The negative inotropic action of catecholamines: role of beta3-adrenoceptors

There is now evidence for the involvement of four beta-adrenoceptor populations in the regulation of cardiac function by catecholamines. Beta1- and beta2-adrenoceptor stimulation classically produces an increase in contractility. A fourth beta-adrenoceptor, as yet uncloned and designated provisionally as a beta4-adrenoceptor, also mediates a positive inotropic effect. Beta3-adrenoceptors, which had been cloned at the end of the eighties, has been extensively studied as a potential target for antiobesity and antidiabetic drugs. Its characterization in the heart has opened new fields of investigations for the understanding of the cardiac adrenergic regulation. This review describes the cardiac electrical and mechanical effects induced by Beta3-adrenoceptor stimulation in different species (including human), as well as the signaling pathway. It also analyzes the role of these receptors in the abnormal responsiveness of catecholamines in heart failure.

Dynamin mediates caveolar sequestration of muscarinic cholinergic receptors and alteration in NO signaling

In cardiac myocytes, agonist binding to muscarinic acetylcholine receptors (mAchRs) leads to the targeting of stimulated receptors to plasmalemmal microdomains termed caveolae. Here, we examined whether this translocation leads to mAchR internalization and alteration in downstream NO signaling. Differential binding of membrane-permeant and -impermeant mAchR radioligands on caveolae-enriched membranes revealed that carbachol stimulation of cardiac myocytes induces sequestration of mAchRs through caveolae fission. GTP but not its non-hydrolyzable analog GTP gamma S drove the further detachment of caveolae from myocyte sarcolemma. Also, incubation of extracts of carbachol-stimulated myocytes with recombinant GTPase dynamin induced mAchR sequestration in budded caveolae, while dominant-negative K44A dynamin inhibited it. These data were confirmed by immunofluorescence microscopy on m2 mAchR-expressing COS cells. Finally, repeated carbachol stimulations of mAchRs co-expressed in COS cells with endothelial nitric oxide synthase (eNOS) and wild-type, but not mutant, dynamin led to a progressive increase in mAchR sequestration and a concurrent stabilization of the inhibitory eNOS-caveolin complex. These findings emphasize the role of caveolae in mAchR trafficking and NO signaling, and suggest that caveolae fission may contribute to G-protein-coupled receptor desensitization.

[Beta 3 adrenergic receptor: physiologic role and potential therapeutic applications]

The characterization in 1989 of the gene encoding the beta 3-adrenoceptor helped to interpret the results of pharmacological experiments on atypical effects of catecholamines distinct from the classical activation of beta 1 and beta 2 adrenoceptors. In rodents, the beta 3 adrenoceptor is abundantly expressed in white adipose tissue where energy is stored in the form of triglycerides and in brown adipose tissue that is specialized for thermogenesis. Treatment of rodents with beta 3 adrenoceptor agonists induces a weight loss related to the stimulation of lipolysis in the two types of tissues. These results led to propose the use of these agonists for the treatment of human obesity and NIDDM. However, the poor lipolytic effect of these agonists in human adipose tissue and the recent discovery of functional beta 3 adrenoceptors in the human heart raise new questions on the therapeutic use of beta 3-adrenoceptor agonists in man. In the human ventricle, these agonists induce a negative inotropic effect. In vessels, stimulation of beta 3-adrenoceptors produces a vasodilation. If these effects are conserved in the failing heart, they could shed a new light on the pathogenic role of the hyperadrenergism associated with cardiac failure, as well as on its treatment with beta-adrenoceptor blockers.

A specific method for measurement of nitric oxide synthase enzymatic activity in peritoneal biopsies

A specific method for measurement of nitric oxide synthase enzymatic activity in peritoneal biopsies.

BACKGROUND

Nitric oxide (NO) is synthesized by NO synthase (NOS) isoforms that are expressed in the peritoneum. Thus far, NOS activity in the peritoneum has been assessed by nonspecific methods. We describe the application of a specific method for determination of NOS activity in rat and human peritoneal biopsies.

METHODS

The L-citrulline assay is based on the stoechiometric production of NO and L-[3H]-citrulline from L-[3H]-arginine by NOS. The assay is technically difficult when applied on small samples with relatively low levels of NOS activity, which required specific procedures for extraction and samples processing. Reaction parameters ensuring assay linearity in the peritoneum were defined. Peritoneum lysates were also used for immunoblot analysis to identify the NOS isoforms involved.

RESULTS

A significant NOS activity is detected in the normal peritoneum because of both Ca2+-dependent and Ca2+-independent NOS. The specificity of NOS activity has been demonstrated by various controls, including the NOS inhibitor L-NMMA. Competition experiments with L-valine and amino acid analyses have reasonably excluded the interference of endogenous arginase and L-arginine, which both might underestimate NOS activity. The procedure is sensitive; it detects a high range of NOS activities as well as the appropriate NOS isoforms in various tissues and conditions, as shown by correlations with immunoblot studies.

CONCLUSIONS

We have adapted and characterized the L-citrulline assay to measure specific NOS activities within the peritoneum. The peritoneum lysate assayed for NOS activity can also be used for characterizing NOS isoform expression by immunoblot analysis.

Regulation of aquaporin-1 and nitric oxide synthase isoforms in a rat model of acute peritonitis

The loss of ultrafiltration (UF) that accompanies acute peritonitis is a common problem in peritoneal dialysis (PD). It has been suggested that changes in nitric oxide (NO)-mediated vascular tone and permeability might be involved in the loss of UF, whereas channel-mediated water permeability should not be affected. This study used a model of acute peritonitis in rats to characterize changes in PD parameters, in correlation with: (1) expression studies of water channel aquaporin-1 and NO synthase (NOS) isoforms and (2) enzymatic assays for NOS in the peritoneum. Compared with controls, rats with peritonitis had a higher removal of plasma urea, a faster glucose absorption, and a loss of UF. Additional changes, including high protein loss, elevated leukocyte counts in dialysate, positive bacterial cultures, edema, and mononuclear infiltrates, were similar to those observed in PD patients with acute peritonitis. Acute peritonitis in rats induced a major increase in total NOS activity, which was inversely correlated with free-water permeability. The increased NOS activity was mediated by both inducible (Ca2+-independent) and endothelial (Ca2+-dependent) NOS isoforms and was reflected by increased peritoneal staining for nitrotyrosine. In contrast, aquaporin-1 expression was unchanged in rats with peritonitis. These findings cast light on the pathophysiology of permeability changes and loss of UF that characterize acute peritonitis. In particular, these data suggest that a local production of NO, mediated by different NOS isoforms, might play a key role in these changes.

Hypercholesterolemia decreases nitric oxide production by promoting the interaction of caveolin and endothelial nitric oxide synthase

Hypercholesterolemia is a central pathogenic factor of endothelial dysfunction caused in part by an impairment of endothelial nitric oxide (NO) production through mechanisms that remain poorly characterized. The activity of the endothelial isoform of NO synthase (eNOS) was recently shown to be modulated by its reciprocal interactions with the stimulatory Ca2+-calmodulin complex and the inhibitory protein caveolin. We examined whether hypercholesterolemia may reduce NO production through alteration of this regulatory equilibrium. Bovine aortic endothelial cells were cultured in the presence of serum obtained from normocholesterolemic (NC) or hypercholesterolemic (HC) human volunteers. Exposure of endothelial cells to the HC serum upregulated caveolin abundance without any measurable effect on eNOS protein levels. This effect of HC serum was associated with an impairment of basal NO release paralleled by an increase in inhibitory caveolin-eNOS complex formation. Similar treatment with HC serum significantly attenuated the NO production stimulated by the calcium ionophore A23187. Accordingly, higher calmodulin levels were required to disrupt the enhanced caveolin-eNOS heterocomplex from HC serum-treated cells. Finally, cell exposure to the low-density lipoprotein (LDL) fraction alone dose-dependently reproduced the inhibition of basal and stimulated NO release, as well as the upregulation of caveolin expression and its heterocomplex formation with eNOS, which were unaffected by cotreatment with antioxidants. Together, our data establish a new mechanism for the cholesterol-induced impairment of NO production through the modulation of caveolin abundance in endothelial cells, a mechanism that may participate in the pathogenesis of endothelial dysfunction and the proatherogenic effects of hypercholesterolemia.

The negative inotropic effect of beta3-adrenoceptor stimulation is mediated by activation of a nitric oxide synthase pathway in human ventricle

Beta1- and beta2-adrenoceptors in heart muscle cells mediate the catecholamine-induced increase in the force and frequency of cardiac contraction. Recently, in addition, we demonstrated the functional expression of beta3-adrenoceptors in the human heart. Their stimulation, in marked contrast with that of beta1- and beta2-adrenoceptors, induces a decrease in contractility through presently unknown mechanisms. In the present study, we examined the role of a nitric oxide (NO) synthase pathway in mediating the beta3-adrenoceptor effect on the contractility of human endomyocardial biopsies. The negative inotropic effects of a beta3-adrenoceptor agonist, BRL 37344, and also of norepinephrine in the presence of alpha- and beta1-2-blockade were inhibited both by a nonspecific blocker of NO, methylene blue, and two NO synthase (NOS) inhibitors, L-N-monomethyl-arginine and L-nitroarginine-methyl ester. The effect of the NOS inhibitors was reversed by an excess of L-arginine, the natural substrate of NOS, but not by D-arginine. Moreover, the effects of the beta3-adrenoceptor agonist on contractility were associated with parallel increases in the production of NO and intracellular cGMP, which were also inhibited by NOS inhibitors. Immunohistochemical staining of human ventricular biopsies showed the expression of the endothelial constitutive (eNOS), but not the inducible (iNOS) isoform of NOS in both ventricular myocytes and endothelial cells. These results demonstrate that beta3-adrenoceptor stimulation decreases cardiac contractility through activation of an NOS pathway. Changes in the expression of this pathway may alter the balance between positive and negative inotropic effects of catecholamines on the heart potentially leading to myocardial dysfunction.

Reversible S-nitrosation of creatine kinase by nitric oxide in adult rat ventricular myocytes

We have previously demonstrated that the nitric oxide (NO) donor S-nitroso-N-acetylcysteine (SNAC) reversibly decreases the activity of creatine kinase (CK) in an isolated rat heart preparation, markedly suppressing myocardial contractile responsiveness to an inotropic challenge. We wished to further examine the role of exogenous and endogenous sources of NO species on S-nitrosation of CK and subsequent enzyme activity in adult rat ventricular myocytes (ARVM). Two S-nitrosothiol groups were formed in the CK dimer after nitrosation of rabbit skeletal muscle CK in solution. CK inactivation due to S-nitrosation was time- and concentration-dependent in solution and in ARVM lysate for both NO donors S-nitroso-N-acetylpenicillamine (SNAP) and SNAC, and was rapidly reversible with the sulfhydryl dithiothreitol (DTT). Similarly, SNAC or SNAP dose-dependently decreased CK activity in intact ARVM, which was further attenuated by increasing the metabolic activity of the cells with electrical pacing for 1 h. Co-cultures of ARVM with interleukin 1 beta (IL-1 beta)- and interferon gamma (IFN gamma)-pretreated cardiac microvascular endothelial cells (CMEC) caused no detectable decline in myocyte CK activity. Increasing GSH levels attenuated the decline in myocyte CK activity with SNAC, while decreases in myocyte GSH levels enhanced the inhibitory effect of SNAC on intact myocyte CK activity. These data indicate that the degree of inhibition of cardiac myocyte CK by NO is dependent on the extent of myocyte metabolic activity and the intracellular GSH content.

Hypoleptinemia in patients with anorexia nervosa: loss of circadian rhythm and unresponsiveness to short-term refeeding

Leptin is a protein encoded by the ob gene that is expressed in adipocytes and regulates eating behavior via neuroendocrine mechanisms. Plasma leptin levels have been shown to correlate with weight and body fat in normal, obese and anorexic subjects. In the last of these populations, the dynamic profile of plasma leptin levels during short-term refeeding has never been assessed. We thus investigated basal plasma leptin levels in 29 female patients with anorexia nervosa (AN) (age 21.9 +/- 1.4 years, body mass index (BMI) 15.2 +/- 0.3 kg/m2) and in 80 normal female controls (age 21.2 +/- 0.2 years, BMI 20.3 +/- 0.3 kg/m2, mean +/- S.E.M.). Basal plasma leptin levels in AN were decreased by 77% compared with controls (2.5 +/- 0.2 vs 11.1 +/- 0.7 ng/ml, P < 0.0001). In both AN subjects and controls, plasma leptin levels correlated significantly with BMI (r2 = 0.448, P < 0.0001 and r2 = 0.339, P < 0.0001 respectively). Five AN patients (four female, one male, age 22.0 +/- 4.7 years, BMI 14.2 +/- 0.4 kg/m2, body fat 4.3 +/- 0.9 kg or 11.0 +/- 1.9% of body weight, basal metabolic rate (BMR) 958 +/- 122 kcal/day) were studied during a 3-day refeeding period and compared with eight control subjects (two male, six female, age 25.7 +/- 1.2 years, BMI 21.3 +/- 0.8 kg/m2, body fat 15.1 +/- 0.9 kg or 24.6 +/- 1.7%, BMR 1455 +/- 78 kcal/day) submitted to 36-h fasting. The amount of calories administered was based on BMR + 20% (carbohydrate 60%, protein 17%, fat 23%). In contrast to the rise in leptin levels that occurred during refeeding after a prolonged fast period in normal subjects, plasma leptin levels remained low and unchanged throughout the 3 days of renutrition in AN patients. The circadian rhythm of leptin was also completely abolished. This contrasted with the preserved circadian variations of cortisol, whose mean levels were increased. In conclusion, we confirmed that plasma leptin levels are low in AN and correlate with body weight. We further demonstrated that plasma leptin levels do not respond to short-term refeeding in anorexic patients in whom circadian variations are not restored, which suggests that the acute regulation of leptin by positive changes in energy balance is not preserved under a critical threshold of body fat.

Regulation of cardiac myocyte contractile function by inducible nitric oxide synthase (iNOS): mechanisms of contractile depression by nitric oxide

Inflammatory cytokines have been implicated in the reversible depression of cardiac contractile function accompanying local or systemic immune stimulation. Incubation of cardiac myocytes with soluble components in the supernatant from cultured rat lung macrophages activated with endotoxin decreases their contractile response to beta-adrenergic stimulation through the induction of iNOS and the subsequent production of nitric oxide by these cells. In the present study, we characterize the mechanisms underlying NO's attenuation of adrenergic responsiveness in cardiac myocytes. iNOS was induced in cultured ventricular myocytes from adult rats by incubation for 20 h with conditioned medium from lipopolysaccharide (LPS)-activated macrophages. iNOS induction did not induce any alteration in beta-adrenergic receptor density or affinity, Galphai protein abundance, or adenylyl cyclase activity in cultured myocytes. Myocyte exposure to activated macrophage-conditioned medium markedly attenuated the elevation of cAMP in response to isoproterenol (Iso, 2 nM). Induction of iNOS with the macrophage-conditioned medium also potentiated the Iso-induced increase in myocyte cGMP. This cGMP increase was totally abolished by NOS inhibitors. NOS inhibition also returned the attenuated cAMP response to 2 nM Iso to levels observed in control cells. Pre-incubation of the cells in isobutylmethylxanthine (IBMX), a phosphodiesterase inhibitor, also partly reversed the attenuation of cAMP increase with 2 nM Iso in cells expressing iNOS. Brief (15 min) exposure of myocytes to the NO donor, S-nitrosoacetylcysteine (SNAC, 100 micro M) which produced a three-fold increase in intracellular cGMP, also decreased by half the contractile response of cardiac myocytes to Iso (2 nM). We conclude that NO endogenously produced by iNOS decreases the intracellular levels of cAMP in response to beta-adrenergic stimulation in isolated cardiac myocytes, in part through a cGMP-mediated mechanism. This effect may participate in the NO-dependent depression of cardiac function following cytokine exposure.

Nitric oxide synthases and cardiac muscle. Autocrine and paracrine influences

The different cell types comprising cardiac muscle express one or more of the three isoforms (neuronal NOS, or nNOS; inducible NOS, or iNOS; and endothelial NOS, or eNOS) of nitric oxide synthase (NOS). nNOS is expressed in orthosympathetic nerve terminals and regulates the release of catecholamines in the heart. eNOS constitutively expressed in endothelial cells inhibits contractile tone and the proliferation of underlying vascular smooth muscle cells, inhibits platelet aggregation and monocyte adhesion, promotes diastolic relaxation, and decreases O2 consumption in cardiac muscle through paracrinally produced NO. eNOS is also constitutively expressed in cardiac myocytes from rodent and human species, where it autocrinally opposes the inotropic action of catecholamines after muscarinic cholinergic and beta-adrenergic receptor stimulation. iNOS gene transcription and protein expression are induced in all cell types after exposure to a variety of inflammatory cytokines. Aside from participating in the immune defense against intracellular microorganisms and viruses, the large amounts of NO produced autocrinally or paracrinally mediate the vasoplegia and myocardial depression characteristic of systemic immune stimulation and promote cell death through apoptosis. In cardiac myocytes, NO may regulate L-type calcium current and contraction through activation of cGMP-dependent protein kinase and cGMP-modulated phosphodiesterases. Other mechanisms independent of cGMP elevations may operate through interaction of NO with heme proteins, non-heme iron, or free thiol residues on target signaling proteins, enzymes, or ion channels. Given the multiplicity of NOS isoforms expressed in cardiac muscle and of the potential molecular targets for the NO produced, tight molecular regulation of NOS expression and activity at the transcriptional and posttranscriptional level appear to be needed to coordinate the many roles of NO in heart function in health and disease.

Regulation of bFGF expression and ANG II secretion in cardiac myocytes and microvascular endothelial cells

Basic fibroblast growth factor (bFGF; fibroblast growth factor-2) and angiotensin II (ANG II), among other peptide signaling autacoids (cytokines), are known to regulate the phenotypic adaptation of cardiac muscle to physiological stress. The cell type(s) in cardiac muscle responsible for ANG II synthesis and secretion and the role of endogenous cytokines in the regulation of bFGF induction remain unclear. With the use of confluent, serum-starved, low-passage cultures of cardiac microvascular endothelial cells (CMEC), ANG II could be detected in cellular lysates and in medium conditioned by these cells with the use of high-performance liquid chromatography followed by radioimmunoassay. The secretion of angiotensins by individual CMEC could be detected with a cell-blot assay technique. ANG II secretion was decreased by brefeldin A, an agent that interrupts constitutive and regulated secretory pathways for peptide autacoid/ hormone synthesis, suggesting de novo synthesis, activation, and secretion of angiotensins by CMEC. In primary isolates of adult rat ventricular myocytes (ARVM) and CMEC, ANG II, acting at ANG II type 1 receptors in both cell types, was found to increase bFGF mRNA levels measured by ribonuclease protection assay. Endothelin-1 (ET-1), which is known to be synthesized by CMEC, and bFGF itself, which has been detected in both ARVM and CMEC, increased bFGF transcript levels in both cell types. Interleukin-1beta (IL-1beta), which like ANG II and ET-1 is known to activate mitogen-activated protein kinases in both ARVM and CMEC, increased bFGF mRNA levels only in cardiac myocytes. Thus cytokines such as ANG II, ET-1, bFGF, and IL-1beta locally generated by cellular constituents of cardiac muscle, including CMEC, regulate bFGF mRNA levels in a cell type-specific manner.

Cardiac endothelium and tissue growth

In recent years, a number of peptide and nonpeptide signalling autacoids have been implicated in the regulation of cardiac myocyte growth as well as vasculogenesis and angiogenesis, and in the function of cardiac muscle following development. In this review, we first examine the evidence for a role for specific cytokines during cardiac ontogeny, including fibroblast factors, TGFbeta, neuregulins, and gp/30-mediated signalling pathways. Evidence is also reviewed for a role for local, intracardiac generation of endothelins and angiotensins in the regulation of cardiac muscle function and adaptation to physiologic stress. The role of cardiac myocytes in regulating angiogenesis in the developed heart is then reviewed briefly. Finally, the effects of the endogenous generation of nitric oxides by the inflammatory cytokine-inducible isoform of nitric oxide synthase (iNOS or NOS2) within the heart are also reviewed.

Inhibition of nitric oxide synthase augments myocardial contractile responses to beta-adrenergic stimulation

Recent in vitro evidence suggests a role for nitric oxide (NO) in the modulation of myocardial contractility. The specific role of NO in the control of cardiac function in vivo, however, remains unclear. We investigated the effect of NO synthase (NOS) inhibition on myocardial contractility in response to beta-adrenergic stimulation in autonomically blocked dogs. Intracoronary infusions of dobutamine (1-50 micrograms/min) and isoproterenol (0.1 and 0.5 microgram/min) were performed before and after the intracoronary administration of the specific NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME). Intracoronary dobutamine resulted in a dose-dependent increase in peak first derivative of pressure (dP/dtmax) to a maximum of 195 +/- 10% (P < 0.001). After inhibition of NOS with intracoronary L-NAME at rates of 0.1 and 1 mg/min, the response to dobutamine was significantly enhanced with dP/dtmax, increasing 276 +/- 17 and 317 +/- 26%, respectively (P < 0.001). Intracoronary isoproterenol resulted in a maximum increase in dP/dtmax of 116 +/- 15% (P < 0.001) that further increased to 154 +/- 17 and 157 +/- 18% after NOS inhibition with 0.1 and 1 mg/min L-NAME, respectively (both P < 0.002). L-NAME had no effect on baseline dP/dtmax but did produce a reduction in myocardial guanosine 3',5'-cyclic monophosphate content. These results suggest a role for NO in the control of myocardial contractility in response to beta-adrenergic stimulation in vivo.

The role of the NO pathway in the control of cardiac function

Nitric oxide (NO) acts as an autocrine- and paracrine-acting signaling autacoid that, among other functions, has been shown to regulate cardiac contractile responsiveness to beta-adrenergic and muscarinic cholinergic agonists. Nitric oxide (NO) is formed by the oxidation of one of two equivalent guanidino nitrogens in L-arginine by O2 to form NO and L-citrulline. This reaction is catalyzed by a family of enzymes termed NO synthases. Three distinct isoforms of NOS have been identified, each the product of a separate gene. Cellular constituents of cardiac muscle, including ventricular myocytes as well as microvascular endothelial cells, have been shown to express the "endothelial constitutive" isoform of NO synthase (ecNOS or NOS3) in vivo, and both cell types also express the NO synthase isoform induced by specific inflammatory cytokines (iNOS or NOS2) in vivo and in vitro. While NO-dependent intracellular signalling in cardiac myocytes clearly involves the activation of guanylate cyclase and downstream signalling by cGMP, there is accumulating evidence that non-cGMP-dependent regulatory signalling events are also initiated by NO. In addition, decreased contractile responsiveness of cardiac myocytes to beta-adrenergic agonists, following induction of NOS2 by inflammatory cytokines, requires the presence of insulin and the co-induction of enzymes responsible for production of tetrahydrobiopterin, a NOS co-factor. Inappropriate or excessive production of NO by cardiac myocytes and by microvascular endothelial cells likely contributes to the cardiac contractile dysfunction characteristic of the systemic inflammatory response syndrome and cardiac allograft rejection.

Nitric oxide synthase (NOS3)-mediated cholinergic modulation of Ca2+ current in adult rabbit atrioventricular nodal cells

We examined the role of endogenous NO in the autonomic regulation of atrioventricular (AV) nodal function by studying spontaneous action potentials (SAPs) and L-type Ca2+ current (ICa-L) in isolated single AV nodal cells from adult rabbit hearts. Both the perforated and the membrane-ruptured patch-clamp techniques in the whole-cell configuration were used under conditions known to alter NO production. Three NO donors, 3-morpholinosydnonimine (SIN-1, 0.1 mmol/L), S-nitroso-acetylcysteine (0.1 mmol/L), and sodium nitroprusside (0.1 mmol/L), suppressed the beta-adrenergic agonist isoproterenol (ISO, 1 mumol/L)-stimulated increase in ICa-L. SIN-1 also decreased the frequency and amplitude of SAPs. In cells in which ICa-L had been previously attenuated by the muscarinic agonist carbamylcholine (CCh, 1 mumol/L), SIN-1 had no additive effect. CCh activated an acetylcholine-sensitive outward K+ current (IK(ACh)) in AV nodal cells, in addition to the ICa-L inhibition. Intracellular dialysis with the NO synthase inhibitor N-monomethyl-L-arginine (L-NMMA, 0.5 mmol/L) blocked CCh-induced, but not SIN-1-induced, ICa.L attenuation. However, intracellular dialysis with methylene blue (20 mumol/L), which inhibits NO-mediated activation of guanylyl cyclase and cGMP production, blocked the effects of both CCh and SIN-1 on ICa-L. In these cells, neither L-NMMA nor methylene blue affected the CCh-activated IK(ACh). Direct application of cGMP (10 mumol/L) via internal dialysis significantly inhibited ISO-stimulated ICa-L. In AV nodal cells internally perfused with either a nonhydrolyzable cAMP analogue, 8-Br-cAMP (0.5 mmol/L), or a high concentration of cAMP (0.5 mmol/L), CCh did not inhibit, ICa-L but still activated IK(ACh). CCh-induced ICa-L attenuation could be abolished or quickly reversed by the nonselective phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (20 mumol/L). However, CCh still significantly suppressed ISO-stimulated ICa-L after the cGMP-inhibited PDE isozyme (PDE3) had been selectively inhibited by milrinone (5 mumol/L). Immunohistochemical staining identified the presence of the endothelial constitutive NO synthase (ecNOS or NOS3) in both single AV nodal cells in vitro and in cryostat sections of AV nodal tissue in situ. These results demonstrate that endogenous NO is involved in the muscarinic cholinergic attenuation of ICa-L in AV nodal cell; the mechanism likely involves the cGMP-stimulated PDE.

Nitric oxide inhibits creatine kinase and regulates rat heart contractile reserve

Cardiac myocytes express both constitutive and cytokine-inducible nitric oxide syntheses (NOS). NO and its congeners have been implicated in the regulation of cardiac contractile function. To determine whether NO could affect myocardial energetics, 31P NMR spectroscopy was used to evaluate high-energy phosphate metabolism in isolated rat hearts perfused with the NO donor S-nitrosoacetylcysteine (SNAC). All hearts were exposed to an initial high Ca2+ (3.5 mM) challenge followed by a recovery period, and then, either in the presence or absence of SNAC, to a second high Ca2+ challenge. This protocol allowed us to monitor simultaneously the effect of SNAC infusion on both contractile reserve (i.e., baseline versus high workload contractile function) and high-energy phosphate metabolism. The initial high Ca2+ challenge caused the rate-pressure product to increase by 74 +/- 5% in all hearts. As expected, ATP was maintained as phosphocreatine (PCr) content briefly dropped and then returned to baseline during the subsequent recovery period. Control hearts responded similarLy to the second high Ca2+ challenge, but SNAC-treated hearts did not demonstrate the expected increase in rate-pressure product. In these hearts, ATP declined significantly during the second high Ca2+ challenge, whereas phosphocreatine did not differ from controls, suggesting that phosphoryl transfer by creatine kinase (CK) was inhibited. CK activity, measured biochemically, was decreased by 61 +/- 13% in SNAC-treated hearts compared to controls. Purified CK in solution was also inhibited by SNAC, and reversal could be accomplished with DTT, a sulfhydryl reducing agent. Thus, NO can regulate contractile reserve, possibly by reversible nitrosothiol modification of CK.

Nitric oxide synthase (NOS3) and contractile responsiveness to adrenergic and cholinergic agonists in the heart. Regulation of NOS3 transcription in vitro and in vivo by cyclic adenosine monophosphate in rat cardiac myocytes

Cardiac myocytes express the nitric oxide synthase isoform originally identified in constitutive nitric oxide synthase cells (NOS3), which mediates the attenuation by muscarinic cholinergic agonists of beta-adrenergic stimulation of L-type calcium current and contractility in these cells. However, calcium current and contractility in these cells. However, the reciprocal regulation of NOS3 activity in myocytes by agents that elevate cAMP has not been reported. In this study, we show that NOS3 and mRNA and protein levels in cardiac myocytes are reduced both in vitro after treatment with cAMP elevating drugs, and in vivo after 3 d of treatment with milrinone, a type III cAMP phosphodiesterase inhibitor. This effect on NOS3 activity by cAMP is cell type specific because treatment of cardiac microvascular endothelial cells in vitro or in vivo did not decrease NOS3 mRNA or protein in these cells. NOS3 downregulation in myocytes appeared to be at the level of transcription since there was no modification of NOS3 mRNA half-life by agents that increase intracellular cAMP. To determine the functional effects of NOS3 downregulation, we examined the contractile responsiveness of isolated electrically paced ventricular myocytes, isolated from animals that had been treated in vivo with milrinone, to the beta-adrenergic agonist isoproterenol and the muscarinic cholinergic agonist carbamylcholine. There was no difference in baseline contractile function in cells that had been pretreated with cAMP elevating agents compared to controls, but cells exposed to milrinone in vivo exhibited an accentuation in their contractile responsiveness to isoproterenol compared to controls and a loss of responsiveness to carbamylcholine. Downregulation of myocyte NOS3 by sustained elevation of cAMP may have important implications for the regulation of myocardial contractile state by the autonomic nervous system.

Frequency-dependent activation of a constitutive nitric oxide synthase and regulation of contractile function in adult rat ventricular myocytes

Cardiac myocytes have recently been shown to express a constitutive Ca(2+)-sensitive isoform of NO synthase (NOS3), although the mechanism(s) responsible for activation of NOS3 and its physiological function remain to be determined. Since the activity of NOS3 is known to be regulated in part by the intracellular Ca2+ activity ([Ca2+]i) in endothelial cells, we determined whether increasing myocyte [Ca2+]i by uniform electric field pacing was accompanied by an increase in NOS3 activity, detected as nitrite accumulation in the medium. A higher [Ca2+]i with increasing pacing frequencies was shown to be accompanied by a time-dependent accumulation of nitrite in medium that bathed adult rat ventricular myocytes stimulated at 3 Hz. Nitrite release by paced cells was significantly attenuated by treatment with either the NO synthase inhibitor nitro-L-arginine (L-NA, 1 mmol/L) or the intracellular Ca2+ chelator BAPTA-AM (20 mumol/L). Paced myocytes also exhibited a frequency- and time-dependent increase in intracellular cGMP content that could be inhibited significantly by either L-NA or the soluble guanylate cyclase inhibitor LY83583 (5 mumol/L). To determine whether the increase in NOS3 activity with pacing affected contractile function, myocytes were sequentially paced at frequencies from 0.5 to 3 Hz. Methylene blue, L-NA, and LY83583 all increased the amplitude of shortening of myocytes paced at 3 Hz. Furthermore, a significantly greater positive inotropic response to high extracellular Ca2+ (3 mmol/L) was demonstrated by myocytes pretreated with L-NA compared with control cells. These data indicate that myocyte NOS3 activity is regulated in part by [Ca2+]i, whether induced by changes in pacing frequency or [Ca2+]o, and depresses myocyte contractile responsiveness to higher stimulation frequencies.

Regulation of cytokine-inducible nitric oxide synthase in cardiac myocytes and microvascular endothelial cells. Role of extracellular signal-regulated kinases 1 and 2 (ERK1/ERK2) and STAT1 alpha

Adult rat ventricular myocytes and cardiac microvascular endothelial cells (CMEC) both express an inducible nitric oxide synthase (iNOS or NOS2) following exposure to soluble inflammatory mediators. However, NOS2 gene expression is regulated differently in response to specific cytokines in each cell type. Interleukin-1 beta (IL-1 beta) induces NOS2 in both, whereas interferon gamma (IFN gamma) induces NOS2 expression in myocytes but not in CMEC. Therefore, we examined the specific signal transduction pathways that could regulate NOS2 mRNA levels, including activation of 44- and 42-kDa mitogenactivated protein kinases (MAPKs; ERK1/ERK2) and STAT1 alpha, a transcriptional regulatory protein linked to cell membrane receptors. Although IL-1 beta treatment increased ERK1/ERK2 activities in both cell types, IFN gamma activated these MAPKs only in myocytes. STAT1 alpha phosphorylation, consistent with IFN gamma-induced signaling, was readily apparent in both cell types, and binding of activated STAT1 alpha from cytoplasmic or nuclear fractions from IFN gamma-treated adult myocytes to a sis-inducible element could be demonstrated by gel-shift assay. The farnesyl transferase inhibitor BZA-5B blocked activation of ERK1/ERK2 and induction of NOS2 by IFN gamma and IL-1 beta in myocytes. IL-1 beta and IFN gamma-induced NOS2 gene expression in myocytes was also down-regulated by both protein kinase C (PKC) desensitization and by the PKC inhibitor bisindolylmaleimide, implicating PKC-linked activation of Ras or Raf in the induction of NOS2 by IL-1 beta and IFN gamma in cardiac muscle cells. In CMEC, the MAPK kinase inhibitor PD 98059 blocked activation of ERK1/ERK2 and down-regulated IL-1 beta-mediated NOS2 induction, whereas activation of ERK2 in the absence of cytokines by okadaic acid, an inhibitor of phosphoserine protein phosphatases, also induced NOS2 mRNA. These data demonstrate that ERK1/ERK2 activation appears to be necessary for the induction of NOS2 by IL-1 beta and IFN gamma in cardiac myocytes and CMEC. In the absence of ERK1/ERK2 activation by IFN gamma in CMEC, phosphorylation of STAT1 alpha is not sufficient for NOS2 gene expression. These overlapping yet distinct cellular responses to specific cytokines may serve to target NOS2 gene expression to specific cells or regions within the heart and also provide for rapid escalation of NO production if required for host defense.

Glucocorticoids increase osteopontin expression in cardiac myocytes and microvascular endothelial cells. Role in regulation of inducible nitric oxide synthase

In heart muscle, the cytokine-inducible isoform of nitric oxide synthase (NOS2) is expressed in both cardiac myocytes and microvascular endothelial cells (CMEC). mRNA levels for both NOS2 and for osteopontin, a multifunctional extracellular matrix phosphoprotein containing and RGD integrin binding domain, are increased in cardiac muscle following intraperitoneal injection of adult rats with lipopolysaccharide. In vitro, interleukin-1 beta and interferon-gamma increased osteopontin mRNA levels in CMEC as well as NOS2 expression in both CMEC and cardiac myocytes. However, osteopontin mRNA levels in heart muscle in vivo, and in cardiac myocytes and CMEC in vitro, also are increased 10-30-fold by the synthetic glucocorticoid dexamethasone, an agent that suppresses cytokine induction of NOS2 in both cell types. The hexapeptide GRGDSP, which interrupts binding of RGD-containing proteins to cell surface integrins, increased NOS2 mRNA, while a synthetic osteopontin peptide analogue decreased NOS2 mRNA and protein levels in both cytokine-pretreated cardiac myocytes and CMEC cultures. Also, transfection with a full-length antisense-osteopontin cDNA in cytokine-pretreated CMEC decreased endogenous osteopontin mRNA and increased NOS2 mRNA levels. These results suggest that osteopontin could regulate the location and extent of NOS2 induction in the heart. Increased expression of osteopontin also may be one mechanism by which glucocorticoids suppress NOS2 activity in cardiac myocytes and microvascular endothelial cells.