Phosphorylation of the inositol 1,4,5-trisphosphate receptor. Cyclic GMP-dependent protein kinase mediates cAMP and cGMP dependent phosphorylation in the intact rat aorta

Calcium signaling in endothelial and vascular smooth muscle cells: sex differences and the influence of estrogens and androgens

Calcium signaling in vascular endothelial cells (ECs) and smooth muscle cells (VSMCs) is essential for the regulation of vascular tone. However, the changes to intracellular Ca2+ concentrations are often influenced by sex differences. Furthermore, a large body of evidence shows that sex hormone imbalance leads to dysregulation of Ca2+ signaling and this is a key factor in the pathogenesis of cardiovascular diseases. In this review, the effects of estrogens and androgens on vascular calcium-handling proteins are discussed, with emphasis on the associated genomic or nongenomic molecular mechanisms. The experimental models from which data were collected were also considered. The review highlights 1) in female ECs, transient receptor potential vanilloid 4 (TRPV4) and mitochondrial Ca2+ uniporter (MCU) enhance Ca2+-dependent nitric oxide (NO) generation. In males, only transient receptor potential canonical 3 (TRPC3) plays a fundamental role in this effect. 2) Female VSMCs have lower cytosolic Ca2+ levels than males due to differences in the activity and expression of stromal interaction molecule 1 (STIM1), calcium release-activated calcium modulator 1 (Orai1), calcium voltage-gated channel subunit-α1C (CaV1.2), Na+-K+-2Cl- symporter (NKCC1), and the Na+/K+-ATPase. 3) When compared with androgens, the influence of estrogens on Ca2+ homeostasis, vascular tone, and incidence of vascular disease is better documented. 4) Many studies use supraphysiological concentrations of sex hormones, which may limit the physiological relevance of outcomes. 5) Sex-dependent differences in Ca2+ signaling mean both sexes ought to be included in experimental design.

The Role of NO/sGC/cGMP/PKG Signaling Pathway in Regulation of Platelet Function

Circulating blood platelets are controlled by stimulatory and inhibitory factors, and a tightly regulated equilibrium between these two opposing processes is essential for normal platelet and vascular function. NO/cGMP/ Protein Kinase G (PKG) pathways play a highly significant role in platelet inhibition, which is supported by a large body of studies and data. This review focused on inconsistent and controversial data of NO/sGC/cGMP/PKG signaling in platelets including sources of NO that activate sGC in platelets, the role of sGC/PKG in platelet inhibition/activation, and the complexity of the regulation of platelet inhibitory mechanisms by cGMP/PKG pathways. In conclusion, we suggest that the recently developed quantitative phosphoproteomic method will be a powerful tool for the analysis of PKG-mediated effects. Analysis of phosphoproteins in PKG-activated platelets will reveal many new PKG substrates. A future detailed analysis of these substrates and their involvement in different platelet inhibitory pathways could be a basis for the development of new antiplatelet drugs that may target only specific aspects of platelet functions.

PDE-Mediated Cyclic Nucleotide Compartmentation in Vascular Smooth Muscle Cells: From Basic to a Clinical Perspective

Cardiovascular diseases are important causes of mortality and morbidity worldwide. Vascular smooth muscle cells (SMCs) are major components of blood vessels and are involved in physiologic and pathophysiologic conditions. In healthy vessels, vascular SMCs contribute to vasotone and regulate blood flow by cyclic nucleotide intracellular pathways. However, vascular SMCs lose their contractile phenotype under pathological conditions and alter contractility or signalling mechanisms, including cyclic nucleotide compartmentation. In the present review, we focus on compartmentalized signaling of cyclic nucleotides in vascular smooth muscle. A deeper understanding of these mechanisms clarifies the most relevant axes for the regulation of vascular tone. Furthermore, this allows the detection of possible changes associated with pathological processes, which may be of help for the discovery of novel drugs.

Cyclic GMP-Dependent Regulation of Vascular Tone and Blood Pressure Involves Cysteine-Rich LIM-Only Protein 4 (CRP4)

The cysteine-rich LIM-only protein 4 (CRP4), a LIM-domain and zinc finger containing adapter protein, has been implicated as a downstream effector of the second messenger 3',5'-cyclic guanosine monophosphate (cGMP) pathway in multiple cell types, including vascular smooth muscle cells (VSMCs). VSMCs and nitric oxide (NO)-induced cGMP signaling through cGMP-dependent protein kinase type I (cGKI) play fundamental roles in the physiological regulation of vascular tone and arterial blood pressure (BP). However, it remains unclear whether the vasorelaxant actions attributed to the NO/cGMP axis require CRP4. This study uses mice with a targeted deletion of the CRP4 gene (CRP4 KO) to elucidate whether cGMP-elevating agents, which are well known for their vasorelaxant properties, affect vessel tone, and thus, BP through CRP4. Cinaciguat, a NO- and heme-independent activator of the NO-sensitive (soluble) guanylyl cyclase (NO-GC) and NO-releasing agents, relaxed both CRP4-proficient and -deficient aortic ring segments pre-contracted with prostaglandin F2α. However, the magnitude of relaxation was slightly, but significantly, increased in vessels lacking CRP4. Accordingly, CRP4 KO mice presented with hypotonia at baseline, as well as a greater drop in systolic BP in response to the acute administration of cinaciguat, sodium nitroprusside, and carbachol. Mechanistically, loss of CRP4 in VSMCs reduced the Ca2+-sensitivity of the contractile apparatus, possibly involving regulatory proteins, such as myosin phosphatase targeting subunit 1 (MYPT1) and the regulatory light chain of myosin (RLC). In conclusion, the present findings confirm that the adapter protein CRP4 interacts with the NO-GC/cGMP/cGKI pathway in the vasculature. CRP4 seems to be part of a negative feedback loop that eventually fine-tunes the NO-GC/cGMP axis in VSMCs to increase myofilament Ca2+ desensitization and thereby the maximal vasorelaxant effects attained by (selected) cGMP-elevating agents.

Braylin induces a potent vasorelaxation, involving distinct mechanisms in superior mesenteric and iliac arteries of rats

Arterial hypertension is a risk factor for various cardiovascular and renal diseases, representing a major public health challenge. Although a wide range of treatment options are available for blood pressure control, many hypertensive individuals remain with uncontrolled hypertension. Thus, the search for new substances with antihypertensive potential becomes necessary. Coumarins, a group of polyphenolic compounds derived from plants, have attracted intense interest due to their diverse pharmacological properties, like potent antihypertensive activities. Braylin (6-methoxyseselin) is a coumarin identified in the Zanthoxylum tingoassuiba species, described as a phosphodiesterase-4 (PDE4) inhibitor. Although different coumarin compounds have been described as potent antihypertensive agents, the activity of braylin on the cardiovascular system has yet to be investigated. To investigate the vasorelaxation properties of braylin and its possible mechanisms of action, we performed in vitro studies using superior mesenteric arteries and the iliac arteries isolated from rats. In this study, we demonstrated, for the first time, that braylin induces potent vasorelaxation, involving distinct mechanisms from two different arteries, isolated from rats. A possible inhibition of phosphodiesterase, altering the cyclic adenosine monophosphate (cAMP)/cAMP-dependent protein kinase (PKA) pathway, may be correlated with the biological action of braylin in the mesenteric vessel, while in the iliac artery, the biological action of braylin may be correlated with increase of cyclic guanosine monophosphate (cGMP), followed by BKCa, Kir, and Kv channel activation. Together, these results provide evidence that braylin can represent a potential therapeutic use in preventing and treating cardiovascular diseases.

Ion channels as effectors of cyclic nucleotide pathways: Functional relevance for arterial tone regulation

Numerous mediators and drugs regulate blood flow or arterial pressure by acting on vascular tone, involving cyclic nucleotide intracellular pathways. These signals lead to regulation of several cellular effectors, including ion channels that tune cell membrane potential, Ca²⁺ influx and vascular tone. The characterization of these vasocontrictive or vasodilating mechanisms has grown in complexity due to i) the variety of ion channels that are expressed in both vascular endothelial and smooth muscle cells, ii) the heterogeneity of responses among the various vascular beds, and iii) the number of molecular mechanisms involved in cyclic nucleotide signalling in health and disease. This review synthesizes key data from literature that highlight ion channels as physiologically relevant effectors of cyclic nucleotide pathways in the vasculature, including the characterization of the molecular mechanisms involved. In smooth muscle cells, cation influx or chloride efflux through ion channels are associated with vasoconstriction, whereas K⁺ efflux repolarizes the cell membrane potential and mediates vasodilatation. Both categories of ion currents are under the influence of cAMP and cGMP pathways. Evidence that some ion channels are influenced by CN signalling in endothelial cells will also be presented. Emphasis will also be put on recent data touching a variety of determinants such as phosphodiesterases, EPAC and kinase anchoring, that complicate or even challenge former paradigms.

Evaluation of some prostaglandins modulators on rat corpus cavernosum in-vitro: Is relaxation negatively affected by COX-inhibitors?

INTRODUCTION

Prostaglandins (PGs) play an important role in corpus cavernosum relaxation, as evidenced by alprostadil being used as a drug for erectile dysfunction. Reports about the effect of cyclooxygenase (COX) inhibitors on erectile function are highly contradictory.

AIM

To compare the potential effects of some COX inhibitors with varying COX-1/COX-2 selectivities (indomethacin, ketoprofen and diclofenac) with that of the selective COX-2 inhibitor (DFU) on corpus cavernosal tone in-vitro. The role played by PGE₁, PGI₂-analogue and PGE₄ receptor (EP₄)-agonist in controlling corpus cavernosum function and the modulation of their action by sildenafil is also studied.

METHODS

Organ bath experiments were performed using isolated rat corpus cavernosum. Direct relaxations and changes to electric field stimulation (EFS, 2-16 Hz, 60 V, 0.8 ms, 10 s train)-induced relaxation by the effect of the selected drugs were studied. Strips were precontracted using phenylephrine (PE, 10^-5 M). Results are expressed as mean ± SEM of 5-9 rats.

RESULTS

Alprostadil, iloprost and L902688 (selective EP₄ agonist) induced direct relaxation where L902688 showed greater relaxant effect. Sildenafil potentiated the E_max of alprostadil and iloprost but not L902688. EFS and acetylcholine (ACh)-induced relaxations were significantly potentiated in presence of indomethacin, ketoprofen and diclofenac (20, 100 μM) but not in presence of selective COX-2 inhibitor (DFU, 1 μM). GR32191B (Thromboxane A₂ receptor antagonist, 10^-6 M) significantly reduced the potentiatory effect of indomethacin. Only diclofenac succeeded to potentiate sodium nitroprusside (SNP)-induced relaxation.

CONCLUSIONS

EP₄ receptors may play an important nitric oxide (NO)/cGMP-independent role in corpus cavernosal relaxation. Nonselective COX inhibitors seem of no harm concerning cavernosal tissue relaxation, possibly because they inhibit the synthesis of the highly contracting mediator thromboxane A₂.

Effects of the NO/soluble guanylate cyclase/cGMP system on the functions of human platelets

Platelets are circulating sentinels of vascular integrity and are activated, inhibited, or modulated by multiple hormones, vasoactive substances or drugs. Endothelium- or drug-derived NO strongly inhibits platelet activation via activation of the soluble guanylate cyclase (sGC) and cGMP elevation, often in synergy with cAMP-elevation by prostacyclin. However, the molecular mechanisms and diversity of cGMP effects in platelets are poorly understood and sometimes controversial. Recently, we established the quantitative human platelet proteome, the iloprost/prostacyclin/cAMP/protein kinase A (PKA)-regulated phosphoproteome, and the interactions of the ADP- and iloprost/prostacyclin-affected phosphoproteome. We also showed that the sGC stimulator riociguat is in vitro a highly specific inhibitor, via cGMP, of various functions of human platelets. Here, we review the regulatory role of the cGMP/protein kinase G (PKG) system in human platelet function, and our current approaches to establish and analyze the phosphoproteome after selective stimulation of the sGC/cGMP pathway by NO donors and riociguat. Present data indicate an extensive and diverse NO/riociguat/cGMP phosphoproteome, which has to be compared with the cAMP phosphoproteome. In particular, sGC/cGMP-regulated phosphorylation of many membrane proteins, G-proteins and their regulators, signaling molecules, protein kinases, and proteins involved in Ca²⁺ regulation, suggests that the sGC/cGMP system targets multiple signaling networks rather than a limited number of PKG substrate proteins.

Augmentation of cGMP/PKG pathway and colonic motility by hydrogen sulfide

Hydrogen sulfide (H2S), like nitric oxide (NO), causes smooth muscle relaxation, but unlike NO, does not stimulate soluble guanylyl cyclase (sGC) activity and generate cyclic guanosine 5'-monophosphate (cGMP). The aim of this study was to investigate the interplay between NO and H2S in colonic smooth muscle. In colonic smooth muscle from rabbit, mouse, and human, l-cysteine, substrate of cystathionine-γ-lyase (CSE), or NaHS, an H2S donor, inhibited phosphodiesterase 5 (PDE5) activity and augmented the increase in cGMP levels, IP3 receptor phosphorylation at Ser1756 (measured as a proxy for PKG activation), and muscle relaxation in response to NO donor S-nitrosoglutathione (GSNO), suggesting augmentation of cGMP/PKG pathway by H2S. The inhibitory effect of l-cysteine, but not NaHS, on PDE5 activity was blocked in cells transfected with CSE siRNA or treated with CSE inhibitor d,l-propargylglycine (dl-PPG), suggesting activation of CSE and generation of H2S in response to l-cysteine. H2S levels were increased in response to l-cysteine, and the effect of l-cysteine was augmented by GSNO in a cGMP-dependent protein kinase-sensitive manner, suggesting augmentation of CSE/H2S by cGMP/PKG pathway. As a result, GSNO-induced relaxation was inhibited by dl-PPG. In flat-sheet preparation of colon, l-cysteine augmented calcitonin gene-related peptide release in response to mucosal stimulation, and in intact segments, l-cysteine increased the velocity of pellet propulsion. These results demonstrate that in colonic smooth muscle, there is a novel interplay between NO and H2S. NO generates H2S via cGMP/PKG pathway, and H2S, in turn, inhibits PDE5 activity and augments NO-induced cGMP levels. In the intact colon, H2S promotes colonic transit.NEW & NOTEWORTHY Hydrogen sulfide (H2S) and nitric oxide (NO) are important regulators of gastrointestinal motility. The studies herein provide the cross talk between NO and H2S signaling to mediate smooth muscle relaxation and colonic transit. H2S inhibits phosphodiesterase 5 activity to augment cGMP levels in response to NO, which, in turn, via cGMP/PKG pathway, generates H2S. These studies suggest that interventions targeted at restoring NO and H2S homeostasis within the smooth muscle may provide novel therapeutic approaches to mitigate motility disorders.

Cardiac inositol 1,4,5-trisphosphate receptors

Calcium is a second messenger that regulates almost all cellular functions. In cardiomyocytes, calcium plays an integral role in many functions including muscle contraction, gene expression, and cell death. Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are a family of calcium channels that are ubiquitously expressed in all tissues. In the heart, IP₃Rs have been associated with regulation of cardiomyocyte function in response to a variety of neurohormonal agonists, including those implicated in cardiac disease. Notably, IP₃R activity is thought to be essential for mediating the hypertrophic response to multiple stimuli including endothelin-1 and angiotensin II. In this review, we will explore the functional implications of IP₃R activity in the heart in health and disease.

Probabilistic encoding of stimulus strength in astrocyte global calcium signals

Astrocyte calcium signals can range in size from subcellular microdomains to waves that spread through the whole cell (and into connected cells). The differential roles of such local or global calcium signaling are under intense investigation, but the mechanisms by which local signals evolve into global signals in astrocytes are not well understood, nor are the computational rules by which physiological stimuli are transduced into a global signal. To investigate these questions, we transiently applied receptor agonists linked to calcium signaling to primary cultures of cerebellar astrocytes. Astrocytes repetitively tested with the same stimulus responded with global signals intermittently, indicating that each stimulus had a defined probability for triggering a response. The response probability varied between agonists, increased with agonist concentration, and could be positively and negatively modulated by crosstalk with other signaling pathways. To better understand the processes determining the evolution of a global signal, we recorded subcellular calcium "puffs" throughout the whole cell during stimulation. The key requirement for puffs to trigger a global calcium wave following receptor activation appeared to be the synchronous release of calcium from three or more sites, rather than an increasing calcium load accumulating in the cytosol due to increased puff size, amplitude, or frequency. These results suggest that the concentration of transient stimuli will be encoded into a probability of generating a global calcium response, determined by the likelihood of synchronous release from multiple subcellular sites.

Regulation of Gβγi-dependent PLC-β3 activity in smooth muscle: inhibitory phosphorylation of PLC-β3 by PKA and PKG and stimulatory phosphorylation of Gαi-GTPase-activating protein RGS2 by PKG

In gastrointestinal smooth muscle, agonists that bind to Gi-coupled receptors activate preferentially PLC-β3 via Gβγ to stimulate phosphoinositide (PI) hydrolysis and generate inositol 1,4,5-trisphosphate (IP3) leading to IP3-dependent Ca(2+) release and muscle contraction. In the present study, we identified the mechanism of inhibition of PLC-β3-dependent PI hydrolysis by cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG). Cyclopentyl adenosine (CPA), an adenosine A1 receptor agonist, caused an increase in PI hydrolysis in a concentration-dependent fashion; stimulation was blocked by expression of the carboxyl-terminal sequence of GRK2(495-689), a Gβγ-scavenging peptide, or Gαi minigene but not Gαq minigene. Isoproterenol and S-nitrosoglutathione (GSNO) induced phosphorylation of PLC-β3 and inhibited CPA-induced PI hydrolysis, Ca(2+) release, and muscle contraction. The effect of isoproterenol on all three responses was inhibited by PKA inhibitor, myristoylated PKI, or AKAP inhibitor, Ht-31, whereas the effect of GSNO was selectively inhibited by PKG inhibitor, Rp-cGMPS. GSNO, but not isoproterenol, also phosphorylated Gαi-GTPase-activating protein, RGS2, and enhanced association of Gαi3-GTP and RGS2. The effect of GSNO on PI hydrolysis was partly reversed in cells (i) expressing constitutively active GTPase-resistant Gαi mutant (Q204L), (ii) phosphorylation-site-deficient RGS2 mutant (S46A/S64A), or (iii) siRNA for RGS2. We conclude that PKA and PKG inhibit Gβγi-dependent PLC-β3 activity by direct phosphorylation of PLC-β3. PKG, but not PKA, also inhibits PI hydrolysis indirectly by a mechanism involving phosphorylation of RGS2 and its association with Gαi-GTP. This allows RGS2 to accelerate Gαi-GTPase activity, enhance Gαβγi trimer formation, and inhibit Gβγi-dependent PLC-β3 activity.

Tadalafil therapy and severe chronic foot wound resolution

We report an unanticipated medication effect resulting in near-complete wound healing in a patient after beginning tadalafil therapy of 20 mg thrice daily. This patient was treated for 50 years with state-of-the art acute and then chronic wound interventions after a traumatic farm machinery accident which resulted in a devascularised foot wound. This infection was both life- and limb-threatening. The patient had undergone multiple vascular and plastic surgeries and antimicrobial therapies in addition to hospitalisations for sepsis. Limb amputation was being considered, when his urologist placed him on daily phosphodiesterase-5 inhibitor therapy, prior to unrelated urologic surgery. Remarkably, his foot wound underwent near-complete resolution and has been stable for 2 years.

Inhibition of RhoA-dependent pathway and contraction by endogenous hydrogen sulfide in rabbit gastric smooth muscle cells

Inhibitory neurotransmitters, chiefly nitric oxide and vasoactive intestinal peptide, increase cyclic nucleotide levels and inhibit muscle contraction via inhibition of myosin light chain (MLC) kinase and activation of MLC phosphatase (MLCP). H2S produced as an endogenous signaling molecule synthesized mainly from l-cysteine via cystathionine-γ-lyase (CSE) and cystathionine-β-synthase (CBS) regulates muscle contraction. The aim of this study was to analyze the expression of CSE and H2S function in the regulation of MLCP activity, 20-kDa regulatory light chain of myosin II (MLC20) phosphorylation, and contraction in isolated gastric smooth muscle cells. Both mRNA expression and protein expression of CSE, but not CBS, were detected in smooth muscle cells of rabbit, human, and mouse stomach. l-cysteine, an activator of CSE, and NaHS, a donor of H2S, inhibited carbachol-induced Rho kinase and PKC activity, Rho kinase-sensitive phosphorylation of MYPT1, PKC-sensitive phosphorylation of CPI-17, and MLC20 phosphorylation and sustained muscle contraction. The inhibitory effects of l-cysteine, but not NaHS, were blocked upon suppression of CSE expression by siRNA or inhibition of its activity by dl-propargylglycine (PPG) suggesting that the effect of l-cysteine is mediated via activation of CSE. Glibenclamide, an inhibitor of KATP channels, had no effect on the inhibition of contraction by H2S. Both l-cysteine and NaHS had no effect on basal cAMP and cGMP levels but augmented forskolin-induced cAMP and SNP-induced cGMP formation. We conclude that both endogenous and exogenous H2S inhibit muscle contraction, and the mechanism involves inhibition of Rho kinase and PKC activities and stimulation of MLCP activity leading to MLC20 dephosphorylation and inhibition of muscle contraction.

Phosphodiesterase types 3 and 4 regulate the phasic contraction of neonatal rat bladder smooth myocytes via distinct mechanisms

Activation of the cyclic AMP (cAMP) pathway reduces bladder contractility. However, the role of phosphodiesterase (PDE) families in regulating this function is poorly understood. Here, we compared the contractile function of the cAMP hydrolyzing PDEs in neonatal rat bladder smooth myocytes. RT-PCR and Western blotting analysis revealed that several isoforms of PDE1-4 were expressed in neonatal rat bladder. While 8-methoxymethyl-3-isobutyl-1-methylxanthine (a PDE1 inhibitor) and BAY-60-7550 (a PDE2 inhibitor) had no effect on the carbachol-enhanced phasic contractions of bladder strips, cilostamide (Cil, a PDE3 inhibitor) and Ro-20-1724 (Ro, a PDE4 inhibitor) significantly reduced these contractions. This inhibitory effect of Ro was blunted by the PKA inhibitor H-89, while the inhibitory effect of Cil was strongly attenuated by the PKG inhibitor KT 5823. Application of Ro in single bladder smooth myocytes resulted in an increase in Ca(2+) spark frequency but a decrease both in Ca(2+) transients and in sarcoplasmic reticulum (SR) Ca(2+) content. In contrast, Cil had no effect on these events. Furthermore, Ro-induced inhibition of the phasic contractions was significantly blocked by ryanodine and iberiotoxin. Taken together, PDE3 and PDE4 are the main PDE isoforms in maintaining the phasic contractions of bladder smooth myocytes, with PDE4 being functionally more active than PDE3. However, their roles are mediated through different mechanisms.

Structural basis for cyclic-nucleotide selectivity and cGMP-selective activation of PKG I

Cyclic guanosine monophosphate (cGMP) and cyclic AMP (cAMP)-dependent protein kinases (PKG and PKA) are closely related homologs, and the cyclic nucleotide specificity of each kinase is crucial for keeping the two signaling pathways segregated, but the molecular mechanism of cyclic nucleotide selectivity is unknown. Here, we report that the PKG Iβ C-terminal cyclic nucleotide binding domain (CNB-B) is highly selective for cGMP binding, and we have solved crystal structures of CNB-B with and without bound cGMP. These structures, combined with a comprehensive mutagenic analysis, allowed us to identify Leu296 and Arg297 as key residues that mediate cGMP selectivity. In addition, by comparing the cGMP bound and unbound structures, we observed large conformational changes in the C-terminal helices in response to cGMP binding, which were stabilized by recruitment of Tyr351 as a "capping residue" for cGMP. The observed rearrangements of the C-terminal helices provide a mechanical insight into release of the catalytic domain and kinase activation.

Role of cyclic nucleotide-dependent actin cytoskeletal dynamics:Ca(2+)](i) and force suppression in forskolin-pretreated porcine coronary arteries

Initiation of force generation during vascular smooth muscle contraction involves a rise in intracellular calcium ([Ca(2+)]i) and phosphorylation of myosin light chains (MLC). However, reversal of these two processes alone does not account for the force inhibition that occurs during relaxation or inhibition of contraction, implicating that other mechanisms, such as actin cytoskeletal rearrangement, play a role in the suppression of force. In this study, we hypothesize that forskolin-induced force suppression is dependent upon changes in actin cytoskeletal dynamics. To focus on the actin cytoskeletal changes, a physiological model was developed in which forskolin treatment of intact porcine coronary arteries (PCA) prior to treatment with a contractile agonist resulted in complete suppression of force. Pretreatment of PCA with forskolin suppressed histamine-induced force generation but did not abolish [Ca(2+)]i rise or MLC phosphorylation. Additionally, forskolin pretreatment reduced filamentous actin in histamine-treated tissues, and prevented histamine-induced changes in the phosphorylation of the actin-regulatory proteins HSP20, VASP, cofilin, and paxillin. Taken together, these results suggest that forskolin-induced complete force suppression is dependent upon the actin cytoskeletal regulation initiated by the phosphorylation changes of the actin regulatory proteins and not on the MLC dephosphorylation. This model of complete force suppression can be employed to further elucidate the mechanisms responsible for smooth muscle tone, and may offer cues to pathological situations, such as hypertension and vasospasm.

The oligopeptide DT-2 is a specific PKG I inhibitor only in vitro, not in living cells

BACKGROUND AND PURPOSE

cGMP is involved in the regulation of many cellular processes including cardiac and smooth muscle contractility, aldosterone synthesis and inhibition of platelet activation. Intracellular effects cGMP are mediated by cGMP-dependent PKs, cGMP-regulated PDEs and cGMP-gated ion channels. PKG inhibitors are widely used to discriminate PKG-specific effects. They can be divided into cyclic nucleotide-binding site inhibitors such as Rp-phosphorothioate analogues (Rp-cGMPS), ATP-binding site inhibitors such as KT5823, and substrate binding site inhibitors represented by the recently described DT-oligopeptides. As it has been shown that Rp-cGMPS and KT5823 have numerous non-specific effects, we analysed the pharmacological properties of the oligopeptide (D)-DT-2 described as a highly specific, membrane-permeable, PKG inhibitor.

EXPERIMENTAL APPROACH

Specificity and potency of (D)-DT-2 to inhibit PKG activity was evaluated using biochemical assays in vitro and by substrate phosphorylation analysis in various cell types including human platelets, rat mesangial cells and rat neonatal cardiomyocytes.

KEY RESULTS

Despite potent inhibition of PKGI in vitro, (D)-DT-2 lost specificity for PKG in cell homogenates and particularly in living cells, as demonstrated by phosphorylation of different substrates. Instead, (D)-DT-2 modulated activity of other kinases including ERK, p38, PKB and PKC, thereby inducing unpredicted and often opposing functional effects.

CONCLUSIONS AND IMPLICATIONS

We conclude that DT-oligopeptides, as other inhibitors, cannot be used to specifically inhibit PKG in intact cells. Therefore, no specific pharmacological PKG inhibitors are available, and reliable studies of PKG signalling can only be made by using RNA knockdown or genetic deletion methods.

cGMP-dependent protein kinases (cGK)

cGMP-dependent protein kinases (cGK) are serine/threonine kinases that are widely distributed in eukaryotes. Two genes-prkg1 and prkg2-code for cGKs, namely, cGKI and cGKII. In mammals, two isozymes, cGKIα and cGKIβ, are generated from the prkg1 gene. The cGKI isozymes are prominent in all types of smooth muscle, platelets, and specific neuronal areas such as cerebellar Purkinje cells, hippocampal neurons, and the lateral amygdala. The cGKII prevails in the secretory epithelium of the small intestine, the juxtaglomerular cells, the adrenal cortex, the chondrocytes, and in the nucleus suprachiasmaticus. Both cGKs are major downstream effectors of many, but not all, signalling events of the NO/cGMP and the ANP/cGMP pathways. cGKI relaxes smooth muscle tone and prevents platelet aggregation, whereas cGKII inhibits renin secretion, chloride/water secretion in the small intestine, the resetting of the clock during early night, and endochondral bone growth. This chapter focuses on the involvement of cGKs in cardiovascular and non-cardiovascular processes including cell growth and metabolism.

Inositol trisphosphate receptors in smooth muscle cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of tetrameric intracellular calcium (Ca(2+)) release channels that are located on the sarcoplasmic reticulum (SR) membrane of virtually all mammalian cell types, including smooth muscle cells (SMC). Here, we have reviewed literature investigating IP(3)R expression, cellular localization, tissue distribution, activity regulation, communication with ion channels and organelles, generation of Ca(2+) signals, modulation of physiological functions, and alterations in pathologies in SMCs. Three IP(3)R isoforms have been identified, with relative expression and cellular localization of each contributing to signaling differences in diverse SMC types. Several endogenous ligands, kinases, proteins, and other modulators control SMC IP(3)R channel activity. SMC IP(3)Rs communicate with nearby ryanodine-sensitive Ca(2+) channels and mitochondria to influence SR Ca(2+) release and reactive oxygen species generation. IP(3)R-mediated Ca(2+) release can stimulate plasma membrane-localized channels, including transient receptor potential (TRP) channels and store-operated Ca(2+) channels. SMC IP(3)Rs also signal to other proteins via SR Ca(2+) release-independent mechanisms through physical coupling to TRP channels and local communication with large-conductance Ca(2+)-activated potassium channels. IP(3)R-mediated Ca(2+) release generates a wide variety of intracellular Ca(2+) signals, which vary with respect to frequency, amplitude, spatial, and temporal properties. IP(3)R signaling controls multiple SMC functions, including contraction, gene expression, migration, and proliferation. IP(3)R expression and cellular signaling are altered in several SMC diseases, notably asthma, atherosclerosis, diabetes, and hypertension. In summary, IP(3)R-mediated pathways control diverse SMC physiological functions, with pathological alterations in IP(3)R signaling contributing to disease.

Cyclic nucleotide-dependent relaxation pathways in vascular smooth muscle

Vascular smooth muscle tone is controlled by a balance between the cellular signaling pathways that mediate the generation of force (vasoconstriction) and release of force (vasodilation). The initiation of force is associated with increases in intracellular calcium concentrations, activation of myosin light-chain kinase, increases in the phosphorylation of the regulatory myosin light chains, and actin-myosin crossbridge cycling. There are, however, several signaling pathways modulating Ca(2+) mobilization and Ca(2+) sensitivity of the contractile machinery that secondarily regulate the contractile response of vascular smooth muscle to receptor agonists. Among these regulatory mechanisms involved in the physiological regulation of vascular tone are the cyclic nucleotides (cAMP and cGMP), which are considered the main messengers that mediate vasodilation under physiological conditions. At least four distinct mechanisms are currently thought to be involved in the vasodilator effect of cyclic nucleotides and their dependent protein kinases: (1) the decrease in cytosolic calcium concentration ([Ca(2+)]c), (2) the hyperpolarization of the smooth muscle cell membrane potential, (3) the reduction in the sensitivity of the contractile machinery by decreasing the [Ca(2+)]c sensitivity of myosin light-chain phosphorylation, and (4) the reduction in the sensitivity of the contractile machinery by uncoupling contraction from myosin light-chain phosphorylation. This review focuses on each of these mechanisms involved in cyclic nucleotide-dependent relaxation of vascular smooth muscle under physiological conditions.

Kinetics of relaxation by cGMP/cGKI signaling in fundus smooth muscle

cGMP-dependent kinase I (cGKI) is a major mediator of smooth muscle relaxation and exists in two isoforms, α and β. Both isoforms are supposed to mediate their effects via different intracellular signaling pathways. To verify this concept, the kinetics of relaxation mediated by either isoform was analyzed in gastric fundus smooth muscle from mice. Muscles from mice that express selectively the Iα or Iβ isoform of cGKI in smooth muscle (sm-cGKIα or sm-cGKIβ mice) were compared to muscles from conventional cGKI(-/-) mice. Fundus muscles were contracted by carbachol and then relaxed by 8-Br-cGMP or by electrical field stimulation (EFS). The time course of relaxation by 8-Br-cGMP was not different between muscles from sm-cGKIα and sm-cGKIβ mice. EFS induced a fast transient relaxation in muscles from sm-cGKIα and sm-cGKIβ mice that was blocked by the NO synthase inhibitor L-NAME. Recovery from this relaxation was about 4-times slower in muscles from sm-cGKIα mice than in muscles from sm-cGKIβ mice. The different kinetic of recovery from relaxation after EFS in sm-cGKIα and sm-cGKIβ mice suggests that different signaling pathways exist for each cGKI isoform in vivo in fundus muscles.

Nitric oxide enhances melanogenesis of alpaca skin melanocytes in vitro by activating the MITF phosphorylation

Ultraviolet (UV) B radiation can cause skin-tanning via the synthesis of melanin which is synthesized by specific tyrosinase and tyrosinase-related enzymes expressed in melanocytes. It is reported that several melanogenic factors are released from keratinocytes and other cells surrounding melanocytes in the skin following UV radiation. Some of them are reported to up-regulate tyrosinase gene expression through a different pathway, but most regulate tyrosinase via microphthalmia-associated transcription factor (MITF). It is unknown whether an NO-induced pathway regulates melanogenesis via MITF in vitro. In this study, we investigated this problem because it is important for our understanding of how to enhance the coat color of alpaca. We set up three groups for experiments using alpaca melanocytes: the control cultures were allowed a total of 5 days growth; the UV group cultures were also allowed 5 days of growth like the control group, but were then irradiated once everyday with 312 mJ/cm(2) of UVB; the UV + L-NAME group was the same as the UV group, but with the addition of 300 μM L-NAME every 6 h. To determine the NO inhibition effect, NO product was measured. To determine the effect of NO on MITF, the expression levels of the MITF gene and protein were measured by immunofluorescence, quantitative real-time PCR and western immunoblotting. To determine the influence of NO on MITF phosphorylation, phosphorylated MITF protein (p-MITF) was measured by western immunoblotting. To determine the effect of NO on melanogenesis, the melanin content was measured. The results provide exciting new evidence that NO can enhance melanogenesis in alpaca skin melanocytes by stimulating MITF phosphorylation.

Differences between and within human parotid saliva and nasal mucus cAMP and cGMP in normal subjects and in patients with taste and smell dysfunction

BACKGROUND

We previously described some of the moieties in human saliva and nasal mucus including cyclic nucleotides. However, comparison of levels of these latter moieties in saliva and nasal mucus has not been performed and meaning of differences found has not been discussed.

PURPOSE

To compare the levels of cAMP and cGMP in saliva and nasal mucus and to describe the differences in their concentrations and function.

METHODS

cAMP and cGMP in saliva and nasal mucus were compared in normal subjects and patients with taste and smell dysfunction by use of a spectrophotometric colorimetric ELISA.

RESULTS

Both cAMP and cGMP were present in saliva and nasal mucus of normals and patients with levels of both moieties lower in patients than in normals. In normals, cAMP is 6½ times higher in saliva than in nasal mucus whereas cGMP in nasal mucus is 2½ times higher than in saliva. In patients, these differences persist but are less robust. In normals, within saliva, cAMP is 9½ times higher than cGMP whereas within nasal mucus cAMP is half the level of cGMP. In patients, within saliva, these differences persist but at variable differences.

CONCLUSIONS

Both saliva and nasal mucus cAMP and cGMP play roles in taste and smell function, and differences in their concentrations may offer insight into these roles. In nasal mucus, cGMP may be more relevant than cAMP in activity of olfactory epithelial cell function. In saliva, cAMP may be more relevant as a growth factor in taste bud function than cGMP.

Feedback control through cGMP-dependent protein kinase contributes to differential regulation and compartmentation of cGMP in rat cardiac myocytes

RATIONALE

We have shown recently that particulate (pGC) and soluble guanylyl (sGC) cyclases synthesize cGMP in different compartments in adult rat ventricular myocytes (ARVMs).

OBJECTIVE

We hypothesized that cGMP-dependent protein kinase (PKG) exerts a feedback control on cGMP concentration contributing to its intracellular compartmentation.

METHODS AND RESULTS

Global cGMP levels, cGMP-phosphodiesterase (PDE) and pGC enzymatic activities were determined in purified ARVMs. Subsarcolemmal cGMP signals were monitored in single cells by recording the cGMP-gated current (I(CNG)) in myocytes expressing the wild-type rat olfactory cyclic nucleotide-gated (CNG) channel. Whereas the NO donor S-nitroso-N-acetyl-penicillamine (SNAP) (100 μmol/L) produced little effect on I(CNG), the response increased 2-fold in the presence of the PKG inhibitors KT5823 (50 nmol/L) or DT-2 (2 μmol/L). The effect of KT5823 was abolished in the presence of the nonselective cyclic nucleotide PDE inhibitor 3-isobutyl-1-methylxantine (IBMX) (100 μmol/L) or the selective cGMP-PDE5 inhibitor sildenafil (100 nmol/L). PKG inhibition also potentiated the effect of SNAP on global cGMP levels and fully blocked the increase in cGMP-PDE5 activity. In contrast, PKG inhibition decreased by ≈50% the I(CNG) response to ANP (10 and 100 nmol/L), even in the presence of IBMX. Conversely, PKG activation increased the I(CNG) response to ANP and amplified the stimulatory effect of ANP on pGC activity.

CONCLUSIONS

PKG activation in adult cardiomyocytes limits the accumulation of cGMP induced by NO donors via PDE5 stimulation but increases that induced by natriuretic peptides. These findings support the paradigm that cGMP is not uniformly distributed in the cytosol and identifies PKG as a key component in this process.

Linking structure to function: Recent lessons from inositol 1,4,5-trisphosphate receptor mutagenesis

Great insight has been gained into the structure and function of the inositol 1,4,5 trisphosphate receptor (InsP(3)R) by studies employing mutagenesis of the cDNA encoding the receptor. Notably, early studies using this approach defined the key constituents required for InsP(3) binding in the N-terminus and the membrane spanning regions in the C-terminal domain responsible for channel formation, targeting and function. In this article we evaluate recent studies which have used a similar approach to investigate key residues underlying the in vivo modulation by select regulatory factors. In addition, we review studies defining the structural requirements in the channel domain which comprise the conduction pathway and are suggested to be involved in the gating of the channel.

Nitric oxide enhances the sensitivity of alpaca melanocytes to respond to alpha-melanocyte-stimulating hormone by up-regulating melanocortin-1 receptor

Nitric oxide (NO) and alpha-melanocyte-stimulating hormone (alpha-MSH) have been correlated with the synthesis of melanin. The NO-dependent signaling of cellular response to activate the hypothalamopituitary proopiomelanocortin system, thereby enhances the hypophysial secretion of alpha-MSH to stimulate alpha-MSH-receptor responsive cells. In this study we investigated whether an NO-induced pathway can enhance the ability of the melanocyte to respond to alpha-MSH on melanogenesis in alpaca skin melanocytes in vitro. It is important for us to know how to enhance the coat color of alpaca. We set up three groups for experiments using the third passage number of alpaca melanocytes: the control cultures were allowed a total of 5 days growth; the UV group cultures like the control group but the melanocytes were then irradiated everyday (once) with 312 mJ/cm(2) of UVB; the UV+L-NAME group is the same as group UV but has the addition of 300 microM L-NAME (every 6h). To determine the inhibited effect of NO produce, NO produces were measured. To determine the effect of the NO to the key protein and gene of alpha-MSH pathway on melanogenesis, the key gene and protein of the alpha-MSH pathway were measured by quantitative real-time PCR and Western immunoblotting. The results provide exciting new evidence that NO can enhance alpha-MSH pathway in alpaca skin melanocytes by elevated MC1R. And we suggest that the NO pathway may more rapidly cause the synthesis of melanin in alpaca skin under UV, which at that time elevates the expression of MC1R and stimulates the keratinocytes to secrete alpha-MSH to enhance the alpha-MSH pathway on melanogenesis. This process will be of considerable interest in future studies.

Role of cGMP-PKG signaling in the protection of neonatal rat cardiac myocytes subjected to simulated ischemia/reoxygenation

Nitric oxide (NO) and B-type natriuretic peptide (BNP) are protective against ischemia-reperfusion injury as they increase intracellular cGMP level via activation of soluble (sGC) or particulate guanylate cyclases (pGC), respectively. The aim of the present study was to examine if the cGMP-elevating mediators, NO and BNP, share a common downstream signaling pathway via cGMP-dependent protein kinase (PKG) in cardiac cytoprotection. Neonatal rat cardiac myocytes in vitro were subjected to 2.5 h simulated ischemia (SI) followed by 2 h reoxygenation. Cell viability was tested by trypan blue exclusion assay. PKG activity of cardiac myocytes was assessed by phospholamban (PLB) phosphorylation determined by western blot. Cell death was 34 +/- 2% after SI/reoxygenation injury in the control group. cGMP-inducing agents significantly decreased irreversible cell injury: the cGMP analog 8-bromo-cGMP (8-Br-cGMP, 10 nM) decreased it to 13 +/- 1% (p < 0.001), the direct NO-donor S-nitroso-N-acetylpenicillamine (SNAP, 1 microM) to 18 +/- 6% (p < 0.05) and BNP (10 nM) to 12 +/- 2% (p < 0.001), respectively. This protective effect was abolished by the selective PKG inhibitor KT-5823 (600 nM) in each case. As PLB is not a unique reporter for PKG activity since it is also phosphorylated by protein kinase A (PKA), we examined PLB phosphorylation in the presence of the PKA inhibitor KT-5720 (1 microM). The ratio of pPLB/PLB significantly increased after administration of both BNP and 8-Br-cGMP under ischemic conditions, which was abolished by the PKG inhibitor. This is the first demonstration that elevated cGMP produced either by the sGC activator SNAP or the pGC activator BNP exerts cytoprotective effects via a common downstream signaling pathway involving PKG activation.

Oxidant sensing by protein kinases a and g enables integration of cell redox state with phosphoregulation

The control of vascular smooth muscle contractility enables regulation of blood pressure, which is paramount in physiological adaptation to environmental challenges. Maintenance of stable blood pressure is crucial for health as deregulation (caused by high or low blood pressure) leads to disease progression. Vasotone is principally controlled by the cyclic nucleotide dependent protein kinases A and G, which regulate intracellular calcium and contractile protein calcium sensitivity. The classical pathways for activation of these two kinases are well established and involve the formation and activation by specific cyclic nucleotide second messengers. Recently we reported that both PKA and PKG can be regulated independently of their respective cyclic nucleotides via a mechanism whereby the kinases sense cellular oxidant production using redox active thiols. This novel redox regulation of these kinases is potentially of physiological importance, and may synergise with the classical regulatory mechanisms.

Modulation of vascular sarco/endoplasmic reticulum calcium ATPase in cardiovascular pathophysiology

Endothelial dysfunction associated with decreased nitric oxide (NO) bioactivity is a major feature of vascular diseases such as atherosclerosis or diabetes. Sodium nitroprusside (SNP)-induced relaxation is entirely dependent on cyclic guanosine monophosphate (cGMP) and preserved in atherosclerosis, suggesting that smooth muscle response to NO donor is intact. However, NO gas activates both cGMP-dependent and -independent signal pathways in vascular smooth muscle cells, and oxidative stress associated with vascular diseases selectively impairs cGMP-independent relaxation to NO. Sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA), which regulates intracellular Ca(2+) levels by pumping Ca(2+) into store, is a major cGMP-independent target for NO. Physiological levels of reactive nitrogen species (RNS) S-glutathiolate SERCA at Cys674 to increase its activity, and the augmentation of RNS in vascular diseases irreversibly oxidizes Cys674 or nitrates tyrosine residues at Tyr296-Tyr297, which are associated with loss of function. S-glutathiolation of various proteins by NO can explain redox-sensitive cGMP-independent actions, and oxidative inactivation of target proteins for NO can be associated with the pathogenesis of cardiovascular diseases. Oxidative inactivation of SERCA is also implicated with dysregulation of smooth muscle migration, promotion of platelet aggregation, and impairment of cardiac function, which can be implicated with restenosis, pathological angiogenesis, thrombosis, as well as heart failure. Analysis of posttranslational oxidative modifications of SERCA and the preservation of SERCA function can be novel strategies against cardiovascular diseases associated with oxidative stress.

Central role of guanylyl cyclase in natriuretic peptide signaling in hypertension and metabolic syndrome

Studied for nearly 30 years for its ability to control many parameters, such as vascular smooth muscle cell relaxation, heart fibrosis, and kidney function, the natriuretic peptide (NP) system is now considered to be a key element in several other major metabolic pathways. After stimulation by NPs, natriuretic peptide receptors (NPR) convert GTP to the second messenger cGMP. In addition to its vasodilatory effects and natriuretic and diuretic functions, cGMP has been positively associated with fat cell function, apoptosis, and NPR expression/activity modulation. The NP system is also closely linked to metabolic syndrome (MetS) progression and obesity control. A new era is now on its way targeting the NP system to not only treat high blood pressure, but to also assist in the fight against the obesity pandemic. Here, we summarize recent data on the role of NPs in hypertension and MetS.

cGK substrates

Signalling of cGK (cGMP-dependent protein kinases) are mediated through phosphorylation of specific substrates. Several substrates of cGKI and cGKII were identified meanwhile. Some cGKI substrates are specifically regulated by the cGKIalpha or the cGKIbeta isozyme. In various cells and tissues, different cGK substrates exist that are essential for the regulation of diverse functions comprising tissue contractility, cell motility, cell contact, cellular secretion, cell proliferation, and cell differentiation. On the molecular level, cGKI substrates fulfill various cellular functions regulating e.g. the intracellular calcium and potassium concentration, the calcium sensitivity, and the organisation of the intracellular cytoskeleton. cGKII substrates are involved e.g. in chloride transport, sodium/proton transport and transcriptional regulation. The understanding of cGK signalling and function depends strongly on the identification of further specific substrates. In the last years, diverse approaches ranging from biochemistry to genetic deletion lead to the identification and establishment of several substrates, which raised new insights in the molecular mechanisms of cGK functions and elucidated new cellular cGK functions. However, the analysis of the dynamic signalling of cGK in tissues and cells will be necessary to discover new signalling pathways and substrates.

Regulation of inositol 1,4,5-trisphosphate-induced Ca2+ release by reversible phosphorylation and dephosphorylation

The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) is a universal intracellular Ca2+-release channel. It is activated after cell stimulation and plays a crucial role in the initiation and propagation of the complex spatio-temporal Ca2+ signals that control cellular processes as different as fertilization, cell division, cell migration, differentiation, metabolism, muscle contraction, secretion, neuronal processing, and ultimately cell death. To achieve these various functions, often in a single cell, exquisite control of the Ca2+ release is needed. This review aims to highlight how protein kinases and protein phosphatases can interact with the IP3R or with associated proteins and so provide a large potential for fine tuning the Ca2+-release activity and for creating efficient Ca2+ signals in subcellular microdomains.

Non-genomic vascular actions of female sex hormones: physiological implications and signalling pathways

1. Epidemiological studies indicate a lower incidence of coronary heart disease in premenopausal women compared with age-matched men and post-menopausal women. Accumulating evidence suggests that this cardiovascular protection observed in premenopausal women is at least partially attributed to the direct action of oestrogens on the vascular system. 2. Research focused on vascular actions of 17beta-oestradiol indicates that this female sex hormone favourably modulates vascular reactivity at physiological concentrations. The vascular actions of 17beta-oestradiol appear independent of its genomic actions. Both endothelium-dependent and -independent signalling cascades have been implicated in the vascular effects of 17beta-oestradiol. 3. However, clinical trials on hormone-replacement therapy argue against a role of oestrogens in preventing the development of coronary heart disease. Supplementation with oestrogen is also complicated with the increased risk of breast and endometrial cancer. Hence, a better understanding of the vascular actions of 17beta-oestradiol will serve to enhance our understanding of its role in coronary heart disease.

Role of glycolytically generated ATP for CaMKII-mediated regulation of intracellular Ca2+ signaling in bovine vascular endothelial cells

The role of glycolytically generated ATP in Ca(2+)/calmodulin-dependent kinase II (CaMKII)-mediated regulation of intracellular Ca(2+) signaling was examined in cultured calf pulmonary artery endothelial (CPAE) cells. Exposure of cells (extracellular Ca(2+) concentration = 2 mM) to glycolytic inhibitors 2-deoxy-D-glucose (2-DG), pyruvate (pyr) + beta-hydroxybutyrate (beta-HB), or iodoacetic acid (IAA) caused an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)). CaMKII inhibitors (KN-93, W-7) triggered a similar increase of [Ca(2+)](i). The rise of [Ca(2+)](i) was characterized by a transient spike followed by a small sustained plateau of elevated [Ca(2+)](i). In the absence of extracellular Ca(2+) 2-DG caused an increase in [Ca(2+)](i), suggesting that inhibition of glycolysis directly triggered release of Ca(2+) from intracellular endoplasmic reticulum (ER) Ca(2+) stores. The inositol-1,4,5-trisphosphate receptor (IP(3)R) inhibitor 2-aminoethoxydiphenyl borate abolished the KN-93- and 2-DG-induced Ca(2+) response. Ca(2+) release was initiated in peripheral cytoplasmic processes from which activation propagated as a [Ca(2+)](i) wave toward the central region of the cell. Focal application of 2-DG resulted in spatially confined elevations of [Ca(2+)](i). Propagating [Ca(2+)](i) waves were preceded by [Ca(2+)](i) oscillations and small, highly localized elevations of [Ca(2+)](i) (Ca(2+) puffs). Inhibition of glycolysis with 2-DG reduced the KN-93-induced Ca(2+) response, and vice versa during inhibition of CaMKII 2-DG-induced Ca(2+) release was attenuated. Similar results were obtained with pyr + beta-HB and W-7. Furthermore, 2-DG and IAA caused a rapid increase of intracellular Mg(2+) concentration, indicating a concomitant drop of cellular ATP levels. In conclusion, CaMKII exerts a profound inhibition of ER Ca(2+) release in CPAE cells, which is mediated by glycolytically generated ATP, possibly through ATP-dependent phosphorylation of the IP(3)R.

Role of L-type calcium channels and PKC in active tone development in rabbit coronary artery

The present study investigated active tone development in isolated ring segments of rabbit epicardial coronary artery. Endothelium-denuded (E-) or endothelium-intact (E+) vessels treated with the NO synthase inhibitor N(omega)-nitro-L-arginine (100 microM) developed active tone, which was enhanced by stretch and reversed by the NO donor sodium nitroprusside (SNP; IC(50)=9 nM). Nifedipine abolished active tone and the contractile response to phorbol dibutyrate (PDBu; 10 nM) with the same potency (IC(50)=8 nM), whereas 300 nM PDBu responses were only partially blocked by nifedipine. The classical and novel PKC inhibitors GF-109203X (IC(50)=1-2 microM) and chelerythrine (IC(50)=4-5 microM) and the classical PKC inhibitor Gö-6976 (IC(50)=0.3-0.4 microM) blocked both active tone and 10 nM PDBu responses with similar potency. Active tone development was associated with depolarization of membrane potential (E(m)) and a shift to the left of the E(m)-vs.-contraction relationship determined by varying extracellular potassium. The depolarization and leftward shift were reversed by either chelerythrine (10 microM) or SNP (30 nM). PDBu (100-300 nM) increased peak L-type calcium channel (Ca(v)) currents in isolated coronary myocytes, and this effect was reversed by chelerythrine (1 microM) or Gö-6976 (200 nM). SNP (500 nM) reduced Ca(v) currents only in the presence of the PKA blocker 8-bromo-2'-O-monobutyryl-cAMPS, Rp isomer (10 microM). In conclusion, active tone development in coronary artery is suppressed by basal NO release and is dependent on both enhanced Ca(v) activity and classical PKC activity. Both E(m)-dependent and -independent processes contribute to contraction. Our results suggest that one E(m)-independent process is direct enhancement of Ca(v) current by PKC.

Proliferating cholangiocytes: a neuroendocrine compartment in the diseased liver

In the last 15 years, the intrahepatic biliary tree has become the object of extensive studies, which highlighted the extraordinary biologic properties of cholangiocytes involved in bile formation, proliferation, injury repair, fibrosis, angiogenesis, and regulation of blood flow. Proliferation is a "typical" property of cholangiocytes and is key as a mechanism of repair responsible for maintaining the integrity of the biliary tree. Cholangiocyte proliferation occurs virtually in all pathologic conditions of liver injury where it is associated with inflammation, regeneration, and repair, thus conditioning the evolution of liver damage. Interestingly, proliferating cholangiocytes acquire the phenotype of neuroendocrine cells, and secrete different cytokines, growth factors, neuropeptides, and hormones, which represent potential mechanisms for cross talk with other liver cells. Many studies suggest the generation of a neuroendocrine compartment in the injured liver, mostly constituted by cells with cholangiocyte features, which functionally conditions the progression of liver disease. These insights on cholangiocyte pathophysiology will provide new potential strategies for the management of chronic liver diseases. The purpose of this review is to summarize the recent findings on the mechanisms regulating cholangiocyte proliferation and the significance of the neuroendocrine regulation of cholangiocyte biology.

Inhibition of Gαq-dependent PLC-β1 activity by PKG and PKA is mediated by phosphorylation of RGS4 and GRK2

In smooth muscle of the gut, Gq-coupled receptor agonists activate preferentially PLC-β1 to stimulate phosphoinositide (PI) hydrolysis and inositol 1,4,5-trisphosphate (IP3) generation and induce IP3-dependent Ca2+ release. Inhibition of Ca2+ mobilization by cAMP- (PKA) and cGMP-dependent (PKG) protein kinases reflects inhibition of PI hydrolysis by both kinases and PKG-specific inhibitory phosphorylation of IP3 receptor type I. The mechanism of inhibition of PLC-β1-dependent PI hydrolysis has not been established. Neither Gq nor PLC-β1 was directly phosphorylated by PKA or PKG in gastric smooth muscle cells. However, both kinases 1) phosphorylated regulator of G protein signaling 4 (RGS4) and induced its translocation from cytosol to plasma membrane, 2) enhanced ACh-stimulated association of RGS4 and Gαq·GTP and intrinsic Gαq·GTPase activity, and 3) inhibited ACh-stimulated PI hydrolysis. RGS4 phosphorylation and inhibition of PI hydrolysis were blocked by selective PKA and PKG inhibitors. Expression of RGS4(S52A), which lacks a PKA/PKG phosphorylation site, blocked the increase in GTPase activity and the decrease in PI hydrolysis induced by PKA and PKG. Blockade of PKA-dependent effects was only partial. Selective phosphorylation of G protein-coupled receptor kinase 2 (GRK2), which contains a RGS domain, by PKA augmented ACh-stimulated GRK2:Gαq·GTP association; both effects were blocked in cells expressing GRK2(S685A), which lacks a PKA phosphorylation site. Inhibition of PI hydrolysis induced by PKA was partly blocked in cells expressing GRK2(S685A) and completely blocked in cells coexpressing GRK2(S685A) and RGS4(S52A) or Gαq(G188S), a Gαq mutant that binds GRK2 but not RGS4. The results demonstrate that inhibition of PLC-β1-dependent PI hydrolysis by PKA is mediated via stimulatory phosphorylation of RGS4 and GRK2, leading to rapid inactivation of Gαq·GTP. PKG acts only via phosphorylation of RGS4.

IRAG mediates NO/cGMP-dependent inhibition of platelet aggregation and thrombus formation

Defective regulation of platelet activation/aggregation is a predominant cause for arterial thrombosis, the major complication of atherosclerosis triggering myocardial infarction and stroke. A central regulatory pathway conveying inhibition of platelet activation/aggregation is nitric oxide (NO)/cyclic GMP (cGMP) signaling by cGMP-dependent protein kinase I (cGKI). However, the regulatory cascade downstream of cGKI mediating platelet inhibition is still unclear. Here, we show that the inositol-1,4,5-trisphosphate receptor-associated cGMP kinase substrate (IRAG) is abundantly expressed in platelets and assembled in a macrocomplex together with cGKIbeta and the inositol-1,4,5-trisphosphate receptor type I (InsP3RI). cGKI phosphorylates IRAG at Ser664 and Ser677 in intact platelets. Targeted deletion of the IRAG-InsP3RI interaction in IRAGDelta12/Delta12 mutant mice leads to a loss of NO/cGMP-dependent inhibition of fibrinogen-receptor activation and platelet aggregation. Intracellular calcium transients were not affected by DEA/NO or cGMP in mutant platelets. Furthermore, intravital microscopy shows that NO fails to prevent arterial thrombosis of the injured carotid artery in IRAGDelta12/Delta12 mutants. These findings reveal that interaction between IRAG and InsP3RI has a central role in NO/cGMP-dependent inhibition of platelet aggregation and in vivo thrombosis.

Administration of r-VEGF-A prevents hepatic artery ligation-induced bile duct damage in bile duct ligated rats

The hepatic artery, through the peribiliary plexus, nourishes the intrahepatic biliary tree. During obstructive cholestasis, the nutritional demands of intrahepatic bile ducts are increased as a consequence of enhanced proliferation; in fact, the peribiliary plexus (PBP) displays adaptive expansion. The effects of hepatic artery ligation (HAL) on cholangiocyte functions during cholestasis are unknown, although ischemic lesions of the biliary tree complicate the course of transplanted livers and are encountered in cholangiopathies. We evaluated the effects of HAL on cholangiocyte functions in experimental cholestasis induced by bile duct ligation (BDL). By using BDL and BDL + HAL rats or BDL + HAL rats treated with recombinant-vascular endothelial growth factor-A (r-VEGF-A) for 1 wk, we evaluated liver morphology, the degree of portal inflammation and periductular fibrosis, microcirculation, cholangiocyte apoptosis, proliferation, and secretion. Microcirculation was evaluated using a scanning electron microscopy vascular corrosion cast technique. HAL induced in BDL rats 1) the disappearance of the PBP, 2) increased apoptosis and impaired cholangiocyte proliferation and secretin-stimulated ductal secretion, and 3) decreased cholangiocyte VEGF secretion. The effects of HAL on the PBP and cholangiocyte functions were prevented by r-VEGF-A, which, by maintaining the integrity of the PBP and cholangiocyte proliferation, prevents damage of bile ducts following ischemic injury.

Signaling for contraction and relaxation in smooth muscle of the gut

Phosphorylation of Ser19 on the 20-kDa regulatory light chain of myosin II (MLC20) by Ca2+/calmodulin-dependent myosin light-chain kinase (MLCK) is essential for initiation of smooth muscle contraction. The initial [Ca2+]i transient is rapidly dissipated and MLCK inactivated, whereas MLC20 and muscle contraction are well maintained. Sustained contraction does not reflect Ca2+ sensitization because complete inhibition of MLC phosphatase activity in the absence of Ca2+ induces smooth muscle contraction. This contraction is suppressed by staurosporine, implying participation of a Ca2+-independent MLCK. Thus, sustained contraction, as with agonist-induced contraction at experimentally fixed Ca2+ concentrations, involves (a) G protein activation, (b) regulated inhibition of MLC phosphatase, and (c) MLC20 phosphorylation via a Ca2+-independent MLCK. The pathways that lead to inhibition of MLC phosphatase by G(q/13)-coupled receptors are initiated by sequential activation of Galpha(q)/alpha13, RhoGEF, and RhoA, and involve Rho kinase-mediated phosphorylation of the regulatory subunit of MLC phosphatase (MYPT1) and/or PKC-mediated phosphorylation of CPI-17, an endogenous inhibitor of MLC phosphatase. Sustained MLC20 phosphorylation is probably induced by the Ca2+-independent MLCK, ZIP kinase. The pathways initiated by G(i)-coupled receptors involve sequential activation of Gbetagamma(i), PI 3-kinase, and the Ca2+-independent MLCK, integrin-linked kinase. The last phosphorylates MLC20 directly and inhibits MLC phosphatase by phosphorylating CPI-17. PKA and PKG, which mediate relaxation, act upstream to desensitize the receptors (VPAC2 and NPR-C), inhibit adenylyl and guanylyl cyclase activities, and stimulate cAMP-specific PDE3 and PDE4 and cGMP-specific PDE5 activities. These kinases also act downstream to inhibit (a) initial contraction by inhibiting Ca2+ mobilization and (b) sustained contraction by inhibiting RhoA and targets downstream of RhoA. This increases MLC phosphatase activity and induces MLC20 dephosphorylation and muscle relaxation.

Activation of the cGMP/PKG pathway inhibits electrical activity in rabbit urethral interstitial cells of Cajal by reducing the spatial spread of Ca2+ waves

In the present study we used a combination of patch clamping and fast confocal Ca2+ imaging to examine the effects of activators of the nitric oxide (NO)/cGMP pathway on pacemaker activity in freshly dispersed ICC from the rabbit urethra, using the amphotericin B perforated patch configuration of the patch-clamp technique. The nitric oxide donor, DEA-NO, the soluble guanylyl cyclase activator YC-1 and the membrane-permeant analogue of cGMP, 8-Br-cGMP inhibited spontaneous transient depolarizations (STDs) and spontaneous transient inward currents (STICs) recorded under current-clamp and voltage-clamp conditions, respectively. Caffeine-evoked Cl- currents were unaltered in the presence of SP-8-Br-PET-cGMPs, suggesting that activation of the cGMP/PKG pathway does not block Cl- channels directly or interfere with Ca2+ release via ryanodine receptors (RyR). However, noradrenaline-evoked Cl- currents were attenuated by SP-8-Br-PET-cGMPs, suggesting that activation of cGMP-dependent protein kinase (PKG) may modulate release of Ca2+ via IP3 receptors (IP3R). When urethral interstitial cells (ICC) were loaded with Fluo4-AM (2 microm), and viewed with a confocal microscope, they fired regular propagating Ca2+ waves, which originated in one or more regions of the cell. Application of DEA-NO or other activators of the cGMP/PKG pathway did not significantly affect the oscillation frequency of these cells, but did significantly reduce their spatial spread. These effects were mimicked by the IP3R blocker, 2-APB (100 microm). These data suggest that NO donors and activators of the cGMP pathway inhibit electrical activity of urethral ICC by reducing the spatial spread of Ca2+ waves, rather than decreasing wave frequency.

Adrenergic receptor agonists prevent bile duct injury induced by adrenergic denervation by increased cAMP levels and activation of Akt

Loss of parasympathetic innervation after vagotomy impairs cholangiocyte proliferation, which is associated with depressed cAMP levels, impaired ductal secretion, and enhanced apoptosis. Agonists that elevate cAMP levels prevent cholangiocyte apoptosis and restore cholangiocyte proliferation and ductal secretion. No information exists regarding the role of adrenergic innervation in the regulation of cholangiocyte function. In the present studies, we investigated the role of adrenergic innervation on cholangiocyte proliferative and secretory responses to bile duct ligation (BDL). Adrenergic denervation by treatment with 6-hydroxydopamine (6-OHDA) during BDL decreased cholangiocyte proliferation and secretin-stimulated ductal secretion with concomitant increased apoptosis, which was associated with depressed cholangiocyte cAMP levels. Chronic administration of forskolin (an adenylyl cyclase activator) or beta(1)- and beta(2)-adrenergic receptor agonists (clenbuterol or dobutamine) prevented the decrease in cholangiocyte cAMP levels, maintained cholangiocyte secretory and proliferative activities, and decreased cholangiocyte apoptosis resulting from adrenergic denervation. This was associated with enhanced phosphorylation of Akt. The protective effects of clenbuterol, dobutamine, and forskolin on 6-OHDA-induced changes in cholangiocyte apoptosis and proliferation were partially blocked by chronic in vivo administration of wortmannin. In conclusion, we propose that adrenergic innervation plays a role in the regulation of biliary mass and cholangiocyte functions during BDL by modulating intracellular cAMP levels.

Natriuretic peptides, their receptors, and cyclic guanosine monophosphate-dependent signaling functions

Natriuretic peptides are a family of structurally related but genetically distinct hormones/paracrine factors that regulate blood volume, blood pressure, ventricular hypertrophy, pulmonary hypertension, fat metabolism, and long bone growth. The mammalian members are atrial natriuretic peptide, B-type natriuretic peptide, C-type natriuretic peptide, and possibly osteocrin/musclin. Three single membrane-spanning natriuretic peptide receptors (NPRs) have been identified. Two, NPR-A/GC-A/NPR1 and NPR-B/GC-B/NPR2, are transmembrane guanylyl cyclases, enzymes that catalyze the synthesis of cGMP. One, NPR-C/NPR3, lacks intrinsic enzymatic activity and controls the local concentrations of natriuretic peptides through constitutive receptor-mediated internalization and degradation. Single allele-inactivating mutations in the promoter of human NPR-A are associated with hypertension and heart failure, whereas homozygous inactivating mutations in human NPR-B cause a form of short-limbed dwarfism known as acromesomelic dysplasia type Maroteaux. The physiological effects of natriuretic peptides are elicited through three classes of cGMP binding proteins: cGMP-dependent protein kinases, cGMP-regulated phosphodiesterases, and cyclic nucleotide-gated ion channels. In this comprehensive review, the structure, function, regulation, and biological consequences of natriuretic peptides and their associated signaling proteins are described.

Rapid modulatory effect of estradiol on acetylcholine-induced Ca2+ signal is mediated through cyclic-GMP cascade in LHRH-releasing GT1-7 cells

Hypothalamic luteinizing hormone-releasing hormone neurons (LHRH) form the final pathway for the central control of reproduction through the release of LHRH into the pituitary-hypothalamic system. We previously found that LHRH-producing GT1-7 cells respond to acetylcholine (ACh) with an increase in intracellular calcium ([Ca2+]i) through activation of muscarinic receptors. This effect is acutely modulated by 17beta-estradiol in a manner compatible with specific membrane binding sites. Because increasing evidence suggests that second messengers are involved in the rapid action of estradiol, the aim of the present study was to identify the pathway underlying estrogen actions on ACh-induced Ca2+ signals. 8-Bromoguanosine 3',5'-cyclic monophosphate (10 microm) and C-type natriuretic peptide (10 microm) mimicked the effect of estradiol. On the contrary, neither dibutyryl cAMP (100 microm), forskolin (100 nm or 10 microm), or sodium nitroprusside (10 microm) induced any modification of [Ca2+]i in response to ACh. The effect of estradiol on calcium transients was totally blocked by two different cGMP-dependent protein kinase (PKG) inhibitors. In addition, phosphorylation of inositol 1,4,5-triphosphate (IP3) receptor was rapidly induced by estradiol but totally blocked when the cells were pretreated with a PKG inhibitor. We conclude that physiological concentrations of estradiol reduce ACh-induced Ca2+ transients via a mechanism involving a membrane-associated guanylate cyclase, which finally induces a PKG-dependent IP3 receptor phosphorylation that modifies calcium release from the endoplasmic reticulum.

Effects of cGMP on coordination of vascular smooth muscle cells of rat mesenteric small arteries

OBJECTIVE

We tested the hypothesis that cGMP can induce a state of only partial coordination of vascular smooth muscle cells (VSMC).

METHODS

This was done by studying the concentration-dependent effect of 8Br-cGMP on isometric and isobaric force development of noradrenaline-activated segments of rat mesenteric small arteries in which the endothelium was removed. We further measured the concentration-dependent effect of 8Br-cGMP on VSMC membrane potential, spatially resolved [Ca(2+)](i) and VSMC membrane conductance.

RESULTS

With 300 microM 8Br-cGMP, coordinated [Ca(2+)](i) activity and vasomotion were seen as previously reported. At 10-30 microM 8Br-cGMP, beating isometric tension oscillations were seen. Isobaric recordings revealed oscillations with different frequencies in different parts of the arteries. At these (10-30 microM) 8Br-cGMP concentrations, membrane potential oscillations did not always concur with isometric tension oscillations, and [Ca(2+)](i) oscillations were only synchronized locally within groups of cells. 8Br-cGMP concentration-dependently decreased the frequency of vasomotion and, in unsynchronized hyperpolarized VSMC, the frequency of [Ca(2+)](i) waves.

CONCLUSION

Our results demonstrated that cGMP can cause a partial coordination of the VSMC in the vascular wall (and at high concentrations near complete coordination). Furthermore, the cGMP concentration-dependent decrease of Ca(2+) wave frequency and of vasomotion frequency suggests that cGMP modifies oscillatory Ca(2+) release from the sarcoplasmic reticulum and supports the suggestion that this oscillatory release paces vasomotion.

Inhibition of carbachol-evoked oscillatory currents by the NO donor sodium nitroprusside in guinea-pig ileal myocytes

The effect of sodium nitroprusside (SNP) on carbachol (CCh)-evoked inward cationic current (Icat) oscillations in guinea-pig ileal longitudinal myocytes was investigated using the whole-cell patch-clamp technique and permeabilized longitudinal muscle strips. SNP (10 microm) completely inhibited I(cat) oscillations evoked by 1 microm CCh. 1H-(1,2,4) Oxadiazole [4,3-a] quinoxaline-1-one (ODQ; 1 microm) almost completely prevented the inhibitory effect of SNP on Icat oscillations. 8-Bromo-guanosine 3',5'-cyclic monophosphate (8-Br-cGMP; 30 microm) in the pipette solution completely abolished Icat oscillations. However, a pipette solution containing Rp-8-Br-cGMP (30 microm) almost completely abolished the inhibitory effect of SNP on Icat oscillations. When the intracellular calcium concentration ([Ca2+]i) was held at a resting level using BAPTA (10 mm) and Ca2+ (4.6 microm) in the pipette solution, CCh (1 microm) evoked only the sustained component of Icat without any oscillations and SNP did not affect the current. A high concentration of inositol 1,4,5-trisphosphate (IP3; 30 microm) in the patch pipette solutions significantly reduced the inhibitory effect of SNP (10 microm) on Icat oscillations. SNP significantly inhibited the Ca2+ release evoked by either CCh or IP3 but not by caffeine in permeabilized preparations of longitudinal muscle strips. These results suggest that the inhibitory effects of SNP on Icat oscillations are mediated, in part, by functional modulation of the IP3 receptor, and not by the inhibition of cationic channels themselves or by muscarinic receptors in the plasma membrane. This inhibition seems to be mediated by an increased cGMP concentration in a protein kinase G-dependent manner.