The formation and metabolism of cyclic GMP

A dominant variant in the PDE1C gene is associated with nonsyndromic hearing loss

Identification of genes with variants causing non-syndromic hearing loss (NSHL) is challenging due to genetic heterogeneity. The difficulty is compounded by technical limitations that in the past prevented comprehensive gene identification. Recent advances in technology, using targeted capture and next-generation sequencing (NGS), is changing the face of gene identification and making it possible to rapidly and cost-effectively sequence the whole human exome. Here, we characterize a five-generation Chinese family with progressive, postlingual autosomal dominant nonsyndromic hearing loss (ADNSHL). By combining population-specific mutation arrays, targeted deafness genes panel, whole exome sequencing (WES), we identified PDE1C (Phosphodiesterase 1C) c.958G>T (p.A320S) as the disease-associated variant. Structural modeling insights into p.A320S strongly suggest that the sequence alteration will likely affect the substrate-binding pocket of PDE1C. By whole-mount immunofluorescence on postnatal day 3 mouse cochlea, we show its expression in outer (OHC) and inner (IHC) hair cells cytosol co-localizing with Lamp-1 in lysosomes. Furthermore, we provide evidence that the variant alters the PDE1C hydrolytic activity for both cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Collectively, our findings indicate that the c.958G>T variant in PDE1C may disrupt the cross talk between cGMP-signaling and cAMP pathways in Ca2+ homeostasis.

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.

Guanylyl cyclase (GC)-A and GC-B activities in ventricles and cardiomyocytes from failed and non-failed human hearts: GC-A is inactive in the failed cardiomyocyte

Cardiomyocytes release atrial natriuretic peptide (ANP) and B-type natriuretic peptide to stimulate processes that compensate for the failing heart by activating guanylyl cyclase (GC)-A. C-type natriuretic peptide is also elevated in the failing heart and inhibits cardiac remodeling by activating the homologous receptor, GC-B. We previously reported that GC-A is the most active membrane GC in normal mouse ventricles while GC-B is the most active membrane GC in failing ventricles due to increased GC-B and decreased GC-A activities. Here, we examined ANP and CNP-specific GC activity in membranes obtained from non-failing and failing human left ventricles and in membranes from matched cardiomyocyte-enriched pellet preparations. Similar to our findings in the murine study, we found that CNP-dependent GC activity was about half of the ANP-dependent GC activity in the non-failing ventricular and was increased in the failing ventricle. ANP and CNP increased GC activity 9- and 5-fold in non-failing ventricles, respectively. In contrast to the mouse study, in failing human ventricles, ANP-dependent activity was unchanged compared to non-failing values whereas CNP-dependent activity increased 35% (p=0.005). Compared with ventricular membranes, basal GC activity was reduced an order of magnitude in membranes derived from myocyte-enriched pellets from non-failing ventricles. ANP increased GC activity 2.4-fold but CNP only increased GC activity 1.3-fold. In contrast, neither ANP nor CNP increased GC activity in equivalent preparations from failing ventricles. We conclude that: 1) GC-B activity is increased in non-myocytes from failing human ventricles, possibly as a result of increased fibrosis, 2) human ventricular cardiomyocytes express low levels of GC-A and much lower levels or possibly no GC-B, and 3) GC-A in cardiomyocytes from failing human hearts is refractory to ANP stimulation.

Neuronal differentiation of NG108-15 cells has impact on nitric oxide- and membrane (natriuretic peptide receptor-A) cyclic GMP-generating proteins

Cyclic GMP (cGMP), produced in response to either nitric oxide (NO) or certain peptides, controls important neuronal functions. NG108-15 cells were used to characterize the expression of NO- and cGMP-generating proteins and to identify potential alterations associated with neuronal differentiation (neurite outgrowth). We find that these cells contain exclusively neuronal NO synthase (nNOS) isoforms as well as both NO- (soluble guanylyl cyclase, sGC) and natriuretic peptide- (natriuretic peptide receptor-A, NPR-A) responsive cGMP-producing enzymes. The sGC beta(1) subunit (unlike protein phosphatase 2A subunits) is highly membrane-associated. Membrane concentrations of NPR-A and nNOS, but not sGC beta(1) protein are up-regulated with neuronal differentiation. Intriguingly, the rate of hormone-induced cGMP production by NPR-A is significantly diminished in differentiated cells. These findings support roles for NPR-A, the common receptor of atrial (ANP) and B-type (BNP) natriuretic peptide in mature neurons and provide evidence for pronounced changes in neuronal submembrane cGMP signalling during neuronal differentiation.

Phosphodiesterase type 5: expanding roles in cardiovascular regulation

Phosphodiesterase type 5A (PDE5A) selectively hydrolyzes cyclic GMP. Inhibitors of PDE5A such as sildenafil are widely used to treat erectile dysfunction, but growing evidence supports important roles for the enzyme in both the vasculature and heart. In disorders such as cardiac failure, PDE5A upregulation may contribute to a decline in cGMP and protein kinase G signaling, exacerbating dysfunction. PDE5A plays an important role in the pulmonary vasculature where its inhibition benefits patients with pulmonary hypertension. In the heart, PDE5A signaling appears compartmentalized, and its inhibition is cardioprotective against ischemia-reperfusion and antracycline toxicity, blunts acute adrenergic contractile stimulation, and can suppress chronic hypertrophy and dysfunction attributable to pressure-overload. In this review, we discuss the molecular biology, pharmacology, and physiology of PDE5A, mechanisms of vascular and cardiac regulation, and recent evidence supporting the utility of selective PDE5A inhibition for the treatment of cardiovascular disorders.

Atrial natriuretic peptide-dependent photolabeling of a regulatory ATP-binding site on the natriuretic peptide receptor-A

The natriuretic peptide receptor-A (NPR-A) is composed of an extracellular ligand-binding domain, a transmembrane-spanning domain, a kinase homology domain (KHD) and a guanylyl cyclase domain. Because the presence of ATP or adenylylimidodiphosphate reduces atrial natriuretic peptide (ANP) binding and is required for maximal guanylyl cyclase activity, a direct interaction of ATP with the receptor KHD domain is plausible. Therefore, we investigated whether ATP interacts directly with a binding site on the receptor by analyzing the binding of a photoaffinity analog of ATP to membranes from human embryonic kidney 293 cells expressing the NPR-A receptor lacking the guanylyl cyclase moiety (DeltaGC). We demonstrate that this receptor (NPR-A-DeltaGC) can be directly labeled by 8-azido-3'-biotinyl-ATP and that labeling is highly increased following ANP treatment. The mutant receptor DeltaKC, which does not contain the KHD, is not labeled. Photoaffinity labeling of the NPR-A-DeltaGC is reduced by 50% in the presence of 550 microm ATP, and competition curve fitting studies indicate a Hill slope of 2.2, suggestive of cooperative binding. This approach demonstrates directly that the interaction of ANP with its receptor modulates the binding of ATP to the KHD, probably through a conformational change in the KHD. In turn, this conformational change is essential for maximal activity. In addition, the ATP analog, 8-azido-adenylylimidodiphosphate, inhibits guanylyl cyclase activity but increases ANP binding to the extracellular domain. These results suggest that the KHD regulates ANP binding and guanylyl cyclase activity independently.

Discovery of some of the biological effects of nitric oxide and its role in cell signaling

The role of nitric oxide in cellular signaling in the past 22 years has become one of the most rapidly growing areas in biology with more than 20,000 publications to date. Nitric oxide is a gas and free radical with an unshared electron that can regulate an ever-growing list of biological processes. In many instances nitric oxide mediates its biological effects by activating guanylyl cyclase and increasing cyclic GMP synthesis from GTP. However, the list of effects of nitric oxide that are independent of cyclic GMP is also growing at a rapid rate. For example, nitric oxide can interact with transition metals such as iron, thiol groups, other free radicals, oxygen, superoxide anion, unsaturated fatty acids and other molecules. Some of these reactions result in the oxidation of nitric oxide to nitrite and nitrate to terminate its effect, while other reactions can lead to altered protein structure, function, and/or catalytic capacity. These diverse effects of nitric oxide that are either cyclic GMP dependent or independent can alter and regulate important physiological and biochemical events in cell regulation and function. Nitric oxide can function as an intracellular messenger, an autacoid, a paracrine substance, a neurotransmitter, or as a hormone that can be carried to distant sites for effects. Thus, it is a unique simple molecule with an array of signaling functions. However, as with any messenger molecule, there can be too little or too much of the substance and pathological events result. Some of the methods to regulate either nitric oxide formation, metabolism, or function have been in clinical use for more than a century as with the use of organic nitrates and nitroglycerin in angina pectoris that was initiated in the 1870's. Current and future research with nitric oxide and cyclic GMP will undoubtedly expand the clinicians' therapeutic armamentarium to manage a number of important diseases by perturbing nitric oxide and cyclic GMP formation and metabolism. Such promise and expectations have obviously fueled the interests in these signaling molecules for a growing list of potential therapeutic applications.

Implications of PDE4 structure on inhibitor selectivity across PDE families

Phosphodiesterases (PDEs) control cellular concentrations of cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP). PDE4 and PDE5 selectively hydrolyze cAMP and cGMP, respectively. PDE family members share approximately 25% sequence identity within a conserved catalytic domain of about 300 amino acids. Crystal structure analysis of PDE4's catalytic domain identifies two metal-binding sites: a high-affinity site and a low-affinity site, which probably bind zinc (Zn2+) and magnesium (Mg2+), respectively. Absolute conservation among the PDEs of two histidine and two aspartic acid residues for divalent metal binding suggests the importance of these amino acids in catalysis. Although active sites of PDEs are apparently structurally similar, PDE4 is specifically inhibited by selective inhibitors such as rolipram, while PDE5 is preferentially blocked by sildenafil. Modeling interactions of the PDE5 inhibitor sildenafil with the PDE4 active site may help explain inhibitor selectivity and provide useful information for the design of new inhibitors.

The crystal structure of AMP-bound PDE4 suggests a mechanism for phosphodiesterase catalysis

Cyclic nucleotide phosphodiesterases (PDEs) regulate the intracellular concentrations of cyclic 3',5'-adenosine and guanosine monophosphates (cAMP and cGMP, respectively) by hydrolyzing them to AMP and GMP, respectively. Family-selective inhibitors of PDEs have been studied for treatment of various human diseases. However, the catalytic mechanism of cyclic nucleotide hydrolysis by PDEs has remained unclear. We determined the crystal structure of the human PDE4D2 catalytic domain in complex with AMP at 2.4 A resolution. In this structure, two divalent metal ions simultaneously interact with the phosphate group of AMP, implying a binuclear catalysis. In addition, the structure suggested that a hydroxide ion or a water bridging two metal ions may serve as the nucleophile for the hydrolysis of the cAMP phosphodiester bond.

A disulfide-bridged mutant of natriuretic peptide receptor-A displays constitutive activity. Role of receptor dimerization in signal transduction

Natriuretic peptide receptor-A (NPR-A), a particulate guanylyl cyclase receptor, is composed of an extracellular domain (ECD) with a ligand binding site, a transmembrane spanning, a kinase homology domain (KHD), and a guanylyl cyclase domain. Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), the natural agonists, bind and activate the receptor leading to cyclic GMP production. This receptor has been reported to be spontaneously dimeric or oligomeric. In response to agonists, the KHD-mediated guanylate cyclase repression is removed, and it is assumed that ATP binds to the KHD. Since NPR-A displays a pair of juxtamembrane cysteines separated by 8 residues, we hypothesized that the removal of one of those cysteines would leave the other unpaired and reactive, thus susceptible to form an interchain disulfide bridge and to favor the dimeric interactions. Here we show that NPR-AC423S mutant, expressed mainly as a covalent dimer, increases the affinity of pBNP for this receptor by enhancing a high affinity binding component. Dimerization primarily depends on ECD since a secreted NPR-A C423S soluble ectodomain (ECDC423S) also documents a covalent dimer. ANP binding to the unmutated ECD yields up to 80-fold affinity loss as compared with the membrane receptor. However, the ECD C423S mutation restores a high binding affinity. Furthermore, C423S mutation leads to cellular constitutive activation (20-40-fold) of basal catalytic production of cyclic GMP by the full-length mutant. In vitro particulate guanylyl cyclase assays demonstrate that NPR-AC423S displays an increased sensitivity to ATP treatment alone and that the effect of ANP + ATP joint treatment is cumulative instead of synergistic. Finally, the cellular and particulate guanylyl cyclase assays indicate that the receptor is desensitized to agonist stimulation. We conclude the following: 1) dimers are functional units of NPR-A guanylyl cyclase activation; and 2) agonists are inducing dimeric contact of the juxtamembranous region leading to the removal of the KHD-mediated guanylyl cyclase repression, hence allowing catalytic activation.

Phosphorylation of the kinase homology domain is essential for activation of the A-type natriuretic peptide receptor

Natriuretic peptide receptor A (NPR-A) is the biological receptor for atrial natriuretic peptide (ANP). Activation of the NPR-A guanylyl cyclase requires ANP binding to the extracellular domain and ATP binding to a putative site within its cytoplasmic region. The allosteric interaction of ATP with the intracellular kinase homology domain (KHD) is hypothesized to derepress the carboxyl-terminal guanylyl cyclase catalytic domain, resulting in the synthesis of the second messenger, cyclic GMP. Here, we show that phosphorylation of the KHD is essential for receptor activation. Using a combination of phosphopeptide mapping techniques, we have identified six residues within the ATP-binding domain (S497, T500, S502, S506, S510, and T513) which are phosphorylated when NPR-A is expressed in HEK 293 cells. Mutation of any one of these Ser or Thr residues to Ala caused reductions in the receptor phosphorylation state, the number and pattern of phosphopeptides observed in tryptic maps, and ANP-dependent guanylyl cyclase activity. The reductions were not explained by decreases in NPR-A protein levels, as indicated by immunoblot analysis and determinations of cyclase activity in the presence of detergent. Conversion of Ser-497 to Ala resulted in the most dramatic decrease in cyclase activity (approximately 20% of wild-type activity), but conversion to an acidic residue (Glu), which mimics the charge of the phosphoserine moiety, had no effect. Simultaneous mutation of five of the phosphorylation sites to Ala resulted in a dephosphorylated receptor which was unresponsive to hormone and had potent dominant negative inhibitory activity. We conclude that phosphorylation of the KHD is absolutely required for hormone-dependent activation of NPR-A.

Phosphorylation-dependent regulation of the guanylyl cyclase-linked natriuretic peptide receptor B: dephosphorylation is a mechanism of desensitization

C-type natriuretic peptide (CNP) binds the guanylyl cyclase-linked natriuretic peptide receptor B (NPR-B) and stimulates marked elevations of the intracellular signaling molecule, cGMP. Here, the essential role of phosphorylation in the hormonal activation and deactivation of this receptor is described. Exposure of NIH3T3 fibroblasts overexpressing NPR-B (3T3-NPR-B) to CNP resulted in time-dependent decreases in both subsequent CNP-dependent cGMP elevations in whole cells and hormone-dependent guanylyl cyclase activity assayed in crude membranes. NPR-B isolated from resting 3T3-NPR-B cells was phosphorylated on serine and threonine residues, and exposure to CNP resulted in a time-dependent dephosphorylation and desensitization of the receptor. Immunoblot analysis and guanylyl cyclase activity assayed with the general activators Mn2+ and Triton X-100 indicated that these reductions were not due to receptor degradation. Tryptic phosphopeptide mapping analysis suggested that CNP treatment caused a complete dephosphorylation of approximately one-half of the NPR-B population. In vitro dephosphorylation of crude 3T3-NPR-B membranes with purified protein phosphatase 2A was highly correlated with losses in CNP- but not Mn2+- and Triton X-100-dependent guanylyl cyclase activity. Taken together, these data indicate that the catalytic activity of NPR-B is tightly coupled to its phosphorylation state and that dephosphorylation is a mechanism of desensitization.

Disruption of the guanylyl cyclase-C gene leads to a paradoxical phenotype of viable but heat-stable enterotoxin-resistant mice

Heat-stable enterotoxins (STa), which cause an acute secretory diarrhea, have been suggested to mediate their actions through the guanylyl cyclase-C (GC-C) receptor. The GC-C gene was disrupted by insertion of neo into exon 1 and subsequent homologous recombination. GC-C null mice contained no detectable GC-C protein. Intestine mucosal guanylyl cyclase activity was approximately 16-fold higher in wild-type mice than in the GC-C null mice, and STa-stimulable guanylyl cyclase activity was absent in the null animals. Thus, GC-C is the major cyclase activity present in the intestine, and also completely accounts for the STa-induced elevations of cGMP. Gavage with STa resulted in marked fluid accumulation within the intestine of wild-type and heterozygous suckling mice, but GC-C null animals were resistant. In addition, infection with enterotoxigenic bacteria that produce STa led to diarrhea and death in wild-type and heterozygous mice, while the null mice were protected. Cholera toxin, in contrast, continued to cause diarrhea in GC-C null mice, demonstrating that the cAMP signaling pathway remained intact. Markedly different diets (high carbohydrate, fat, or protein) or the inclusion of high salt (K+, Na+) in the drinking water or diet also did not severely affect the null animals. Given that GC-C is a major intestinal receptor in all mammals, the pressure to retain a functional GC-C in the face of diarrhea-inflicted mortality remains unexplained. Therefore, GC-C likely provides a protective effect against stressors not yet tested, possibly pathogens other than noninvasive enterotoxigenic bacteria.

Mucor dimorphism

Mucor dimorphism has interested microbiologists since the time of Pasteur. When deprived of oxygen, these fungi grow as spherical, multipolar budding yeasts. In the presence of oxygen, they propagate as branching coenocytic hyphae. The ease with which these morphologies can be manipulated in the laboratory, the diverse array of morphopoietic agents available, and the alternative developmental fates that can be elicited from a single cell type (the sporangiospore) make Mucor spp. a highly propitious system in which to study eukaryotic cellular morphogenesis. The composition and organization of the cell wall differ greatly in Mucor yeasts and hyphae. The deposition of new wall polymers is isodiametric in yeasts and apically polarized in hyphae. Current research has focused on the identity and control of enzymes participating in wall synthesis. An understanding of how the chitosome interacts with appropriate effectors, specific enzymes, and the plasma membrane to assemble chitin-chitosan microfibrils and to deposit them at the proper sites on the cell exterior will be critical to elucidating dimorphism. Several biochemical and physiological parameters have been reported to fluctuate in a manner that correlates with Mucor morphogenesis. The literature describing these has been reviewed critically with the intent of distinguishing between causal and casual connections. The advancement of molecular genetics has afforded powerful new tools that researchers have begun to exploit in the study of Mucor dimorphism. Several genes, some encoding products known to correlate with development in Mucor spp. or other fungi, have been cloned, sequenced, and examined for transcriptional activity during morphogenesis. Most have appeared in multiple copies displaying independent transcriptional control. Selective translation of stored mRNA molecules occurs during sporangiospore germination. Many other correlates of Mucor morphogenesis, presently described but not yet explained, should prove amenable to analysis by the emerging molecular technology.

Haem-dependent activation of guanylate cyclase and cyclic GMP formation by endogenous nitric oxide: a unique transduction mechanism for transcellular signaling

The interaction between nitric oxide (NO) synthesized in one cell and the haem group of cytosolic guanylate cyclase located in target cells to form NO-haem-guanylate cyclase represents a unique signal transduction mechanism that links extracellular stimuli to the synthesis of cyclic GMP in nearby target cells. Autacoids, neurotransmitters, and macrophage- and neutrophil-activating factors interact with selective extracellular receptors to trigger formation of NO from L-arginine. NO may be viewed as a second messenger. The NO diffuses into adjacent target cells and causes haem-dependent activation of guanylate cyclase, thereby stimulating cyclic GMP accumulation. Guanylate cyclase-bound haem serves as a transducer in transferring the signal from NO to guanylate cyclase. Cyclic GMP acts as a third messenger in causing vascular smooth muscle relaxation, inhibition of platelet aggregation and adhesion, and modulation of macrophage, neutrophil, and other phagocytic cell functions. The unique physical and chemical properties of NO allow it to function as an intercellular modulator within a localized environment. This intercellular or transcellular signaling mechanism involving a common signal transduction mechanism permits the rapid initiation of localized complementary cellular functions leading to increased local blood flow, inhibition of local thrombosis, and modulation of phagocytosis and cytotoxicity.

Guanylate cyclase receptor family

The plasma membrane forms of guanylate cyclase contain a highly conserved catalytic domain, which is also conserved in the soluble form of the enzyme and in mammalian adenylate cyclase. A protein kinase-like domain lies to the amino-terminal side of the catalytic domain and appears to be required for signaling via cGMP; it might also signal, itself, through phosphotransferase activity. This domain is present in the growth factor receptors, but appears not to be a component of other guanylate cyclases or adenylate cyclases. A single transmembrane domain then separates the cyclase catalytic and protein kinase-like domains from the putative ligand-binding domain. At least two plasma membrane forms of gunaylate cyclase (i.e., GC-A and GC-B) have now been identified, and their ligand specificities appear to be distinctly different. The tissue/cellular distribution of this family of receptors is now of potential importance, since specific agonists might differentially regulate physiological processes via the secondary messenger, cGMP, dependent on cellular distribution of the receptors.

Cyclic GMP and the second messenger hypothesis

A plasma membrane form of guanylate cyclase appears to contain a single transmembrane domain that divides the protein into a highly conserved intracellular domain and a variable extracellular domain. Various extracellular peptides can bind directly to guanylate cyclase to increase the production of the second messenger, cyclic GMP.

The protein kinase domain of the ANP receptor is required for signaling

A plasma membrane form of guanylate cyclase is a cell surface receptor for atrial natriuretic peptide (ANP). In response to ANP binding, the receptor-enzyme produces increased amounts of the second messenger, guanosine 3',5'-monophosphate. Maximal activation of the cyclase requires the presence of adenosine 5'-triphosphate (ATP) or nonhydrolyzable ATP analogs. The intracellular region of the receptor contains at least two domains with homology to other proteins, one possessing sequence similarity to protein kinase catalytic domains, the other to regions of unknown function in a cytoplasmic form of guanylate cyclase and in adenylate cyclase. It is now shown that the protein kinase-like domain functions as a regulatory element and that the second domain possesses catalytic activity. When the kinase-like domain was removed by deletion mutagenesis, the resulting ANP receptor retained guanylate cyclase activity, but this activity was independent of ANP and its stimulation by ATP was markedly reduced. A model for signal transduction is suggested in which binding of ANP to the extracellular domain of its receptor initiates a conformational change in the protein kinase-like domain, resulting in derepression of guanylate cyclase activity.

Stimulation-induced changes in cyclic nucleotide content by isolated rat parotid acini: effects of calcium

In the present study we investigated the relation between receptor activation and the acinar formation of cyclic nucleotides as a function of time, including effects of calcium. The experiments were performed using collagenase-isolated rat parotid acini and the cyclic nucleotide content was determined using a specific cAMP binding protein and cGMP antibody. Under physiological conditions, activation of beta-adrenoceptors increased cAMP content 27 fold within 60 sec. This response was not dependent upon the presence of extracellular calcium nor could it be mimicked by the calcium ionophore A23187. The adrenaline-induced activation of the alpha-adrenergic receptors lead to a 19% increase in cGMP content within 15 sec. This response was abolished in the absence of extracellular calcium. Pure activation of the alpha-1-adrenoceptors by phenylephrine resulted in a 62% increase in cGMP content within 15 sec. Finally, carbachol-induced activation of the cholinoceptors lead to a 50% transient increase in cGMP content within 7 sec. In the absence of calcium (EGTA present), the basal cGMP level was increased and in addition, the carbachol-induced cGMP formation was slower in onset and amounted to only 23% within 60 sec. The carbachol-induced cGMP formation could be mimicked by the calcium ionophore A23187 or by addition of calcium to acini preincubated in a calcium free buffer containing A23187. In conclusion, stimulation-induced formation of cyclic nucleotides under various conditions proved to be very rapid, well regulated, and sensitive processes.

The effects of nitroprusside and putative agonists on guanylate cyclase activity in squid giant axons

cGMP content of axoplasm from the giant axon of Loligo forbesi was investigated after subjecting the axon to various treatments. Repetitive electrical stimulation or depolarisation by high K+ caused no change in cGMP content. Glutamate and serotonin were also without effect. The nicotinic agonist carbachol (100 microM) increased cGMP levels by 90% (n = 5). A large transient elevation of cGMP content was evoked by external nitroprusside (10 nM-20 microM in intact axons. Nitroprusside injected into both extruded axoplasm and intact axons also increased cGMP content, the stimulation being considerably higher in intact axons where the axolemma was also present. Nitroprusside was also active in axons where the soluble cytoplasmic components were washed out by internal perfusion.

Retention of a functional resact receptor in isolated sperm plasma membranes

Resact, a peptide obtained from eggs, causes a change in the Mr, and a loss of 32P from a plasma membrane protein identified as guanylate cyclase. Here, a resact analog (125I-[Tyr1, Ser8] resact) was synthesized and shown to bind to isolated sperm membranes. Resact, but not speract, competed with the radiolabeled ligand for binding. When membranes were prepared under appropriate conditions, guanylate cyclase remained at Mr 160,000; the incubation of membranes with gamma-32P-ATP resulted in the formation of 32P-labeled guanylate cyclase. The addition of resact to the membranes caused a shift in the Mr, a complete loss of 32P, and a 70% reduction in guanylate cyclase activity within 1 min; resact had an ED 50 at 100 nM concentration. Speract failed to cause any of these effects. This represents the first demonstration of receptor-mediated responses of isolated sperm membranes identical to those seen in the intact cell.

Simultaneous determination of guanine nucleotides in rat brain by high-performance liquid chromatography with dual-electrochemical detection

A new, sensitive, and specific assay method for guanine nucleotides using high-performance liquid chromatography with dual-electrochemical detection was developed. GTP, GDP, GMP, and cyclic GMP were separated with reversed-phase "ion-pair" chromatography and detected by a dual-electrochemical detector. Only guanine nucleotides among all purine and pyrimidine nucleotides responded to the electrochemical detector at 0.95 V. The peak heights for these guanine nucleotides were linear at concentrations between 0.5 pmol and 1 nmol. The regional distribution of these guanine nucleotides in the rat brain was studied by this new assay method.

The mechanisms of endoreduplication

Chinese hamster cells, Don line, were treated with concanavalin A (ConA), calcium ionophore A23187 (A23187), colchicine, sodium fluoride (NaF), 6-thiopurine, dibutyryl cyclic AMP (db-cAMP), and other nucleotides, alone or in combination. A23187 itself did not induce endoreduplication but did so in combination with ConA. NaF could induce endoreduplication and the combination of NaF and ConA showed a synergistic effect. db-cAMP suppressed the inducing activity of ConA. The findings that various chemicals are inducers of endoreduplication and that synergistic effects appear on combined treatments suggest that various mechanisms of induction of endoreduplication may exist. The chemical nature of the inducers and the suppressor, db-cAMP, implies that blocking of the phosphorylation of some cellular components may be involved as a main mechanism. Analysis of the endoreduplication cell cycle indicated that cells treated with reagents in S require a longer cell cycle than those treated in G2 and that the length of the lag period between induction treatment and the initiation of S, the length of S, and the length of G2 war variable. The inducers, induction mechanisms, and the cell cycle of endoreduplication seem to vary; however, the essence of endoreduplication is the omission of mitotic events by connecting S and G1.

Distinct regulations by calcium of cyclic GMP levels and catecholamine secretion in isolated bovine adrenal chromaffin cells

The effects of various calcium-dependent secretagogues on cyclic GMP levels and catecholamine (CA) secretion were measured in a preparation of bovine adrenal chromaffin cells. The secretory effect of acetylcholine (ACh; 8--10 fold stimulation) was mimicked by nicotine but not muscarine. Three--five fold stimulations of cyclic GMP levels were also obtained with ACh and muscarine but not nicotine. High concentration of K+, and the ionophore A23187, also elevated cyclic GMP levels. However, secretion produced by veratridine, ouabain, and the ionophore X537A was not accompanied by any rise in cyclic GMP levels. Removal of extracellular calcium significantly decreased both basal levels of CA secretion and of cyclic GMP and completely abolished their stimulation by ACh. The half-maximal effects of calcium on the cholinergic stimulations of cyclic GMP levels and of CA secretion were observed at 0.2 and 2.5 mM, respectively. Substitution of Ca2+ by Sr2+ was more effective in maintaining the cyclic GMP response than the secretory response. The calcium channel blockers Co2+, Mg2+ and Ni2+ inhibited the cholinergic stimulation of cyclic GMP more than that of CA release. On the other hand, the organic calcium channel blockers, verapamil and methoxyverapamil (D--600) were more effective antagonists of the secretory response. These data indicate that the cholinergic stimulations of CA secretion and of cyclic GMP levels in bovine adrenal chromaffin cells are regulated by calcium via two distinct mechanisms.

Variations of cyclic AMP and cyclic GMP contents of the liver and of brown and white adipose tissues of the rat during the first week of cold exposure

The variation in the amounts of cyclic AMP and cyclic GMP were studied in white and brown adipose tissues and in the liver of rats during the first week of cold exposure (5 degrees C). In white fat, only a small increase in cAMP was observed on the first day. In brown fat, parallel decreases in cAMP and cGMP contents were induced which might be related to a large mobilization of tissue fatty acids. In the liver, cold exposure barely affected the cAMP content but the level of cGMP was markedly increased. These results are discussed with regard to the respective role of these different tissues in cold-induced energetic substrate mobilization.

Tension and cyclic GMP changes in potassium depolarized rabbit colon muscle

Potassium depolarization of rabbit colon muscle elicited a contraction consisting of 2 distinct phases, an initial rapid phasic contraction and a tonic contracture. The tonic contraction was, in contrast to the phasic contraction, dependent on the extracellular calcium for its development. There was a correlation between the tension development and the increase of the cyclic GMP level in the K+-depolarized muscle. Experimental conditions which abolished the tonic contracture, viz glucose omission and treatment with Ca2+-antagonists (verapamil, SKF 525A) also inhibited the cyclic GMP response. The changes of the cyclic GMP levels were Ca2+-dependent. K+-ions also changes the cyclic AMP content an effect which was atropine sensitive. From the experimental data obtained in this investigation we suggest that the co-variation of the tension and the cyclic GMP level in the depolarized colon muscle might depend on oscillations in a common intracellular factor, probably Ca2+.

Alterations of pulmonary vascular resistance and pulmonary cyclic nucleotide metabolism in the hypoxic pig

Hypoxia leads to an elevation of the pulmonary vascular resistance (pvR) in pigs. This alteration of the hemodynamic was accompanied by a slight increase of intraparenchymatous concentrations of cyclic GMP (cGMP), while cycle AMP (cAMP) levels were not different from values, measured during normoxia. beta-adrenergic agents are potent inhibitors of the hypoxia induced vasoconstriction. This effect was associated with an increase of intraparenchymatous cAMP-concentrations and a decrease of cGMP-levels. The regression analysis between pvR versus the ratio cGMP/cAMP revealed a significant linear correlation. These data may indicate, that cAMP and cGMP are involved in hypoxia induced vasoconstriction and its pharmacological inhibition.

Activation of liver guanylate cyclase by bile salts and contaminants in crude secretin and pancreozymin preparations

Crude preparations of secretin or pancreozymin increased and at higher concentrations decreased guanylate cyclase (GTP pyophosphate-lyase, EC 4.6.1.2) activity from soluble and particulate fractions of rat liver homogenates. Partially purified and synthetic secretin were without effect as was the biologically active octapeptide fragment of pancreozymin. The active contaminants in these preparations survived boiling, saponification, and treatment with phospholipase A, trypsin and neuraminidase C. The activity was extractable with chloroform/methanol and did not survive ashing. Eight bile salt contaminants in crude secretin were obtained with thin-layer chromatography. Two of the contaminating bile salts that increased liver particulate guanylate cyclase activity were identified as taurodeoxycholate and either glycochenodeoxycholate or glycodeoxycholate; taurocholate was inhibitory. The sodium salts of cholate, deoxycholate, chenodeoxycholate and their glycine-or taurine-conjugated forms either increased or decreased particulate and soluble rat liver guanylate cyclase activity depending upon their concentration. Thus, the previously reported stimulatory and inhibitory effects of secretin and pancreozymin preparations on guanylate cyclase activity are probable attributable to their bile salt contaminants.