Muscarinic cholinergic inhibition of beta-adrenergic stimulation of phospholamban phosphorylation and Ca2+ transport in guinea pig ventricles

Regulation of myocardial contraction as revealed by intracellular Ca2+ measurements using aequorin

Of the ions involved in myocardial function, Ca2+ is the most important. Ca2+ is crucial to the process that allows myocardium to repeatedly contract and relax in a well-organized fashion; it is the process called excitation-contraction coupling. In order, therefore, for accurate comprehension of the physiology of the heart, it is fundamentally important to understand the detailed mechanism by which the intracellular Ca2+ concentration is regulated to elicit excitation-contraction coupling. Aequorin was discovered by Shimomura, Johnson and Saiga in 1962. By taking advantage of the fact that aequorin emits blue light when it binds to Ca2+ within the physiologically relevant concentration range, in the 1970s and 1980s, physiologists microinjected it into myocardial preparations. By doing so, they proved that Ca2+ transients occur upon membrane depolarization, and tension development (i.e., actomyosin interaction) subsequently follows, dramatically advancing the research on cardiac excitation-contraction coupling.

Protein kinase A: A potential marker of sympathovagal imbalance in heart failure

Mitigation of cardiac autonomic dysregulation by neuromodulation technologies is emerging as a new therapeutic modality of heart failure (HF). This recent progress has necessitated the identification of a biomarker for the quantification of sympathovagal balance, the potential target of 'neuromodulation' strategies. The currently available autonomic nervous system assessment parameters do not truly reflect the sympathovagal balance of the ventricle. Protein kinase A (PKA) is an intracellular enzyme that plays a major role in the pathophysiology of functional and structural ventricular remodeling in HF. Interestingly, sympathetic and parasympathetic activations exert reciprocal influence on the activity of PKA. The current review attempts to evaluate the potential concept and feasibility of using in vitro assessment of PKA activity as a marker of sympathovagal balance in HF.

Autonomic Regulation of the Goldfish Intact Heart

Autonomic regulation plays a central role in cardiac contractility and excitability in numerous vertebrate species. However, the role of autonomic regulation is less understood in fish physiology. Here, we used Goldfish as a model to explore the role of autonomic regulation. A transmural electrocardiogram recording showed perfusion of the Goldfish heart with isoproterenol increased the spontaneous heart rate, while perfusion with carbamylcholine decreased the spontaneous heart rate. Cardiac action potentials obtained via sharp microelectrodes exhibited the same modifications of the spontaneous heart rate in response to isoproterenol and carbamylcholine. Interestingly, the duration of the cardiac action potentials lengthened in the presence of both isoproterenol and carbamylcholine. To evaluate cardiac contractility, the Goldfish heart was perfused with the Ca²⁺ indicator Rhod-2 and ventricular epicardial Ca²⁺ transients were measured using Pulsed Local Field Fluorescence Microscopy. Following isoproterenol perfusion, the amplitude of the Ca²⁺ transient significantly increased, the half duration of the Ca²⁺ transient shortened, and there was an observable increase in the velocity of the rise time and fall time of the Ca²⁺ transient, all of which are compatible with the shortening of the action potential induced by isoproterenol perfusion. On the other hand, carbamylcholine perfusion significantly reduced the amplitude of the Ca²⁺ transient and increased the half duration of the Ca²⁺ transient. These results are interesting because the effect of carbamylcholine is opposite to what happens in classically used models, such as mouse hearts, and the autonomic regulation of the Goldfish heart is strikingly similar to what has been observed in larger mammalian models resembling humans.

Multisite phosphorylation of the cardiac ryanodine receptor: a random or coordinated event?

Many proteins are phosphorylated at more than one phosphorylation site to achieve precise tuning of protein function and/or integrate a multitude of signals into the activity of one protein. Increasing the number of phosphorylation sites significantly broadens the complexity of molecular mechanisms involved in processing multiple phosphorylation sites by one or more distinct kinases. The cardiac ryanodine receptor (RYR2) is a well-established multiple phospho-target of kinases activated in response to β-adrenergic stimulation because this Ca²⁺ channel is a critical component of Ca²⁺ handling machinery which is responsible for β-adrenergic enhancement of cardiac contractility. Our review presents a selective overview of the extensive, often conflicting, literature which focuses on identifying reliable lines of evidence to establish if multiple RYR2 phosphorylation is achieved randomly or in a specific sequence, and whether phosphorylation at individual sites is functionally specific and additive or similar and can therefore be substituted.

Transmural Autonomic Regulation of Cardiac Contractility at the Intact Heart Level

The relationship between cardiac excitability and contractility depends on when Ca²⁺ influx occurs during the ventricular action potential (AP). In mammals, it is accepted that Ca²⁺ influx through the L-type Ca²⁺ channels occurs during AP phase 2. However, in murine models, experimental evidence shows Ca²⁺ influx takes place during phase 1. Interestingly, Ca²⁺ influx that activates contraction is highly regulated by the autonomic nervous system. Indeed, autonomic regulation exerts multiple effects on Ca²⁺ handling and cardiac electrophysiology. In this paper, we explore autonomic regulation in endocardial and epicardial layers of intact beating mice hearts to evaluate their role on cardiac excitability and contractility. We hypothesize that in mouse cardiac ventricles the influx of Ca²⁺ that triggers excitation–contraction coupling (ECC) does not occur during phase 2. Using pulsed local field fluorescence microscopy and loose patch photolysis, we show sympathetic stimulation by isoproterenol increased the amplitude of Ca²⁺ transients in both layers. This increase in contractility was driven by an increase in amplitude and duration of the L-type Ca²⁺ current during phase 1. Interestingly, the β-adrenergic increase of Ca²⁺ influx slowed the repolarization of phase 1, suggesting a competition between Ca²⁺ and K⁺ currents during this phase. In addition, cAMP activated L-type Ca²⁺ currents before SR Ca²⁺ release activated the Na⁺-Ca²⁺ exchanger currents, indicating Ca_v1.2 channels are the initial target of PKA phosphorylation. In contrast, parasympathetic stimulation by carbachol did not have a substantial effect on amplitude and kinetics of endocardial and epicardial Ca²⁺ transients. However, carbachol transiently decreased the duration of the AP late phase 2 repolarization. The carbachol-induced shortening of phase 2 did not have a considerable effect on ventricular pressure and systolic Ca²⁺ dynamics. Interestingly, blockade of muscarinic receptors by atropine prolonged the duration of phase 2 indicating that, in isolated hearts, there is an intrinsic release of acetylcholine. In addition, the acceleration of repolarization induced by carbachol was blocked by the acetylcholine-mediated K⁺ current inhibition. Our results reveal the transmural ramifications of autonomic regulation in intact mice hearts and support our hypothesis that Ca²⁺ influx that triggers ECC occurs in AP phase 1 and not in phase 2.

How Ca2+ influx is attenuated in the heart during a "fight or flight" response

Bazmi and Escobar highlight a recent investigation of the mechanisms that regulate Ca²⁺ influx during sympathetic stimulation.

Phospholamban and cardiac function: a comparative perspective in vertebrates

Phospholamban (PLN) is a small phosphoprotein closely associated with the cardiac sarcoplasmic reticulum (SR). Dephosphorylated PLN tonically inhibits the SR Ca-ATPase (SERCA2a), while phosphorylation at Ser16 by PKA and Thr17 by Ca(2+) /calmodulin-dependent protein kinase (CaMKII) relieves the inhibition, and this increases SR Ca(2+) uptake. For this reason, PLN is one of the major determinants of cardiac contractility and relaxation. In this review, we attempted to highlight the functional significance of PLN in vertebrate cardiac physiology. We will refer to the huge literature on mammals in order to describe the molecular characteristics of this protein, its interaction with SERCA2a and its role in the regulation of the mechanic and the electric performance of the heart under basal conditions, in the presence of chemical and physical stresses, such as β-adrenergic stimulation, response to stretch, force-frequency relationship and intracellular acidosis. Our aim is to provide the basis to discuss the role of PLN also on the cardiac function of nonmammalian vertebrates, because so far this aspect has been almost neglected. Accordingly, when possible, the literature on PLN will be analysed taking into account the nonuniform cardiac structural and functional characteristics encountered in ectothermic vertebrates, such as the peculiar and variable organization of the SR, the large spectrum of response to stresses and the disaptive absence of crucial proteins (i.e. haemoglobinless and myoglobinless species).

Genetic disruption of G proteins, G(i2)alpha or G(o)alpha, does not abolish inotropic and chronotropic effects of stimulating muscarinic cholinoceptors in atrium

BACKGROUND AND PURPOSE

Classically, stimulation of muscarinic cholinoceptors exerts negative inotropic and chronotropic effects in the atrium of mammalian hearts. These effects are crucial to the vagal regulation of the heart beat. This effect is assumed to be mediated via GTP binding (G) proteins, because they can be abolished by Pertussis toxin. However, it is unknown which G proteins are involved.

EXPERIMENTAL APPROACH

We studied contractility in isolated left or right atrium from genetically manipulated mice with deletion of one of two G proteins, either of the alpha subunit of G(i2) protein (G(i2)alpha) or of the alpha subunit of G(o) protein (G(o)alpha). Preparations were stimulated with carbachol alone or after pretreatment with the beta-adrenoceptor agonist isoprenaline. For comparison, the effects of carbachol on L-type Ca(2+)-channels in isolated ventricular cardiomyocytes were studied.

KEY RESULTS

The negative inotropic and chronotropic effects of carbachol alone or in the presence of isoprenaline were identical in atria from knockout or wild-type mice. However, the effect of carbachol on isoprenaline-activated L-type Ca(2+)-channel in isolated ventricular cardiomyocytes was greatly attenuated in both types of knockout mice studied.

CONCLUSIONS AND IMPLICATIONS

These data imply that there is either redundancy of G proteins for signal transduction or that Pertussis toxin-sensitive proteins other than G(i2)alpha and G(o)alpha mediate the vagal stimulation in the atrium. Moreover, different G proteins mediate the effect of carbachol in ventricle compared with atrium.

Simulation analysis of intracellular Na+ and Cl− homeostasis during β1-adrenergic stimulation of cardiac myocyte

To quantitatively understand intracellular Na+ and Cl- homeostasis as well as roles of Na+/K+ pump and cystic fibrosis transmembrane conductance regulator Cl- channel (ICFTR) during the beta1-adrenergic stimulation in cardiac myocyte, we constructed a computer model of beta1-adrenergic signaling and implemented it into an excitation-contraction coupling model of the guinea-pig ventricular cell, which can reproduce membrane excitation, intracellular ion changes (Na+, K+, Ca2+ and Cl-), contraction, cell volume, and oxidative phosphorylation. An application of isoproterenol to the model cell resulted in the shortening of action potential duration (APD) after a transient prolongation, the increases in both Ca2+ transient and cell shortening, and the decreases in both Cl- concentration and cell volume. These results are consistent with experimental data. Increasing the density of ICFTR shortened APD and augmented the peak amplitudes of the L-type Ca2+ current (ICaL) and the Ca2+ transient during the beta1-adrenergic stimulation. This indirect inotropic effect was elucidated by the increase in the driving force of ICaL via a decrease in plateau potential. Our model reproduced the experimental data demonstrating the decrease in intracellular Na+ during the beta-adrenergic stimulation at 0 or 0.5 Hz electrical stimulation. The decrease is attributable to the increase in Na+ affinity of Na+/K+ pump by protein kinase A. However it was predicted that Na+increases at higher beating rate because of larger Na+ influx through forward Na+/Ca2+ exchange. It was demonstrated that dynamic changes in Na+ and Cl- fluxes remarkably affect the inotropic action of isoproterenol in the ventricular myocytes.

Lack of role for nitric oxide in cholinergic modulation of myocardial contractility in vivo

Despite intensive investigation, the role of nitric oxide (NO) in cholinergic modulation of myocardial contractility remains unresolved. The left anterior descending coronary artery of 34 anesthetized, open-chest dogs was perfused via an extracorporeal circuit. Segmental shortening (SS) was measured with ultrasonic crystals and coronary blood flow (CBF) was measured with an ultrasonic flow transducer. An intracoronary infusion of ACh (20 microg/min) was performed, with CBF held constant, under baseline and during dobutamine, CaCl(2), or amrinone at doses increasing SS by approximately 50% (10 microg/min, 15 mg/min, and 300 microg/min ic, respectively). ACh-induced responses during dobutamine were also assessed following treatment with the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 300 microg/min ic for 15 min). The effects of sodium nitroprusside (SNP; 80 microg/min ic), an exogenous NO donor, bradykinin (2.5 microg/min ic), a nonmuscarinic releaser of endothelial NO, and bilateral vagal stimulation (before and after L-NAME) were evaluated during dobutamine. ACh had no effect on SS under baseline or during CaCl(2), but it decreased SS during dobutamine or amrinone (-23 +/- 4% and -30 +/- 5%, respectively). Vagal stimulation also reduced SS during dobutamine. L-NAME did not alter the ACh- or vagal-induced decreases in SS during dobutamine. Neither SNP nor bradykinin affected SS during dobutamine. In conclusion, ACh and vagal stimulation have a negative inotropic effect during stimulation of the beta-adrenergic receptors that is independent of NO. The persistence of this effect during amrinone suggests that a mechanism downstream from adenylate cyclase is involved.

Inhibition of spontaneous beta 2-adrenergic activation rescues beta 1-adrenergic contractile response in cardiomyocytes overexpressing beta 2-adrenoceptor

Cardiac-specific overexpression of the human beta(2)-adrenergic receptor (AR) in transgenic mice (TG4) enhances basal cardiac function due to ligand-independent spontaneous beta(2)-AR activation. However, agonist-mediated stimulation of either beta(1)-AR or beta(2)-AR fails to further enhance contractility in TG4 ventricular myocytes. Although the lack of beta(2)-AR response has been ascribed to an efficient coupling of the receptor to pertussis toxin-sensitive G(i) proteins in addition to G(s), the contractile response to beta(1)-AR stimulation by norepinephrine and an alpha(1)-adrenergic antagonist prazosin is not restored by pertussis toxin treatment despite a G(i) protein elevation of 1.7-fold in TG4 hearts. Since beta-adrenergic receptor kinase, betaARK1, activity remains unaltered, the unresponsiveness of beta(1)-AR is not caused by betaARK1-mediated receptor desensitization. In contrast, pre-incubation of cells with anti-adrenergic reagents such as muscarinic receptor agonist, carbachol (10(-5)m), or a beta(2)-AR inverse agonist, ICI 118,551 (5 x 10(-7)m), to abolish spontaneous beta(2)-AR signaling, both reduce the base-line cAMP and contractility and, surprisingly, restore the beta(1)-AR contractile response. The "rescued" contractile response is completely reversed by a beta(1)-AR antagonist, CGP 20712A. Furthermore, these results from the transgenic animals are corroborated by in vitro acute gene manipulation in cultured wild type adult mouse ventricular myocytes. Adenovirus-directed overexpression of the human beta(2)-AR results in elevated base-line cAMP and contraction associated with a marked attenuation of beta(1)-AR response; carbachol pretreatment fully revives the diminished beta(1)-AR contractile response. Thus, we conclude that constitutive beta(2)-AR activation induces a heterologous desensitization of beta(1)-ARs independent of betaARK1 and G(i) proteins; suppression of the constitutive beta(2)-AR signaling by either a beta(2)-AR inverse agonist or stimulation of the muscarinic receptor rescues the beta(1)-ARs from desensitization, permitting agonist-induced contractile response.

Protein dephosphorylation rates in myocytes after isoproterenol withdrawal

Dephosphorylation of substrates for cyclic AMP-dependent protein kinase is essential for reversing the effects of this enzyme. It has been proposed that the relevant phosphatase(s) is stimulated by muscarinic cholinergic agonists, thereby accentuating cholinergic antagonism of beta-adrenergic agonists in the heart. To test this hypothesis, dephosphorylation of the three major substrates of cardiac cyclic AMP-dependent protein kinase (phospholamban, troponin-I, and C-protein) was examined. In isolated myocytes, isoproterenol caused concentration-dependent phosphorylation of these three proteins. Simultaneous exposure to acetylcholine with the isoproterenol caused a rightward shift in the concentration-response curve that was similar for protein phosphorylation in myocytes and for the inotropic response of the intact heart. The addition of propranolol after exposure to isoproterenol resulted in protein dephosphorylation, the onset of which was accelerated by acetylcholine. However, acetylcholine did not affect the rate of dephosphorylation for any of the proteins, indicating that phosphatase activity in cardiac muscle is not enhanced by acetylcholine.

Phosphatase inhibitor cantharidin blocks adenosine A(1) receptor anti-adrenergic effect in rat cardiac myocytes

Experiments were performed to examine whether the protein phosphatase inhibitor cantharidin blocks the anti-adrenergic effect of adenosine A(1) receptor stimulation. In electrically stimulated adult rat ventricular myocytes loaded with the intracellular calcium concentration ([Ca(2+)](i)) indicator fluo-3, isoproterenol (10 nM) increased systolic [Ca(2+)](i) by 46%, increased twitch amplitude by 56%, and increased total cellular cAMP content by 140%. The adenosine A(1) receptor agonist 2-chloro-N(6)-cyclopentlyadenosine (CCPA) reduced isoproterenol-stimulated [Ca(2+)](i) and contractility by 87 and 80%, respectively, but reduced cAMP content by only 18%. Cantharidin had no effects on myocyte [Ca(2+)](i), contractility, or cAMP in the absence or presence of isoproterenol but blocked the effects of CCPA on [Ca(2+)](i) and contractility by approximately 44%. Cantharidin had no effect on CCPA attenuation of isoproterenol-induced increases in cAMP. Pretreatment with CCPA also reduced the increase in contractile parameters produced by the direct cAMP-dependent protein kinase A (PKA) activator 8-bromocAMP. These results suggest that activation of protein phosphatases mediate, in part, the anti-adrenergic effect of adenosine A(1) receptor activation in ventricular myocardium.

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

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

Dissociation of the effects of forskolin and dibutyryl cAMP on force of contraction and phospholamban phosphorylation in human heart failure

Forskolin and dibutyryl cyclic adenosine monophosphate (cAMP) stimulate force of contraction independent of beta-adrenoceptor stimulation. We studied their effects on force of contraction and phosphorylation of regulatory proteins in isolated electrically driven trabeculae carneae from failing human ventricles. The phosphorylation state of the regulatory protein phospholamban was studied because its phosphorylation usually faithfully follows contractility. For comparison, the phosphorylation state of the inhibitory subunit of troponin was studied. The phosphorylation state was inferred from in vitro phosphorylation of homogenates with cAMP-dependent protein kinase in the presence of radioactive gamma[32P]ATP Proteins were separated by electrophoresis, and radioactivity in the proteins of interest was quantified. The maximal positive inotropic effects occurred at 30 microM forskolin and were attenuated in comparison with the maximal effects to dibutyryl cAMP (1 mM). Both forskolin and dibutyryl cAMP enhanced phospholamban phosphorylation. However, phospholamban phosphorylation in intact trabeculae treated with 30 microM forskolin and 1 mM dibutyryl cAMP was comparable. It is suggested that phospholamban phosphorylation can be dissociated from inotropy at least in isolated trabeculae from failing human hearts.

Phospholamban: protein structure, mechanism of action, and role in cardiac function

A comprehensive discussion is presented of advances in understanding the structure and function of phospholamban (PLB), the principal regulator of the Ca2+-ATPase of cardiac sarcoplasmic reticulum. Extensive historical studies are reviewed to provide perspective on recent developments. Phospholamban gene structure, expression, and regulation are presented in addition to in vitro and in vivo studies of PLB protein structure and activity. Applications of breakthrough experimental technologies in identifying PLB structure-function relationships and in defining its interaction with the Ca2+-ATPase are also highlighted. The current leading viewpoint of PLB's mechanism of action emerges from a critical examination of alternative hypotheses and the most recent experimental evidence. The potential physiological relevance of PLB function in human heart failure is also covered. The interest in PLB across diverse biochemical disciplines portends its continued intense scrutiny and its potential exploitation as a therapeutic target.

Deferoxamine blocks interactions of fluoride and carbachol in isolated mammalian cardiac preparations

In papillary muscles, carbachol reduced the positive inotropic effects of isoprenaline (10 nmol/l). The negative inotropic effects of carbachol in isoprenaline-stimulated guinea pig papillary muscles were attenuated by additionally applied sodium fluoride (3 mmol/l). These effects of sodium fluoride were blocked by deferoxamine (200 micromol/l). In guinea pig left atria, sodium fluoride alone greatly reduced force of contraction. These effects in atria were blocked by 200 micromol/l deferoxamine, and positive inotropic effects of sodium fluoride were observed. It is suggested that the cardiac effects of muscarinic M2 receptor agonists in the ventricle involve, at least in part, the activation of phosphatases which are blocked by fluoride and reactivated by deferoxamine.

Postnatal changes in contractile time parameters, calcium regulatory proteins, and phosphatases

Compared with isolated electrically driven neonatal ventricular preparations, the total time of contraction, the time to peak tension, and the time of relaxation were decreased to approximately 50% in adult ventricular preparations. The expression of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) was increased to 133% at the protein level and to 154% at the mRNA level in adult vs. neonatal ventricular preparations, whereas phospholamban was unchanged at both the protein and mRNA levels. Moreover, Ca2+ uptake was increased to 180% in adult vs. neonatal ventricular preparations. Phospholamban phosphorylation was enhanced in adult vs. neonatal ventricular preparations. In adult ventricular preparations, phosphatase activity was reduced to 53% of neonatal preparations, the protein levels of the immunologically detectable catalytic subunits of protein phosphatase types 1 and 2A were reduced to 28 and 61% of neonatal preparations, respectively, and the mRNA levels of type 1alpha, 1beta, 1gamma, 2Aalpha, and 2Abeta phosphatase isoforms were decreased to 69, 68, 54, 67, and 63%, respectively. We conclude that in the adult rat heart, the shortened time parameters of contraction can be explained by an elevated expression of SERCA. In addition, an increased phosphorylation state of phospholamban due to reduced phosphatase activity may be involved.

Cardiac protein phosphorylation: functional and pathophysiological correlates

Protein phosphorylation acts a pivotal mechanism in regulating the contractile state of the heart by modulating particular levels of autonomic control on cardiac force/length relationships. Early studies of changes in cardiac protein phosphorylation focused on key components of the excitation-coupling process, namely phospholamban of the sarcoplasmic reticulum and myofibrillar troponin I. In more recent years the emphasis has shifted towards the identification of other phosphoproteins, and more importantly, the delineation of the mechanistic and signaling pathways regulating the various known phosphoproteins. In addition to cAMP- and Ca(2+)-calmodulin-dependent kinase processes, these have included regulation by protein kinase C and the ever-emerging family of growth factor-related kinases such as the tyrosine-, mitogen- and stress-activated protein kinases. Similarly, the role of protein dephosphorylation by protein phosphatases has been recognized as integral in modulating normal cardiac cellular function. Recent studies involving a variety of cardiovascular pathologies have demonstrated that changes in the phosphorylation states of key cardiac regulatory proteins may underlie cardiac dysfunction in disease states. The emphasis of this comprehensive review will be on discussing the role of cardiac phosphoproteins in regulating myocardial function and pathophysiology based not only on in vitro data, but more importantly, from ex vivo experiments with corroborative physiological and biochemical evidence.

Transgenic approaches to define the functional role of dual site phospholamban phosphorylation

Phospholamban is a critical regulator of the sarcoplasmic reticulum Ca2+-ATPase activity and myocardial contractility. Phosphorylation of phospholamban occurs on both Ser16 and Thr17 during isoproterenol stimulation. To determine the physiological significance of dual site phospholamban phosphorylation, we generated transgenic models expressing either wild-type or the Ser16 --> Ala mutant phospholamban in the cardiac compartment of the phospholamban knockout mice. Transgenic lines with similar levels of mutant or wild-type phospholamban were studied in parallel. Langendorff perfusion indicated that the basal hyperdynamic cardiac function of the knockout mouse was reversed to the same extent by reinsertion of either wild-type or mutant phospholamban. However, isoproterenol stimulation was associated with much lower responses in the contractile parameters of mutant phospholamban compared with wild-type hearts. These attenuated responses were due to lack of phosphorylation of mutant phospholamban, assessed in 32P labeling perfusion experiments. The lack of phospholamban phosphorylation in vivo was not due to conversion of Ser16 to Ala, since the mutated phospholamban form could serve as substrate for the calcium-calmodulin-dependent protein kinase in vitro. These findings indicate that phosphorylation of Ser16 is a prerequisite for Thr17 phosphorylation in phospholamban, and prevention of phosphoserine formation results in attenuation of the beta-agonist stimulatory responses in the mammalian heart.

Characterization of biochemical effects of CGS 21680C, an A2-adenosine receptor agonist, in the mammalian ventricle

Effects of a putative A2-adenosine receptor agonist 2-[(p-2-carboxyethyl)-phenethylamino]-5'-N-ethyl-carboxamide-adeno sine (CGS 21680C) on force of contraction, protein phosphorylation, cyclic adenosine monophosphate (cAMP) content, and the activity of phosphodiesterase (PDE) isoenzymes in guinea pig ventricular (GPV) preparations were studied. CGS 21680C (1-100 microM) did not affect force of contraction in isolated electrically driven papillary muscles and was ineffective in increasing phosphorylation of phospholamban (PLB) and the inhibitory subunit of troponin (TnI) in [32P]-labeled GPV cardiomyocytes. However, under the same conditions, CGS 21680C (10 microM) increased cAMP content from 4.3 +/- 0.2 to 13.0 +/- 0.6 pmol/mg protein, and this effect was completely abolished by A2-adenosine receptor antagonist 9-chloro-2-(2-furanyl)-5,6-dihydro-1,2,4-triazolo-(1,5-c)quinazolin++ +-5-imine (CGS 15943A). CGS 21680C (10 microM) inhibited PDE isoenzymes I, II, III, IV by 7.0, 8.3, 4.7, and 23.2%, respectively. Similarly, rolipram (100 microM), a selective PDE IV inhibitor, increased cAMP content from 4.4 +/- 0.3 to 7.2 +/- 0.3 pmol/mg protein without affecting the phosphorylation state of PLB and TnI. We conclude that CGS 21680C increases cAMP content in GPV cardiomyocytes by activation of adenylyl cyclase or in part by inhibition of PDE IV activity. The increase in cAMP content by CGS 21680C or rolipram is ineffective in increasing phosphorylation of PLB and TnI. These results support the concept of compartments for cAMP or protein kinase A or both in cardiomyocytes that are not coupled to phosphorylation and contractility.

Effects of adrenergic and muscarinic agonist stimulation on IP3 and cyclic nucleotide levels in the pressure overloaded rat heart

In this study, the dynamic interrelationships between myocardial functional state and changes in the second messenger content in pressure-overloaded hypertrophied hearts were investigated. Forty-three rat hearts were used after partial clamping of the abdominal aorta. The isolated hearts were perfused with Krebs-Henseleit buffer and allocated to perfusion for 20 s or 40 min as controls (n = 12); or with noradrenaline (10(-6) mol l-1, n = 11); carbachol (3 x 10(-7) mol l-1, n = 9); or noradrenaline plus carbachol (10(-6) mol l-1 + 3 x 10(-7) mol l-1, respectively, n = 11). maxdP/dt increased more than 2-fold already after 20 s on noradrenaline stimulation, followed by a significant increase in cAMP. After 40 min, maxdP/dt was lower than the maximal value, although higher than controls. cAMP was also decreased, but still significantly higher than controls. Perfusion with noradrenaline plus carbachol produced the same changes in maxdP/dt as those seen after noradrenaline stimulation alone, but failed to increase cAMP content after both 20 s and 40 min. The inositol trisphosphate (IP3) content was increased 40 min of control perfusion (p < 0.05). Noradrenaline and carbachol, separately, produced an increase in IP3 content already after 20 s (p < 0.05). The combination of noradrenaline plus carbachol also produced an increase of IP3 (p < 0.05; compared to controls), but to a lesser extent when compared either to noradrenaline or carbachol (p < 0.05). After 40 min of perfusion, IP3 was in the same range regardless of added agonist(s) and still slightly above control level (p < 0.05). The early increase in maxdP/dt induced by noradrenaline or the combination of noradrenaline plus carbachol was not paralleled by a decrease in ATP content. This was also the case upon addition of carbachol alone. However, after 40 min of agonistic perfusion, ATP levels were substantially decreased. In conclusion, myocardial IP3 content in pressure-overloaded hypertrophied hearts was not different from that of sham-operated hearts. After agonistic stimulation, an early increase in IP3 formation was seen. Attenuation of the IP3 response by combined stimulation with noradrenaline and carbachol was initially present in pressure-overloaded hypertrophied hearts. After 40 min no attenuation was found for either IP3 or for cAMP content, suggestive of induction of a desensitization.

Different sympathetic-parasympathetic interactions on sinus rate and atrioventricular conduction in dog hearts

We investigated the sympathetic-parasympathetic interactions involved in SA nodal pacemaker activity and AV conductivity in the anesthetized dog heart. Stimulation of the intracardiac parasympathetic nerves to the SA nodal region (SAPS) and stimulation of the intracardiac parasympathetic nerves to the AV nodal region (AVPS) induced negative chronotropic and dromotropic responses, respectively. Cardiac sympathetic stimulation, aminophylline, 3-isobutyl-1-methylxanthine (IBMX, a relatively pure nonselective phosphodiesterase inhibitor) and methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-p iridine-5-carboxylate (Bay k 8644, a Ca2+ channel agonist) increased sinus rate and decreased AV conduction time. Sympathetic stimulation augmented the negative chronotropic response to SAPS but not the negative dromotropic response to AVPS, IBMX augmented both responses, Bay k 8644 augmented the chronotropic response and attenuated the dromotropic response, and aminophylline did not affect the chronotropic response to SAPS and inhibited the dromotropic response to AVPS. Additionally, when Bay k 8644 directly given via the AV node artery decreased AV conduction time, it attenuated the negative dromotropic response to AVPS and carbachol injected into the AV node artery. These results suggest that the differential sympathetic-parasympathetic interactions on sinus rate and AV conduction are at least partly induced by an interaction between changes in slow inward Ca2+ current or intracellular Ca2+ and the cardiac effects of acetylcholine in the heart in situ.

Mechanism of action of sarcoplasmic reticulum calcium-uptake activators--discrimination between sarco(endo)plasmic reticulum Ca2+ ATPase and phospholamban interaction

The Ca2+ uptake by the sarcoplasmic reticulum (SR) can be affected by direct modulation of the Ca2+ pump or by removing the inhibitory effect of dephosphorylated phospholamban. The effect of these mechanisms was assessed using ellagic acid and 1-(3,4-dimethoxyphenyl)-3-dodecanone. Both compounds (30 micromol/l) enhanced SR-Ca2+ uptake in rabbit cardiomyocytes by 65.3 +/- 13% and 44.3 +/- 6.7% for 1-(3,4-dimethoxyphenyl)-3-dodecanone and ellagic acid, respectively (at pCa 6.2). A similar effect was observed in cardiac SR microsomes (59.5 +/- 7.4% and 45.1 +/- 6.7) with 30 micromol/l 1-(3,4-dimethodoxyphenyl)-3-dodecanone and ellagic acid, respectively. 1-(3,4-Dimethoxyphenyl)-3-dodecanone increased Ca2+ storage by cardiac SR microsomes mainly at high [Ca2+] with a 57% increase of Vmax, whereas ellagic acid increased Vmax to a smaller extent (22%) and stimulated Ca2+ uptake at lower [Ca2+] with a leftward-shift of the pCa/ATPase relationship by pCa 0.24. Ellagic acid also differed from 1-(3,4-dimethoxylphenyl)-3-dodecanone in that it produced a Ca2+ sensitizing effect only in cardiac SR microsomes (by pCa 0.3) whereas 1-(3,4-dimethoxyphenyl)-3-dodecanone stimulated the ATPase, at saturating Ca2+, in both cardiac and skeletal muscle SR vesicles. It is suggested that 1-(3,4-dimethoxyphenyl)-3-dodecanone stimulates directly the Ca2+-ATPase activity, in contrast to ellagic acid which enhances the cardiac SR-Ca2+ uptake by interacting with phospholamban, as confirmed by the lack of additive effect between ellagic acid and monoclonal antibodies raised against phospholamban. 1-(3,4-dimethoxyphenyl)-3-dodecanone and ellagic acid constitute attractive pharmacological tools to investigate the functional consequences of enhancing SR Ca2+, uptake by affecting different mechanisms.

Quantification of the cAMP response element binding protein in ventricular nuclear protein from failing and nonfailing human hearts

Alterations in the expression of myocardial regulatory proteins (e.g. beta-adrenoceptor, inhibitory G-proteins) in human heart failure are associated with excessive stimulation of the cAMP signalling pathway by endogenous catecholamines. The transcription factor cAMP response element binding protein (CREB) mediates cAMP-dependent transcriptional activation and is expressed in the human heart. Here, CREB protein was immunologically quantified in ventricular nuclear protein preparations from nonfailing donor hearts (n = 8) and from failing hearts transplanted due to dilative (n = 10) or ischemic cardiomyopathy (n = 6). CREB expression was unchanged in ventricular nuclei from failing hearts compared to the nonfailing controls suggesting that expressional alterations in human heart failure cannot be explained by altered expression of CREB.

In vivo evidence of positive inotropism of EMD 57033 through calcium sensitization

The previous separation of the racemic cardiotonic thiadiazinone derivative EMD 53998 yielded two enantiomers with different pharmacologic properties: EMD 57,033, a potent Ca2+ sensitizer with some residual phosphodiesterase III (PDE III) inhibition, and EMD 57,439, a pure PDE III inhibitor. Although numerous in vitro studies demonstrated the ability of EMD 57,033 to increase the responsiveness of cardiac contractile proteins to Ca2+, in vivo evidence for such an action is lacking. Because there is no possibility of directly proving Ca2+ sensitization in vivo, we attempted to exclude PDE III inhibition as a major contributing component of the positive inotropic action of EMD 57,033. In anesthetized rats, EMD 57,033 increased left ventricular (LV) first derivative of change in systolic pressure over time (dP/dt max) without affecting blood pressure. In contrast, the PDE III-inhibitory enantiomer EMD 57,439 decreased blood pressure. The pattern of hemodynamic effects in anesthetized dogs revealed similar differences between EMD 57,033 and PDE inhibitors. Thus the increase in LV dP/dt max in response to EMD 57,033 was not accompanied by changes of heart rate and blood pressure. As expected for PDE inhibitors, pimobendan and milrinone increased cardiac contractile force in dogs, concomitant, however, with tachycardia, hypotension, and a decrease in total peripheral resistance. When regional contractility was measured separately in two different areas of the dog myocardium, the positive inotropic action of the PDE inhibitors pimobendan and milrinone was antagonized by local coronary infusion of acetylcholine. The cardiotonic effect of the Ca2+ sensitizer EMD 57,033 was entirely resistant to inhibition by acetylcholine. In conscious dogs, beta-blockade markedly attenuated the increase in LV dP/dt max produced by two different doses of the PDE III inhibitor EMD 57,439. In contrast, a dose of EMD 57,033 equieffective in positive inotropic action with the lower dose of EMD 57,439 remained unaffected by < b tau-blockade. We concluded (a) that EMD 57,033 increases cardiac contractile force in two species in vivo, (b) that this action is independent of the cardiac cyclic adenosine monophosphate (AMP) system, (c) that EMD 57,033 does not reduce blood pressure and increase heart rate, an action indicative of PDE inhibition, and (d) that, on the basis of numerous previous in vitro findings, the mechanism of action of EMD 57,033, also in vivo, is consistent with sensitization of the cardiac myofibrils to Ca2+. Of special importance is the finding that this Ca2+ sensitizer at appropriate doses may be able to improve systolic function without adverse effects on diastolic function, as indicated by a slight decrease in left ventricular end-diastolic pressure.

Muscarinic regulation of the L-type calcium current in isolated cardiac myocytes

Muscarinic agonists regulate the L-type calcium current in isolated cardiac myocytes. The second messengers pathways involved in this regulation are discussed briefly, with particular emphasis on the involvement of cAMP and cGMP pathways.

Negative chronotropic and inotropic effects exerted by diadenosine hexaphosphate (AP6A) via A1-adenosine receptors

1. Diadenosine hexaphosphate (AP6A) exerts vasoconstrictive effects. The purpose of this study was to investigate whether AP6A has any effect on cardiac function. 2. The effects of AP6A (0.1-100 microM) on cardiac contractility and frequency were studied in guinea-pig and human isolated cardiac preparations. Furthermore, the effects of AP6A on the amplitude of the L-type calcium current, on the adenosine 3':5'-cyclic monophosphate (cyclic AMP) content and on the phosphorylation of regulatory phosphoproteins, i.e. phospholamban and troponin inhibitor, were investigated in guinea-pig isolated ventricular myocytes. 3. In isolated spontaneously beating right atria of the guinea-pig AP6A exerted a negative chronotropic effect and reduced the rate of contraction maximally by 35% (IC20 = 35 microM). 4. In isolated electrically driven left atria of the guinea-pig AP6A exerted a negative inotropic effect and reduced force of contraction maximally by 23% (IC20 = 70 microM). 5. In isolated electrically driven papillary muscles of the guinea-pig AP6A alone was ineffective, but attenuated isoprenaline-stimulated force of contraction maximally by 23% (IC20 = 60 microM). Furthermore, AP6A attenuated the relaxant effect of isoprenaline. 6. In human isolated electrically driven ventricular preparations AP6A alone was ineffective, but attenuated isoprenaline-stimulated force of contraction by maximally 42% (IC20 = 18 microM). Moreover, AP6A attenuated the relaxant effect of isoprenaline. 7. All these effects of AP6A were abolished by the selective A1-adenosine receptor antagonist 1,3-dipropyl-cyclopentyl-xanthine (DPCPX, 0.3 microM), whereas the M-cholinoceptor antagonist atropine (10 microM) and the P2-purinoceptor antagonist suramin (300 microM) failed to abolish the effects of AP6A. 8. AP6A 100 microM had no effect on the amplitude of the L-type calcium current, but attenuated isoprenaline-stimulated L-type calcium current. The maximum of the current-voltage relationship (I-V curve) was shifted to the left by isoprenaline and additional application of AP6A shifted the I-V curve back to the right to the control value. The phosphorylation state of phospholamban and the troponin inhibitor was unchanged by AP6A alone, but was markedly attenuated by AP6A in the presence of isoprenaline. Cyclic AMP levels remained unchanged by AP6A, even after stimulation with isoprenaline. 9. In summary, AP6A exerts negative chronotropic and inotropic effects in guinea-pig and human cardiac preparations. These effects are mediated via A1-adenosine receptors as all effects were sensitive to the selective A1-adenosine receptor antagonist DPCPX. Furthermore, the effects of AP6A on cyclic AMP levels, protein phosphorylation and the L-type calcium current are in accordance with stimulation of A1-adenosine receptors.

PACAP induces bradycardia in guinea-pig heart by stimulation of atrial cholinergic neurones

Based on previous studies which indicated that pituitary adenylate cyclase activating peptide (PACAP) acts as a positive inotropic and chronotropic substance in different species via the cAMP signal transduction pathway, the objective of the present work was to investigate cAMP-regulated myocardial key proteins in response to PACAP in isolated ventricular cells of the guinea pig. Surprisingly, the two molecular forms of PACAP, PACAP(1-27) and PACAP(1-38), showed no effect on intracellular cAMP-levels, L-type Ca2+ channel current or phosphorylation of troponin inhibitor (TnI) and phospholamban (PLB). Additionally, inotropy of isolated guinea-pig ventricular strips was not affected by the neuropeptide. However, in isolated spontaneously beating guinea-pig atria, PACAP(1-27) and PACAP(1-38), but not VIP induced severe bradycardia in a dose-dependent manner. This effect could be prevented by preincubation with the PACAP receptor antagonist PACAP(6-38), by atropine and by omega-conotoxin, a blocker of neuronal N-type Ca2+ channels. PACAP stimulates release of [3H]-labelled acetylcholine. Only preparations showing an increase in [3H]acetylcholine release developed bradycardia, indicating a causal relationship between both phenomena. It was concluded that PACAP exerts no influence on guinea-pig ventricular tissue, but induces negative chronotropic effects in isolated guinea-pig atria by stimulation of acetylcholine release from parasympathetic neurons via PACAP type 1 receptors.

Myocardial contractile response and IP3, cAMP and cGMP interrelationships

An experimental study in the perfused working normal and pressure overloaded rat heart. A mini review based on a doctoral thesis.

Sodium fluoride attenuates the negative inotropic effects of muscarinic M2 and adenosine receptor agonists

Sodium fluoride increased the force of contraction in isolated guinea-pig papillary muscles concentration dependently, starting at 3 mmol/1. Sodium fluoride inhibited phosphorylase phosphatase activity in homogenates from guinea pig hearts, starting at 1 mmol/1. The positive inotropic effect of 3 mmol/1 sodium fluoride was not accompanied by an increase in cAMP content in guinea-pig papillary muscles. In papillary muscles, carbachol or (-)-N(6)-phenylisopropyladenosine reduced the positive inotropic effect of isoprenaline (10 nmol/1) or the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (60 mu mol/1). These negative inotropic effects of carbachol and (-)-N(6)-phenylisopropyladenosine were attenuated by additional sodium fluoride (3 mmol/l). It is concluded that sodium fluoride can impair the signal transduction of muscarinic M2 (carbachol) and adenosine receptor (-)-N(6)-phenylisopropyladenosine) agonists. This effect of sodium fluoride could support the hypothesis that the cardiac effects of muscarinic M2 and adenosine receptor agonists involve, at least in part, the activation of phosphatases.

Regulation of phospholamban and troponin-I phosphorylation in the intact rat cardiomyocytes by adrenergic and cholinergic stimuli: roles of cyclic nucleotides, calcium, protein kinases and phosphatases and depolarization

Protein phosphorylation was investigated in [32P]-labeled cardiomyocytes isolated from adult rat heart ventricles. The beta-adrenergic stimulation (by isoproterenol, ISO) increased the phosphorylation of inhibitory subunit of troponin (TN-I), C-protein and phospholamban (PLN). Such stimulation was largely mediated by increased adenylyl cyclase (AC) activity, increased myoplasmic cyclic AMP and increased cyclic AMP dependent protein kinase (A-kinase)-catalyzed phosphorylation of these proteins in view of the following observations: (a) dibutyryl-and bromo-derivatives of cyclic AMP mimicked the stimulatory effect of ISO on protein phosphorylation while (b) Rp-cyclic AMP was found to attenuate ISO-dependent stimulation. Unexpectedly, 8-bromo cyclic GMP was found to markedly increase TN-I and PLN phosphorylation. Both beta 1- and beta 2-adrenoceptors were present and ISO binding to either receptor was found to stimulate myocyte AC. However, the stimulation of the beta 2-AR only marginally increased while the stimulation of beta 1-AR markedly increased PLN phosphorylation. Other stimuli that increase tissue cyclic AMP levels also increased PLN and TN-I phosphorylation and these included isobutylmethylxanthine (non-specific phosphodiesterase inhibitor), milrinone (inhibits cardiotonic inhibitable phosphodiesterase, sometimes called type III or IV) and forskolin (which directly stimulates adenylyl cyclase). Cholinergic agonists acting on cardiomyocyte M2-muscarinic receptors that are coupled to AC via pertussis toxin(PT)-sensitive G proteins inhibited AC and attenuated ISO-dependent increases in PLN and TN-I phosphorylation. The in vivo PT treatment, which ADP-ribosylated Gi-like protein(s) in the myocytes, markedly attenuated muscarinic inhibitory effect on PLN and TN-I phosphorylation on one hand and, increased the beta-adrenergic stimulation, on the other. Controlled exposure of isolated myocytes to N-ethyl maleimide, also led to the findings similar to those seen following the PT treatment. Exposure of myocytes to phorbol, 12-myristate, 13-acetate (PMA) increased the protein phosphorylation, augmenting the stimulation by ISO, and such augmentation was antagonized by propranolol suggesting modulation of the beta-adrenoceptor coupled AC pathway by PMA. Okadaic acid (OA) exposure of myocytes also increased protein phosphorylation with the results supporting the roles for type 1 and 2A protein phosphatases in the dephosphorylation of PLN and TN-I. Interestingly OA treatment attenuated the muscarinic inhibitory effect which was restored by subsequent brief exposure of myocytes to PMA. While the stimulation of alpha adrenoceptors exerted little effect on the phosphorylation of PLN and TN-I, inactivation of alpha adrenoceptors by chloroethylclonidine (CEC), augmented beta-adrenergically stimulated phosphorylation. KCl-dependent depolarization of myocytes was observed to potentiate ISO-dependent increase in phosphorylation (incubation period 15 sec to 1 min) as well as to accelerate the time-dependent decline in this phosphorylation seen upon longer incubation. Verapamil decreased ISO-stimulated protein phosphorylation in the depolarized myocytes. Depolarization was found to have little effect on the muscarinic inhibitory action on phosphorylation. Prior treatment of myocytes with PMA, was found to augment ISO-stimulated protein phosphorylation in the depolarized myocytes. Such augmented increases were completely blocked by propranolol. Forskolin also stimulated PLN and TN-I phosphorylation. Prior exposure of myocytes to forskolin followed by incubation in the depolarized and polarized media showed that PLN was dephosphorylated more rapidly in the depolarized myocytes. The results support the view that both cyclic AMP and calcium signals cooperatively increase the rates of phosphorylation of TN-I and PLN in the depolarized cardiomyocytes during beta-adrenergic stimulation. (ABSTRACT TRUNCATED)

Carbachol-induced increase in inositol trisphosphate (IP3) content is attenuated by adrenergic stimulation in the isolated working rat heart

The interrelated responses of concomitant adrenergic and muscarinic receptor stimulation on second messengers and mechanical activity in the isolated perfused working rat heart were studied. The hearts were perfused with Krebs-Henseleit buffer in a modified Langendorff apparatus. The hearts were perfused with noradrenaline (10(-6) mol L-1, n = 20), with carbachol (3 x 10(-7) mol L-1, n = 11) or with noradrenaline plus carbachol (n = 20) in the above-mentioned concentrations. The hearts were frozen at 20 s, 30 s and 40 min after addition of noradrenaline and noradrenaline plus carbachol and at 20 s and 40 min after addition of carbachol. Five hearts were freeze-clamped directly after preperfusion and another five hearts after 40 min of perfusion and used as controls. Myocardial cAMP increased at 20 s and 40 min after noradrenaline perfusion. In contrast to this cAMP was unchanged at 20 s and decreased at 40 min after perfusion with noradrenaline plus carbachol. IP3 content increased after 20 s of carbachol- and after 40 min of noradrenaline perfusion (P < 0.05). However, noradrenaline plus carbachol did not induced any significant increase in IP3 content after 20 s and 30 s, but after 40 min a decrease below basal level was found (P < 0.05). Noradrenaline stimulation attenuated muscarinic agonist induced IP3 formation. A reciprocity existed in that noradrenaline induced IP3 formation was attenuated by carbachol. No direct relationship was observed between the IP3 response and contractility, also valid for cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of various drugs on the myocardial inotropic response

1. The signal transduction process mediated by cyclic AMP that leads to the characteristic positive inotropic effect (PIE) in association with a positive lusitropic effect (acceleration of rate of twitch relaxation) has been well established. Relationships between accumulation of cyclic AMP, changes in intracellular Ca2+ transients and the PIE differ, however, depending on the mechanism of particular drugs that affect different steps in the metabolism of cyclic AMP. Selective partial agonists of beta 1-adrenoceptors and inhibitors of phosphodiesterase (PDE) III cause the accumulation of less cyclic AMP for a given PIE than does isoproterenol. In addition, in aequorin-microinjected canine ventricular muscle, selective inhibitors of PDE III, OPC 18790 and Org 9731, produced smaller decreases in the responsiveness of myofilaments to Ca2+ ions than isoproterenol, while a partial agonist of beta 1-adrenoceptors, denopamine, elicits a decrease in Ca2+ responsiveness of the same extent as does isoproterenol. 2. Activation of myocardial alpha 1-adrenoceptors, as well as stimulation of receptors for endothelin and angiotensin II, which accelerates hydrolysis of phosphoinositide (PI) to result in production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) are associated with very similar inotropic regulation: (1) the dependence on the species of animals of induction of the PIE; (2) an excellent correlation between the extent of acceleration of hydrolysis of PI and the PIE; (3) isometric contraction curves associated with a negative lusitropic effect; (4) the PIE associated with increases in myofibrillar responsiveness to Ca2+ ions; and (5) the selective inhibition of the PIE by an activator of protein kinase C (PKC), phorbol 12,13-dibutyrate (PDBu), with little effect on the PIE of isoproterenol and Bay k 8644. 3. A novel class of cardiotonic agents, namely, Ca2+ sensitizers such as EMD 53998 and Org 30029, act on the Ca(2+)-binding site of troponin C, increasing the affinity of these sites for Ca2+ ions, or at the actin-myosin interface to facilitate the cycling of cross-bridges. These agents produce a PIE with little change or decrease in Ca2+ transients and may bring about a significant breakthrough in the development of drugs for reversal of myocardial failure in the treatment of congestive heart failure.

Physiologic and pharmacologic factors that affect myocardial relaxation

Evaluation of the myocardial relaxation has become important in the last years. An impaired relaxation may precede contractile dysfunctions and even cause heart failure. To treat this impaired lusitropism it is necessary to properly assess the lusitropic state of the heart and understand how drugs affect the cellular mechanisms underlying myocardial relaxation (sarcoplasmic reticulum function, Ca2+ fluxes through the sarcolemma and myofilament Ca2+ sensitivity). Current information regarding these issues is provided in this review. The relative usefulness of the mechanical parameters used to evaluate the lusitropic state of the heart in experimental models applied in pharmacology will also be discussed.

Cyclic AMP metabolism in intact rat ventricular cardiac myocytes: interaction of carbachol with isoproterenol and 3-isobutyl-1-methylxanthine

Experiments were carried out to elucidate the characteristics of regulation of cyclic AMP levels in intact myocardial cells. For this purpose, the influence of isoproterenol, a nonselective cyclic nucleotide phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX) and carbachol on cyclic AMP levels was investigated in isolated rat cardiac myocytes. The extent of cyclic AMP accumulation induced by isoproterenol was much less than that produced by IBMX: submaximal concentrations of isoproterenol and IBMX elevated the cyclic AMP level 2.4- and 4.8-fold of the control level, respectively. Both agents in combination increased the cyclic AMP level markedly 48-fold. Carbachol inhibited the cyclic AMP accumulation induced by isoproterenol, IBMX and their combination by 30%, 60% and 80% of the respective response. The extent of inhibition produced by carbachol of the cyclic AMP accumulation induced by IBMX + isoproterenol was smaller than that caused by propranolol, and carbachol produced only a marginal additional inhibitory action to that of propranolol, implying that carbachol does not affect the process of cyclic AMP degradation. The present findings indicate that in intact cardiac myocytes the rate of cyclic AMP degradation catalyzed by PDE may be a crucial process of cyclic AMP turnover. This view is supported by the observations that the inhibitory action of carbachol on the effect of isoproterenol was less than that on the effect of IBMX, and that the inhibitory action of carbachol was markedly enhanced by the simultaneous presence of IBMX.

A1-adenosine receptor-mediated inhibition of isoproterenol-stimulated protein phosphorylation in ventricular myocytes. Evidence against a cAMP-dependent effect

cAMP content and protein phosphorylation were determined in unlabeled and 32P-labeled guinea pig ventricular myocytes. Isoproterenol (10 nM, 37 degrees C, 10 seconds) increased cAMP content (236%) and phospholamban (265%) and troponin I (135%) phosphorylation in ventricular myocytes. When isoproterenol (0-300 nM) and the A1-adenosine receptor agonist (-)-N6-phenylisopropyladenosine (PIA, 1 microM) or the A1- and A2-adenosine receptor agonist 5'-(N-ethylcarboxamido)-adenosine (NECA, 1 microM) were administered simultaneously, both adenosine receptor agonists attenuated phospholamban phosphorylation to approximately the same extent (40%). The EC50 value for isoproterenol to phosphorylate phospholamban was 8 +/- 1 nM (n = 3), which increased to 31 +/- 4 nM (n = 3) in the presence of PIA or NECA. IC50 values for PIA or NECA to decrease the phosphorylation of phospholamban were 30 or 32 nM in 10 nM isoproterenol-stimulated cells and 80 or 85 nM in 30 nM isoproterenol-stimulated cells. Both adenosine receptor agonists failed to inhibit the phosphorylation of troponin I. However, acetylcholine (2 microM) in the presence of 10 nM isoproterenol inhibited phosphorylation of phospholamban as well as troponin I in ventricular cells. These effects were antagonized by 10 microM atropine. The effects of PIA and NECA on phosphorylation were antagonized by the A1-selective adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (1 microM) but not by the A2-selective adenosine receptor antagonist 9-chloro-2-(2-furanyl)-5,6-dihydro-1,2,4,triazolo(1,5-c)quinazolin -5-imine (1 microM). PIA and NECA did not reduce cAMP levels in isoproterenol-stimulated cells. We conclude that phospholamban phosphorylation was inhibited by A1-adenosine receptor activation and that these effects on phospholamban phosphorylation cannot be explained by a reduction in cAMP levels.

Protein kinase C phosphorylates a 15 kDa protein but not phospholamban in intact rat cardiac myocytes

In the present study the effects of the protein kinase C activator 12-O-tetradecanoylphorbol 13-acetate (TPA) as well as the alpha- and beta-adrenoceptor agonists methoxamine and isoproterenol on protein phosphorylation of intact rat cardiac myocytes were investigated. TPA, isoproterenol and methoxamine were shown to stimulate phosphorylation of a 15 kDa protein. EC50 for TPA and isoproterenol were 4 x 10(-8) M and 5 x 10(-9) M respectively. The time course of phosphorylation by TPA and isoproterenol greatly differed, revealing a maximal phosphorylation (2.9-fold) after 10 min and 1 min respectively. Cell fractionation showed a significant enrichment of the 15 kDa protein in a crude membrane preparation. While the 15 kDa protein was the only phosphoprotein stimulated by TPA and methoxamine, isoproterenol additionally enhanced the 32Pi incorporation into four proteins corresponding to 6 kDa (phospholamban), 28 kDa, 97 kDa and 140 kDa. Furthermore, dephosphorylation of a 21 kDa substrate upon beta-adrenoceptor stimulation was observed. Phospholamban phosphorylation was effectively (max. 9.1-fold) stimulated by isoproterenol (EC50 of 5 x 10(-9) M), reaching a maximal phosphorylation state within 1 min. The present study clearly demonstrates: (1) TPA stimulates the phosphorylation of a membrane-localized 15 kDa protein and this effect can be mimicked by both isoproterenol and methoxamine; (2) TPA, in contrast to isoproterenol, does not change the phosphorylation state of phospholamban. Whilst phospholamban under in vitro conditions is known to be a substrate for protein kinase C, it does not appear to be accessible for the enzyme in intact cardiac myocytes.

Signal transduction by G proteins in cardiac tissues

The role of G proteins in mediating the responses of the heart to circulating catecholamines and to the influences of the autonomic nervous system is of special interest to cardiologists. It is evident that G proteins are essential links in the cascade of biochemical events that ensure when neurotransmitters and hormones interact with receptors on myocardial cells. It is likely [corrected] that dysfunction of G proteins plays a role in cardiovascular pathophysiology. With current methodologies, especially molecular biological and recombinant DNA techniques, and with transgenic animal models that can relate physiological function and specific gene dosage, some cardiovascular diseases may be traced to G protein-related defects.

Role of cAMP in the functional interaction of carbachol with different cAMP elevating agents in rabbit atrium

The muscarinic agonist carbachol antagonized positive inotropic responses of rabbit left atria to the beta-adrenoceptor agonist isoproterenol, the adenylate cyclase activator forskolin and the phosphodiesterase inhibitor IBMX. Carbachol also reduced cAMP levels elevated by isoproterenol, but had no significant effect on cAMP levels in the presence of either forskolin or IBMX. Pre-treatment of rabbits with a dose of pertussis toxin which completely blocked the reduction by carbachol of isoproterenol-induced increases in cAMP, also blocked the reversal by carbachol of positive inotropic responses to isoproterenol, but only partially attenuated the antagonism by carbachol of inotropic responses to forskolin and IBMX. These data suggest that antagonism by carbachol of forskolin and IBMX-induced increases in cAMP levels does not play an important role in the functional interaction of carbachol with these cAMP-elevating agents.

Cardiac responses to VIP and VIP-ergic-cholinergic interaction in isolated dog heart preparations

Whereas i.v. administration of vasoactive intestinal peptide (VIP) to support dogs increased heart rate and decreased systemic blood pressure, sinus rate and contractile force increased in isolated right atria perfused with blood from the support dogs. VIP injected intraarterially into isolated atria induced dose-dependent positive chronotropic and inotropic effects. Intracardiac parasympathetic nerve stimulation attenuated the positive cardiac responses to VIP, but neither propranolol, imipramine, nor tetrodotoxin influenced the responses to VIP. VIP given to isolated left ventricles also increased the contractile force in a dose-dependent manner. However, VIP induced a greater maximum atrial contractility than ventricular contractility. This may indicate that VIP receptor density in the ventricle was lower than in the atrium, as it has recognized that VIP-ergic nerves innervate the right atrium more densely than the left ventricle. We therefore suggest that the positive cardiac responses to VIP, together with the VIP-ergic innervation in dog hearts and vagal activation, attenuate the VIP-mediated responses at site(s) in the cyclic AMP cascade.

Mechanisms for muscarinic inhibition of somatostatin release from canine fundic D cells

We undertook the present studies to explore the mechanisms by which carbachol inhibits the release of somatostatin-like immunoreactivity (SLI) from D cells. D cells were isolated from canine fundic mucosa by collagenase/EDTA dispersion followed by counterflow elutriation. Carbachol inhibited the release of SLI induced by forskolin, dibutyryl 3':5' cyclic adenosine monophosphate (cAMP), pentagastrin (PG), and 12-0-tetradecanoyl-phorbol-13-acetate in a fashion that could be prevented by pertussis toxin (PT) pretreatment of the D cells. Pertussis toxin also prevented the carbachol-induced inhibition of forskolin-stimulated cAMP generation and PG-stimulated [Ca2+]i mobilization. These data indicate that pertussis toxin sensitive inhibitory guanine nucleotide binding proteins mediate many of carbachol's inhibitory actions on D cells.

Muscarinic receptor stimulation and cyclic AMP-dependent effects in guinea-pig ventricular myocardium

1. The effect of carbachol on force of contraction, contraction duration, intracellular Na+ activity and cyclic AMP content was studied in papillary muscles of the guinea-pig exposed to isoprenaline or the phosphodiesterase inhibitor 3-isobutyl, 1-methyl xanthine (IBMX). The preparations were obtained from reserpine-pretreated animals and were electrically driven at a frequency of 0.2 Hz. 2. Isoprenaline (10 nM) and IBMX (100 microM) produced comparable positive inotropic effects of 9.8 and 9.7 mN, respectively. Carbachol (3 microM) attenuated the inotropic effects by 82% (isoprenaline) and by 79% (IBMX). The shortening of contraction duration which accompanied the positive inotropic effect of isoprenaline (by 14.9%) and of IBMX (by 22.4%) was not significantly affected by 3 microM carbachol. 3. The positive inotropic effect of 10 nM isoprenaline and of 100 microM IBMX was accompanied by an increase in cellular cyclic AMP content of 58 and 114%, respectively. Carbachol (3 microM) failed to reduce significantly the elevated cyclic AMP content of muscles exposed to either isoprenaline or IBMX. 4. In the quiescent papillary muscle, isoprenaline (10 nM) and IBMX (100 microM) reduced the intracellular Na+ activity by 28 and 17%, respectively. This decline was not influenced by the additional application of 3 microM carbachol. 5. The results demonstrate that muscarinic antagonism in guinea-pig ventricular myocardium exposed to cyclic AMP-elevating drugs is restricted to force of contraction. The underlying mechanism does not apparently involve the cytosolic signal molecule cyclic AMP.

Studies on the cellular mechanisms of action of positive and negative inotropic agents

We have reviewed the mechanism by which drugs that elevate cyclic AMP level modify myocardial contractility. We have presented preliminary evidence about the mechanism by which muscarinic agonists antagonize the effects of these drugs. Finally, we suggest that the protein phosphorylation experiments, particularly if done in dispersed myocytes, could be an efficient and cost-effective method of screening drugs which may act by elevating intracellular levels of cyclic AMP.