Nitric oxide regulates the calcium current in isolated human atrial myocytes

Nitric oxide donors alter cardiomyocyte cytokine secretion and cardiac function

OBJECTS

The mechanisms by which nitric oxide produces beneficial/detrimental effects on physiologic function are unclear. In this study, we hypothesized that nitric oxide promotes cyclic guanosine monophosphate (cGMP) formation, which, in turn, promotes cardiomyocyte secretion of inflammatory cytokines as well as accumulation of intracellular Na+/Ca2+; these factors contribute to altered cardiac contractile function.

DESIGN

Laboratory study.

SETTING

Medical Center.

SUBJECTS

Adult Sprague Dawley rats weighing 325-350 g.

INTERVENTIONS

Cardiomyocytes were prepared by collagenase perfusion of rat hearts; cells were plated (5 x 10(4) cells/microtiter well) and challenged with either vehicle or nitric oxide donor (S-nitroso-N-acetyl-penicillamine [SNAP] or PAPA NONOATE, 3-[2-Hydroxy-2-nitroso-1-propythdrazinol]-1-propanamine], NOC-15 [PAPA-NO], 0.3 or 1.0 mM of each nitric oxide donor) in the presence/absence of methylene blue (10 microM/L to inhibit cGMP). After 3 hrs, supernatants were collected to measure nitrite/nitrate (nitric oxide), cytokines (tumor necrosis factor-alpha, interleukin-1beta, interleukin-6), and cGMP levels; cells were then loaded with a fluorescent indicator (Fura-2AM or sodium-binding benzofurzan isophthalate) to measure myocyte Ca2+ or Na+, respectively. Parallel experiments included the addition of nitric oxide donor (0.3 or 1.0 mM SNAP or PAPA-NO) to perfused hearts in presence or absence of the methylene blue to examine cGMP-mediated effects on myocardial contraction-relaxation, while other experiments determined a) potential lipopolysaccharide contamination of myocyte preparations; and b) whether a cGMP analogue recapitulated the effects of nitric oxide donors on cytokine secretion.

MEASUREMENTS AND MAIN RESULTS

Nitric oxide donors produced a dose-dependent increase in cGMP levels in myocyte supernatants as well as an increase in myocyte cytokine secretion, increased myocyte loading of Na+/Ca2+, and produced myocardial contractile dysfunction. Addition of the cGMP analog, 8-bromo-cGMP, recapitulated the effects of nitric oxide donors on myocyte cytokine secretion. Nitric oxide donor-related effects were ablated by pretreatment of myocytes or isolated hearts with methylene blue. Treatment of myocytes with recombinant bactericidal/permeability-increasing protein to scavenge lipopolysaccharide confirmed that cytokine responses to nitric oxide donors were not related to lipopolysaccharide contamination of myocyte preparations.

CONCLUSIONS

We suggest that nitric oxide synthesis in injury and disease promotes cGMP formation, which, in turn, modulates cardiac contraction/relaxation by a) altering cardiomyocyte secretion of inflammatory cytokines and b) altering myocyte handling of Na+/Ca2+.

Phenylephrine acts via IP3-dependent intracellular NO release to stimulate L-type Ca2+ current in cat atrial myocytes

This study determined the effects of alpha1-adrenergic receptor (alpha1-AR) stimulation by phenylephrine (PE) on L-type Ca2+ current (I(Ca,L)) in cat atrial myocytes. PE (10 microm) reversibly increased I(Ca,L) (51.3%; n = 40) and shifted peak I(Ca,L) activation voltage by -10 mV. PE-induced stimulation of I(Ca,L) was blocked by each of 1 microm prazocin, 10 microm L-NIO, 10 microm W-7, 10 microm ODQ, 2 microm H-89 or 10 microm LY294002, and was unaffected by 10 microm chelerythrine or incubating cells in pertussis toxin (PTX). PE-induced stimulation of I(Ca,L) also was inhibited by each of 10 microm ryanodine or 5 microm thapsigargin, by blocking IP3 receptors with 2 microm 2-APB or 10 microm xestospongin C or by intracellular dialysis of heparin. In field-stimulated cells, PE increased intracellular NO (NOi) production. PE-induced NOi release was inhibited by each of 1 microm prazocin, 10 microm L-NIO, 10 microm W-7, 10 microm LY294002, 2 microm H-89, 10 microm ryanodine, 5 microm thapsigargin, 2 microm 2-APB or 10 microm xestospongin C, and unchanged by PTX. PE (10 microm) increased phosphorylation of Akt, which was inhibited by LY294002. Confocal microscopy showed that PE stimulated NOi release from subsarcolemmal sites and this was prevented by 2 mm methyl-beta-cyclodextrin, an agent that disrupts caveolae formation. PE also increased local, subsarcolemmal SR Ca2+ release via IP3-dependent signalling. Electron micrographs of atrial myocytes show peripheral SR cisternae in close proximity to clusters of caveolae. We conclude that in cat atrial myocytes PE acts via alpha1-ARs coupled to PTX-insensitive G-protein to release NOi, which in turn stimulates I(Ca,L). PE-induced NOi release requires stimulation of both PI-3K/Akt and IP3-dependent Ca2+ signalling. NO stimulates I(Ca,L) via cGMP-mediated cAMP-dependent PKA signalling. IP3-dependent Ca2+ signalling may enhance local SR Ca2+ release required to activate Ca2+-dependent eNOS/NOi production from subsarcolemmal caveolae sites.

Effects of cyclic GMP and its protein kinase on the contraction of ventricular myocytes from hearts after cardiopulmonary arrest

Hearts undergoing cardiopulmonary arrest and resuscitation have depressed function and may have changes in signal transduction. We hypothesized that the cyclic GMP (cGMP) signaling pathway would be altered in the post-resuscitation heart. This was studied in ventricular myocytes from 7 anesthetized open-chest rabbits. Cardiopulmonary arrest was achieved for 10 min through ventricular fibrillation and respirator shutdown. After cardiopulmonary arrest, respiration was resumed, the heart was defibrillated, and the heart recovered for 15 min. Seven additional rabbits served as controls. Myocyte function was measured via a video edge detector. Myocytes were treated with 8-bromo-cGMP (10(-5)-10(-6) mol/L) followed by KT5823 (10(-6) mol/L, cGMP protein kinase inhibitor). The baseline percent shortening was significantly depressed in the cardiac arrest myocytes compared with control (3.3 +/- 0.1 vs. 5.5 +/- 0.3%). Treatment with 8-Br-cGMP similarly and dose-dependently reduced cell contraction in both cardiac arrest (-24%) and control (-25%) myocytes. The negative effect of 8-Br-cGMP was partially reversed by KT5823 in control myocytes, but not in the arrest group, indicating reduced involvement of cGMP protein kinase. Multiple proteins were specifically phosphorylated when cGMP was present, but the degree of phosphorylation was significantly less in myocytes after cardiac arrest. The data suggested that the basal contraction was reduced, but the functional response to 8-Br-cGMP was preserved in myocytes from cardiopulmonary arrested hearts. The results also indicated that the action of cGMP appeared to be mainly through non-cGMP protein kinase pathways in the post-resuscitation heart.

Species- and tissue-dependent effects of NO and cyclic GMP on cardiac ion channels

Biochemical studies have established the presence of a NO pathway in the heart, including sources of NO and various effectors. Several cardiac ion channels have been shown to be modified by NO, such as L-type Ca(2+), ATP-sensitive K(+), and pacemaker f-channels. Some of these effects are mediated by cGMP, through the activity of three main proteins: the cGMP-dependent protein kinase (PKG), the cGMP-stimulated phosphodiesterase (PDE2) and the cGMP-inhibited PDE (PDE3). Other effects appear independent of cGMP, as for instance the NO modulation of the ryanodine receptor-Ca(2+) channel. In the case of the cardiac L-type Ca(2+) channel current (I(Ca,L)), both cGMP-dependent and cGMP-independent effects have been reported, with important tissue and species specificity. For instance, in rabbit sinoatrial myocytes, NO inhibits the beta-adrenergic stimulation of I(Ca,L) through activation of PDE2. In cat and human atrial myocytes, NO potentiates the cAMP-dependent stimulation of I(Ca,L) through inhibition of PDE3. In rabbit atrial myocytes, NO enhances I(Ca,L) in a cAMP-independent manner through the activation of PKG. In ventricular myocytes, NO exerts opposite effects on I(Ca,L): an inhibition mediated by PKG in mammalian myocytes but by PDE2 in frog myocytes; a stimulation attributed to PDE3 inhibition in frog ventricular myocytes but to a direct effect of NO in ferret ventricular myocytes. Finally, NO can also regulate cardiac ion channels by a direct action on G-proteins and adenylyl cyclase.

Ca2+ currents in cardiac myocytes: Old story, new insights

Calcium is a ubiquitous second messenger which plays key roles in numerous physiological functions. In cardiac myocytes, Ca2+ crosses the plasma membrane via specialized voltage-gated Ca2+ channels which have two main functions: (i) carrying depolarizing current by allowing positively charged Ca2+ ions to move into the cell; (ii) triggering Ca2+ release from the sarcoplasmic reticulum. Recently, it has been suggested than Ca2+ channels also participate in excitation-transcription coupling. The purpose of this review is to discuss the physiological roles of Ca2+ currents in cardiac myocytes. Next, we describe local regulation of Ca2+ channels by cyclic nucleotides. We also provide an overview of recent studies investigating the structure-function relationship of Ca2+ channels in cardiac myocytes using heterologous system expression and transgenic mice, with descriptions of the recently discovered Ca2+ channels alpha(1D) and alpha(1E). We finally discuss the potential involvement of Ca2+ currents in cardiac pathologies, such as diseases with autoimmune components, and cardiac remodeling.

Nitric oxide-mediated regulation of connexin43 expression and gap junctional intercellular communication in mesangial cells

This study investigated a potential role of nitric oxide (NO) in the regulation of gap junctional intercellular communication (GJIC). Incubation of mesangial cells (MC) with NO donor S-nitroso-N-acetylpenicillamine (SNAP) enhanced both basal and 8-bromo-cAMP-stimulated GJIC as well as expression of gap junction protein connexin43 (Cx43). This potentiating action of SNAP on Cx43 expression was mimicked by two other NO donors and significantly blocked by soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-alpha]quinoxalin-1-1. Guanosine 3',5'-cyclic monophosphate (cGMP) analogue 8-bromo-cGMP exerted an effect similar to NO, whereas another cGMP analogue, 8-pCPT-cGMP, which selectively activates cGMP-dependent kinase without affecting cGMP-inhibited phosphodiesterase (PDE3), had no effect. Moreover, the synergistic action of NO on Cx43 expression was completely prevented by protein kinase A inhibitor H89 but not by cGMP-dependent kinase inhibitor Rp-8-Br-PET-cGMP. These results suggested a possible involvement of NO-cAMP interaction via cGMP-mediated inhibition of PDE3. Indeed, PDE3 inhibitor cilostamide caused potentiation of 8-bromo-cAMP-elicited elevations of Cx43 expression that is similar to the effect of SNAP, and an elevation of intracellular cAMP was detected in SNAP-treated cells. With the use of genetically engineered reporter MC that express secreted alkaline phosphatase under the control of the cAMP response element, significant potentiation of cAMP-elicited activation of cAMP response element by SNAP was found. This effect was abrogated in the presence of PDE3 inhibitor cilostamide. Taken together, the results suggest that NO is involved in the control of GJIC and Cx43 expression. This effect of NO is due to activation of protein kinase A via cGMP-dependent inhibition of PDE3 activity.

Negative feedback exerted by cAMP-dependent protein kinase and cAMP phosphodiesterase on subsarcolemmal cAMP signals in intact cardiac myocytes: an in vivo study using adenovirus-mediated expression of CNG channels

Intracardiac cAMP levels are modulated by hormones and neuromediators with specific effects on contractility and metabolism. To understand how the same second messenger conveys different information, mutants of the rat olfactory cyclic nucleotide-gated (CNG) channel alpha-subunit CNGA2, encoded into adenoviruses, were used to monitor cAMP in adult rat ventricular myocytes. CNGA2 was not found in native myocytes but was strongly expressed in infected cells. In whole cell patch-clamp experiments, the forskolin analogue L-858051 (L-85) elicited a non-selective, Mg2+ -sensitive current observed only in infected cells, which was thus identified as the CNG current (ICNG). The beta-adrenergic agonist isoprenaline (ISO) also activated ICNG, although the maximal efficiency was approximately 5 times lower than with L-85. However, ISO and L-85 exerted a similar maximal increase of the L-type Ca2+ current. The use of a CNGA2 mutant with a higher sensitivity for cAMP indicated that this difference is caused by the activation of a localized fraction of CNG channels by ISO. cAMP-dependent protein kinase (PKA) blockade with H89 or PKI, or phosphodiesterase (PDE) inhibition with IBMX, dramatically potentiated ISO- and L-85-stimulated ICNG. A similar potentiation of beta-adrenergic stimulation occurred when PDE4 was blocked, whereas PDE3 inhibition had a smaller effect (by 2-fold). ISO and L-85 increased total PDE3 and PDE4 activities in cardiomyocytes, although this effect was insensitive to H89. However, in the presence of IBMX, H89 had no effect on ISO stimulation of ICNG. This study demonstrates that subsarcolemmal cAMP levels are dynamically regulated by a negative feedback involving PKA stimulation of subsarcolemmal cAMP-PDE.

Nitric oxide control of cardiac function: is neuronal nitric oxide synthase a key component?

Nitric oxide (NO) has been shown to regulate cardiac function, both in physiological conditions and in disease states. However, several aspects of NO signalling in the myocardium remain poorly understood. It is becoming increasingly apparent that the disparate functions ascribed to NO result from its generation by different isoforms of the NO synthase (NOS) enzyme, the varying subcellular localization and regulation of NOS isoforms and their effector proteins. Some apparently contrasting findings may have arisen from the use of non-isoform-specific inhibitors of NOS, and from the assumption that NO donors may be able to mimic the actions of endogenously produced NO. In recent years an at least partial explanation for some of the disagreements, although by no means all, may be found from studies that have focused on the role of the neuronal NOS (nNOS) isoform. These data have shown a key role for nNOS in the control of basal and adrenergically stimulated cardiac contractility and in the autonomic control of heart rate. Whether or not the role of nNOS carries implications for cardiovascular disease remains an intriguing possibility requiring future study.

High and low gain switches for regulation of cAMP efflux concentration: distinct roles for particulate GC- and soluble GC-cGMP-PDE3 signaling in rabbit atria

This study tests the hypothesis that particulate (p) guanylyl cyclase (GC) and soluble (s) GC are involved in the distinct roles for the regulation of cGMP-PDE-cAMP signaling and of mechanical and secretory functions in the heart. Experiments were performed in perfused beating rabbit atria. C-type natriuretic peptide (CNP) and SIN-1, an NO donor, or BAY 41-2272 (BAY), a direct activator for sGC, were used to activate pGC and sGC, respectively. CNP and SIN-1 increased cGMP and cAMP efflux in a concentration-dependent manner. Increase in cAMP was a function of cGMP. The changes in cAMP efflux concentration in terms of cGMP were much more prominent in the atria treated with CNP than in the atria treated with SIN-1. Increase in cAMP efflux concentration was blocked by milrinone but not changed by EHNA. BAY increased cGMP but not cAMP in a concentration-dependent manner. CNP and SIN-1 decreased atrial stroke volume and myocytic ANP release. The decreases in terms of cGMP efflux concentration were much more prominent in the atria treated with CNP than in the atria treated with SIN-1 or BAY. Milrinone accentuated GC agonist-induced decreases in atrial stroke volume and ANP release. In the presence of ODQ, SIN-1 or BAY induced effects were not observed. These data suggest that pGC and sGC activations have distinct roles via cGMP-PDE3-cAMP signaling in the cardiac atrium: high and low gain switches, respectively, for the regulation of cAMP levels and contractile and secretory functions.

Production and role of extracellular guanosine cyclic 3', 5' monophosphate in sodium uptake in human proximal tubule cells

The present study was designed to determine the capability of human renal proximal tubule (RPT) to generate and export guanosine cyclic 3', 5' monophosphate (cGMP) in response to direct stimulation of soluble guanylyl cyclase by nitric oxide (NO) donors. In addition, we investigated whether cGMP extrusion from human RPT cells is required for inhibition of cellular sodium uptake. RPT cells were cultured from fresh human kidneys (normotensive subjects, n=4, mean age 65+/-4.7 years, 3 men, 1 woman; hypertensive patients, n=6, mean age 64+/-6.1 years, 4 men, 2 women) after unilateral nephrectomy. The fluorescence dye Sodium Green was employed to determine cytoplasmic Na+ concentration. In the presence of the Na+/K+ ATPase inhibitor ouabain, fluorescence was monitored at the appropriate wavelength (excitation 485 nm, emission 535 nm). Nitric oxide donor, S-nitroso-N-acetylpenicillamine (SNAP, 10(-4) M), increased both intracellular and extracellular cGMP (from 1.26+/-0.21 to 88.7+/-12.6 pmol/mg protein and from 0.58+/-0.10 to 9.24+/-1.9 pmol/mL, respectively, P<0.01) and decreased cellular Na+ uptake by 37.4+/-6.8% (P<0.05) compared with control. The effects of SNAP on cGMP production were similar in normotensive and hypertensive subjects. The increases in intracellular and extracellular cGMP concentration because of SNAP were blocked completely by soluble guanylyl cyclase inhibitor ODQ (1-H-[1,2,4] oxadiazolo [4,2-alpha] quinoxalin-1-one). Probenecid, an organic anion transport inhibitor, augmented the SNAP (10(-6) M)-induced increase in intracellular cGMP accumulation (from 4.9+/-0.9 to 9.8+/-1.5 pmol/mg protein, P<0.05), abrogated the SNAP-induced increase in extracellular cGMP extrusion (from 1.07+/-0.4 to 0.37+/-0.1 pmol/L, P<0.05) and blocked the SNAP-induced reduction in cellular Na+ uptake. Neither intracellular nor extracellular cGMP were influenced by l-arginine, the metabolic precursor of NO, or N(G)-nitro-L-arginine methyl ester, an inhibitor of NO synthase. After exogenous administration of cGMP (10(-5) M) or its membrane-permeable analogue 8-Br-cGMP (10(-5) M), only 8-Br-cGMP crossed the cell membrane to increase intracellular cGMP (from 1.36+/-0.19 to 289.7+/-29.4 pmol/mg protein, P<0.01). However, both cGMP and 8-Br-cGMP were effective in decreasing cellular Na+ uptake. In conclusion, human RPT cells contain soluble guanylyl cyclase and are able to generate and export cGMP in response to NO. Because human RPT cells do not themselves contain constitutive NO synthase, the NO-generating cGMP must be derived from sources outside the human RPT. The cGMP cellular export system is critical in the regulation of RPT cellular Na+ absorption in humans.

Signaling mechanisms that mediate nitric oxide production induced by acetylcholine exposure and withdrawal in cat atrial myocytes

Fluorescence microscopy and the NO-sensitive indicator 4,5-diaminofluorescein were used to determine the effects of acetylcholine (ACh) on intracellular NO (NOi) in cat atrial myocytes. Field stimulation (1 Hz) of cells or exposure of quiescent cells to ACh (1 to 10 micromol/L) had no effect on NOi. However, in field-stimulated cells, ACh exposure increased NOi, and ACh withdrawal elicited an additional, prominent increase in NOi production. During ACh exposure, addition of 1 micromol/L atropine increased NOi production similar to ACh withdrawal. ACh-induced increases in NOi were reduced by prior exposure to 1 mmol/L extracellular Ca2+ ([Ca2+]o) and prevented by 0.5 mmol/L [Ca2+]o, 1 micromol/L verapamil, 1 micromol/L atropine, 10 micromol/L L-N5-(1-iminoethyl)ornithine, 10 micromol/L W-7, or incubating cells in pertussis toxin or 10 micromol/L LY294002 (inhibits phosphatidylinositol 3-kinase). Switching to 0.5 mmol/L [Ca2+]o during ACh withdrawal prevented the additional increase in NOi. ACh exposure increased phosphorylation (Ser473) of protein kinase B (Akt), and this effect was blocked by LY294002 and unaffected in low (0.5 mmol/L) [Ca2+]o. Confocal microscopy revealed that ACh exposure increased NOi at local subsarcolemmal sites, and ACh withdrawal additionally increased NOi by recruiting additional subsarcolemmal release sites. Disruption of caveolae by 2 mmol/L methyl-beta-cyclodextrin abolished ACh-induced NOi production. We conclude that in cat atrial myocytes, ACh stimulates NOi release from local subsarcolemmal sites. ACh-induced increases in NOi requires both muscarinic receptor-mediated Gi protein/phosphatidylinositol 3-kinase/Akt signaling and voltage-activated Ca2+ influx for stimulation of calmodulin-dependent endothelial NO synthase activity. Increases in NOi elicited by ACh withdrawal result from the recovery of Ca2+ influx after ACh inhibition. NO signaling elicited by ACh withdrawal stimulates rapid recovery from cholinergic atrial inhibition.

Cyclic GMP phosphodiesterases and regulation of smooth muscle function

Cyclic GMP (cGMP) made in response to atrial natriuretic peptide (ANP) or nitric oxide (NO) is an important regulator of short-term changes in smooth muscle tone and longer-term responses to chronic drug treatment or proliferative signals. The ability of smooth muscle cells (SMCs) to utilize different combinations of phosphodiesterase (PDE) isozymes allows cGMP to mediate these multiple processes. For example, PDE5 as a major cGMP-hydrolyzing PDE effectively controls the development of smooth muscle relaxation. In order for contraction to occur, PDE5 is activated and cGMP falls. Conversely, blockade of PDE5 activity allows the relaxation cycle to be prolonged and enhanced. A recently shown direct activation of PDE5 by cGMP binding to the GAF A domain suggests that this regulatory site might be a target for new drug development. The calcium surge associated with vasoconstrictor initiated contraction also activates a calcium/calmodulin-dependent PDE (PDE1A). Together, PDE5 and PDE1A lower cGMP sufficiently to allow contraction. Longer term, both PDE5 and PDE1A mRNA are induced by chronic stimulation of guanylyl cyclase. This induction is a major cause of the tolerance that develops to NO-releasing drugs. Finally, high levels of cGMP or cAMP also act as a brake to attenuate the proliferative response of SMCs to many mitogens. After vessel damage, in order for SMC proliferation to occur, the levels of cGMP and cAMP must be decreased. In humans, this decrease is caused in large part by induction of another Ca2+/calmodulin-dependent PDE (PDE1C) that allows the brake to be released and proliferation to start.

Muscarinic regulation of cardiac ion channels

The parasympathetic component of the autonomic nervous system plays an important role in the physiological regulation of cardiac function by exerting significant influence over the initiation as well as propagation of electrical impulses, in addition to being able to regulate contractile force. These effects are mediated in whole or in part through changes in ion channel activity that occur in response to activation of M(2) muscarinic cholinergic receptors following release of the neurotransmitter acetylcholine. The coupling of M(2) receptor activation to most changes in cardiac ion channel function can be explained by one of two general paradigms. The first involves direct G protein-dependent regulation of ion channel activity. The second involves indirect regulation of ion channel activity through modulation of cAMP-dependent responses. This review focuses on recent advances in our understanding of the mechanisms by which M(2) muscarinic receptor activation both inhibits and facilitates cAMP-dependent ion channel responses in the heart.

Cardiac endothelial-myocardial signaling: its role in cardiac growth, contractile performance, and rhythmicity

Experimental work during the past 15 years has demonstrated that endothelial cells in the heart play an obligatory role in regulating and maintaining cardiac function, in particular, at the endocardium and in the myocardial capillaries where endothelial cells directly interact with adjacent cardiomyocytes. The emerging field of targeted gene manipulation has led to the contention that cardiac endothelial-cardiomyocytal interaction is a prerequisite for normal cardiac development and growth. Some of the molecular mechanisms and cellular signals governing this interaction, such as neuregulin, vascular endothelial growth factor, and angiopoietin, continue to maintain phenotype and survival of cardiomyocytes in the adult heart. Cardiac endothelial cells, like vascular endothelial cells, also express and release a variety of auto- and paracrine agents, such as nitric oxide, endothelin, prostaglandin I(2), and angiotensin II, which directly influence cardiac metabolism, growth, contractile performance, and rhythmicity of the adult heart. The synthesis, secretion, and, most importantly, the activities of these endothelium-derived substances in the heart are closely linked, interrelated, and interactive. It may therefore be simplistic to try and define their properties independently from one another. Moreover, in relation specifically to the endocardial endothelium, an active transendothelial physicochemical gradient for various ions, or blood-heart barrier, has been demonstrated. Linkage of this blood-heart barrier to the various other endothelium-mediated signaling pathways or to the putative vascular endothelium-derived hyperpolarizing factors remains to be determined. At the early stages of cardiac failure, all major cardiovascular risk factors may cause cardiac endothelial activation as an adaptive response often followed by cardiac endothelial dysfunction. Because of the interdependency of all endothelial signaling pathways, activation or disturbance of any will necessarily affect the others leading to a disturbance of their normal balance, leading to further progression of cardiac failure.

Acute antihypertrophic actions of bradykinin in the rat heart: importance of cyclic GMP

The antihypertrophic action of angiotensin (Ang)-converting enzyme (ACE) inhibitors in the heart is attributed in part to potentiation of bradykinin. Bradykinin prevents hypertrophy of cultured cardiomyocytes by releasing nitric oxide (NO) from endothelial cells, which increases cardiomyocyte guanosine 3'5'-cyclic monophosphate (cyclic GMP). It is unknown whether cyclic GMP is essential for the action of bradykinin, or whether findings in isolated cardiomyocytes apply in whole hearts, in the presence of other cell types and mechanical/dynamic activity. We now examine the contribution of cyclic GMP to the antihypertrophic action of bradykinin in cardiomyocytes and perfused hearts. In adult rat isolated cardiomyocytes cocultured with bovine aortic endothelial cells, the inhibitory action of bradykinin (10 micromol/L) against Ang II (1 micromol/L)-induced [3H]phenylalanine incorporation was abolished by the soluble guanylyl cyclase inhibitor [1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one (10 micromol/L). In Langendorff-perfused rat hearts, Ang II (10 nmol/L)-induced increases in [3H]phenylalanine incorporation and atrial natriuretic peptide mRNA expression were prevented by bradykinin (100 nmol/L), the NO donor sodium nitroprusside (3 micromol/L), and the ACE inhibitor ramiprilat (100 nmol/L). The acute antihypertrophic action of bradykinin was accompanied by increased left ventricular cyclic GMP, and the ramiprilat effect was attenuated by HOE 140 (1 micromol/L, a B2-kinin receptor antagonist) or [1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one (100 nmol/L). In conclusion, bradykinin exerts a direct inhibitory action against the acute hypertrophic response to Ang II in rat isolated hearts, and elevation of cardiomyocyte cyclic GMP may be an important antihypertrophic mechanism used by bradykinin and ramiprilat in the heart.

Potentiation of slow component of delayed rectifier K(+) current by cGMP via two distinct mechanisms: inhibition of phosphodiesterase 3 and activation of protein kinase G

1. Regulation of the slowly activating component of delayed rectifier K(+) current (I(Ks)) by intracellular guanosine 3'5' cyclic monophosphate (cGMP) was investigated in guinea-pig sino-atrial (SA) node cells using the whole-cell patch-clamp method. 2. When a cell was dialyzed with pipette solution containing 100 micro M cGMP, I(Ks) started to gradually increase and reached a maximum increase of a factor of 2.37 +/- 0.39 (n = 4) about 10-15 min after rupture of patch membrane. Atrial natriuretic peptide (ANP, 100 nM) also potentiated I(Ks), consistent with intracellular cGMP-induced enhancement of I(Ks). 3. Bath application of a selective blocker of the cGMP-inhibited phosphodiesterase (PDE3) milrinone (100 microM) enhanced I(Ks) by a factor of 1.50 +/- 0.09 (n = 4) but failed to further enhance I(Ks) after a maximum stimulation by intracellular cGMP (100 microM), suggesting that blockade of PDE3 activity is involved in the enhancement of I(Ks). A potent but nonspecific PDE inhibitor 3-isobutyl-1-methylxanthine (IBMX, 100 microM) further increased I(Ks) stimulated by 100 microM milrinone, indicating that PDE subtypes other than PDE3 are also involved in the regulation of basal I(Ks) in guinea-pig SA node cells. 4. Bath application of 100 microM 8-bromoguanosine 3'5' cyclic monophosphate (8-Br-cGMP) increased I(Ks) by a factor of 1.48 +/- 0.11 (n = 5) and this stimulatory effect was totally abolished by cGMP-dependent protein kinase (PKG) inhibitor KT-5823 (500 nM), suggesting that the activation of PKG also mediates cGMP-induced potentiation of I(Ks). 5. These results strongly suggest that intracellular cGMP potentiates I(Ks) not only by blocking PDE3 but also by activating PKG in guinea-pig SA node cells.

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

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

Effect of SIN-1 in rat ventricular myocytes: interference with beta-adrenergic stimulation

We have examined the effects of the nitric oxide (NO) donor, 3-morpholino-sydnonimine (SIN-1), on Ca(2+) transients, L-type Ca(2+) current (I(Ca,L)), and cGMP/cAMP content in electrically-stimulated rat ventricular myocytes in the absence and presence of the beta-adrenergic stimulation with isoproterenol. SIN-1 had no effect at low concentrations, but decreased the amplitude of electrically-induced Ca(2+) transients at higher concentrations. SIN-1 attenuated the increase in Ca(2+) transients induced by isoproterenol in a concentration-dependent manner. SIN-1 Also reduced the amplitude of caffeine-induced Ca(2+) transients, and the increase in I(Ca,L) induced by isoproterenol. These effects of SIN-1 were associated with an increased cGMP and a decreased cAMP content in ventricular myocytes in either the absence or presence of isoproterenol. These data suggest that the inhibitory effect of SIN-1 on basal and beta-adrenergic stimulated Ca2+ signal in ventricular myocytes could be due to the depression in the SR function and I(Ca,L), possibly mediated by a cGMP/cAMP-dependent mechanism. Taken together, the present study supports the idea that NO acts as an inhibitory modulator of the cardiac function during pathological conditions associated with an abnormal production of NO such as septic shock.

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

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

Beta 2-adrenergic receptor signaling acts via NO release to mediate ACh-induced activation of ATP-sensitive K+ current in cat atrial myocytes

In atrial myocytes, an initial exposure to isoproterenol (ISO) acts via cAMP to mediate a subsequent acetylcholine (ACh)-induced activation of ATP-sensitive K(+) current (I(K,ATP)). In addition, beta-adrenergic receptor (beta-AR) stimulation activates nitric oxide (NO) release. The present study determined whether the conditioning effect of beta-AR stimulation acts via beta(1)- and/or beta(2)-ARs and whether it is mediated via NO signaling. 0.1 microM ISO plus ICI 118,551 (ISO-beta(1)-AR stimulation) or ISO plus atenolol (ISO-beta(2)-AR stimulation) both increased L-type Ca(2+) current (I(Ca,L)) markedly, but only ISO-beta(2)-AR stimulation mediated ACh-induced activation of I(K,ATP). 1 microM zinterol (beta(2)-AR agonist) also increased I(Ca,L) and mediated ACh-activated I(K,ATP). Inhibition of NO synthase (10 microM L-NIO), guanylate cyclase (10 microM ODQ), or cAMP-PKA (50 microM Rp-cAMPs) attenuated zinterol-induced stimulation of I(Ca,L) and abolished ACh-activated I(K,ATP). Spermine-NO (100 microM; an NO donor) mimicked beta(2)-AR stimulation, and its effects were abolished by Rp-cAMPs. Intracellular dialysis of 20 microM protein kinase inhibitory peptide (PKI) abolished zinterol-induced stimulation of I(Ca,L). Measurements of intracellular NO ([NO](i)) using the fluorescent indicator DAF-2 showed that ISO-beta(2)-AR stimulation or zinterol increased [NO](i). L-NIO (10 microM) blocked ISO- and zinterol-induced increases in [NO](i). ISO-beta(1)-AR stimulation failed to increase [NO](i). Inhibition of G(i)-protein by pertussis toxin significantly inhibited zinterol-mediated increases in [NO](i). Wortmannin (0.2 microM) or LY294002 (10 microM), inhibitors of phosphatidylinositol 3'-kinase (PI-3K), abolished the effects of zinterol to both mediate ACh-activated I(K,ATP) and stimulate [NO](i). We conclude that both beta(1)- and beta(2)-ARs stimulate cAMP. beta(2)-ARs act via two signaling pathways to stimulate cAMP, one of which is mediated via G(i)-protein and PI-3K coupled to NO-cGMP signaling. Only beta(2)-ARs acting exclusively via NO signaling mediate ACh-induced activation of I(K,ATP). NO signaling also contributes to beta(2)-AR stimulation of I(Ca,L). The differential effects of beta(1)- and beta(2)-ARs can be explained by the coupling of these two beta-ARs to different effector signaling pathways.

Regulation of cardiac and smooth muscle Ca(2+) channels (Ca(V)1.2a,b) by protein kinases

High voltage-activated Ca(2+) channels of the Ca(V)1.2 class (L-type) are crucial for excitation-contraction coupling in both cardiac and smooth muscle. These channels are regulated by a variety of second messenger pathways that ultimately serve to modulate the level of contractile force in the tissue. The specific focus of this review is on the most recent advances in our understanding of how cardiac Ca(V)1.2a and smooth muscle Ca(V)1.2b channels are regulated by different kinases, including cGMP-dependent protein kinase, cAMP-dependent protein kinase, and protein kinase C. This review also discusses recent evidence regarding the regulation of these channels by protein tyrosine kinase, calmodulin-dependent kinase, purified G protein subunits, and identification of possible amino acid residues of the channel responsible for kinase regulation.

Effects of nitric oxide synthase inhibition on Basal function and the force-frequency relationship in the normal and failing human heart in vivo

BACKGROUND

Nitric oxide (NO) exerts autocrine/paracrine effects on cardiac function, including alterations of the inotropic state. In vitro studies suggest that NO modulates the myocardial force-frequency relationship. Basal left ventricular (LV) contractility is depressed and the force-frequency relationship is blunted in human heart failure, and it is speculated that an increase in NO production is involved.

METHODS AND RESULTS

We compared the effects of intracoronary NO synthase inhibition with N(G)-monomethyl-L-arginine (L-NMMA; 25 micromol/min) on basal LV function and the response to incremental atrial pacing in patients with dilated cardiomyopathy (n=11; mean age, 51 years) and in control subjects with atypical chest pain and normal cardiac function (n=7; mean age, 54 years). In controls, L-NMMA significantly reduced basal LV dP/dt(max) (from 1826 to 1578 mm Hg/s; P<0.002), but had no effect on heart rate, mean aortic pressure, or right atrial pressure. Pacing-induced increases in LV dP/dt(max) were unaltered by L-NMMA. In patients with dilated cardiomyopathy, L-NMMA had no effect on baseline LV dP/dt(max) (from 1313 to 1337 mm Hg/s; P=NS). The blunted pacing-induced rise in LV dP/dt(max) in these patients was unaltered by L-NMMA.

CONCLUSION

Endogenous NO has a small baseline positive inotropic effect in the normal human heart, which is lost in heart failure patients. NO does not significantly influence the force-frequency relationship in either the normal or failing human heart in vivo. Because this study was performed in patients with moderate heart failure, whether the findings apply to subjects with more severe heart failure requires further investigation.

Mechanisms of L-type Ca(2+) current downregulation in rat atrial myocytes during heart failure

Downregulation of the L-type Ca(2+) current (I(Ca)) is an important determinant of the electrical remodeling of diseased atria. Using a rat model of heart failure (HF) due to ischemic cardiopathy, we studied I(Ca) in isolated left atrial myocytes with the whole-cell patch-clamp technique and biochemical assays. I(Ca) density was markedly reduced (1.7+/-0.1 pA/pF) compared with sham-operated rats (S) (4.1+/-0.2 pA/pF), but its gating properties were unchanged. Calcium channel alpha(1C)-subunit quantities were not significantly different between S and HF. The beta-adrenergic agonist isoproterenol (1 micromol/L) had far greater stimulatory effects on I(Ca) in HF than in S (2.5- versus 1-fold), thereby suppressing the difference in current density. Dialyzing cells with 100 micromol/L cAMP or pretreating them with the phosphatase inhibitor okadaic acid also increased I(Ca) and suppressed the difference in density between S and HF. Intracellular cAMP content was reduced more in HF than in S. The phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine had a greater effect on I(Ca) in HF than in S (76.0+/-11.2% versus 15.8+/-21.2%), whereas the inhibitory effect of atrial natriuretic peptide on I(Ca) was more important in S than in HF (54.1+/-4.8% versus 24.3+/-8.8%). Cyclic GMP extruded from HF myocytes was enhanced compared with S (55.8+/-8.0 versus 6.2+/-4.0 pmol. mL(-1)). Thus, I(Ca) downregulation in atrial myocytes from rats with heart failure is caused by changes in basal cAMP-dependent regulation of the current and is associated with increased response to catecholamines.

Characterization of effects of endothelin-1 on the L-type Ca2+ current in human atrial myocytes

The effects of endothelin-1 (ET-1) on the L-type Ca2+ current (I(Ca)) were examined in whole cell patch-clamped human atrial myocytes. Depending on the initial current density, ET-1 (10 nM) increased the amplitude of I(Ca) by 99 +/- 7% or decreased it by 33 +/- 2%. The stimulatory effect predominated on current of low density (2.3 +/- 0.2 pA/pF), whereas I(Ca) of higher density (5.8 +/- 0.3 pA/pF) was inhibited by ET-1. After I(Ca) stimulation by 1 microM isoproterenol, ET-1 always inhibited the current by 32 +/- 7% (P < 0.05), an effect that was suppressed by pretreating myocytes with pertussis toxin. Atrial natriuretic peptide (ANP) inhibited I(Ca) (41 +/- 3%) by reducing intracellular cAMP concentration. In ANP-treated myocytes, the stimulatory effect of ET-1 on I(Ca) predominated (52 +/- 7%). The inhibitory effect of ET-1 on I(Ca) was blocked by the ET(A) antagonist BQ-123, whereas the stimulatory effect was suppressed by the ET(B) agonist BQ-788. We conclude that ET-1 has opposite effects on I(Ca) depending on the baseline amplitude of current, and both subtype ET receptors are implicated in the signal transduction pathways.

Effects of angiotensin-(1-7) on forearm circulation in normotensive subjects and patients with essential hypertension

Previous animal studies have shown that angiotensin (Ang)-(1-7) is a biologically active component of the renin-angiotensin system, acting as a vasoactive agent, and may play a role in the blood pressure regulation. There is little information, however, on the effect of Ang-(1-7) on human circulation or the mechanism of its action. To investigate the effect of Ang-(1-7) on forearm circulation and to determine whether this effect is altered in patients with essential hypertension, we measured change in forearm blood flow using venous occlusion plethysmography in response to intra-arterial infusion of Ang-(1-7) (10(-10), 10(-9), and 10(-8) mol/min; for 5 minutes) in normotensive control subjects (n=8) and patients with essential hypertension (n=8). Infusion of Ang-(1-7) significantly increased the forearm blood flow response in a dose-dependent manner in both normotensive control subjects (28.7+/-9.7%, at 10(-8) mol/min; P<0.05) and hypertensive patients (31.8+/-15.2%, at 10(-8) mol/min; P<0.05). The vasodilatory effect of Ang-(1-7) was similar in the two groups. Intra-arterial infusion of N(G)-monomethyl-L-arginine, a nitric oxide synthesis inhibitor, did not alter the forearm blood flow response to Ang-(1-7) in either group. These findings suggest that Ang-(1-7) causes vasodilation in forearm circulation of normotensive subjects and patients with essential hypertension through a pathway that is independent of nitric oxide synthesis.

Local response of L-type Ca(2+) current to nitric oxide in frog ventricular myocytes

1. The regulation of L-type Ca(2+) current (I(Ca)) by the two nitric oxide (NO) donors sodium nitroprusside (SNP, 1 microM to 1 mM) and (+/-)-S-nitroso-N-acetylpenicillamine (SNAP, 3 or 10 microM) was investigated in frog ventricular myocytes using double voltage clamp and double-barrelled microperfusion techniques. 2. SNP and SNAP depressed the isoprenaline (ISO, 10-100 nM)- or forskolin (FSK, 1 microM)-mediated stimulation of I(Ca) via cGMP activation of the cGMP-stimulated phosphodiesterase (PDE2). Complete inhibition of the ISO (100 nM) response was observed at 1 mM SNP. 3. When SNP was applied locally, i.e. to one-half of the cell, and ISO to the whole cell, the response of I(Ca) to ISO was strongly antagonized in the cell half exposed to SNP (up to 100 % inhibition at 1 mM SNP) but a relatively small depression was observed in the other half of the cell (only 20 % inhibition at 1 mM SNP). 4. The NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (carboxy-PTIO, 1 mM) reversed the local effect of SNAP (3 microM) on FSK-stimulated I(Ca) when applied to the same side as the NO donor, but had no effect when applied to the other side of the cell. 5. A local application of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA, 30 microM), a selective inhibitor of PDE2, fully reversed the local effect of SNP (100 microM) or SNAP (10 microM) on I(Ca) but had no effect on the distant response. 6. When EHNA was applied on the distant side, with SNP (1 mM) and ISO (100 nM) applied locally, the distant effect of SNP was fully reversed. 7. Our results demonstrate that in frog ventricular myocytes stimulation of guanylyl cyclase by NO leads to a strong local depletion of cAMP near the L-type Ca(2+) channels due to activation of PDE2, but only to a modest reduction of cAMP in the rest of the cell. This may be explained by the existence of a tight microdomain between L-type Ca(2+) channels and PDE2.

Cyclic GMP regulation of the L-type Ca(2+) channel current in human atrial myocytes

1. The regulation of the L-type Ca(2+) current (I(Ca)) by intracellular cGMP was investigated in human atrial myocytes using the whole-cell patch-clamp technique. 2. Intracellular application of 0.5 microM cGMP produced a strong stimulation of basal I(Ca) (+64 +/- 5 %, n = 60), whereas a 10-fold higher cGMP concentration induced a 2-fold smaller increase (+36 +/- 8 %, n = 35). 3. The biphasic response of I(Ca) to cGMP was not mimicked by the cGMP-dependent protein kinase (PKG) activator 8-bromoguanosine 3',5' cyclic monophosphate (8-bromo-cGMP, 0.5 or 5 microM), and was not affected by the PKG inhibitor KT 5823 (100 nM). 4. In contrast, cGMP stimulation of I(Ca) was abolished by intracellular perfusion with PKI (10 microM), a selective inhibitor of the cAMP-dependent protein kinase (PKA). 5. Selective inhibition of the cGMP-inhibited phosphodiesterase (PDE3) by extracellular cilostamide (100 nM) strongly enhanced basal I(Ca) in control conditions (+78 +/- 13 %, n = 7) but had only a marginal effect in the presence of intracellular cGMP (+22 +/- 7 % in addition to 0.5 microM cGMP, n = 11; +20 +/- 22 % in addition to 5 microM cGMP, n = 7). 6. Application of erythro-9-[2-hydroxy-3-nonyl]adenine (EHNA, 30 microM), a selective inhibitor of the cGMP-stimulated phosphodiesterase (PDE2), fully reversed the secondary inhibitory effect of 5 microM cGMP on I(Ca) (+99 +/- 16 % stimulation, n = 7). 7. Altogether, these data indicate that intracellular cGMP regulates basal I(Ca) in human atrial myocytes in a similar manner to NO donors. The effect of cGMP involves modulation of the cAMP level and PKA activity via opposite actions of the nucleotide on PDE2 and PDE3.

Nitric oxide augments voltage-activated calcium currents of crustacea (Idotea baltica) skeletal muscle

Invertebrate skeletal muscle contraction is regulated by calcium influx through voltage-dependent calcium channels in the sarcolemmal membrane. In present study we investigated the effects of nitric oxide (NO) donors on calcium currents of single skeletal muscle fibres from the marine isopod, Idotea baltica, using two-electrode voltage clamp recording techniques. The NO donors, S-nitrosocysteine, S-nitroso-N-acetyl-penicillamine or hydroxylamine reversibly increased calcium inward currents in a time dependent manner. The increase of the current was prevented by methylene blue. Our experiments suggest that NO increases calcium inward currents. NO, by acting on calcium ion channels in the sarcolemmal membrane, therefore, may directly be involved in the modulation of muscle contraction.

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

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

gamma-tocopherol partially protects insulin-secreting cells against functional inhibition by nitric oxide

Preceding the onset of type 1 diabetes mellitus, pancreatic islets are infiltrated by macrophages secreting interleukin-1beta (IL-1beta) which induces beta-cell apoptosis and exerts inhibitory actions on islet beta-cell insulin secretion. IL-1beta seems to act chiefly through induction of nitric oxide (NO) synthesis. Hence, IL-1beta and NO have been implicated as key effector molecules in type 1 diabetes mellitus. In this paper, the influence of endogenously produced and exogenously delivered NO on the regulation of cell proliferation, cell viability and discrete parts of the stimulus-secretion coupling in insulin-secreting RINm5F cells was investigated. Because vitamin E may delay diabetes onset in animal models, we also investigated whether tocopherols may protect beta-cells from the suppressive actions of IL-1 and NO in vitro. To this end, the impact of NO on insulin secretory responses to activation of phospholipase C (by carbamylcholine), protein kinase C (by phorbol ester), adenylyl cyclase (by forskolin), and Ca(2+) influx through voltage-activated Ca(2+) channels (by K(+)-induced depolarization) was monitored in culture after treatment with IL-1beta or by co-incubation with the NO donor spermine-NONOate. It was found that cell proliferation, viability, insulin production and the stimulation of insulin release evoked by carbamylcholine and phorbol ester were impeded by IL-1beta or spermine-NONOate, whereas the hormone output by the other secretagogues was not altered by NO. Pretreatment with gamma-tocopherol (but not alpha-tocopherol) afforded a partial protection against the inhibitory effects of NO, whereas specifically inhibiting inducible NO synthase with N-nitro-L-arginine completely reversed the IL-1beta effects. In contrast, inhibiting guanylyl cyclase with ODQ (1H-[1,2, 4]oxadiazolo[4,3-alpha]-quinoxaline-1-one) or blocking low voltage-activated Ca(2+) channels with NiCl(2) failed to influence the actions of NO. In conclusion, our data show that NO inhibits growth and insulin secretion in RINm5F cells, and that gamma-tocopherol may partially prevent this. The results suggest that phospholipase C or protein kinase C may be targeted by NO. In contrast, cGMP or low voltage-activated Ca(2+) channels appear not to mediate the toxicity of NO in these cells. These adverse effects of NO on the beta-cell, and the protection by gamma-tocopherol, may be of importance for the development of the impaired insulin secretion characterizing type 1 diabetes mellitus, and offer possibilities for intervention in this process.

New arylpiperazine derivatives as antagonists of the human cloned 5-HT(4) receptor isoforms

New derivatives of arylpiperazine 9 were designed from ML 10302, a potent 5-HT(4) receptor agonist in the gastrointestinal system. Compounds were synthesized by condensation of a number of available arylpiperazines or heteroarylpiperazines with 2-bromoethyl 4-amino-5-chloro-2-methoxybenzoate. They were evaluated in binding assays on the recently cloned human 5-HT(4(e)) isoform stably expressed in C6 glial cells with [(3)H]GR 113808 as the radioligand. The affinity values (K(i)) depended upon the substituent on the aromatic ring. A chlorine atom produced a marked drop in activity (K(i) > 100 nM), while a m-methoxy group gave a compound with nanomolar affinity (K(i) = 3 nM). The most potent compounds were the heterocyclic derivatives with pyrimidine, pyrazine, pyridazine, or pyridine moieties (compounds 9r, 9t, 9u, 9x, respectively). K(i) values for 9a and 9r were determined for the 5-HT(4(a)), 5-HT(4(b)), 5-HT(4(c)), and 5-HT(4(d)) receptor isoforms transiently expressed in COS cells. The results indicated that the compounds were not selective. They produced an inhibition of the 5-HT-stimulated cyclic AMP synthesis in the C6 glial cells stably expressing the 5-HT(4(e)) receptor and shifted the 5-HT concentration-effect curve on adenylyl cyclase activity with pK(D) values of 7.44 and 8.47, respectively. In isolated human atrial myocytes, 9r antagonized the stimulatory effect of 5-HT on the L-type calcium current (I(Ca)) with a K(D) value of 0.7 nM.

Laminin acts via beta 1 integrin signalling to alter cholinergic regulation of L-type Ca(2+) current in cat atrial myocytes

A perforated patch recording method was used to determine how plating cells on laminin (20 microg ml(-1); >2 h) alters cholinergic regulation of L-type Ca(2+) current (I(Ca,L)) in atrial myocytes. Acetylcholine (ACh; 1 microm)-induced inhibition of basal I(Ca,L) was not different between cells on glass and laminin. However, stimulation of I(Ca,L) elicited by ACh withdrawal was significantly smaller in cells on laminin (10 +/- 2 %) than on glass (48 +/- 5 %) (P < 0.001). Stimulation of I(Ca,L) induced by either spermine-NO (200 microm), milrinone (10 microm), IBMX (100 microm) or forskolin (1 microm) was significantly smaller in cells plated on laminin than on glass. However, stimulation of I(Ca,L) by 100 microm 8-CPT-cAMP or intracellular dialysis with 50 microM cAMP was not different between cells plated on laminin or glass. Basal, forskolin- and IBMX-stimulated cAMP content was significantly smaller in cells plated on laminin than on glass. Stimulation of I(Ca,L) by ACh withdrawal was significantly smaller in cells plated on an alpha beta 1-integrin antibody (10 +/- 4 %) than on glass (3 +/- 6 %; P < 0.001). In cells on laminin, prior exposure to 100 microg ml-1 YIGSR, a laminin receptor-binding peptide, restored ACh-induced stimulation of I(Ca,L) (58 +/- 14 %)laminin alone (7 +/- 2 %; P < 0. 05). Addition of 20 microm cytochalasin D or 1 microM latrunculin A, agents that prevent actin polymerization, to cells on laminin restored ACh-induced stimulation of I(Ca,L). We conclude that laminin binding to beta 1 integrins acts in association with the actin-based cytoskeleton to attenuate adenylate cyclase activity. As a result, laminin inhibits NO-mediated stimulation of I(Ca,L) elicited by ACh withdrawal. Laminin-integrin signalling may be relevant to changes in autonomic regulation that occur during cardiac development and/or disease.

Shortening of cardiac action potentials in endotoxic shock in guinea pigs is caused by an increase in nitric oxide activity and activation of the adenosine triphosphate-sensitive potassium channel

OBJECTIVE

To investigate the roles of nitric oxide and adenosine triphosphate (ATP)-sensitive potassium channels (KATP) in the shortening of cardiac action potential in endotoxic shock.

DESIGN

Prospective animal study with concurrent controls.

SETTING

University animal research laboratory.

SUBJECTS

Adult Hartley guinea pigs, weighing 300-400 g.

INTERVENTIONS

Guinea pigs were anesthetized and mechanically ventilated for 6 hrs. Lipopolysaccharide (LPS) or saline (sham group) were given intravenously. Drug effects were examined at the end of 6 hrs.

MEASUREMENTS AND MAIN RESULTS

Plasma nitrate concentration was measured hourly, while guanosine 3',5'-cyclic monophosphate (cGMP) content and action potential duration at 90% of repolarization (APD90) of papillary muscle were examined every 2 hrs in the 6-hr endotoxemia in both the sham and the LPS-treated groups. The basal levels of these three variables showed no difference in the two groups. In the sham group, these variables did not change significantly (n = 14 for plasma nitrate determination; n = 5 for cGMP content measurement; n = 5-14 for APD90 measurement; all p > .05). But in the LPS-treated group, both plasma nitrate concentration and cGMP content of papillary muscle showed time-dependent increases and they were significantly higher than those in the sham group (at the 6th hr, plasma nitrate: 42.6 +/- 7.7 vs. 21.8 +/- 3.1 micromol/L, both n = 14, p < .01; cGMP: 1.52 +/- 0.15 vs. 0.73 +/- 0.08 pmol/mg protein, both n = 5, p < .01). In contrast, APD90 revealed a time-dependent decrease compared with that in the sham group (at the 6th hr, 137.1 +/- 52 vs. 188.2 +/- 4.8 msecs, both n = 14, p < .001). In the following 60-min in vitro recording of action potentials after the end of 6-hr endotoxemia, the shortened APD90 in the LPS-treated group did not recover and remained shorter compared with that in the sham group, in which the APD90 showed no significant changes (at the 60th min, 165.1 +/- 5.7 vs. 200.2 +/- 3.8 msecs, each n = 14, p < .01). However, in the presence of glibenclamide, a specific KATP blocker (100 micromol/L; n = 10), the APD90 could be reversed almost completely to the same value as that in the sham group (n = 14) (196.6 +/- 3.5 vs. 200.2 +/- 3.8 msecs; p > .05), despite glibenclamide having no effect on the APD90 in the sham group. In the LPS-treated group, NG-nitro-L-arginine methyl ester (1 mmol/L; n = 4), methylene blue (10 micromol/L; n = 5), and aminoguanidine (100 micromol/L; n = 4) significantly prolonged the shortened APD90 (192.5 +/- 3.1, 195.0 +/- 3.3, and 176.5 +/- 3.3 msecs, respectively; p < .01, p < .01, and p < .05, respectively, compared with that without these agents, 165.1 +/- 5.7 msecs, n = 14). These agents had negligible effects on the APD90 in the sham group (all p > .05). Furthermore, 8-bromoguanosine-3',5'-cyclic monophosphate (500 micromol/L; n = 5) decreased APD in intact papillary muscle (mean reduction of APD90, 13.5 +/- 3.5%, n = 5; p < .05), an effect abolished by pretreatment with glibenclamide (100 micromol/L; n = 5) that did not have an effect by itself.

CONCLUSIONS

In this experimental model, we provide reasonably convincing evidence to suggest that in endotoxic shock, an increase in nitric oxide activity may activate KATP, which plays a major role in the shortening of APD, presumably through a cGMP-dependent pathway.

Tyrosine kinase and protein kinase C regulate L-type Ca(2+) current cooperatively in human atrial myocytes

The effects of tyrosine protein kinases (TK) on the L-type Ca(2+) current (I(Ca)) were examined in whole cell patch-clamped human atrial myocytes. The TK inhibitors genistein (50 microM), lavendustin A (50 microM), and tyrphostin 23 (50 microM) stimulated I(Ca) by 132 +/- 18% (P < 0.001), 116 +/- 18% (P < 0.05), and 60 +/- 6% (P < 0.001), respectively. After I(Ca) stimulation by genistein, external application of isoproterenol (1 microM) caused an additional increase in I(Ca). Dialyzing the cells with a protein kinase A inhibitor suppressed the effect of isoproterenol on I(Ca) but not that of genistein. Inhibition of protein kinase C (PKC) by pretreatment of cells with 100 nM staurosporine or 100 nM calphostin C prevented the effects of genistein on I(Ca). The PKC activator phorbol 12-myristate 13-acetate (PMA), after an initial stimulation (75 +/- 17%, P < 0.05), decreased I(Ca) (-36 +/- 5%, P < 0.001). Once the inhibitory effect of PMA on I(Ca) had stabilized, genistein strongly stimulated the current (323 +/- 25%, P < 0.05). Pretreating myocytes with genistein reduced the inhibitory effect of PMA on I(Ca). We conclude that, in human atrial myocytes, TK inhibit I(Ca) via a mechanism that involves PKC.

C-type natriuretic peptide inhibits ANP secretion and atrial dynamics in perfused atria: NPR-B-cGMP signaling

The purpose of the present experiments was to define the role of C-type natriuretic peptide (CNP) in the regulation of atrial secretion of atrial natriuretic peptide (ANP) and atrial stroke volume. Experiments were performed in perfused beating and nonbeating quiescent atria, single atrial myocytes, and atrial membranes. CNP suppressed in a dose-related fashion the increase in atrial stroke volume and ANP secretion induced by atrial pacing. CNP caused a right shift in the positive relationships between changes in the secretion of ANP and atrial stroke volume or translocation of the extracellular fluid (ECF), which indicates the suppression of atrial myocytic release of ANP into the paracellular space. The effects of CNP on the secretion and contraction were mimicked by 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP). CNP increased cGMP production in the perfused atria, and the effects of CNP on the secretion of ANP and atrial dynamics were accentuated by pretreatment with an inhibitor of cGMP phosphodiesterase, zaprinast. An inhibitor of the biological natriuretic peptide receptor (NPR), HS-142-1, attenuated the effects of CNP. The suppression of ANP secretion by CNP and 8-BrcGMP was abolished by a depletion of extracellular Ca(2+) in nonbeating atria. Natriuretic peptides increased cGMP production in atrial membranes with a rank order of potency of CNP > BNP > ANP, and the effect was inhibited by HS-142-1. CNP and 8-BrcGMP increased intracellular Ca(2+) concentration transients in single atrial myocytes, and mRNAs for CNP and NPR-B were expressed in the rabbit atrium. From these results we conclude that atrial ANP release and stroke volume are controlled by CNP via NPR-B-cGMP mediated signaling, which may in turn act via regulation of intracellular Ca(2+).

Electropharmacological effects of UK-1745, a novel cardiotonic drug, in guinea-pig ventricular myocytes

Effects of (2RS, 3SR)-2-aminomethyl-2,3,7,8-tetrahydro-2,3,5,8, 8-pentamethyl-6H-furo-[2,3-e] indol-7-one hydrochloride (UK-1745), a novel cardiotonic drug with beta-adrenoceptor blocking property and antiarrhythmic activity, on the action potentials of isolated papillary muscles and the membrane currents of single ventricular myocytes of guinea pigs were examined and compared with those of milrinone using conventional microelectrode and patch-clamp techniques. In papillary muscles, UK-1745 (3-100 microM) produced a mild positive inotropic response and depressed the maximum upstroke velocity of the action potential (V(max)) at 100 microM. Milrinone, a phosphodiesterase III inhibitor, markedly shortened the action potential duration with a significant increase in developed tension. In enzymatically-dissociated ventricular myocytes, UK-1745 failed to increase the L-type Ca(2+) current (I(Ca)) at 10 and 30 microM and rather decreased I(Ca) at 100 microM. The high concentration of UK-1745 slightly inhibited the delayed rectifier K(+) current (I(K)). Although UK-1745 antagonized the isoproterenol-induced increase in I(Ca), it potentiated the histamine-induced increase in I(Ca). On the other hand, milrinone per se significantly increased I(Ca) and markedly enhanced the isoproterenol-induced increase in I(Ca). It can be concluded that UK-1745 is a unique cardiotonic drug possessing beta-adrenoceptor blocking and weak phosphodiesterase-inhibitory actions in addition to direct inhibitory actions on the Na(+), Ca(2+) and K(+) channels with its high concentrations.

Positive and negative inotropic effects of NO donors in atrial and ventricular fibres of the frog heart

1. The cardiac effects of the NO donors sodium nitroprusside (SNP), S-nitroso-N-acetyl-penicillamine (SNAP) and 3-morpholino-sydnonimine (SIN-1) were studied in frog fibres to evaluate the contribution of cyclic GMP-dependent mechanisms. 2. SNP and SNAP (0.1-100 microM) reduced the force of contraction in a concentration-dependent manner in atrial and ventricular fibres. This effect was associated with a reduction in the time to peak (TTP) and the time for half-relaxation of contraction (T). 3. SIN-1 (100 microM) also reduced the force of contraction in two-thirds of the atrial fibres. However, it exerted a positive inotropic effect in the remaining atrial fibres, as well as in most ventricular fibres. 4. The guanylyl cyclase inhibitor 1H-[1,2,4]oxidiazolo[4,3-a]quinoxaline-1-one (ODQ, 10 microM) antagonized the negative inotropic effects of SIN-1 (50 microM) and SNAP (25 microM) but had no effect on the positive inotropic response to SIN-1 (100 microM). 5. In the presence of SIN-1, superoxide dismutase (SOD, 50-200 U ml-1) either potentiated the negative inotropic effect or turned the positive inotropic effect of the drug into a negative effect. SOD had no effects when applied alone or in the presence of SNAP. 6. 6-Anilino-5,8-quinolinedione (LY 83583, 3-30 microM), a superoxide anion generator also known as a cyclic GMP-lowering agent, exerted a positive inotropic effect, which was antagonized by SOD (200-370 U ml-1) but not by ODQ (10 microM). 7. We conclude that SNP, SNAP and SIN-1 exert cyclic GMP-dependent negative inotropic effects, which are attributed to the generation of NO. In addition, SIN-1 and LY 83583 exert cyclic GMP-independent positive inotropic effects, which require the generation of superoxide anion.

Prolonged sinus node recovery time in humans after the intracoronary administration of a nitric oxide synthase inhibitor

In vitro studies indicate that nitric oxide synthase (NOS) inhibitors alter sinus node automaticity. Moreover, whereas the systemic delivery of N(G)-monomethyl-L-arginine (L-NMMA), a NOS inhibitor, results in sinus bradycardia and arterial hypertension, its intracoronary administration has little effect on sinus heart rate. Therefore whether L-NMMA directly alters sinus node function in humans is not known. By using a crossover design, we evaluated the effect of intracoronary L-NMMA (20 micromol/min x 10 min) on corrected sinus node recovery time (CSNRT), heart rate, mean arterial blood pressure, electrocardiographic intervals, and coronary artery blood flow in nine men and 13 women aged 48+/-12 years. All were in sinus rhythm and had normal baseline CSNRTs. Baseline measurements were made during a dextrose infusion, and then L-NMMA was administered, and these parameters remeasured. In 11 patients, the infusions were near the origin of the sinus node artery (Concordant), whereas in the remaining 11, they were into the opposite coronary circulation (Discordant). After L-NMMA, significant prolongations in CSNRT were seen in Concordant (p < 0.001) and Discordant patients (p < 0.05), but were most pronounced in the Concordant group (p < 0.05). Although a significant reduction in coronary artery blood flow and nonsignificant changes in blood pressure and heart rate were observed after L-NMMA, these changes were not related to changes in CSNRT (r2 < or = 0.2; p > or = 0.2). These data support the notion that NO is a modifier of human sinus node automaticity.

Muscarinic regulation of Ca2+ signaling in mammalian atrial and ventricular myocardium

The differential regulation of the contractility of mammalian atrial and ventricular myocardium upon activation of muscarinic receptors can be ascribed, for the most part, to alterations in intracellular Ca2+ transients. However, alterations in myofibrillar sensitivity to Ca2+ ions also contribute to such regulation. In atrial muscle, the following actions are all associated with the corresponding alterations in the amplitude of Ca2+ transients in the same direction as those in the strength of the contractile force: (1) the direct inhibitory action on the basal force of contraction; (2) the increase (recovery) in force that is induced during the prolonged stimulation of muscarinic receptors; and (3) the rebound increase in force induced by washout of muscarinic receptor agonists. In addition, for a given decrease in force induced by muscarinic receptor stimulation in atrial muscle, the amplitude of Ca2+ transients is decreased to a smaller extent than the decrease in amplitude induced by reduction of extracellular Ca2+ concentration ([Ca2+]o), an indication that muscarinic receptor stimulation might increase myofibrillar sensitivity to Ca2+ ions simultaneously with the reduction in the amplitude of Ca2+ transients during induction of the direct inhibitory action. In mammalian ventricular myocardium, the direct inhibitory action of muscarinic receptor stimulation exhibits a wide range of species-dependent variation. A pronounced direct inhibitory action is induced in ferret papillary muscle, which is also associated with a definite increase in myofibrillar sensitivity to Ca2+ ions. By contrast, in the ventricular myocardium of other species including the rabbit and the dog, muscarinic receptor stimulation scarcely affects the baseline Ca2+ transients and the force, but it results in a pronounced decrease in Ca2+ transients and force when applied in the presence of beta-adrenoceptor stimulation, a phenomenon known as 'accentuated antagonism' or the 'indirect inhibitory action' of muscarinic receptor stimulation in mammalian ventricular myocardium. During induction of the indirect inhibitory action in mammalian ventricular myocardium, muscarinic receptor stimulation reverses all the effects induced by beta-adrenoceptor stimulation, including the increase in Ca2+ transients, the positive inotropic and lusitropic effects, and the decrease in myofibrillar sensitivity to Ca2+ ions. The relationship between the amplitude of Ca2+ transients and force is unaffected during induction of the indirect inhibitory action in rabbit and dog ventricular myocardium. The direct and indirect inhibitory actions of muscarinic receptor stimulation on Ca2+ transients have clearly different dependences on frequency: the former is more pronounced at a higher rate of stimulation, while the latter is more pronounced at a lower rate. The more complex interaction of muscarinic receptor and beta-adrenoceptor stimulation in mammalian atrial muscle and ferret ventricular muscle might be explained by the contribution of both the direct and the indirect regulatory mechanisms to the interaction.

Activation of distinct cAMP-dependent and cGMP-dependent pathways by nitric oxide in cardiac myocytes

Nitric oxide (NO) donors were recently shown to produce biphasic contractile effects in cardiac tissue, with augmentation at low NO levels and depression at high NO levels. We examined the subcellular mechanisms involved in the opposing effects of NO on cardiac contraction and investigated whether NO modulates contraction exclusively via guanylyl cyclase (GC) activation or whether some contribution occurs via cGMP/PKG-independent mechanisms, in indo 1-loaded adult cardiac myocytes. Whereas a high concentration of the NO donor S-nitroso-N-acetylpenicillamine (SNAP, 100 micromol/L) significantly attenuated contraction amplitude by 24.4+/-4.5% (without changing the Ca2+ transient or total cAMP), a low concentration of SNAP (1 micromol/L) significantly increased contraction amplitude (38+/-10%), Ca2+ transient (26+/-10%), and cAMP levels (from 6.2 to 8.5 pmol/mg of protein). The negative contractile response of 100 micromol/L SNAP was completely abolished in the presence of the specific blocker of PKG KT 5823 (1 micromol/L); the positive contractile response of 1 micromol/L SNAP persisted, despite the presence of the selective inhibitor of GC 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 micromol/L) alone, but was completely abolished in the presence of ODQ plus the specific inhibitory cAMP analog Rp-8-CPT-cAMPS (100 micromol/L), as well as by the NO scavenger oxyhemoglobin. Parallel experiments in cell suspensions showed significant increases in adenylyl cyclase (AC) activity at low concentrations (0.1 to 1 micromol/L) of SNAP (AC, 18% to 20% above basal activity). We conclude that NO can regulate both AC and GC in cardiac myocytes. High levels of NO induce large increases in cGMP and a negative inotropic effect mediated by a PKG-dependent reduction in myofilament responsiveness to Ca2+. Low levels of NO increase cAMP, at least in part, by a novel cGMP-independent activation of AC and induce a positive contractile response.

Characterization of the cyclic nucleotide phosphodiesterase subtypes involved in the regulation of the L-type Ca2+ current in rat ventricular myocytes

The effects of several phosphodiesterase (PDE) inhibitors on the L-type Ca current (I(Ca)) and intracellular cyclic AMP concentration ([cAMP]i) were examined in isolated rat ventricular myocytes. The presence of mRNA transcripts encoding for the different cardiac PDE subtypes was confirmed by RT-PCR. IBMX (100 microM), a broad-spectrum PDE inhibitor, increased basal I(Ca) by 120% and [cAMP]i by 70%, similarly to a saturating concentration of the beta-adrenoceptor agonist isoprenaline (1 microM). However, MIMX (1 microM), a PDE1 inhibitor, EHNA (10 microM), a PDE2 inhibitor, cilostamide (0.1 microM), a PDE3 inhibitor, or Ro20-1724 (0.1 microM), a PDE4 inhibitor, had no effect on basal I(Ca) and little stimulatory effects on [cAMP]i (20-30%). Each selective PDE inhibitor was then tested in the presence of another inhibitor to examine whether a concomitant inhibition of two PDE subtypes had any effect on I(Ca) or [cAMP]i. While all combinations tested significantly increased [cAMP]i (40-50%), only cilostamide (0.1 microM)+ Ro20-1724 (0.1 microM) produced a significant stimulation of I(Ca) (50%). Addition of EHNA (10 microM) to this mix increased I(Ca) to 110% and [cAMP]i to 70% above basal, i.e. to similar levels as obtained with IBMX (100 microM) or isoprenaline (1 microM). When tested on top of a sub-maximal concentration of isoprenaline (1 nM), which increased I(Ca) by (approximately 40% and had negligible effect on [cAMP]i, each selective PDE inhibitor induced a clear stimulation of [cAMP]i and an additional increase in I(Ca). Maximal effects on I(Ca) were approximately 8% for MIMX (3 microM), approximately 20% for EHNA (1-3 microM), approximately 30% for cilostamide (0.3-1 microM) and approximately 50% for Ro20-1724 (0.1 microM). Our results demonstrate that PDE1-4 subtypes regulate I(Ca) in rat ventricular myocytes. While PDE3 and PDE4 are the dominant PDE subtypes involved in the regulation of basal I(Ca), all four PDE subtypes determine the response of I(Ca) to a stimulus activating cyclic AMP production, with the rank order of potency PDE4>PDE3>PDE2>PDE1.

Enhancement of neuronal survival by 6R-tetrahydrobiopterin

Recently, we have reported that 6R-tetrahydrobiopterin activates Ca2+ channels in neuronal cells independently of its cofactor activities. Several reports indicate that depolarization-induced activation of Ca2+ channels enhances neuronal survival. Here, we investigated the effects of 6R-tetrahydrobiopterin on survival of differentiated PC12 cells. Depletion of serum and nerve growth factor caused cell death, which was prevented by high potassium. 6R-Tetrahydrobiopterin also prevented death of PC12 cells cultured without serum and nerve growth factor in a dose-related manner at physiological concentrations (1-100 microM). However, surviving cells cultured with 6R-tetrahydrobiopterin showed undifferentiated form. 6S-Tetrahydrobiopterin, a diastereoisomer of 6R-tetrahydrobiopterin, also had a cell-surviving effect, but it was less potent as compared with that of 6R-tetrahydrobiopterin. The cell-surviving effect of 6R-tetrahydrobiopterin was eliminated by a Ca2+ channel blocker, but persisted in the presence of an inhibitor for tyrosine hydroxylase, dopamine, L-DOPA, an inhibitor for nitric oxide synthase and a nitric oxide generator. The effect of 6R-tetrahydrobiopterin was mimicked by a cyclic-AMP analogue and inhibited by an inhibitor for protein kinase A. Ca2+ channel activity was preserved but dopamine-releasing activity was disturbed in surviving cells cultured with 6R-tetrahydrobiopterin. 6R-Tetrahydrobiopterin had no effect on mitogen-activated protein kinase. These findings suggest that, independently of its cofactor activities and mitogen-activated protein kinase cascade, 6R-tetrahydrobiopterin enhances survival of PC12 cells by activating Ca2+ channels via the cyclic-AMP-protein kinase A pathway, but that 6R-tetrahydrobiopterin does not preserve neuronal character induced by nerve growth factor.

Acute effect of tumor necrosis factor-alpha is minimal on mechanics but significant on energetics in blood-perfused canine left ventricles

OBJECTIVES

We hypothesized that tumor necrosis factor-alpha (TNF-alpha) acutely alters left ventricular mechanoenergetics in blood-perfused hearts. To test this hypothesis, we examined the relation between left ventricular mechanics and energetics, both before and after infusion of TNF-alpha.

DESIGN

Prospective, experimental study.

SETTING

Research laboratory.

SUBJECTS

Nine isolated, blood-perfused canine hearts.

INTERVENTIONS

Recombinant human TNF-alpha (90 microg/min) was infused into the coronary circulation of the isolated hearts for 20 mins.

MEASUREMENTS AND MAIN RESULTS

In the isolated, cross-circulated, blood-perfused canine left ventricles, left ventricular contractility was assessed through measurement of end-systolic elastance (Ees). Energetics were examined in terms of the end-systolic pressure-volume area-myocardial oxygen consumption (MVo2) relation. TNF-alpha concentration in coronary venous blood was >1000 ng/mL throughout the experiments. Nevertheless, infusion of TNF-alpha barely affected contractility acutely, i.e., there was a minimal decrease during the infusion (8.1+/-2.8% at 10 mins, p < .01) and a minimal increase after the infusion (11.2+/-2.5% at 10 mins, p< .01). Neither did the TNF-alpha infusion affect the slope of the end-systolic pressure-volume area-MVo2 relation. This finding indicated that the chemomechanical conversion efficiency remained unchanged. However, TNF-alpha infusion significantly increased the oxygen cost of contractility by 40% (1.25+/-0.13 vs. 1.75+/-0.24 mL oxygen.mL/mm Hg/beat, p< .05), indicating that MVo2 for the excitation-contraction coupling increased.

CONCLUSIONS

TNF-alpha minimally alters left ventricular mechanics, but significantly changes energetics. The latter effect may result from changes in intracellular calcium handling.

Coordinated role of vasoactive intestinal peptide and nitric oxide in cardioprotection

The present study sought to examine the interrelationship between nitric oxide (NO) and vasoactive intestinal peptide (VIP) in myocardial protection. Isolated rat hearts were perfused for 15 min with buffer only (Group I); 0.3 mM VIP (Group II); 3 mM L-arginine (a precursor of NO) (Group III); VIP and aminoguanidine (iNOS blocker) (Group IV); or L-arginine plus VIP 10-28 (VIP inhibitor) (Group V). Each heart was then made globally ischemic for 30 min followed by 2 h reperfusion. Both VIP and NO were found to provide cardioprotection during ischemia and reperfusion. However, the beneficial effects of VIP and NO were reduced by inhibition of NO and VIP, respectively, suggesting that cardioprotection by VIP is modulated by NO and vice versa. The results of this study suggested a coordinated regulation by cardioprotection by NO and VIP.

Increased expression of constitutive nitric oxide synthase III, but not inducible nitric oxide synthase II, in human heart failure

OBJECTIVES

The purpose of the present study was to examine the expression of the endothelial-type nitric oxide synthase (NOS III) and the inducible-type NOS (NOS II) in human myocardium and their regulation in heart failure from patients with different etiologies.

BACKGROUND

In heart failure, plasma levels of nitrates were found to be elevated. However, data on myocardial NOS expression in heart failure are conflicting.

METHODS

Using RNase protection analysis and Western blotting, the expression of NOS III and NOS II was investigated in ventricular myocardium from nonfailing (NF) hearts (n=5) and from failing hearts of patients with idiopathic dilated cardiomyopathy (dCMP, n=14), ischemic cardiomyopathy (iCMP, n=9) or postmyocarditis cardiomyopathy (mCMP, n=7). Furthermore, immunohistochemical studies were performed to localize NOS III and NOS II within the ventricular myocardium.

RESULTS

In failing human hearts, NOS III mRNA levels were increased to 180% in dCMP, 200% in iCMP and to 210% in mCMP as compared to NF hearts. Similarly, in Western blots (using constitutively expressed beta-tubulin as a reference) NOS III protein expression was increased about twofold in failing compared to NF hearts. Immunohistochemical studies with a selective antibody to NOS III showed no obvious differences in the staining of the endothelium of cardiac blood vessels from NF and failing human hearts. However, NOS III-immunoreactivity in cardiomyocytes was significantly more intense in failing compared to NF hearts. Low expression of NOS II mRNA was detected in only 2 of 30 failing human hearts and was not found in NF hearts. Inducible-type NOS protein was undetectable in either group.

CONCLUSIONS

We conclude that the increased NOS III expression in the ventricular myocardium of failing human hearts may contribute to the contractile dysfunction observed in heart failure and/or may play a role in morphologic alterations such as hypertrophy and apoptosis of cardiomyocytes.