrement, whereas responses to muscarinic agonists exhibit a profound and prolonged desensitization. Remarkably, muscarinic agonists also cause long-term (>30 min) suppression (LTS) of cGMP responses elicited by NO. Biochemical measurements reveal that rat sympathetic neurons also exhibit LTS of cGMP, suggesting that LTS is a widespread mechanism that may contribute to synaptic plasticity.

Reexamination of opioid stimulation of cGMP formation in cell lines of neuronal origin

1. The present study reexamines a previous notion on opioid stimulation of cyclic GMP (cGMP) formation and the retraction of the original findings. 2. The effect of opioid agonists on cGMP accumulation in two cell lines of neuronal origin was measured. The proportion of cGMP stimulation in NG108-15 neuroblastoma x glioma hybrid cells resembled the proportion of [Ca2+]in elevation by opioids in this culture. The failure of opioids to stimulate cGMP formation in SK-N-SH human neuroblastoma coincided with the lack of cGMP stimulation by other Ca2+ mobilizing agents in these cells. The nitric oxide donor nitroprusside elevated cGMP in both cell lines. 3. The implication of the opioid-Ca(2+)-NO-cGMP cellular pathway for opioid activity in vivo is discussed.

Anti-diuresis in the blood-feeding insect Rhodnius prolixus Stål: antagonistic actions of cAMP and cGMP and the role of organic acid transport

Secretion of primary urine by upper Malpighian tubules of the blood-sucking insect Rhodnius prolixus has recently been shown to be inhibited by cyclic GMP (cGMP). In the present work, we have demonstrated that cGMP has effects antagonistic to those of cAMP in Rhodnius tubules and have further characterized the effects of cGMP on tubular secretion. Cyclic GMP inhibited secretion at all concentrations from 5x10(-6) to 10(-3)M, though this inhibition was partially or wholly reversed by large (2mM) doses of cAMP. While sub-maximal concentrations of cGMP did not significantly alter [K(+)] and [Na(+)] of secreted fluid, high external [cGMP] reduced secretion to minimal levels and caused [K(+)] and [Na(+)] to approach pre-stimulation levels. Cyclic GMP does not appear to affect the permeability of the lower Malpighian tubule to water. Both cAMP and cGMP likely enter tubule cells by way of an organic acid transporter whose activity is induced by feeding. Sensitivity of the tubules to exogenous cGMP and cAMP, which is assumed to be a function of transport activity, reaches a peak approximately 5 days after the blood meal and declines rapidly thereafter. Transport of anions into upper tubules involves at least two different transporters: one for acylamides (e.g., p-aminohippuric acid) and another for sulphonates (e.g., amaranth, phenol red). Amaranth and phenol red blocked the actions of both cGMP and cAMP, whereas p-aminohippuric acid was without effect. This suggests that cyclic nucleotides enter by way of the sulphonate transporter.

Presence and biological activity of C-type natriuretic peptide-dependent guanylate cyclase-coupled receptor in the penile corpus cavernosum

PURPOSE

To investigate the presence of C-type natriuretic peptide 1-22 (CNP)-dependent guanylyl cyclase (GC)-coupled receptor and its biological function in the penile erectile smooth muscle.

MATERIALS AND METHODS

Experiments have been done in rabbit and rat to detect cyclic GMP (cGMP) generation by the activation of particulate GC by natriuretic peptides (NPs) in cavernosal membrane, to localize precise receptor using a quantitative in vitro autoradiography of the snap frozen sections, to define natriuretic peptide receptor (NPR) mRNA using a reverse transcriptase-polymerase chain reaction (RT-PCR) technique and to monitor changes of erectile smooth muscle tone by NPs in the penile tissue strips.

RESULTS

Productions of cGMP by particulate GC in the corpus cavernosum membranes of rabbit and rat were stimulated by CNP, atrial natriuretic peptide 1-28 (ANP) and brain natriuretic peptide 1-26 (BNP) with a rank order of potency of CNP > BNP > ANP. HS-142-1, a selective antagonist for the GC-coupled NPR, inhibited the CNP-stimulated cGMP production in corpus cavernosal membrane of rabbit and rat. Specific 125I-(Tyr[0])-CNP bindings were localized in the corpus cavernosal smooth muscle of rabbit with Kd of 19.92+/-3.38 nM. and Bmax of 734.64+/-139.63 amol./mm2. B-subtype of NPR mRNA was detected in the penile corpus cavernosum of rat using RT-PCR technique. CNP relaxed the smooth muscle contracted by Nomega-nitro-L-arginine methylester (L-NAME).

CONCLUSIONS

These results suggest for the first time that CNP modulates the erectile smooth muscle tone of penis by predominant activation of B-subtype of NPR with augmentation of cGMP production via particulate GC.

Synergistic activation of soluble guanylate cyclase by YC-1 and carbon monoxide: implications for the role of cleavage of the iron-histidine bond during activation by nitric oxide

BACKGROUND

Nitric oxide (.NO) is used in biology as both an intercellular signaling agent and a cytotoxic agent. In signaling, submicromolar quantities of .NO stimulate the soluble isoform of guanylate cyclase (sGC) in the receptor cell. .NO increases the Vmax of this heterodimeric hemoprotein up to 400-fold by interacting with the heme moiety of sGC to form a 5-coordinate complex. Carbon monoxide (CO) binds to the heme to form a 6-coordinate complex, but only activates the enzyme 5-fold, YC-1 is a recently discovered compound that relaxes vascular smooth muscle by stimulating sGC.

RESULTS

In the presence of YC-1, CO activates sGC to the same specific activity as attained with .NO. YC-1 did not affect the NO-stimulated activity. The on-rate (kon) and off-rate (koff) of CO for binding to sGC in the presence of YC-1 were determined by stopped-flow spectrophotometry. Neither the kon nor the koff varied from values previously obtained in the absence of YC-1, indicating that YC-1 has no effect on the affinity of CO for the heme. In the presence of YC-1, the visible spectrum of the sGC-CO complex has a Soret peak at 423 nm, indicating the complex is 6-coordinate.

CONCLUSIONS

YC-1 has no effect on the affinity of CO for the heme of sGC. In the presence of YC-1, maximal activation of sGC by CO is achieved by formation of a 6-coordinate complex between CO and the heme indicating that cleavage of the Fe-His bond is not required for maximal activation of sGC.

Developing grasshopper neurons show variable levels of guanylyl cyclase activity on arrival at their targets

The ability of certain grasshopper neurons to respond to exogenously applied donors of nitric oxide (NO) by producing cyclic GMP (cGMP) depends on their developmental state. ODQ, a selective blocker of NO-sensitive guanylyl cyclase, blocks cGMP production at 10(-5) M, thus confirming the nature of the response. Experiments in which the distal axon is separated from its proximal stump before application of an NO donor show that guanylyl cyclase is distributed uniformly throughout the neuron. In the locust abdomen, where segments are formed sequentially, the pattern of guanylyl cyclase up-regulation is predictable and sequential from anterior to posterior. There are two patterns of innervation by cGMP-expressing motor neurons. In the first, typified by muscle 187, an innervating neuron begins to be NO responsive on arrival at its muscle and continues to be so over most of the remainder of embryonic development, including the formation of motor end plates. In the second, typified by a neuron innervating muscle 191, the neuron extends well along the muscle, apparently laying down a number of sites of contact with it, before it becomes NO responsive. In both patterns, however, NO responsiveness marks the neuron's transition from growth cone elongation to the production of lateral branches. Individual muscles receive innervation from multiple motor neurons, some of which express transient NO sensitivity during development and others which do not. With the exception of the leg motor neuron SETi, the first motor neuron to reach any muscle is usually not NO responsive. We suggest that cGMP plays a role in, or reflects, the early stages of communication between a target and specific innervating neurons.

The role of nitric oxide in hepatic metabolism

Nitric oxide (NO) may regulate hepatic metabolism directly by causing alterations in hepatocellular (hepatocyte and Kupffer cell) metabolism and function or indirectly as a result of its vasodilator properties. Its release from the endothelium can be elicited by numerous autacoids such as histamine, vasoactive intestinal peptide, adenosine, ATP, 5-HT, substance P, bradykinin, and calcitonin gene-related peptide. In addition, NO may be released from the hepatic vascular endothelium, platelets, nerve endings, mast cells, and Kupffer cells as a response to various stimuli such as endotoxemia, ischemia-reperfusion injury, and circulatory shock. It is synthesized by nitric oxide synthase (NOS), which has three distinguishable isoforms: NOS-1 (ncNOS), a constitutive isoform originally isolated from neuronal sources; NOS-2 (iNOS), an inducible isoform that may generate large quantities of NO and may be induced in a variety of cell types throughout the body by the action of inflammatory stimuli such as tumor necrosis factor and interleukin (IL)-1 and -6; and NOS-3 (ecNOS), a constitutive isoform originally located in endothelial cells. Another basis for differentiation between the constitutive and inducible enzymes is the requirement for calcium binding to calmodulin in the former. NO is vulnerable to a plethora of biologic reactions, the most important being those involving higher nitrogen oxides (NO2-), nitrosothiol, and nitrosyl iron-cysteine complexes, the products of which (for example, peroxynitrite), are believed to be highly cytotoxic. The ability of NO to react with iron complexes renders the cytochrome P450 series of microsomal enzymes natural targets for inhibition by NO. It is believed that this mechanism provides negative feedback control of NO synthesis. In addition, NO may regulate prostaglandin synthesis because the cyclooxygenases are other hem-containing enzymes. It may also be possible that NO-induced release of IL-1 inhibits cytochrome P450 production, which ultimately renders the liver less resistant to trauma. It is believed that Kupffer cells are the main source of NO during endotoxemic shock and that selective inhibition of this stimulation may have future beneficial therapeutic implications. NO release in small quantities may be beneficial because it has been shown to decrease tumor cell growth and levels of prostaglandin E2 and F2 alpha (proinflammatory products) and to increase protein synthesis and DNA-repair enzymes in isolated hepatocytes. NO may possess both cytoprotective and cytotoxic properties depending on the amount and the isoform of NOS by which it is produced. The mechanisms by which these properties are regulated are important in the maintenance of whole body homeostasis and remain to be elucidated.

Modulation of chromatic difference in receptive field size of H1 horizontal cells in carp retina: dopamine- and APB-sensitive mechanisms

Chromatic aspects of receptive field size in the H1 horizontal cell syncytium of the carp retina were investigated using spectral photostimuli (blue or red) presented in the form of either a pair of a small spot and annulus, or a narrow moving slit. In the light-adapted retina, the receptive field for the blue stimulus was found to be significantly smaller than that for the red, i.e. there was a chromatic difference in the receptive field size. During the course of dark adaptation, the overall receptive field size increased, but the chromatic difference decreased. Immediately after adaptation to bright light, the receptive field sizes were reduced significantly, but the chromatic difference increased, mainly due to a greater reduction in the receptive field for the blue stimulus. Application of dopamine (5 microM) to a dark-adapted retina gradually decreased the receptive field size for both colours, but the chromatic difference became larger, again due to a greater reduction in the receptive field size for the blue stimulus. 2-Amino-4-phosphonobutyrate (APB) applied to light-adapted retinae at a working concentration of 1 mM, greatly expanded the receptive field size and suppressed the chromatic difference due to the effect being greater for the receptive field for the blue stimulus. The effect of APB was slow and cumulative. On the other hand, intracellular injection of cGMP or dibutyryl-cGMP increased the chromatic difference in the receptive field size. It is suggested (i) that the chromatic difference in the receptive field size could be due to a cGMP-coupled, conductance-decreasing receptor mechanism activated by APB; and (ii) that the mechanism is associated with short-wavelength sensitive cone input to the H1 cells and operates in the light-adapted state of the retina.

Metabotropic glutamate receptor-mediated suppression of an inward rectifier current is linked via a cGMP cascade

Glutamate, the neurotransmitter released by photoreceptors, excites horizontal cells and OFF-type bipolar cells by activating ionotropic receptors. This study investigated an additional action of glutamate in which it modulates a voltage-gated ion channel in horizontal cells. We find that glutamate and APB (2-amino-4-phosphonobutyrate) produce a delayed and moderately prolonged suppression of an inward rectifier current (IRK+). This effect is proposed to occur via an APB-sensitive metabotropic glutamate receptor (mGluR) because common agonists for the ionotropic or APB-insensitive mGluRs are ineffective and the APB-insensitive receptor antagonist alpha-methyl-4-carboxyphenylglycine (MCPG) does not block the actions of glutamate or APB. 8-Br-cGMP, 1-methyl-3-isobutylxanthine (IBMX), and atrial natriuretic peptide (ANP) but not 8-Br-cAMP mimic the suppression of IRK+. The effects of glutamate and APB are blocked by protein kinase inhibitors including Rp-8-pCPT-cGMPS, H-8, and H-7 as well as by ATPgammaS. We hypothesize that the APB receptor suppresses IRK+ via upregulation of cGMP and subsequent activation of a cGMP-dependent protein kinase. This pathway is likely regulated by an ATP-dependent phosphorylation. This is a novel signaling pathway for mGluRs and indicates that at least two distinct APB-activated pathways exist in the retina. Functionally, this APB receptor-mediated action found in horizontal cells would provide a means by which spatially restricted changes of glutamate, produced by local illumination of photoreceptors, could regulate IRK+ and consequently the response properties of these neurons. This would serve to adapt selectively retinal regions stimulated by small regions of the visual world.

Investigations into the role of nitric oxide and the large intracranial arteries in migraine headache

Previous studies suggest that nitric oxide (NO) is involved in headaches induced by i.v. infusion of the vasodilator and NO donor glyceryl trinitrate (GTN) in healthy subjects. Extending these studies to sufferers of migraine without aura, it was found that migraineurs experienced a stronger headache than non-migraineurs. In addition, most migraineurs experienced a delayed migraine attack at variable times (mean 5.5 h) after GTN provocation. This biphasic headache response in migraineurs may be linked to hypersensitivity in the NO-cGMP pathway. Thus, compared to controls, migraineurs were found to be more sensitive to GTN-induced intracranial arterial dilatation, which is known to be mediated via liberation of NO and subsequent synthesis of cGMP Furthermore, histamine infusions in migraineurs induced headache responses and intracranial arterial responses resembling those induced by GTN in migraineurs. Histamine is known to liberate NO from the endothelium via stimulation of the H1 receptor, which is present in the large intracranial arteries in man. Because both immediate histamine-induced headache and intracranial arterial dilatation and delayed histamine-induced migraine are blocked by H1-receptor blockade, a likely common pathway for GTN and histamine-induced headaches/migraines and intracranial arterial responses may be via activation of the NO-cGMP pathway. The delay in the development of these experimental migraines may reflect activation of multiple physiological processes. The intracranial arteries of migraineurs were found supersensitive to the vasodilating effect of GTN (exogenous NO). This relates to clinical findings suggesting dilatation of the large intracranial arteries on the headache side during spontaneous migraine attacks. The function of arterial regulatory mechanisms involving NO in migraine was therefore studied. In peripheral arteries, no endothelial dysfunction of NO was found and cardiovascular and intracranial arterial sympathetic function was normal. A mild parasympathetic dysfunction may be involved and may, via denervation supersensitivity, be responsible for the observed supersensitivity to NO. Another possibility is that NO initiates a perivascular neurogenic inflammation with liberation of vasoactive peptides. NO also mediates a variety of other physiological phenomena. One of these, the pain-modulating effect observed in animals, was evaluated in a human study using GTN infusion and measurements of pain thresholds. No definite effects of GTN were demonstrated. The precise mechanisms involved in NO-triggered migraines and which part of the NO-activated cascade that is involved remain to be determined. The possibilities for pharmacological stimulation and/or inhibition of several steps of the NO-activated cascade increase rapidly and soon may be available for human studies.

Requirement for cGMP in nerve cell death caused by glutathione depletion

Glutathione depletion occurs in several forms of apoptosis and is associated with Parkinson's disease and HIV toxicity. The neurotransmitter glutamate kills immature cortical neurons and a hippocampal nerve cell line via an oxidative pathway associated with glutathione depletion. It is shown here that soluble guanylyl cyclase (sGC) activity is required for nerve cell death caused by glutathione depletion. Inhibitors of sGC block glutamate toxicity and a cGMP analogue potentiates cell death. Glutamate also induces an elevation of cGMP that occurs late in the cell death pathway. The resultant cGMP modulates the increase in intracellular calcium that precedes cell death because sGC inhibitors prevent calcium elevation and the cGMP analogue potentiates the increase in intracellular calcium. These results suggest that the final pathway of glutamate induced nerve cell death is through a cGMP-modulated calcium channel.

Cyclic AMP and cyclic GMP concentrations in, and efflux from, preimplantation cattle embryos

Basal embryonic cAMP and cGMP concentrations and cAMP and cGMP accumulation in embryos and in the incubation medium were measured in cattle blastocysts recovered at days 14 (n = 23), 15 (n = 29) and 16 (n = 23) of pregnancy. Cyclic AMP and cGMP concentrations were measured by radioimmunoassay and the results expressed per microgram of protein, which was determined by the Pierce Micro BCA protein assay. Cyclic AMP and cGMP were present on each day. Basal embryonic cAMP was similar on days 14, 15, and 16, at 2.3., 2.5 and 2.6 fmol per microgram protein, respectively, while the concentration of cGMP was higher at day 14 (0.14 fmol per microgram protein than at either day 15 or 16 (0.06 and 0.05 fmol per microgram protein, respectively; P < 0.05), which were similar (P > 0.10). Basal embryonic cAMP concentrations were 15 to 60-fold higher than cGMP concentrations. Following a 2 h culture period in the presence or absence of the phosphodiesterase (PDE) inhibitor, isobutylmethylxanthine (IBMX), the accumulated concentrations of cAMP and cGMP were measured in the embryos and in the incubation medium. IBMX did not affect the concentrations of either cAMP or cGMP in the embryos but increased the concentrations of cAMP (P < 0.005) and cGMP (P < 0.01) in the incubation medium. There was no effect of day on either embryonic or medium cAMP, but both embryonic and medium cGMP were higher at day 14 than at day 15 (P < 0.05) or 16 (P < 0.005). Differences between embryonic and medium cAMP and cGMP accumulation were examined. There was no effect of day or treatment. cGMP accumulation in the medium was higher than in the embryos (P < 0.005) whereas cAMP accumulation in the medium was lower than in the embryos (P < 0.05). Whether the efflux of cAMP and cGMP is active or passive is not clear but it may indicate a possible role in embryo-maternal signalling.

Angiotensin II interacts with nitric oxide-cyclic GMP pathway in the central control of drinking behaviour: mapping with c-fos and NADPH-diaphorase

Recognition of the role of nitric oxide in cell-to-cell communication has changed the concept of traditional neurotransmission. We have shown previously that N-methyl-D-aspartate receptors mediate dipsogenic responses and c-Fos expression induced by intracerebroventricular infusion of angiotensin II. Since these receptors are known to be linked to the nitric oxide-cyclic GMP pathway, the present study explores the contribution of this path to the behavioural and cellular effects of intracerebroventricular angiotensin II by using behavioural testing, NADPH-diaphorase histochemistry and immunocytochemical staining for the immediate-early gene, c-fos. N(G)-nitro-L-arginine methyl ester (125 and 250 microg, intracerebroventricular), an inhibitor of nitric oxide synthase, and Methylene Blue (100 microg), an inhibitor of guanylate cyclase activation, antagonized water intake induced by intracerebroventricular injection of 25 pmol angiotensin II. The effects of N(G)-nitro-L-arginine methyl ester were reversed by co-injection of L-arginine, the substrate for nitric oxide synthase. However, N(G)-nitro-L-arginine methyl ester did not alter the pattern of angiotensin II-induced c-fos expression in the organum vasculosum of the lamina terminalis, median preoptic nucleus, hypothalamic paraventricular nucleus and supraoptic nucleus. Double staining with NADPH-diaphorase histochemistry and c-Fos immunocytochemistry showed that neurons staining for both were localized to the anterior third ventricle. However, only 19-25% of the c-Fos-positive neurons expressed NADPH. There were also substantial numbers of neurons in which angiotensin II induced c-Fos that were NADPH-negative. Extensive co-distribution of NADPH-diaphorase-stained cells and those expressing c-fos in response to intracerebroventricular injection of angiotensin II, especially in the median preoptic nucleus, imply that nitric oxide might participate in the mechanism of angiotensin II-induced drinking behaviour. However, a low rate of co-localization of the two markers to individual cells suggests that angiotensin II stimulated the production of nitric oxide and c-Fos in different populations of neurons. Since our previous results showed that glutamate blockade, but not nitric oxide synthase inhibition, suppressed angiotensin II-induced c-Fos, the experiments reported here further suggest that nitric oxide release is not an essential requirement for the expression of c-fos elicited by angiotensin II. They also provide evidence that the dipsogenic and c-Fos responses to angiotensin II are dissociated at a cellular level.

The nitric oxide system in insects

It is well established that nitric oxide (NO) acts as a signalling molecule in the nervous system of both mammals and insects. In contrast to classical transmitters, the membrane-permeant NO can act on neighbouring targets limited by half-life and diffusion barriers. This type of diffuse signalling seems to be evolutionarily highly conserved and recent findings concerning the characterization and function of the NO system in insects are summarized in this review. Firstly, the properties and the localization of the NO forming enzyme, the NO synthase (NOS), are described. In the nervous system the brain contains by far the highest NOS activity. As an evolutionary peculiarity, a blood-feeding bug exhibits high NOS activity in the salivary glands. Secondly, the soluble guanylate cyclase (sGC), a major target of NO action, and cGMP-regulated enzymes like cGMP-dependent protein kinase and cyclic nucleotide gated channels are described. Anatomical organization of the NO/cGMP system in insects reveals evidence for a cellular separation of the release site and target site of NO, although in the antennal lobes of the locust an exception from this rule exists. Thirdly, the implication of the NO system in neuronal function in insects is described. In the honeybee, the NO/cGMP system in the antennal lobes is implicated in the processing of adaptive mechanisms during chemosensory processing, and recent findings support a specific role of the NO system in memory formation. Discussion of the results in insects with regard to properties and functions of the vertebrate NO system is attempted.

Influence of nitric oxide on transepithelial transport in toad skin: effects of cholinergic agents and morphine

The effects induced by L-arginine (L-Arg) on the short-circuit current and potential difference of Pleurodema thaul skin were investigated. L-Arg, but not D-Arg significantly increased the short-circuit current and potential difference when applied to the serosal surface. The effects of L-Arg were antagonized by amiloride, NG-nitro-methyl-L-arginine (L-NAME) and by methylene blue. Carbachol and acetylcholine induced significant increases of both electrical parameters of the toad skin. These effects of the muscarinic cholinergic drugs were potentiated by a low concentration of L-Arg and antagonized by L-NAME or methylene blue. Carbachol and acetylcholine induced significant increases of both electrical parameters of the toad skin. These effects of the muscarinic cholinergic drugs were potentiated by a low concentration of L-Arg and antagonized by L-NAME or methylene blue. Addition of dibutyryl cyclic guanosyl monophosphate (db cGMP) or dibutyryl cyclic adenosine monophosphate (db cAMP) increased short-circuit current and potential difference. The effects of db cGMP, but not those of db cAMP were antagonized by L-NAME. The consecutive application of db cGMP and db cAMP induced additive effects. These results suggest that L-Arg increases transport in toad skin presumably acting through the formation of nitric oxide, which then stimulates cytoplasmic guanylate cyclase and leads to increased Na+ and K+ transport. The effects of L-Arg and carbachol were antagonized by acute application of morphine; however, a rebound response was observed when carbachol or noradrenaline were given after prolonged exposure of the skin to morphine, which suggests an adaptive response of the skin involving both cGMP and cAMP. Responses to both nucleotides were unchanged by morphine.

Stable analogues of cyclic AMP but not cyclic GMP sensitize unmyelinated primary afferents in rat skin to heat stimulation but not to inflammatory mediators, in vitro

The aim of this investigation was to evaluate the role played by cyclic nucleotides in the transduction of inflammatory pain and hyperalgesia. Unmyelinated afferents (n = 79) were exposed to stable analogues of cyclic AMP and cyclic GMP, to inflammatory mediators and to Methylene Blue, an inhibitor of guanylyl cyclase. Analogues of cyclic AMP at a concentration of 1 mM (n = 9) but not 10 microM (n = 16) sensitized nociceptor responses to noxious heat and enhanced interstimulus activity. In addition. mechanical thresholds were moderately, but significantly lowered after superfusion of the cyclic AMP analogue (1 mM). Addition of 10 microM cyclic AMP analogue to a mixture of excitatory inflammatory mediators (serotonin, histamine, bradykinin and prostaglandin E2, 10 microM each) did not further increase nociceptor activity (n = 15), in contrast to a previous report that cAMP sensitized bradykinin responses. Cyclic GMP analogues (10 microM, 1 mM) did not alter heat sensitivity or mechanical thresholds of polymodal C-fibres, nor did they enhance the ongoing activity that resulted from repeated heat stimulation. After inhibition of guanylyl cyclase with Methylene Blue, cyclic GMP analogues (1-10 microM) did not alter nociceptor responses evoked by application of the mixture of inflammatory mediators. The findings indicate that polymodal nociceptor sensitization and excitation is independent of cyclic GMP. Cyclic AMP can obviously contribute to the increased heat sensitivity of inflamed tissue, whereas cyclic GMP might be of importance in the recruitment of "silent" nociceptors.

Characterization of guanylyl cyclase in purified myelin

This study was undertaken to characterize the enzymatic properties of the particulate guanylyl cyclase previously shown to be present at a high level of activity in purified rat brain myelin. Significant activation was achieved by both Lubrol-PX and Triton X-100, the latter being somewhat more effective. A pH optimum of 7.8 was observed, compared to 7.4 for microsomes. Employing 1.2 mM GTP with 1% Triton X-100, linearity of response was observed up to 60 min and approximately 1.2 mg of myelin protein. Kinetic analysis revealed Km values of 0.258mM and 0.486mM for myelin and microsomes, respectively, similar values being obtained by Lineweaver-Burke analysis or Direct Linear Plot. Vmax values were 20 and 266 pmol/mg protein/min for myelin and microsomes, respectively. Washing of the myelin with 0.5 M NaCl or 0.1% Na taurocholate did not remove a significant amount of guanylyl cyclase activity, indicating the enzyme to be intrinsic to the myelin sheath.

Inhibition of nitric oxide synthase impairs a distinct form of long-term memory in the honeybee, Apis mellifera

Nitric oxide has been shown to be implicated in neural plasticity that underlies processes of learning and memory. In the honeybee, studies on the role of nitric oxide in associative olfactory learning reveal its specific function in memory formation. Inhibition of nitric oxide synthase during olfactory conditioning impairs a distinct long-term memory that is formed as a consequence of multiple learning trials. Acquisition or retrieval of memory or memory formation induced by a single learning trial is not affected by blocking of nitric oxide synthase. This finding provides a first step toward dissection of molecular mechanisms involved in memory formation, in general, and the special function of nitric oxide synthase in particular.

Nitric oxide in coronary artery disease: roles in atherosclerosis, myocardial reperfusion and heart failure

Nitric oxide (NO), derived from the vascular endothelium or other cells of the cardiovascular system, has an important role in physiological regulation of blood flow and has pathophysiological functions in cardiovascular disease. The mechanisms and enzymes involved in the biosynthesis of NO and biological actions of NO, including vasodilatation, cytotoxicity and inflammation, are briefly reviewed. These reactions involving NO cause pathological disturbances of arterial function, coronary blood flow regulation, and may contribute to cardiac myocyte dysfunction. NO and prostacyclin (PGI2), which is also released from the endothelium, act synergistically to inhibit platelet aggregation and adhesion, and in some arteries these mediators also synergise in terms of vasodilatation. In addition, NO is capable of hyperpolarizing vascular smooth muscle, but activation of the endothelium may cause hyperpolarization and may thus promote vasodilatation by an additional mechanism. After myocardial ischemia and reperfusion, production of NO and superoxide radicals represent important mechanisms of cytotoxicity, causing injury to the coronary endothelium and myocytes and compromising ventricular contractile function. Moreover, upon reperfusion endothelium-dependent vasodilatation is impaired and the coronary arteries constrict, leading to irregular myocardial perfusion. This is a consequence of the accumulation of activated leucocytes that we found to generate endogenous inhibitors of NO. These factors have yet to be fully characterised, but clearly they may have a role in irregularities of myocardial reperfusion and cellular injury. Chronic heart failure is associated both with impairment of endothelium-dependent vasodilatation and with excess production of NO via the inducible NO synthase (iNOS), although it is unclear whether the latter assists or compromises ventricular contractile performance under these conditions. Disturbances in the activity of isoforms of NO synthase in the artery wall also accompany the development of atherosclerosis, providing conditions propitious for vasospasm and thrombosis, and perhaps contributing to cell proliferation. Reversing these NO defects with therapeutic agents including angiotensin converting enzyme (ACE) inhibitors offers promise in protecting against some manifestations of vascular disease.

Effects of maturation on cyclic GMP metabolism in ovine carotid arteries

Previous studies suggest that elevated basal levels of cGMP in newborn arteries may help explain why vascular resistance is lower in newborns than adults. To explore the reasons why basal cGMP is higher in neonatal arteries, the present studies examined rates of cGMP synthesis and degradation in newborn and adult ovine common carotid arteries. The measurements were performed in both intact and homogenized arteries, and results were normalized relative to cell water to estimate intracellular concentrations and minimize errors due to compositional differences between newborn and adult arteries. Steady state levels of cGMP measured under baseline conditions averaged 0.11 +/- 0.02 microM in adult arteries and 0.59 +/- 0.11 microM in newborn arteries. These resting cGMP levels were unaffected by endothelium removal. Under baseline conditions, steady state rates of cGMP synthesis (mumol of cGMP/L of cell water/min) were higher in newborn (0.31 +/- 0.06) than in adult (0.15 +/- 0.04) arteries. Maximal rates of cGMP degradation (mumol of cGMP/L of cell water/min) measured in artery homogenates were also much higher in preparations of newborn (106 +/- 6) than of adult (78 +/- 6) arteries. Together, these data suggest that the reason resting cGMP concentrations were higher in newborn than in adult arteries was due at least in part to a higher basal rate of cGMP synthesis in the newborn. Estimates of apparent Km values for PDE were also greater in newborn (2.9 microM) than in adult (1.5 microM) preparations, suggesting that age-related differences in the Km for PDE may also contribute to the elevated basal concentration of cGMP observed in the newborn.

The nitric oxide/cGMP system in the antennal lobe of Apis mellifera is implicated in integrative processing of chemosensory stimuli

The high concentration and the localization of nitric oxide synthase in the olfactory system of both vertebrates and invertebrates suggest that the diffusible messenger nitric oxide plays a central role in the processing of chemosensory information. This paper describes the nitric oxide releasing system in the antenna and the antennal lobes of Apis mellifera using the NADPH diaphorase technique, and analyses the contribution of the nitric oxide system in the neuronal processing of chemosensory signals using a behavioral assay in vivo. In the antenna the strongest NADPH diaphorase staining is found in non-neuronal auxiliary and/or epithelial cells, while the sensory cells and the antennal nerve are stained at a low level. At the major site of chemosensory signal integration, the antennal lobes, the highest nitric oxide synthase activity is located in the glomeruli, which are ideally suited to act as diffusion compartments. We demonstrate that inhibition of nitric oxide synthase in the antennal lobes specifically interferes with neuronal processing of repetitive chemosensory stimuli but does not affect the response to single stimuli, and is independent of parameters such as satiation level, stimulus strength, interstimulus interval and duration of sensory stimuli. Since inhibition of the soluble guanylate cyclase, a major target of nitric oxide, also particularly affects the adaptive component, the physiological effects of nitric oxide appear to be mediated by the action of cGMP. These findings suggest that the nitric oxide/cGMP system in the antennal lobes is a component of the molecular machinery involved in adaptive and/or integrative mechanisms during chemosensory information processing in vivo.

3,4-Diaminopyridine-evoked noradrenaline release in rat hippocampal slices: facilitation by endogenous or exogenous nitric oxide

The involvement of nitric oxide (NO) in the evoked release of noradrenaline (NA) was studied in rat hippocampal slices preincubated with [3H]NA and stimulated with 3,4-diaminopyridine (3,4-DAP; 200 microM) for 2 min. The 3,4-DAP-evoked [3H]overflow was enhanced by the NO synthase substrate L-arginine, but not by D-arginine; it was reduced by the NO synthase inhibitor NG-nitro-L-arginine, which also antagonized the effects of L-arginine. The corresponding nitro derivative of D-arginine was inactive and unable to block the effects of L-arginine. Also drugs known to produce NO in-vitro, like sodium nitroprusside (SNP), 3-morpholino-sydnonimine (SIN-1) and S-nitroso-N-acetylpenicillamine (SNAP) enhanced the 3,4-DAP-evoked NA release. The NO scavenger hemoglobin showed no significant effects when given alone, but reduced or abolished, respectively, the facilitatory effects of SNP, or SNAP and L-arginine. The cyclic GMP derivatives 8-Br-cGMP and Sp-8-p-chlorophenylthioguanosine-3',5'-cyclic monophosphorothioate (Sp-8-pCPT-cGMPS) also acted facilitatory, whereas the corresponding Rp-enantiomer of the latter compound was inactive, but antagonized the effect of Sp-8-pCPT-cGMPS. NA release evoked by 3,4-DAP (10 microM) from rat hippocampus synaptosomes was not affected by L-arginine or NG-nitro-L-arginine but slightly increased by SNAP and Sp-8-pCPT-cGMPS. Antagonists at NMDA, non-NMDA and metabotropic glutamate receptors neither affected the 3,4-DAP-evoked NA release nor the facilitatory effect of L-arginine.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential modulation of voltage-activated conductances by intracellular and extracellular cyclic nucleotides in leech salivary glands

1. Two-electrode voltage clamp was used to study the effects of adenosine 3':5'-cyclic monophosphate (cyclic AMP) and guanosine 3':5'-cyclic monophosphate (cyclic GMP) on voltage-dependent ion channels in salivary gland cells of the leech, Haementeria ghilianii. 2. Intracellular cyclic AMP specifically blocked delayed rectifier K+ channels. This was shown by use of 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor), forskolin (an activator of adenylyl cyclase) and intracellular injection of cyclic AMP and its dibutyryl and 8-bromo analogues. Cyclic AMP appeared to be the second messenger for the putative neuroglandular transmitter, 5-hydroxytryptamine. 3. Intracellular injection of cyclic GMP specifically potentiated high-voltage-activated (HVA) Ca2+ current and the effect was mimicked by zaprinast, an inhibitor of cyclic GMP-dependent phosphodiesterase. 4. Extracellularly, cyclic GMP and cyclic AMP specifically decreased the amplitude and increased the rate of inactivation of HVA Ca2+ current. These effects of the cyclic nucleotides are identical to those known for extracellular ATP, which activates a presumed purinoceptor. The pyrimidine nucleotide, UTP, was almost equipotent to ATP (threshold dose < 10(-6) M), indicative of a vertebrate-type nucleotide receptor. However, suramin (5 x 10(-5) M), a non-specific P2-receptor antagonist, failed to block the effects of 5 x 10(-6) M ATP (higher suramin doses could not be reliably tested because of the depolarization and increase in membrane conductance produced by the drug). 5. Activation of the putative purinoceptor by ATP did not affect inward rectifier Na+/K+ current which is known to be potentiated by intracellular cyclic AMP and reduced by intracellular cyclic GMP. 6. The preparation may provide a useful model for study of nucleotide actions, and interactions, in channel modulation. It has technical advantages such as large cells (1200 microns in diameter) which lack intercellular coupling and may be individually dissected for biochemical studies.

Molecular characterization of two Drosophila guanylate cyclases expressed in the nervous system

We have isolated, by interspecies hybridization, two classes of Drosophila cDNA each encoding a different guanylate cyclase (GC). One of them encodes an alpha subunit homolog of soluble GC, designated DGC alpha 1, and the other encodes a receptor-type GC, designated DrGC. The dgc alpha 1 cDNA encodes a protein of 676 amino acids and maps to 99B. In situ hybridization to adult tissue sections showed that dgc alpha 1 mRNA is found mainly in the cell bodies of the optic lobe, central brain, and thoracic ganglia. The DGC alpha 1 protein was also localized primarily to the nervous system by immunocytochemical staining, consistent with results of in situ hybridization. However, no detectable expression of this protein was found in the retina. The other class of cDNA, drgc, maps to 76C and encodes a 1525-amino acid protein displaying structural features similar to other known receptor-type guanylate cyclases. However, it has a C-terminal 430 amino acid region that has no homology to any known proteins. drgc RNA is expressed at low levels throughout development and in adult heads and bodies. In situ hybridizations to adult tissue sections showed that drgc mRNA is expressed in a wide range of tissues, including the optic lobe, central brain, thoracic ganglia, digestive tract, and the oocyte.

Identification, characterization, and developmental regulation of a receptor guanylyl cyclase expressed during early stages of Drosophila development

Membrane forms of guanylyl cyclase are single-transmembrane proteins that are activated by the binding of specific peptide ligands to their extracellular domains. In this report, we describe the identification and characterization of a Drosophila cDNA clone encoding a protein, DrGC-1, with high sequence identity to members of this family of receptor proteins. The protein contains a single, hydrophobic domain predicted to represent a transmembrane segment separating an extracellular domain with significant sequence identity (30%) to sea urchin egg peptide receptors from intracellular domains containing a protein kinase-like domain followed by a region with high sequence identity (65%) to cyclase catalytic domains found in receptor guanylyl cyclases from both vertebrates and invertebrates. In contrast to other members of this family, DrGC-1 is predicted to contain a carboxyl-terminal extension of 430 residues that has no homology to any described protein. Northern analysis indicates that DrGC-1 transcripts are present at variable levels in all stages of development. In situ hybridization demonstrates that high levels of uniformly distributed transcript are present in 0-2-h embryos. Later in embryogenesis (14-18 h), elevated levels of hybridization appear to be preferentially associated with muscle fibers.

Activation of adenylate cyclase attenuates the hyperpolarization following single action potentials in brain noradrenergic neurons independently of protein kinase A

Afterhyperpolarizations that follow action potentials are a prominent mechanism for the control of neuronal excitability. Such afterhyperpolarizations in many neurons are modulated by a variety of second messenger systems. Here, we examined the regulation of afterhyperpolarizations in noradrenergic locus coeruleus neurons by the adenylate cyclase system. Although superfusion of the adenylate cyclase activator, forskolin, had no effect on hyperpolarizations following trains of action potentials, both forskolin and a membrane permeable analog of cyclic AMP, 8-bromo-cyclic AMP, attenuated the amplitude of afterhyperpolarizations which followed single action potentials of locus coeruleus neurons recorded intracellularly in brain slices. In contrast, superfusion of 1,9-dideoxyforskolin, the forskolin analog that does not activate adenylate cyclase, had no effect on these single action potential afterhyperpolarizations. Co-application of a protein kinase inhibitor (H8, KT5720, staurosporin or Rp-cAMPS) with either forskolin or 8-bromo-cyclic AMP failed to block the reduction of afterhyperpolarization amplitude, but blocked the cyclic AMP-dependent enhancement of opiate responses in the same locus coeruleus neurons. Furthermore, application of a membrane permeable analog of 5'-AMP, 8-bromo-5'-AMP, the cyclic AMP metabolite that does not activate a protein kinase, potently reduced the amplitudes of single action potential afterhyperpolarizations. The afterhyperpolarization amplitude was also reduced in locus coeruleus neurons taken from chronically morphine-treated rats, a treatment known to increase adenylate cyclase activity. These results indicate that elevation of intracellular cyclic AMP or 5'-AMP reduces the single action potential afterhyperpolarization in locus coeruleus neurons. This action may be mediated through a mechanism independent of protein kinase activation.