Muscarinic and beta-adrenergic regulation of heart rate, force of contraction and calcium current is preserved in mice lacking endothelial nitric oxide synthase

Proposal for considerations during human iPSC-derived cardiac organoid generation for cardiotoxicity drug testing

Human iPSC-derived cardiac organoids (hiPSC-COs) for cardiotoxicity drug testing via the variety of cell lines and unestablished protocols may lead to differences in response results due to a lack of criteria for generation period and size. To ensure reliable drug testing, it is important for researchers to set optimal generation period and size of COs according to the cell line and protocol applied in their studies. Hence, we sought to propose a process to establish minimum criteria for the generation duration and size of hiPSC-COs for cardiotoxic drug testing. We generated hiPSC-COs of different sizes based on our protocol and continuously monitored organoids until they indicated a minimal beating rate change as a control that could lead to more accurate beating rate changes on drug testing. Calcium transients and physiological tests to assess the functionality of hiPSC-COs on selected generation period, which showed regular cardiac beating, and immunostaining assays to compare characteristics were performed. We explained the generation period and size that exhibited and maintained regular beating rate changes on hiPSC-COs, and lead to reliable response results to cardiotoxicity drugs. We anticipate that this study will offer valuable insights into considering the appropriate generation period and size of hiPSC-COs ensuring reliable outcomes in cardiotoxicity drug testing.

Region-specific sympatho-adrenergic regulation of specialized vasculature in bone homeostasis and regeneration

Type H vessels couple angiogenesis with osteogenesis, while sympathetic cues regulate vascular and skeletal function. The crosstalk between sympathetic nerves and type H vessels in bone remains unclear. Here, we first identify close spatial connections between sympathetic nerves and type H vessels in bone, particularly in metaphysis. Sympathoexcitation, mimicked by isoproterenol (ISO) injection, reduces type H vessels and bone mass. Conversely, beta-2-adrenergic receptor (ADRB2) deficiency maintains type H vessels and bone mass in the physiological condition. In vitro experiments reveal indirect sympathetic modulation of angiogenesis via paracrine effects of mesenchymal stem cells (MSCs), which alter the transcription of multiple angiogenic genes in endothelial cells (ECs). Furthermore, Notch signaling in ECs underlies sympathoexcitation-regulated type H vessel formation, impacting osteogenesis and bone mass. Finally, propranolol (PRO) inhibits beta-adrenergic activity and protects type H vessels and bone mass against estrogen deficiency. These findings unravel the specialized neurovascular coupling in bone homeostasis and regeneration.

Generation and maturation of human iPSC-derived 3D organotypic cardiac microtissues in long-term culture

Cardiovascular diseases remain the leading cause of death worldwide; hence there is an increasing focus on developing physiologically relevant in vitro cardiovascular tissue models suitable for studying personalized medicine and pre-clinical tests. Despite recent advances, models that reproduce both tissue complexity and maturation are still limited. We have established a scaffold-free protocol to generate multicellular, beating human cardiac microtissues in vitro from hiPSCs-namely human organotypic cardiac microtissues (hOCMTs)-that show some degree of self-organization and can be cultured for long term. This is achieved by the differentiation of hiPSC in 2D monolayer culture towards cardiovascular lineage, followed by further aggregation on low-attachment culture dishes in 3D. The generated hOCMTs contain multiple cell types that physiologically compose the heart and beat without external stimuli for more than 100 days. We have shown that 3D hOCMTs display improved cardiac specification, survival and metabolic maturation as compared to standard monolayer cardiac differentiation. We also confirmed the functionality of hOCMTs by their response to cardioactive drugs in long-term culture. Furthermore, we demonstrated that they could be used to study chemotherapy-induced cardiotoxicity. Due to showing a tendency for self-organization, cellular heterogeneity, and functionality in our 3D microtissues over extended culture time, we could also confirm these constructs as human cardiac organoids (hCOs). This study could help to develop more physiologically-relevant cardiac tissue models, and represent a powerful platform for future translational research in cardiovascular biology.

Dimerization of β2-adrenergic receptor is responsible for the constitutive activity subjected to inverse agonism

Dimerization of beta 2-adrenergic receptor (β₂-AR) has been observed across various physiologies. However, the function of dimeric β₂-AR is still elusive. Here, we revealed that dimerization of β₂-AR is responsible for the constitutive activity of β₂-AR generating inverse agonism. Using a co-immunoimmobilization assay, we found that transient β₂-AR dimers exist in a resting state, and the dimer was disrupted by the inverse agonists. A Gα_s preferentially interacts with dimeric β₂-AR, but not monomeric β₂-AR, in a resting state, resulting in the production of a resting cAMP level. The formation of β₂-AR dimers requires cholesterol on the plasma membrane. The cholesterol did not interfere with the agonist-induced activation of monomeric β₂-AR, unlike the inverse agonists, implying that the cholesterol is a specific factor regulating the dimerization of β₂-AR. Our model not only shows the function of dimeric β₂-AR but also provides a molecular insight into the mechanism of the inverse agonism of β₂-AR.

Muscarinic receptor activation reduces force and arrhythmias in human atria independent of IK,ACh

ABSTRACT

In human hearts, muscarinic receptors (M-R) are expressed in ventricular and atrial tissue, but the acetylcholine-activated potassium current (IK,ACh) is expressed mainly in the atrium. M-R activation decreases force and increases electrical stability in human atrium, but the impact of IK,ACh to both effects remains unclear. We employed a new selective blocker of IK,ACh to elaborate the contribution of IK,ACh to M-R activation-mediated effects in human atrium.Force and action potentials were measured in rat atria and in human right atrial trabeculae. Cumulative concentration-effect curves for norepinephrine-induced force and arrhythmias were measured in the presence of either carbachol (CCh;1µM) or CCh together with the IK,ACh -blocker XAF-1407 (1 µM) or in time-matched controls. To investigate the vulnerability to arrhythmias we performed some experiments also in the presence of cilostamide (0.3µM) and rolipram (1µM), inhibiting PDE3 and PDE4.In rat atria and human right atrial trabeculae, CCh shortened the action potential duration persistently. However, the direct negative inotropy of CCh was only transient in human, but stable in rat atria. In both rat and human atria, the negative inotropic effect was insensitive to blockage of IK,ACh by XAF-1407. In the presence of cilostamide and rolipram about 40% of trabeculae developed arrhythmias when exposed to norepinephrine. CCh prevented these concentration-dependent norepinephrine-induced arrhythmias, again insensitive to XAF-1407. Maximum catecholamine-induced force was not depressed by CCh.In human atrium, both the direct and the indirect negative inotropic effect of CCh are independent of IK,ACh. The same applies to the CCh-mediated suppression of norepinephrine/PDE-inhibition-induced arrhythmias.

Acute Detubulation of Ventricular Myocytes Amplifies the Inhibitory Effect of Cholinergic Agonist on Intracellular Ca2+ Transients

Muscarinic receptors expressed in cardiac myocytes play a critical role in the regulation of heart function by the parasympathetic nervous system. How the structural organization of cardiac myocytes affects the regulation of Ca²⁺ handling by muscarinic receptors is not well-defined. Using confocal Ca²⁺ imaging, patch-clamp techniques, and immunocytochemistry, the relationship between t-tubule density and cholinergic regulation of intracellular Ca²⁺ in normal murine ventricular myocytes and myocytes with acute disruption of the t-tubule system caused by formamide treatment was studied. The inhibitory effect of muscarinic receptor agonist carbachol (CCh, 10 μM) on the amplitude of Ca²⁺ transients, evoked by field-stimulation in the presence of 100 nM isoproterenol (Iso), a β-adrenergic agonist, was directly proportional to the level of myocyte detubulation. The timing of the maximal rate of fluorescence increase of fluo-4, a Ca²⁺-sensitive dye, was used to classify image pixels into the regions functionally coupled or uncoupled to the sarcolemmal Ca²⁺ influx (I_Ca). CCh decreased the fraction of coupled regions and suppressed Ca²⁺ propagation from sarcolemma inside the cell. Formamide treatment reduced I_Ca density and decreased sarcoplasmic reticulum (SR) Ca²⁺ content. CCh did not change SR Ca²⁺ content in Iso-stimulated control and formamide-treated myocytes. CCh inhibited peak I_Ca recorded in the presence of Iso by ∼20% in both the control and detubulated myocytes. Reducing I_Ca amplitude up to 40% by changing the voltage step levels from 0 to –25 mV decreased Ca²⁺ transients in formamide-treated but not in control myocytes in the presence of Iso. CCh inhibited CaMKII activity, whereas CaMKII inhibition with KN93 mimicked the effect of CCh on Ca²⁺ transients in formamide-treated myocytes. It was concluded that the downregulation of t-tubules coupled with the diminished efficiency of excitation–contraction coupling, increases the sensitivity of Ca²⁺ release and propagation to muscarinic receptor-mediated inhibition of both I_Ca and CaMKII activity.

Cyclic nucleotide signaling and pacemaker activity

The sinoatrial node (SAN) is the natural pacemaker of the heart, producing the electrical impulse that initiates every heart beat. Its activity is tightly controlled by the autonomic nervous system, and by circulating and locally released factors. Neurohumoral regulation of heart rate plays a crucial role in the integration of vital functions and influences behavior and ability to respond to changing environmental conditions. At the cellular level, modulation of SAN activity occurs through intracellular signaling pathways involving cyclic nucleotides: cyclic AMP (cAMP) and cyclic GMP (cGMP). In this Review, dedicated to Professor Dario DiFrancesco and his accomplishements in the field of cardiac pacemaking, we summarize all findings on the role of cyclic nucleotides signaling in regulating the key actors of cardiac automatism, and we provide an up-to-date review on cAMP- and cGMP-phosphodiesterases (PDEs), compellingly involved in this modulation.

The cGMP system: components and function

The cyclic guanosine monophosphate (cGMP) signaling system is one of the most prominent regulators of a variety of physiological and pathophysiological processes in many mammalian and non-mammalian tissues. Targeting this pathway by increasing cGMP levels has been a very successful approach in pharmacology as shown for nitrates, phosphodiesterase (PDE) inhibitors and stimulators of nitric oxide-guanylyl cyclase (NO-GC) and particulate GC (pGC). This is an introductory review to the cGMP signaling system intended to introduce those readers to this system, who do not work in this area. This article does not intend an in-depth review of this system. Signal transduction by cGMP is controlled by the generating enzymes GCs, the degrading enzymes PDEs and the cGMP-regulated enzymes cyclic nucleotide-gated ion channels, cGMP-dependent protein kinases and cGMP-regulated PDEs. Part A gives a very concise introduction to the components. Part B gives a very concise introduction to the functions modulated by cGMP. The article cites many recent reviews for those who want a deeper insight.

Adrenergic and Glucocorticoid Receptors in the Pulmonary Health Effects of Air Pollution

Adrenergic receptors (ARs) and glucocorticoid receptors (GRs) are activated by circulating catecholamines and glucocorticoids, respectively. These receptors regulate the homeostasis of physiological processes with specificity via multiple receptor subtypes, wide tissue-specific distribution, and interactions with other receptors and signaling processes. Based on their physiological roles, ARs and GRs are widely manipulated therapeutically for chronic diseases. Although these receptors play key roles in inflammatory and cellular homeostatic processes, little research has addressed their involvement in the health effects of air pollution. We have recently demonstrated that ozone, a prototypic air pollutant, mediates pulmonary and systemic effects through the activation of these receptors. A single exposure to ozone induces the sympathetic–adrenal–medullary and hypothalamic–pituitary–adrenal axes, resulting in the release of epinephrine and corticosterone into the circulation. These hormones act as ligands for ARs and GRs. The roles of beta AR (βARs) and GRs in ozone-induced pulmonary injury and inflammation were confirmed in a number of studies using interventional approaches. Accordingly, the activation status of ARs and GRs is critical in mediating the health effects of inhaled irritants. In this paper, we review the cellular distribution and functions of ARs and GRs, their lung-specific localization, and their involvement in ozone-induced health effects, in order to capture attention for future research.

Multiplicity of Nitric Oxide and Natriuretic Peptide Signaling in Heart Failure

Heart failure (HF) is a common consequence of several cardiovascular diseases and is understood as a vicious cycle of cardiac and hemodynamic decline. The current inventory of treatments either alleviates the pathophysiological features (eg, cardiac dysfunction, neurohumoral activation, and ventricular remodeling) and/or targets any underlying pathologies (eg, hypertension and myocardial infarction). Yet, since these do not provide a cure, the morbidity and mortality associated with HF remains high. Therefore, the disease constitutes an unmet medical need, and novel therapies are desperately needed. Cyclic guanosine-3',5'-monophosphate (cGMP), synthesized by nitric oxide (NO)- and natriuretic peptide (NP)-responsive guanylyl cyclase (GC) enzymes, exerts numerous protective effects on cardiac contractility, hypertrophy, fibrosis, and apoptosis. Impaired cGMP signaling, which can occur after GC deactivation and the upregulation of cyclic nucleotide-hydrolyzing phosphodiesterases (PDEs), promotes cardiac dysfunction. In this study, we review the role that NO/cGMP and NP/cGMP signaling plays in HF. After considering disease etiology, the physiological effects of cGMP in the heart are discussed. We then assess the evidence from preclinical models and patients that compromised cGMP signaling contributes to the HF phenotype. Finally, the potential of pharmacologically harnessing cardioprotective cGMP to rectify the present paucity of effective HF treatments is examined.

Mathematical model for β1-adrenergic regulation of the mouse ventricular myocyte contraction

The β1-adrenergic regulation of cardiac myocyte contraction plays an important role in regulating heart function. Activation of this system leads to an increased heart rate and stronger myocyte contraction. However, chronic stimulation of the β1-adrenergic signaling system can lead to cardiac hypertrophy and heart failure. To understand the mechanisms of action of β1-adrenoceptors, a mathematical model of cardiac myocyte contraction that includes the β1-adrenergic system was developed and studied. The model was able to simulate major experimental protocols for measurements of steady-state force-calcium relationships, cross-bridge release rate and force development rate, force-velocity relationship, and force redevelopment rate. It also reproduced quite well frequency and isoproterenol dependencies for intracellular Ca2+ concentration ([Ca2+]i) transients, total contraction force, and sarcomere shortening. The mathematical model suggested the mechanisms of increased contraction force and myocyte shortening on stimulation of β1-adrenergic receptors is due to phosphorylation of troponin I and myosin-binding protein C and increased [Ca2+]i transient resulting from activation of the β1-adrenergic signaling system. The model was used to simulate work-loop contractions and estimate the power during the cardiac cycle as well as the effects of 4-aminopyridine and tedisamil on the myocyte contraction. The developed mathematical model can be used further for simulations of contraction of ventricular myocytes from genetically modified mice and myocytes from mice with chronic cardiac diseases.

NEW & NOTEWORTHY A new mathematical model of mouse ventricular myocyte contraction that includes the β1-adrenergic system was developed. The model simulated major experimental protocols for myocyte contraction and predicted the effects of 4-aminopyridine and tedisamil on the myocyte contraction. The model also allowed for simulations of work-loop contractions and estimation of the power during the cardiac cycle.

A concise discussion of the regulatory role of cGMP kinase I in cardiac physiology and pathology

The underlying cause of cardiac hypertrophy, fibrosis, and heart failure has been investigated in great detail using different mouse models. These studies indicated that cGMP and cGMP-dependent protein kinase type I (cGKI) may ameliorate these negative phenotypes in the adult heart. Recently, evidence has been published that cardiac mitochondrial BKCa channels are a target for cGKI and that activation of mitoBKCa channels may cause some of the positive effects of conditioning in ischemia/reperfusion injury. It will be pointed out that most studies could not present convincing evidence that it is the cGMP level and the activity cGKI in specific cardiac cells that reduces hypertrophy or heart failure. However, anti-fibrotic compounds stimulating nitric oxide-sensitive guanylyl cyclase may be an upcoming therapy for abnormal cardiac remodeling.

Cardiovascular Disease: An Introduction

Cardiovascular disease (CVD) is a collective term designating all types of affliction affecting the blood circulatory system, including the heart and vasculature, which, respectively, displaces and conveys the blood.

Translational neurocardiology: preclinical models and cardioneural integrative aspects

Neuronal elements distributed throughout the cardiac nervous system, from the level of the insular cortex to the intrinsic cardiac nervous system, are in constant communication with one another to ensure that cardiac output matches the dynamic process of regional blood flow demand. Neural elements in their various 'levels' become differentially recruited in the transduction of sensory inputs arising from the heart, major vessels, other visceral organs and somatic structures to optimize neuronal coordination of regional cardiac function. This White Paper will review the relevant aspects of the structural and functional organization for autonomic control of the heart in normal conditions, how these systems remodel/adapt during cardiac disease, and finally how such knowledge can be leveraged in the evolving realm of autonomic regulation therapy for cardiac therapeutics.

Impact of dietary nitrate on age-related diastolic dysfunction

AIMS

Diastolic dysfunction is highly prevalent, and ageing is the main contributor due to impairments in active cardiac relaxation, ventriculo-vascular stiffening, and endothelial dysfunction. Nitric oxide (NO) affects cardiovascular functions, and NO bioavailability is critically reduced with ageing. Whether replenishment of NO deficiency with dietary inorganic nitrate would offer a novel approach to reverse age-related cardiovascular alterations was not known.

METHODS AND RESULTS

A dietary nitrate supplementation was applied to young (6 month) and old (20 month) wild-type mice for 8 weeks and compared with controls. High-resolution ultrasound, pressure-volume catheter techniques, and isolated heart measurements were applied to assess cardiac diastolic and vascular functions. Cardiac manganese-enhanced magnetic resonance imaging was performed to study the effects of dietary nitrate on myocyte calcium handling. In aged mice with preserved systolic function, dietary nitrate supplementation improved LV diastolic function, arterial compliance, and coronary flow reserve. Mechanistically, improved cardiovascular functions were associated with an accelerated cardiomyocyte calcium handling and augmented NO/cyclic guanosine monophosphate/protein kinase G signalling, while enhanced nitrate reduction was related to age-related differences in the oral microbiome.

CONCLUSION

Dietary inorganic nitrate reverses age-related LV diastolic dysfunction and improves vascular functions. Our results highlight the potential of a dietary approach in the therapy of age-related cardiovascular alterations.

Protection against ventricular fibrillation via cholinergic receptor stimulation and the generation of nitric oxide

KEY POINTS

Animal studies suggest an anti-fibrillatory action of the vagus nerve on the ventricle, although the exact mechanism is controversial. Using a Langendorff perfused rat heart, we show that the acetylcholine analogue carbamylcholine raises ventricular fibrillation threshold (VFT) and flattens the electrical restitution curve. The anti-fibrillatory action of carbamylcholine was prevented by the nicotinic receptor antagonist mecamylamine, inhibitors of neuronal nitric oxide synthase (nNOS) and soluble guanylyl cyclase (sGC), and can be mimicked by the nitric oxide (NO) donor sodium nitroprusside. Carbamylcholine increased NO metabolite content in the coronary effluent and this was prevented by mecamylamine. The anti-fibrillatory action of both carbamylcholine and sodium nitroprusside was ultimately dependent on muscarinic receptor stimulation as all effects were blocked by atropine. These data demonstrate a protective effect of carbamylcholine on VFT that depends upon both muscarinic and nicotinic receptor stimulation, where the generation of NO is likely to be via a neuronal nNOS-sGC dependent pathway.

ABSTRACT

Implantable cardiac vagal nerve stimulators are a promising treatment for ventricular arrhythmia in patients with heart failure. Animal studies suggest the anti-fibrillatory effect may be nitric oxide (NO) dependent, although the exact site of action is controversial. We investigated whether a stable analogue of acetylcholine could raise ventricular fibrillation threshold (VFT), and whether this was dependent on NO generation and/or muscarinic/nicotinic receptor stimulation. VFT was determined in Langendorff perfused rat hearts by burst pacing until sustained VF was induced. Carbamylcholine (CCh, 200 nmol l(-1) , n = 9) significantly (P < 0.05) reduced heart rate from 292 ± 8 to 224 ± 6 b.p.m. Independent of this heart rate change, CCh caused a significant increase in VFT (control 1.5 ± 0.3 mA, CCh 2.4 ± 0.4 mA, wash 1.1 ± 0.2 mA) and flattened the restitution curve (n = 6) derived from optically mapped action potentials. The effect of CCh on VFT was abolished by a muscarinic (atropine, 0.1 μmol l(-1) , n = 6) or a nicotinic receptor antagonist (mecamylamine, 10 μmol l(-1) , n = 6). CCh significantly increased NOx content in coronary effluent (n = 8), but not in the presence of mecamylamine (n = 8). The neuronal nitric oxide synthase inhibitor AAAN (N-(4S)-4-amino-5-[aminoethyl]aminopentyl-N'-nitroguanidine; 10 μmol l(-1) , n = 6) or soluble guanylate cyclase (sGC) inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one; 10 μmol l(-1) , n = 6) prevented the rise in VFT with CCh. The NO donor sodium nitrprusside (10 μmol l(-1) , n = 8) mimicked the action of CCh on VFT, an effect that was also blocked by atropine (n = 10). These data demonstrate a protective effect of CCh on VFT that depends upon both muscarinic and nicotinic receptor stimulation, where the generation of NO is likely to be via a neuronal nNOS/sGC-dependent pathway.

Mechanisms of orthostatic intolerance during heat stress

Heat stress profoundly and unanimously reduces orthostatic tolerance. This review aims to provide an overview of the numerous and multifactorial mechanisms by which this occurs in humans. Potential causal factors include changes in arterial and venous vascular resistance and blood distribution, and the modulation of cardiac output, all of which contribute to the inability to maintain cerebral perfusion during heat and orthostatic stress. A number of countermeasures have been established to improve orthostatic tolerance during heat stress, which alleviate heat stress induced central hypovolemia (e.g., volume expansion) and/or increase peripheral vascular resistance (e.g., skin cooling). Unfortunately, these countermeasures can often be cumbersome to use with populations prone to syncopal episodes. Identifying the mechanisms of inter-individual differences in orthostatic intolerance during heat stress has proven elusive, but could provide greater insights into the development of novel and personalized countermeasures for maintaining or improving orthostatic tolerance during heat stress. This development will be especially impactful in occuational settings and clinical situations that present with orthostatic intolerance and/or central hypovolemia. Such investigations should be considered of vital importance given the impending increased incidence of heat events, and associated cardiovascular challenges that are predicted to occur with the ensuing changes in climate.

Cardiac arrhythmia induced by genetic silencing of 'funny' (f) channels is rescued by GIRK4 inactivation

The mechanisms underlying cardiac automaticity are still incompletely understood and controversial. Here we report the complete conditional and time-controlled silencing of the "funny" current (I_f) by expression of a dominant-negative, non-conductive HCN4-channel subunit (hHCN4-AYA). Heart-specific I_f silencing caused altered [Ca²⁺]_i release and Ca²⁺ handling in the sinoatrial node, impaired pacemaker activity, and symptoms reminiscent of severe human disease of pacemaking. The effects of I_f silencing critically depended on the activity of the autonomic nervous system. We were able to rescue the failure of impulse generation and conduction by additional genetic deletion of cardiac muscarinic G-protein-activated (GIRK4) channels in I_f-deficient mice without impairing heartbeat regulation. Our study establishes the role of f-channels in cardiac automaticity and indicates that arrhythmia related to HCN loss-of-function may be managed by pharmacological or genetic inhibition of GIRK4 channels, thus offering a new therapeutic strategy for the treatment of heart rhythm diseases.

Prostaglandin E2 does not attenuate adrenergic-induced cardiac contractile response

Systemic inflammation may contribute to heart failure. PGE2 was recently suggested to mediate inflammation-induced impairment of cardiac function by desensitizing the murine heart to isoprenaline. Given the magnitude of the reported effect and the potential relevance, we sought to reproduce it in the human heart. Human trabeculae were prepared from the right atrial tissue obtained during heart surgery and from the right ventricle of two explanted human failing hearts. Muscle strips were electrically driven and isometric force development was measured. PGE2 was given at a single concentration (0.1 μM). Norepinephrine was used to activate β1-adrenoceptors, epinephrine to activate β2-adrenoceptors in atrial trabeculae. Isoprenaline was used in ventricular tissue. All patients were in sinus rhythm. Murine ventricular strips were used for comparison and stimulated with isoprenaline. The pharmacological activity of the PGE2 batch was confirmed by assessing concentration-dependent vasoconstriction in murine aorta. We used atrial and ventricular trabeculae from humans. Exposure to PGE2 (15 min) did not affect contractility when compared to time-matched controls. PGE2 neither altered the sensitivity or efficacy of β1- or β2-adrenoceptor-mediated stimulation of force in human atrial or in ventricular trabeculae for nonselective β1- or β2-adrenoceptor-stimulation. Surprisingly, PGE2 also did not affect -logEC50 values or maximum catecholamine-stimulated force in ventricular strips from mice, whereas it induced vasoconstriction in aortic rings with an -logEC50 of 5.0 (n = 6). Our data do not support a role for PGE2 in regulating catecholamine inotropy, neither in mice nor in humans.

Nitric oxide synthases in heart failure

SIGNIFICANCE

The regulation of myocardial function by constitutive nitric oxide synthases (NOS) is important for the maintenance of myocardial Ca(2+) homeostasis, relaxation and distensibility, and protection from arrhythmia and abnormal stress stimuli. However, sustained insults such as diabetes, hypertension, hemodynamic overload, and atrial fibrillation lead to dysfunctional NOS activity with superoxide produced instead of NO and worse pathophysiology.

RECENT ADVANCES

Major strides in understanding the role of normal and abnormal constitutive NOS in the heart have revealed molecular targets by which NO modulates myocyte function and morphology, the role and nature of post-translational modifications of NOS, and factors controlling nitroso-redox balance. Localized and differential signaling from NOS1 (neuronal) versus NOS3 (endothelial) isoforms are being identified, as are methods to restore NOS function in heart disease.

CRITICAL ISSUES

Abnormal NOS signaling plays a key role in many cardiac disorders, while targeted modulation may potentially reverse this pathogenic source of oxidative stress.

FUTURE DIRECTIONS

Improvements in the clinical translation of potent modulators of NOS function/dysfunction may ultimately provide a powerful new treatment for many hearts diseases that are fueled by nitroso-redox imbalance.

cGMP-dependent protein kinases (cGK)

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

Oroxylin a, but not vasopressin, ameliorates cardiac dysfunction of endotoxemic rats

The mortality in septic patients with myocardial dysfunction is higher than those without it. Beneficial effects of flavonoid oroxylin A (Oro-A) on endotoxemic hearts were evaluated and compared with that of arginine vasopressin (AVP) which is used to reverse hypotension in septic patients. Endotoxemia in rats was induced by one-injection of lipopolysaccharides (LPS, 10 mg/kg, i.p.), and hearts were isolated 5-hrs or 16-hrs later. Isolated hearts with constant-pressure or constant-flow mode were examined by Langendorff technique. Rate and force of contractions of isolated atrial and ventricular strips were examined by tissue myography. Isolated endotoxemic hearts were characterized by decreased or increased coronary flow (CF) in LPS-treated-for-5hr and LPS-treated-for-16-hr groups, respectively, with decreased inotropy in both groups. Oro-A-perfusion ameliorated while AVP-perfusion worsened the decreased CF and inotropy in both preparations. Oro-A and AVP, however, did not affect diminished force or rate of contraction of atrial and ventricular strips of endotoxemic hearts. Oro-A-induced CF increase was not affected following coronary endothelium-denudation with saponin. These results suggest that Oro-A ameliorates LPS-depressed cardiac functions by increasing CF, leading to positive inotropy. In contrast, AVP aggravates cardiac dysfunction by decreasing CF. Oro-A is a potentially useful candidate for treating endotoxemia complicated with myocardial dysfunction.

Functional expression and modulation of the L-type Ca2+ current in embryonic heart cells

Voltage-dependent L-type Ca2+ channels (VDCCs) are critically involved in excitation contraction coupling and regulation of the force of contraction. An important mechanism contributing to the adaptation of heart function is modulation of the L-type Ca2+ current (I(Ca-L)) by hormones of the autonomous nervous system. The signaling pathways underlying this regulation in the adult heart are well understood. However, VDCC expression and its regulation in the embryonic heart are less understood. This report therefore provides a short overview of the current knowledge on this topic using embryonic stem cells and the mouse as model systems.

Muscarinic receptor agonists and antagonists: effects on cardiovascular function

Muscarinic receptor activation plays an essential role in parasympathetic regulation of cardiovascular function. The primary effect of parasympathetic stimulation is to decrease cardiac output by inhibiting heart rate. However, pharmacologically, muscarinic agonists are actually capable of producing both inhibitory and stimulatory effects on the heart as well as vasculature. This reflects the fact that muscarinic receptors are expressed throughout the cardiovascular system, even though they are not always involved in mediating parasympathetic responses. In the heart, in addition to regulating heart rate by altering the electrical activity of the sinoatrial node, activation of M₂ receptors can affect conduction of electrical impulses through the atrioventricular node. These same receptors can also regulate the electrical and mechanical activity of the atria and ventricles. In the vasculature, activation of M₃ and M₅ receptors in epithelial cells can cause vasorelaxation, while activation of M₁ or M₃ receptors in vascular smooth muscle cells can cause vasoconstriction in the absence of endothelium. This review focuses on our current understanding of the signaling mechanisms involved in mediating these responses.

Displacement-encoded and manganese-enhanced cardiac MRI reveal that nNOS, not eNOS, plays a dominant role in modulating contraction and calcium influx in the mammalian heart

Within cardiomyocytes, endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) are thought to modulate L-type calcium channel (LTCC) function and sarcoplasmic reticulum calcium cycling, respectively. However, divergent results from mostly invasive prior studies suggest more complex roles. To elucidate the roles of nNOS and eNOS in vivo, we applied noninvasive cardiac MRI to study wild-type (WT), eNOS(-/-), and nNOS(-/-) mice. An in vivo index of LTCC flux (LTCCI) was measured at baseline (Bsl), dobutamine (Dob), and dobutamine + carbacholamine (Dob + CCh) using manganese-enhanced MRI. Displacement-encoded MRI assessed contractile function by measuring circumferential strain (E(cc)) and systolic (dE(cc)/dt) and diastolic (dE(cc)/dt(diastolic)) strain rates at Bsl, Dob, and Dob + CCh. Bsl LTCCI was highest in nNOS(-/-) mice (P < 0.05 vs. WT and eNOS(-/-)) and increased only in WT and eNOS(-/-) mice with Dob (P < 0.05 vs. Bsl). LTCCI decreased significantly from Dob levels with Dob + CCh in all mice. Contractile function, as assessed by E(cc), was similar in all mice at Bsl. With Dob, E(cc) increased significantly in WT and eNOS(-/-) but not nNOS(-/-) mice (P < 0.05 vs. WT and eNOS(-/-)). With Dob + CCh, E(cc) returned to baseline levels in all mice. Systolic blood pressure, measured via tail plethysmography, was highest in eNOS(-/-) mice (P < 0.05 vs. WT and nNOS(-/-)). Mice deficient in nNOS demonstrate increased Bsl LTCC function and an attenuated contractile reserve to Dob, whereas eNOS(-/-) mice demonstrate normal LTCC and contractile function under all conditions. These results suggest that nNOS, not eNOS, plays the dominant role in modulating Ca(2+) cycling in the heart.

Sub-cellular targeting of constitutive NOS in health and disease

Constitutive nitric oxide synthases (NOSs) are ubiquitous enzymes that play a pivotal role in the regulation of myocardial function in health and disease. The discovery of both a neuronal NOS (nNOS) and an endothelial NOS (eNOS) isoform in the myocardium and the availability of genetically modified mice with selective eNOS or nNOS gene deletion have been of crucial importance for understanding the role of constitutive nitric oxide (NO) production in the myocardium. eNOS and nNOS are homologous in structure and utilize the same co-factors and substrates; however, they differ in their subcellular localization, regulation, and downstream signaling, all of which may account for their distinct effects on excitation-contraction coupling. In particular, eNOS-derived NO has been reported to increase left ventricular (LV) compliance, attenuate beta-adrenergic inotropy and enhance parasympathetic/muscarinic responses, and mediate the negative inotropic response to β3 adrenoreceptor stimulation via cGMP-dependent signaling. Conversely, nNOS-derived NO regulates basal myocardial inotropy and relaxation by inhibiting the sarcolemmal Ca(2+) current (I(Ca)) and promoting protein kinase A-dependent phospholamban (PLN) phosphorylation, independent of cGMP. By inhibiting the activity of myocardial oxidase systems, nNOS regulates the redox state of the myocardium and contributes to maintain eNOS "coupled" activity. After myocardial infarction, up-regulation of myocardial nNOS attenuates adverse remodeling and prevents arrhythmias whereas uncoupled eNOS activity in murine models of left ventricular pressure overload accelerates the progress towards heart failure. Here we review the evidence in support of the idea that NOS subcellular localization, mode of activation, and downstream signaling account for the diverse and highly specialized actions of NO in the heart. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

A cell-based biosensor for real-time detection of cardiotoxicity using lensfree imaging

A portable and cost-effective real-time cardiotoxicity biosensor was developed using a CMOS imaging module extracted from a commercially available webcam. The detection system consists of a CMOS imaging module, a white LED and a pinhole. Real-time image processing was conducted by comparing reference and live frame images. To evaluate the engineered system, the effects of two different drugs, isoprenaline and doxorubicin, on the beating rate and beat-to-beat variations of ESC-derived cardiomyocytes were measured. The detection system was used to conclude that the beat-to-beat variability increased under treatment with both isoprenaline and doxorubicin. However, the beating rates increased upon the addition of isoprenaline but decreased for cultures supplemented with doxorubicin. Moreover, the response time for both the beating rates and the beat-to-beat variability of ESC-derived cardiomyocytes under treatment of isoprenaline was shorter than for doxorubicin, although the amount of isoprenaline used in the measurement was three orders of magnitude lower than that of doxorubicin. Given its ability to perform real-time cell monitoring in a simple and inexpensive manner, the proposed system may be useful for a range of cell-based biosensing applications.

Regulation of cardiovascular cellular processes by S-nitrosylation

BACKGROUND

Nitric oxide (NO), a highly versatile signaling molecule, exerts a broad range of regulatory influences in the cardiovascular system that extends from vasodilation to myocardial contractility, angiogenesis, inflammation, and energy metabolism. Considerable attention has been paid to deciphering the mechanisms for such diversity in signaling. S-nitrosylation of cysteine thiols is a major signaling pathway through which NO exerts its actions. An emerging concept of NO pathophysiology is that the interplay between NO and reactive oxygen species (ROS), the nitroso/redox balance, is an important regulator of cardiovascular homeostasis.

SCOPE OF REVIEW

ROS react with NO, limit its bioavailability, and compete with NO for binding to the same thiol in effector molecules. The interplay between NO and ROS appears to be tightly regulated and spatially confined based on the co-localization of specific NO synthase (NOS) isoforms and oxidative enzymes in unique subcellular compartments. NOS isoforms are also in close contact with denitrosylases, leading to crucial regulation of S-nitrosylation.

MAJOR CONCLUSIONS

Nitroso/redox balance is an emerging regulatory pathway for multiple cells and tissues, including the cardiovascular system. Studies using relevant knockout models, isoform specific NOS inhibitors, and both in vitro and in vivo methods have provided novel insights into NO- and ROS-based signaling interactions responsible for numerous cardiovascular disorders.

GENERAL SIGNIFICANCE

An integrated view of the role of nitroso/redox balance in cardiovascular pathophysiology has significant therapeutic implications. This is highlighted by human studies where pharmacologic manipulation of oxidative and nitrosative pathways exerted salutary effects in patients with advanced heart failure. This article is part of a Special Issue entitled Regulation of Cellular Processes by S-nitrosylation.

Phosphodiesterases reduce spontaneous sinoatrial beating but not the 'fight or flight' tachycardia elicited by agonists through Gs-protein-coupled receptors

Cyclic AMP (cAMP) steers the generation of basal heart beat in the sinoatrial node. It also induces sinoatrial tachycardia and increased cardiac force, elicited through activation of Gs-protein-coupled receptors (GsPCRs). Phosphodiesterases (PDEs) hydrolyse cAMP. In the heart mainly PDE3 and PDE4 would be expected to limit those functions, and the PDE isoenzymes do indeed reduce basal sinoatrial beating rate and blunt the positive inotropic effects of agonists, mediated by GsPCRs. By contrast, recent evidence shows that GsPCR-mediated sinoatrial tachycardia is not controlled by PDE1-5. A PDE-resistant cAMP pool in sinoatrial cells, generated through activation of GsPCRs, including β(1)- and β(2)-adrenoceptors, appears to guarantee unrestrained tachycardia during fight or flight stress.

Cardiac electrophysiological effects of nitric oxide

Nitric oxide (NO) synthetized by essentially all cardiac cell types plays a key role in the regulation of cardiac function. Recent evidence shows that NO modulates the activity of cardiac ion channels implicated in the genesis of the cardiac action potential and exerts anti-arrhythmic properties under some circumstances. We review the effects of NO on cardiac ion channels and the signalling pathways, including cGMP-dependent (protein kinase G and cGMP-regulated phosphodiesterases) and cGMP-independent mechanisms (S-nitrosylation and direct effects on G proteins) and finally the role of NO in the genesis of cardiac arrhythmias during ischemia-reperfusion, heart failure, long QT syndrome, atrial fibrillation, and sudden cardiac death.

The exercising heart at altitude

Maximal cardiac output is reduced in severe acute hypoxia but also in chronic hypoxia by mechanisms that remain poorly understood. In theory, the reduction of maximal cardiac output could result from: (1) a regulatory response from the central nervous system, (2) reduction of maximal pumping capacity of the heart due to insufficient coronary oxygen delivery prior to the achievement of the normoxic maximal cardiac output, or (3) reduced central command. In this review, we focus on the effects that acute and chronic hypoxia have on the pumping capacity of the heart, particularly on myocardial contractility and the molecular responses elicited by acute and chronic hypoxia in the cardiac myocytes. Special emphasis is put on the cardioprotective effects of chronic hypoxia.

Targeting cardiac sympatho-vagal imbalance using gene transfer of nitric oxide synthase

Heightened sympathetic excitation and diminished parasympathetic suppression of heart rate, cardiac contractility and vascular tone are all associated with cardiovascular diseases such as hypertension and ischemic heart disease. This phenotype often exists before these disease states have been established and is a strong correlate of mortality in the population. However, the causal role of the autonomic phenotype in the development and maintenance of hypertension and myocardial ischemia remains a subject of debate, as are the mechanisms responsible for regulating sympathovagal balance. Emerging evidence suggests oxidative stress and reactive oxygen species (such as nitric oxide (NO) and superoxide) play important roles in the modulation of autonomic balance, but so far the most important sites of action of these ubiquitous signaling molecules are unclear. In many cases, these mediators have opposing effects in separate tissues rendering conventional pharmacological approaches non-efficacious. Novel techniques have recently been used to augment these signaling pathways experimentally in a targeted fashion to central autonomic nuclei, cardiac neurons, and myocytes using gene transfer of NO synthase. This review article discusses these recent advances in the understanding of the roles of NO and its oxidative metabolites on autonomic imbalance in models of cardiovascular disease.

cGMP regulated protein kinases (cGK)

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

Genesis and regulation of the heart automaticity

The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrical oscillations. The exact cascade of steps initiating the pacemaker cycle in automatic cells has not yet been entirely elucidated. Nevertheless, ion channels and intracellular Ca(2+) signaling are necessary for the proper setting of the pacemaker mechanism. Here, we review the current knowledge on the cellular mechanisms underlying the generation and regulation of cardiac automaticity. We discuss evidence on the functional role of different families of ion channels in cardiac pacemaking and review recent results obtained on genetically engineered mouse strains displaying dysfunction in heart automaticity. Beside ion channels, intracellular Ca(2+) release has been indicated as an important mechanism for promoting automaticity at rest as well as for acceleration of the heart rate under sympathetic nerve input. The potential links between the activity of ion channels and Ca(2+) release will be discussed with the aim to propose an integrated framework of the mechanism of automaticity.

Nitric oxide signaling and the regulation of myocardial function

Nitric oxide, which is produced endogenously within cardiac myocytes by three distinct isoforms of nitric oxide synthase, is a key regulator of myocardial function. This review will focus on the regulation of myocardial function by each nitric oxide synthase isoform during health and disease, with a specific emphasis on the proposed end-targets and signaling pathways.

Endothelial nitric oxide synthase decreases beta-adrenergic responsiveness via inhibition of the L-type Ca2+ current

Signaling via endothelial nitric oxide synthase (NOS3) limits the heart's response to beta-adrenergic (beta-AR) stimulation, which may be protective against arrhythmias. However, mechanistic data are limited. Therefore, we performed simultaneous measurements of action potential (AP, using patch clamp), Ca2+ transients (fluo 4), and myocyte shortening (edge detection). L-type Ca2+ current (ICa) was directly measured by the whole cell ruptured patch-clamp technique. Myocytes were isolated from wild-type (WT) and NOS3 knockout (NOS3-/-) mice. NOS3-/- myocytes exhibited a larger incidence of beta-AR (isoproterenol, 1 microM)-induced early afterdepolarizations (EADs) and spontaneous activity (defined as aftercontractions). We also examined ICa, a major trigger for EADs. NOS3-/- myocytes had a significantly larger beta-AR-stimulated increase in ICa compared with WT myocytes. In addition, NOS3-/- myocytes had a larger response to beta-AR stimulation compared with WT myocytes in Ca2+ transient amplitude, shortening amplitude, and AP duration (APD). We observed similar effects with specific NOS3 inhibition [L-N5-(1-iminoethyl)-ornithine (l-NIO), 10 microM] in WT myocytes as with NOS3 knockout. Specifically, l-NIO further increased isoproterenol-stimulated EADs and aftercontractions. l-NIO also further increased the isoproterenol-stimulated ICa, Ca2+ transient amplitude, shortening amplitude, and APD (all P < 0.05 vs isoproterenol alone). l-NIO had no effect in NOS3-/- myocytes. These results indicate that NOS3 signaling inhibits the beta-AR response by reducing ICa and protects against arrhythmias. This mechanism may play an important role in heart failure, where arrhythmias are increased and NOS3 expression is decreased.

eNOS deficient mice develop progressive cardiac hypertrophy with altered cytokine and calcium handling protein expression

Although studies have shown that endothelial nitric oxide synthase (eNOS) homozygous knockout mice (eNOS-/-) develop left ventricular (LV) hypertrophy, well compensated at least to 24 wks, uncertainty still exists as to the cardiac functional and molecular mechanistic consequences of eNOS deficiency at later time-points. To bridge the gap in existent data, we examined whole hearts from eNOS-/- and age-matched wild-type (WT) control mice ranging in age from 18 to 52 wks for macroscopic and microscopic histopathology, LV mRNA and protein expression using RNA Dot blots and Western blots, respectively, and LV function using isolated perfused work-performing heart preparations. Heart weight to body weight (HW/BW in mg/g) ratio increased significantly as eNOS-/- mice aged (82.2%, P < 0.001). Multi-focal replacement fibrosis and myocyte degeneration/death were first apparent in eNOS-/- mouse hearts at 40 wks. Progressive increases in LV atrial natriuretic factor (ANF) and alpha-skeletal actin mRNA levels both correlated significantly with increasing HW/BW ratio in aged eNOS-/- mice (r = 0.722 and r = 0.648, respectively; P < 0.001). At 52 wks eNOS-/- mouse hearts exhibited basal LV hypercontractility yet blunted beta adrenergic receptor (betaAR) responsiveness that coincided with a significant reduction in the LV ratio of phospholamban to sarcoplasmic reticulum Ca2+-ATPase-2a protein levels and was preceded by a significant upregulation in LV steady-state mRNA and protein levels of the 28 kDa membrane-bound form of tumor necrosis factor-alpha. We conclude that absence of eNOS in eNOS-/- mice results in a progressive concentric hypertrophic cardiac phenotype that is functionally compensated with decreased betaAR responsiveness, and is associated with a potential cytokine-mediated alteration of calcium handling protein expression.

Mechanisms of the negative inotropic effects of sphingosine-1-phosphate on adult mouse ventricular myocytes

Sphingosine-1-phosphate (S1P) induces a transient bradycardia in mammalian hearts through activation of an inwardly rectifying K(+) current (I(K(ACh))) in the atrium that shortens action potential duration (APD) in the atrium. We have investigated probable mechanisms and receptor-subtype specificity for S1P-induced negative inotropy in isolated adult mouse ventricular myocytes. Activation of S1P receptors by S1P (100 nM) reduced cell shortening by approximately 25% (vs. untreated controls) in field-stimulated myocytes. S1P(1) was shown to be involved by using the S1P(1)-selective agonist SEW2871 on myocytes isolated from S1P(3)-null mice. However, in these myocytes, S1P(3) can modulate a somewhat similar negative inotropy, as judged by the effects of the S1P(1) antagonist VPC23019. Since S1P(1) activates G(i) exclusively, whereas S1P(3) activates both G(i) and G(q), these results strongly implicate the involvement of mainly G(i). Additional experiments using the I(K(ACh)) blocker tertiapin demonstrated that I(K(ACh)) can contribute to the negative inotropy following S1P activation of S1P(1) (perhaps through G(ibetagamma) subunits). Mathematical modeling of the effects of S1P on APD in the mouse ventricle suggests that shortening of APD (e.g., as induced by I(K(ACh))) can reduce L-type calcium current and thus can decrease the intracellular Ca(2+) concentration ([Ca(2+)](i)) transient. Both effects can contribute to the observed negative inotropic effects of S1P. In summary, these findings suggest that the negative inotropy observed in S1P-treated adult mouse ventricular myocytes may consist of two distinctive components: 1) one pathway that acts via G(i) to reduce L-type calcium channel current, blunt calcium-induced calcium release, and decrease [Ca(2+)](i); and 2) a second pathway that acts via G(i) to activate I(K(ACh)) and reduce APD. This decrease in APD is expected to decrease Ca(2+) influx and reduce [Ca(2+)](i) and myocyte contractility.

Kinin B1 receptor participates in the control of cardiac function in mice

The kinins have an important role in control of the cardiovascular system. They have been associated with protective effects in the heart tissue. Kinins act through stimulation of two 7-transmembrane G protein-coupled receptors, denoted B(1) and B(2) receptors. However, the physiological relevance of B(1) receptor in the heart has not been clearly established. Using B(1) kinin receptor gene knock-out mice we tested the hypothesis that the B(1) receptor plays an important role in the control of baseline cardiac function. We examined the functional aspects of the intact heart and also in the isolated cardiomyocytes to study intracellular Ca(2+) cycling by using confocal microscopy and whole-cell voltage clamp techniques. We measured heart rate, diastolic and systolic tension, contraction and relaxation rates and, coronary perfusion pressure. Whole-cell voltage clamp was performed to measure L-type Ca(2+) current (I(Ca,L)). The hearts from B(1)(-/-) mice showed smaller systolic tension. The average values for WT and B(1)(-/-) mice were 2.6+/-0.04 g vs. 1.6+/-0.08 g, respectively. This result can be explained, at least in part, by the decrease in the Ca(2+) transient (3.1+/-0.06 vs. 3.4+/-0.09 for B(1)(-/-) and WT, respectively). There was an increase in I(Ca,L) at depolarized membrane potentials. Interestingly, the inactivation kinetics of I(Ca,L) was statistically different between the groups. The coronary perfusion pressure was higher in the hearts from B(1)(-/-) mice indicating an increase in coronary resistance. This result can be explained by the significant reduction of eNOS (NOS-3) expression in the aorta of B(1)(-/-) mice. Collectively, our results demonstrate that B(1) receptor exerts a fundamental role in the mammalian cardiac function.

Mechanisms involved in the regulation of mRNA for M2 muscarinic acetylcholine receptors and endothelial and neuronal NO synthases in rat atria

BACKGROUND AND PURPOSE

Agonists of the M(2) muscarinic acetylcholine receptor (mAChR) increase mRNA for this receptor and mRNA for endothelial and neuronal isoforms of NO synthase (eNOS or nNOS). Here we examine the different signalling pathways involved in such events in rat cardiac atria.

EXPERIMENTAL APPROACH

In isolated atria, the effects of carbachol on mRNA for M(2) receptors, eNOS and nNOS were measured along with changes in phosphoinositide (PI) turnover, translocation of protein kinase C (PKC), NOS activity and atrial contractility.

KEY RESULTS

Carbachol increased mRNA for M(2) receptors, activation of PI turnover, translocation of PKC and NOS activity and decreased atrial contractility. Inhibitors of phospholipase C (PLC), calcium/calmodulin (CaM), NOS and PKC prevented the carbachol-dependent increase in mRNA for M(2) receptors. These inhibitors also attenuated the carbachol induced increase in nNOS- and eNOS-mRNA levels. Inhibition of nNOS shifted the dose response curve of carbachol on contractility to the right, whereas inhibition of eNOS shifted it to the left.

CONCLUSIONS AND IMPLICATIONS

From our results, activation of M(2) receptors induced nNOS and eNOS expression and activation of NOS up-regulated M(2) receptor gene expression. The signalling pathways involved included stimulation of PI turnover via PLC activation, CaM and PKC. nNOS and eNOS mediated opposing effects on the negative inotropic effect in atria, induced by stimulation of M(2) receptors. These results may contribute to a better understanding of the effects and side effects of cholinomimetic treatment in patients with cardiac neuromyopathy.

The emerging role of neuronal nitric oxide synthase in the regulation of myocardial function

The recent discovery of a NOS1 gene product (i.e. a neuronal-like isoform of nitric oxide synthase or nNOS) in the mammalian left ventricular (LV) myocardium has provided a new key for the interpretation of the complex experimental evidence supporting a role for myocardial constitutive nitric oxide (NO) production in the regulation of basal and beta-badrenergic cardiac function. Importantly, nNOS gene deletion has been associated with more severe LV remodelling and functional deterioration in murine models of myocardial infarction, suggesting that nNOS-derived NO may also be involved in the myocardial response to injury. To date, the mechanisms by which nNOS influences myocardial pathophysiology remain incompletely understood. In particular, it seems over simplistic to assume that all aspects of the myocardial phenotype of nNOS knockout (nNOS(-/-)) mice are a direct consequence of lack of NO production from this source. Emerging data showing co-localisation of xanthine oxidoreductase (XOR) and nNOS in the sarcoplasmic reticulum of rodents, and increased XOR activity in the nNOS(-/-) myocardium, suggest that nNOS gene deletion may have wider implications on the myocardial redox state. Similarly, the mechanisms regulating the targeting of myocardial nNOS to different subcellular compartments and the functional consequences of intracellular nNOS trafficking have not been fully established. Whether this information could be translated into a better understanding and management of human heart failure remains the most important challenge for future investigations.

Functional localization of cAMP signalling in cardiac myocytes

The cAMP pathway is of cardinal importance for heart physiology and pathology. The spatial organization of the various components of the cAMP pathway is thought to allow the segregation of functional responses triggered by the different neuromediators and hormones that use this pathway. PDEs (phosphodiesterases) hydrolyse cAMP (and cGMP) and play a major role in this process by preventing cAMP diffusion to the whole cytosol and inadequate target activation. The development of olfactory cyclic nucleotide-gated channels to directly monitor cAMP beneath the plasma membrane in real time allows us to gain new insights into the molecular mechanisms responsible for cAMP homoeostasis and hormonal specificity in cardiac cells. The present review summarizes the recent results we obtained using this approach in adult rat ventricular myocytes. In particular, the role of PDEs in the maintenance of specific cAMP signals generated by beta-adrenergic receptors and other G(s)-coupled receptors will be discussed.

Depression of cardiac L-type calcium current by a scorpion venom fraction M1 following muscarinic receptors interaction involving adenylate cyclase pathway

The effects of a non-toxic fraction, called M1, from Buthus occitanus tunetanus (Bot) scorpion were studied on rat cardiac contraction and calcium transient and current. A decrease in both rate and tension on isolated intact hearts as well as in calcium transient induced by depolarizing 100 K(+) solution on isolated ventricular cardiomyocytes was firstly observed. Studies with the whole cell patch clamp method showed that M1 decreased the L-type calcium current (ICa(L)) in a dose-dependent manner with an IC50 of 0.36 microg/mL and a Hill coefficient of 0.95. This effect was blocked and reversed by the specific muscarinic receptors antagonist atropine, 1 microM, and was completely prevented when cardiomyocytes were pretreated with Pertussis toxin, 1 microg/mL, to block the alpha subunit of the PTX-sensitive G proteins. These results show that M1 fraction of Bot inhibits basal calcium current by interacting with muscarinic receptors and suggest that this inhibition could be attributed to inhibition of adenylate cyclase activity by a mechanism involving PTX-sensitive G proteins.

Reactive Oxygen Species and Autonomic Regulation of Cardiac Excitability

Sympathetic hyper-activity and diminished parasympathetic activity are a consequence of many primary cardiovascular disease states and can trigger arrhythmias. Emerging evidence suggests that reactive oxygen species (ROS) including nitric oxide, superoxide, and peroxynitrite may contribute to cardiac sympathovagal imbalance in the brainstem, peripheral neurons, and in cardiomyocytes since all experience increased oxidative stress as a result of cardiac disease processes and aging. This article reviews the roles of ROS in autonomic dysfunction and arrhythmia. In addition, novel research directed toward finding targets for modulating sympathovagal balance in cardiac disease is discussed.

Direct positive chronotropic action by angiotensin II in the isolated mouse atrium

We observed the direct positive chronotropic effect of angiotensin II in mouse atria and characterized its pharmacological property. C57BL/6J mice were anesthetized with pentobarbital and hearts were quickly excised. Atrial preparations including right and left atrium were isolated and suspended in the organ bath filled with Krebs-Henseleit solution gassed with 95% O2 and 5% CO2. Angiotensin II at concentrations of 10(-10) to 10(-6) M caused concentration-dependent increase in heart rate, and the maximal response was about 13% of that by isoproterenol. The effect was blocked by the selective AT1-receptor antagonist, losartan at concentrations of 10(-6) M, but not by the selective beta-blocker, nadolol at concentration of 10(-5) M. Furthermore, angiotensin I also caused concentration-dependent increase in heart rate, and the effect was blocked by angiotensin converting enzyme (ACE) inhibitor, captopril at concentrations of 10(-6) M. These results suggested that angiotensin I is converted to angiotensin II via ACE system in mice atria, and regulate heart rate through AT1-receptor stimulation, not by beta-adrenergic receptor.

Function of cGMP-Dependent Protein Kinases as Revealed by Gene Deletion

Over the past few years, a wealth of biochemical and functional data have been gathered on mammalian cGMP-dependent protein kinases (cGKs). In mammals, three different kinases are encoded by two genes. Mutant and chimeric cGK proteins generated by molecular biology techniques yielded important biochemical knowledge, such as the function of the NH2-terminal domains of cGKI and cGKII, the identity of the cGMP-binding sites of cGKI, and the substrate specificity of the enzymes. Genetic approaches have proven especially useful for the analysis of the biological functions of cGKs. Recently, some of the in vivo targets and mechanisms leading to changes in neuronal adaptation, smooth muscle relaxation and growth, intestinal water secretion, bone growth, renin secretion, and other important functions have been identified. These data show that cGKs are signaling molecules involved in many biological functions.