Inositol trisphosphate enhances calcium release in skinned cardiac and skeletal muscle

Role of Vasoactive Hormone-Induced Signal Transduction in Cardiac Hypertrophy and Heart Failure

Heart failure is the common concluding pathway for a majority of cardiovascular diseases and is associated with cardiac dysfunction. Since heart failure is invariably preceded by adaptive or maladaptive cardiac hypertrophy, several biochemical mechanisms have been proposed to explain the development of cardiac hypertrophy and progression to heart failure. One of these includes the activation of different neuroendocrine systems for elevating the circulating levels of different vasoactive hormones such as catecholamines, angiotensin II, vasopressin, serotonin and endothelins. All these hormones are released in the circulation and stimulate different signal transduction systems by acting on their respective receptors on the cell membrane to promote protein synthesis in cardiomyocytes and induce cardiac hypertrophy. The elevated levels of these vasoactive hormones induce hemodynamic overload, increase ventricular wall tension, increase protein synthesis and the occurrence of cardiac remodeling. In addition, there occurs an increase in proinflammatory cytokines and collagen synthesis for the induction of myocardial fibrosis and the transition of adaptive to maladaptive hypertrophy. The prolonged exposure of the hypertrophied heart to these vasoactive hormones has been reported to result in the oxidation of catecholamines and serotonin via monoamine oxidase as well as the activation of NADPH oxidase via angiotensin II and endothelins to promote oxidative stress. The development of oxidative stress produces subcellular defects, Ca2+-handling abnormalities, mitochondrial Ca2+-overload and cardiac dysfunction by activating different proteases and depressing cardiac gene expression, in addition to destabilizing the extracellular matrix upon activating some metalloproteinases. These observations support the view that elevated levels of various vasoactive hormones, by producing hemodynamic overload and activating their respective receptor-mediated signal transduction mechanisms, induce cardiac hypertrophy. Furthermore, the occurrence of oxidative stress due to the prolonged exposure of the hypertrophied heart to these hormones plays a critical role in the progression of heart failure.

Emerging Evidence for cAMP-calcium Cross Talk in Heart Atrial Nanodomains Where IP3-Evoked Calcium Release Stimulates Adenylyl Cyclases

Calcium handling is vital to normal physiological function in the heart. Human atrial arrhythmias, eg. atrial fibrillation, are a major morbidity and mortality burden, yet major gaps remain in our understanding of how calcium signaling pathways function and interact. Inositol trisphosphate (IP3) is a calcium-mobilizing second messenger and its agonist-induced effects have been observed in many tissue types. In the atria IP3 receptors (IR3Rs) residing on junctional sarcoplasmic reticulum augment cellular calcium transients and, when over-stimulated, lead to arrhythmogenesis. Recent studies have demonstrated that the predominant pathway for IP3 actions in atrial myocytes depends on stimulation of calcium-dependent forms of adenylyl cyclase (AC8 and AC1) by IP3-evoked calcium release from the sarcoplasmic reticulum. AC8 shows co-localisation with IP3Rs and AC1 appears to be nearby. These observations support crosstalk between calcium and cAMP pathways in nanodomains in atria. Similar mechanisms also appear to operate in the pacemaker region of the sinoatrial node. Here we discuss these significant advances in our understanding of atrial physiology and pathology, together with implications for the identification of potential novel targets and modulators for the treatment of atrial arrhythmias.

IP3-mediated Ca2+ release regulates atrial Ca2+ transients and pacemaker function by stimulation of adenylyl cyclases

Inositol trisphosphate (IP₃) is a Ca²⁺-mobilizing second messenger shown to modulate atrial muscle contraction and is thought to contribute to atrial fibrillation. Cellular pathways underlying IP₃ actions in cardiac tissue remain poorly understood, and the work presented here addresses the question whether IP₃-mediated Ca²⁺ release from the sarcoplasmic reticulum is linked to adenylyl cyclase activity including Ca²⁺-stimulated adenylyl cyclases (AC1 and AC8) that are selectively expressed in atria and sinoatrial node (SAN). Immunocytochemistry in guinea pig atrial myocytes identified colocalization of type 2 IP₃ receptors with AC8, while AC1 was located in close vicinity. Intracellular photorelease of IP₃ by UV light significantly enhanced the amplitude of the Ca²⁺ transient (CaT) evoked by electrical stimulation of atrial myocytes (31 ± 6% increase 60 s after photorelease, n = 16). The increase in CaT amplitude was abolished by inhibitors of adenylyl cyclases (MDL-12,330) or protein kinase A (H89), showing that cAMP signaling is required for this effect of photoreleased IP₃. In mouse, spontaneously beating right atrial preparations, phenylephrine, an α-adrenoceptor agonist with effects that depend on IP₃-mediated Ca²⁺ release, increased the maximum beating rate by 14.7 ± 0.5%, n = 10. This effect was substantially reduced by 2.5 µmol/L 2-aminoethyl diphenylborinate and abolished by a low dose of MDL-12,330, observations which are again consistent with a functional interaction between IP₃ and cAMP signaling involving Ca²⁺ stimulation of adenylyl cyclases in the SAN pacemaker. Understanding the interaction between IP₃ receptor pathways and Ca²⁺-stimulated adenylyl cyclases provides important insights concerning acute mechanisms for initiation of atrial arrhythmias.

NEW & NOTEWORTHY This study provides evidence supporting the proposal that IP₃ signaling in cardiac atria and sinoatrial node involves stimulation of Ca²⁺-activated adenylyl cyclases (AC1 and AC8) by IP₃-evoked Ca²⁺ release from junctional sarcoplasmic reticulum. AC8 and IP₃ receptors are shown to be located close together, while AC1 is nearby. Greater understanding of these novel aspects of the IP₃ signal transduction mechanism is important for future study in atrial physiology and pathophysiology, particularly atrial fibrillation.

Increases in [IP3]i aggravates diastolic [Ca2+] and contractile dysfunction in Chagas' human cardiomyocytes

Chagas cardiomyopathy is the most severe manifestation of human Chagas disease and represents the major cause of morbidity and mortality in Latin America. We previously demonstrated diastolic Ca2+ alterations in cardiomyocytes isolated from Chagas' patients to different degrees of cardiac dysfunction. In addition, we have found a significant elevation of diastolic [Na+]d in Chagas' cardiomyocytes (FCII>FCI) that was greater than control. Exposure of cardiomyocytes to agents that enhance inositol 1,4,5 trisphosphate (IP3) generation or concentration like endothelin (ET-1) or bradykinin (BK), or membrane-permeant myoinositol 1,4,5-trisphosphate hexakis(butyryloxy-methyl) esters (IP3BM) caused an elevation in diastolic [Ca2+] ([Ca2+]d) that was always greater in cardiomyocytes from Chagas' than non- Chagas' subjects, and the magnitude of the [Ca2+]d elevation in Chagas' cardiomyocytes was related to the degree of cardiac dysfunction. Incubation with xestospongin-C (Xest-C), a membrane-permeable selective blocker of the IP3 receptors (IP3Rs), significantly reduced [Ca2+]d in Chagas' cardiomyocytes but did not have a significant effect on non-Chagas' cells. The effects of ET-1, BK, and IP3BM on [Ca2+]d were not modified by the removal of extracellular [Ca2+]e. Furthermore, cardiomyocytes from Chagas' patients had a significant decrease in the sarcoplasmic reticulum (SR) Ca2+content compared to control (Control>FCI>FCII), a higher intracellular IP3 concentration ([IP3]i) and markedly depressed contractile properties compared to control cardiomyocytes. These results provide additional and convincing support about the implications of IP3 in the pathogenesis of Chagas cardiomyopathy in patients at different stages of chronic infection. Additionally, these findings open the door for novel therapeutic strategies oriented to improve cardiac function and quality of life of individuals suffering from chronic Chagas cardiomyopathy (CC).

Inositol 1,4,5-trisphosphate receptors and pacemaker rhythms

Intracellular Ca(2+) plays an important role in the control of the heart rate through the interaction between Ca(2+) release by ryanodine receptors in the sarcoplasmic reticulum (SR) and the extrusion of Ca(2+) by the sodium-calcium exchanger which generates an inward current. A second type of SR Ca(2+) release channel, the inositol 1,4,5-trisphosphate receptor (IP(3)R), can release Ca(2+) from SR stores in many cell types, including cardiac myocytes. However, it is still uncertain whether IP(3)Rs play any functional role in regulating the heart rate. Accumulated evidence shows that IP(3) and IP(3)R are involved in rhythm control in non-cardiac pacemaker tissues and in the embryonic heart. In this review we focus on intracellular Ca(2+) oscillations generated by Ca(2+) release from IP(3)R that initiates membrane depolarization and provides a common mechanism producing spontaneous activity in a range of cells with pacemaker function. Emerging new evidence also suggests that IP(3)/IP(3)Rs play a functional role in normal and diseased hearts and in cardiac rhythm control. Several membrane currents, including a store-operated Ca(2+) current, might be activated by Ca(2+) release from IP(3)Rs. IP(3)/IP(3)R may thus add another dimension to the complex regulation of heart rate.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Emerging roles of inositol 1,4,5-trisphosphate signaling in cardiac myocytes

Inositol 1,4,5-trisphosphate (IP(3)) is a ubiquitous intracellular messenger regulating diverse functions in almost all mammalian cell types. It is generated by membrane receptors that couple to phospholipase C (PLC), an enzyme which liberates IP(3) from phosphatidylinositol 4,5-bisphosphate (PIP(2)). The major action of IP(3), which is hydrophilic and thus translocates from the membrane into the cytoplasm, is to induce Ca(2+) release from endogenous stores through IP(3) receptors (IP(3)Rs). Cardiac excitation-contraction coupling relies largely on ryanodine receptor (RyR)-induced Ca(2+) release from the sarcoplasmic reticulum. Myocytes express a significantly larger number of RyRs compared to IP(3)Rs (~100:1), and furthermore they experience substantial fluxes of Ca(2+) with each heartbeat. Therefore, the role of IP(3) and IP(3)-mediated Ca(2+) signaling in cardiac myocytes has long been enigmatic. Recent evidence, however, indicates that despite their paucity cardiac IP(3)Rs may play crucial roles in regulating diverse cardiac functions. Strategic localization of IP(3)Rs in cytoplasmic compartments and the nucleus enables them to participate in subsarcolemmal, bulk cytoplasmic and nuclear Ca(2+) signaling in embryonic stem cell-derived and neonatal cardiomyocytes, and in adult cardiac myocytes from the atria and ventricles. Intriguingly, expression of both IP(3)Rs and membrane receptors that couple to PLC/IP(3) signaling is altered in cardiac disease such as atrial fibrillation or heart failure, suggesting the involvement of IP(3) signaling in the pathology of these diseases. Thus, IP(3) exerts important physiological and pathological functions in the heart, ranging from the regulation of pacemaking, excitation-contraction and excitation-transcription coupling to the initiation and/or progression of arrhythmias, hypertrophy and heart failure.

Calcium handling in human embryonic stem cell-derived cardiomyocytes

The objective of the current study was to characterize calcium handling in developing human embryonic stem cell-derived cardiomyocytes (hESC-CMs). To this end, real-time polymerase chain reaction (PCR), immunocytochemistry, whole-cell voltage-clamp, and simultaneous patch-clamp/laser scanning confocal calcium imaging and surface membrane labeling with di-8-aminonaphthylethenylpridinium were used. Immunostaining studies in the hESC-CMs demonstrated the presence of the sarcoplasmic reticulum (SR) calcium release channels, ryanodine receptor-2, and inositol-1,4,5-trisphosphate (IP3) receptors. Store calcium function was manifested as action-potential-induced calcium transients. Time-to-target plots showed that these action-potential-initiated calcium transients traverse the width of the cell via a propagated wave of intracellular store calcium release. The hESC-CMs also exhibited local calcium events ("sparks") that were localized to the surface membrane. The presence of caffeine-sensitive intracellular calcium stores was manifested following application of focal, temporally limited puffs of caffeine in three different age groups: early-stage (with the initiation of beating), intermediate-stage (10 days post-beating [dpb]), and late-stage (30-40 dpb) hESC-CMs. Calcium store load gradually increased during in vitro maturation. Similarly, ryanodine application decreased the amplitude of the spontaneous calcium transients. Interestingly, the expression and function of an IP3-releasable calcium pool was also demonstrated in the hESC-CMs in experiments using caged-IP3 photolysis and antagonist application (2 microM 2-Aminoethoxydiphenyl borate). In summary, our study establishes the presence of a functional SR calcium store in early-stage hESC-CMs and shows a unique pattern of calcium handling in these cells. This study also stresses the importance of the functional characterization of hESC-CMs both for developmental studies and for the development of future myocardial cell replacement strategies. Disclosure of potential conflicts of interest is found at the end of this article.

Ontogeny of ingestive behavior

Review of the ontogeny of the controls of independent ingestion reveals that some of the direct and indirect controls of meal size identified in adult rats function in the first three postnatal weeks. The controls appear sequentially and some of them change their potency after they emerge. Indirect controls exerted by metabolism and adiposity do not emerge until the fourth postnatal week or later in the postweaning period. Recent experiments in rats with monogenic obesities involving the leptin and cholecystokinin receptors have demonstrated the usefulness of independent ingestion in the detection of the earliest expression of hyperphagia. Although much remains to be learned about the normal controls of independent ingestion, it is clear that it provides relevant information about the development of normal and abnormal controls of meal size in rodents that is useful for translational research into the controls of meal size in normal and obese children.

Angiotensin II and cardiac excitation-contraction coupling: questions and controversies

Angiotensin II (AngII) is a circulating peptide that produces a positive inotropic effect in the heart in several species, including humans. The subcellular mechanisms involved in producing this effect have been the focus of numerous studies; however, the results of these studies have generated considerable controversy. Although part of the controversy might arise from species and developmental differences, conflicting results have also been reported in the same species. To further complicate the understanding of the cardiac actions of AngII, the binding of the peptide to its transmembrane G-protein-coupled receptors has been shown to activate signalling cascades that involve numerous second messengers. Among these, inositol 1,4,5-triphosphate (IP3) and protein kinase C (PKC) have been shown to have the potential to modulate either one or both of the two basic mechanisms known to increase contractility: (i) an increase in the intracellular Ca2+ concentration ([Ca2+]i); or (ii) an increase in myofilament responsiveness to Ca2+. The aim of this review is to examine the effect of AngII on the fundamental components of cardiac excitation-contraction coupling: calcium currents, Na+/Ca2+ exchange, sarcoplasmic reticulum (SR)-CaZ+ release, calcium transients and contractile proteins. An answer to the following question is sought: Is the positive inotropic effect of AngII due to an increase in [Ca2+]i, to an increase in myofilament responsiveness to Ca2+, or to both?

Modulation ofICa-Lby α1-adrenergic stimulation in rat ventricular myocytes

We found when L-type calcium current (ICa-L) was recorded with the perforated patch-clamp method in rat ventricular myocytes that bath application of phenylephrine (with propranolol) evoked a biphasic response characterized by an initial transient suppression followed by a sustained potentiation. The transient suppression occurred 30–60 s after phenylephrine perfusion and reached peak inhibition at approximately 2 min. The biphasic modulation of ICa-Lwas also elicited by methoxamine, and the effects of phenylephrine were blocked by prazosin, indicating that the responses were mediated through α1-adrenoceptors. Pretreatment of cells with H7 (100 µmol/L), a broad-spectrum protein kinase inhibitor that inhibits both protein kinase C and A, eliminated potentiation but did not affect transient suppression. The transient suppression occurred concurrently with the acceleration of the fast component of ICa-Linactivation. Depletion of intracellular Ca2+stores by ryanodine plus caffeine or thapsigargin eliminated the transient suppression. When ICa-Lwas recorded with whole-cell patch-clamp and with 0.05 mmol/L EGTA in the pipette solution to allow intracellular Ca2+to fluctuate, phenylephrine evoked a transient suppression as in the perforated patch recordings. Heparin, a specific blocker of IP3(inositol 1,4,5-trisphosphate) receptors, eliminated the phenylephrine-induced transient suppression of ICa-Lwhen added to the pipette solution. Intensive chelation of intracellular Ca2+by 5 mmol/L BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid) in the pipette solution also eliminated the phenylephrine-induced transient suppression of ICa-L. We conclude that transient increase in the concentration of intracellular calcium ([Ca2+]i) caused by Ca2+release from intracellular stores underlies the transient suppression of ICa-L, whereas the potentiation of ICa-Lis a result of activation of protein kinases.Key words: Ca2+mobilization, IP3, Ca2+-induced inactivation of Ca2+current, perforated patch-clamp.

Biosensors to measure inositol 1,4,5-trisphosphate concentration in living cells with spatiotemporal resolution

Phosphoinositides participate in many signaling cascades via phospholipase C stimulation, which hydrolyzes phosphatidylinositol 4,5-bisphosphate, producing second messengers diacylglycerol and inositol 1,4,5-trisphosphate (InsP3). Destructive chemical approaches required to measure [InsP3] limit spatiotemporal understanding of subcellular InsP3 signaling. We constructed novel fluorescence resonance energy transfer-based InsP3 biosensors called FIRE (fluorescent InsP3-responsive element) by fusing plasmids encoding the InsP3-binding domain of InsP3 receptors (types 1-3) between cyan fluorescent protein and yellow fluorescent protein sequences. FIRE was expressed and characterized in COS-1 cells, cultured neonatal cardiac myocytes, and incorporated into an adenoviral vector for expression in adult cardiac ventricular myocytes. FIRE-1 exhibits an approximately 11% increase in the fluorescence ratio (F530/F480) at saturating [InsP3] (apparent K(d) = 31.3 +/- 6.7 nm InsP3). In COS-1 cells, neonatal rat cardiac myocytes and adult cat ventricular myocytes FIRE-1 exhibited comparable dynamic range and a 10% increase in donor (cyan fluorescent protein) fluorescence upon bleach of yellow fluorescent protein, indicative of fluorescence resonance energy transfer. In FIRE-1 expressing ventricular myocytes endothelin-1, phenylephrine, and angiotensin II all produced rapid and spatially resolved increases in [InsP3] using confocal microscopy (with free [InsP3] rising to approximately 30 nm). Local entry of intracellular InsP3 via membrane rupture by a patch pipette (containing InsP3)in myocytes expressing FIRE-1 allowed detailed spatiotemporal monitoring of intracellular InsP3 diffusion. Both endothelin-1-induced and direct InsP3 application (via pipette rupture) revealed that InsP3 diffusion into the nucleus occurs with a delay and blunted rise of [InsP3] versus cytosolic [InsP3]. These new biosensors allow studying InsP3 dynamics at high temporal and spatial resolution that will be powerful in under-standing InsP3 signaling in intact cells.

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

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

Defective phosphatidic acid–phospholipase C signaling in diabetic cardiomyopathy

The effects of exogenous phosphatidic acid (PA) on Ca2+ transients and contractile activity were studied in cardiomyocytes isolated from chronic streptozotocin-induced diabetic rats. In control cells, 25 microM PA induced a significant increase in active cell shortening and Ca2+ transients. PA increased IP3 generation in the control cardiomyocytes and its inotropic effects were blocked by a phospholipase C inhibitor. In cardiomyocytes from diabetic rats, PA induced a 25% decrease in active cell shortening and no significant effect on Ca2+ transients. Basal and PA-induced IP3 generation in diabetic rat cardiomyocytes was 3-fold lower as compared to control cells. Sarcolemmal membrane PLC activity was impaired. Insulin treatment of the diabetic animals resulted in a partial recovery of PA responses. Our results, therefore, identify an important defect in the PA-PLC signaling pathway in diabetic rat cardiomyocytes, which may have significant implications for heart dysfunction during diabetes.

Inositol-1,4,5-trisphosphate-dependent Ca(2+) signalling in cat atrial excitation-contraction coupling and arrhythmias

Inositol-1,4,5-trisphosphate (IP(3))-dependent Ca(2+) release represents the major Ca(2+) mobilizing pathway responsible for diverse functions in non-excitable cells. In the heart, however, its role is largely unknown or controversial. In intact cat atrial myocytes, endothelin (ET-1) increased basal [Ca(2+)](i) levels, enhanced action potential-evoked [Ca(2+)](i) transients, caused [Ca(2+)](i) transients with alternating amplitudes (Ca(2+) alternans), and facilitated spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) in the form of Ca(2+) sparks and arrhythmogenic Ca(2+) waves. These effects were prevented by the IP(3) receptor (IP(3)R) blocker aminoethoxydiphenyl borate (2-APB), suggesting the involvement of IP(3)-dependent SR Ca(2+) release. In saponin-permeabilized myocytes IP(3) and the more potent IP(3)R agonist adenophostin increased basal [Ca(2+)](i) and the frequency of spontaneous Ca(2+) sparks. In the presence of tetracaine to eliminate Ca(2+) release from ryanodine receptor (RyR) SR Ca(2+) release channels, IP(3) and adenophostin triggered unique elementary, non-propagating IP(3)R-dependent Ca(2+) release events with amplitudes and kinetics that were distinctly different from classical RyR-dependent Ca(2+) sparks. The effects of IP(3) and adenophostin were prevented by heparin and 2-APB. The data suggest that IP(3)-dependent Ca(2+) release increases [Ca(2+)](i) in the vicinity of RyRs and thus facilitates Ca(2+)-induced Ca(2+) release during excitation-contraction coupling. It is concluded that in the adult mammalian atrium IP(3)-dependent Ca(2+) release enhances atrial Ca(2+) signalling and exerts a positive inotropic effect. In addition, by facilitating Ca(2+) release, IP(3) may also be an important component in the development of Ca(2+)-mediated atrial arrhythmias.

Depressed phosphatidic acid-induced contractile activity of failing cardiomyocytes

The effects of phosphatidic acid (PA), a known inotropic agent, on Ca(2+) transients and contractile activity of cardiomyocytes in congestive heart failure (CHF) due to myocardial infarction were examined. In control cells, PA induced a significant increase (25%) in active cell shortening and Ca(2+) transients. The phospholipase C (PLC) inhibitor, 2-nitro-4-carboxyphenyl N,N-diphenylcarbonate, blocked the positive inotropic action induced by PA, indicating that PA induces an increase in contractile activity and Ca(2+) transients through stimulation of PLC. Conversely, in failing cardiomyocytes there was a loss of PA-induced increase in active cell shortening and Ca(2+) transients. PA did not alter resting cell length. Both diastolic and systolic [Ca(2+)] were significantly elevated in the failing cardiomyocytes. In vitro assessment of the cardiac sarcolemmal (SL) PLC activity revealed that the impaired failing cardiomyocyte response to PA was associated with a diminished stimulation of SL PLC activity by PA. Our results identify an important defect in the PA-PLC signaling pathway in failing cardiomyocytes, which may have significant implications for the depressed contractile function during CHF.

The role of serotonin in ischemic cellular damage and the infarct size-reducing effect of sarpogrelate, a 5-hydroxytryptamine-2 receptor blocker, in rabbit hearts

OBJECTIVE

We aimed to clarify the relation between sarpogrelate (SG), a 5-hydroxytryptamine (5-HT)-2 receptor blocker, and myocardial interstitial serotonin or infarct size during ischemia and reperfusion.

BACKGROUND

In cardiac tissues serotonin is rich in vascular platelets, mast cells, sympathetic nerve endings, and the receptors are present in platelets and cardiomyocytes.

METHODS

The myocardial interstitial serotonin levels were measured using a microdialysis technique during 30-min ischemia with and without SG in in vivo as well as isolated rabbit hearts. Other rabbits underwent 30 min of ischemia and 48 h of reperfusion, and the effect of SG on the infarct size was investigated in the absence and presence of a selective protein kinase C (PKC) inhibitor, chelerythrine (5 mg/kg, intravenously), or a mitochondrial adenosine triphosphate sensitive potassium (KATP) channel blocker, 5-hydroxydecanoate (5-HD) (5 mg/kg, intravenously). In another series, the effect of SG on PKC isoforms in cytosol and membrane fraction was assessed after a 20-min global ischemia in isolated rabbit hearts.

RESULTS

Interstitial serotonin levels were markedly increased during 30-min ischemia in in vivo and isolated hearts, and the increases were inhibited by SG in each. The infarct size was reduced by SG (27 +/- 2% vs. 40 +/- 3% of control). This effect was blocked by chelerythrine and 5-HD, respectively. Sarpogrelate further enhanced the ischemia-induced translocation of PKC-epsilon to the membrane fraction.

CONCLUSIONS

Sarpogrelate reduces the myocardial infarct size by inhibiting the serotonin release followed by enhancement of PKC-epsilon translocation and opening of the mitochondrial KATP channel in ischemic myocytes.

Effect of P(i) on unloaded shortening velocity of slow and fast mammalian muscle fibers

Chemically skinned muscle fibers, prepared from the rat medial gastrocnemius and soleus, were subjected to four sequential slack tests in Ca(2+)-activating solutions containing 0, 15, 30, and 0 mM added P(i). P(i) (15 and 30 mM) had no effect on the unloaded shortening velocity (V(o)) of fibers expressing type IIb myosin heavy chain (MHC). For fibers expressing type I MHC, 15 mM P(i) did not alter V(o), whereas 30 mM P(i) reduced V(o) to 81 plus minus 1% of the original 0 mM P(i) value. This effect was readily reversible when P(i) was lowered back to 0 mM. These results are not compatible with current cross-bridge models, developed exclusively from data obtained from fast fibers, in which V(o) is independent of P(i). The response of the type I fibers at 30 mM P(i) is most likely the result of increased internal drag opposing fiber shortening resulting from fiber type-specific effects of P(i) on cross bridges, the thin filament, or the rate-limiting step of the cross-bridge cycle.

Gastric saline infusion reduces ultrasonic vocalizations and brown fat activity in suckling rat pups

Under standard conditions involving isolation and cooling, it has been documented that intraoral infusion of milk and injection of the intestinal peptide cholecystokinin (CCK) result in an attenuation in ultrasonic vocalizations (USV) emitted by infant rat pups. One of the most effective stimuli in inhibiting ingestion in suckling rat pups is gastric distension, but the effect of gastric distension on USV production has not been reported. In this experiment, we subjected infant rats to intragastric infusion of isotonic saline (2% body weight) to produce a natural level of gastric distension and hydration. We found that this stimulus resulted in a powerful reduction in USV emissions in isolated 10-day-old rats. In a subsequent experiment, we found that gastric saline infusion also diminished brown adipose tissue (BAT) thermogenesis. There were different time courses of the gastric saline infusion effects on BAT thermogenesis and on USV emissions, however, suggesting that these processes may be independently regulated. We hypothesize that this stimulus induces a transient activation of the parasympathetic nervous system, which overrides the sympathetic control of BAT and USV production.

Positive inotropic and negative lusitropic effect of angiotensin II: intracellular mechanisms and second messengers

In the cat ventricle angiotensin II exerts a positive inotropic effect produced by an increase in intracellular calcium associated with a prolongation of relaxation. The signaling cascades involved in these effects as well as the subcellular mechanisms of the negative lusitropic effect are still not clearly defined. The present study was directed to investigate these issues in cat papillary muscles and isolated myocytes. The functional suppression of the sarcoplasmic reticulum (SR) with either 0.5 microm ryanodine or 0.5 microm ryanodine plus 1 microm thapsigargin or the preincubation of the myocytes with the specific inhibitor of the inositol 1,4,5-triphosphate (IP3) receptors [diphenylborinic acid, ethanolamine ester (2-APB), 5-50 microm] did not prevent the positive inotropic effect and the increment in Ca2+ transient produced by 1 microm angiotensin II. In contrast, protein kinase C (PKC) inhibitors, chelerythrine (20 microm) and calphostin C (1 microm) completely inhibited both, the angiotensin II-induced increase in L-type calcium current and positive inotropic effect. The prolongation of half relaxation time produced by 0.5 microm angiotensin II [207+/-15.4 msec (control) to 235+/-19.98 msec (angiotensin II), P<0.05] was completely blunted by PKC inhibition. This antirelaxant effect, which was independent of intracellular pH changes, was associated with a prolongation of the action potential duration and was preserved after either the inhibition of the SR and the SR Ca2+ ATPase (ryanodine plus thapsigargin) or of the reverse mode of the Na+/Ca2+ exchanger (KB-R7943, 5 microm). We conclude that in feline myocardium the positive inotropic and negative lusitropic effects of angiotensin II are both entirely mediated by PKC without any significant participation of the IP3 limb of the phosphatidylinositol/phospholipase C cascade. The results suggest that the antirelaxant effect of angiotensin II might be determined by the decrease in Ca2+ efflux through the Na+/Ca2+ exchanger produced by the angiotensin II-induced prolongation of the action potential duration.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Mechanical stress stimulates phospholipase C activity and intracellular calcium ion levels in neonatal rat cardiomyocytes

To investigate how mechanical stress is sensed by cardiomyocytes and translated to cardiac hypertrophy, cardiomyocytes were subjected to stretch while measuring phospholipase C (PLC) and phospholipase D (PLD) activities and levels of intracellular calcium ions ([Ca2+]i) and pH. In stretched cardiomyocytes, PLC activity increased 2-fold after 30 min, whereas PLD activity hardly increased at all. Mechanical stress induced by prodding or by cell stretch increased [Ca2+](i)by a factor 5.2 and 4, respectively. Gadolinium chloride (stretch-activated channel blocker) attenuated the prodding-induced and stretch-induced [Ca2+](i)rise by about 50%. Blockade of ryanodine receptors by a combination of Ruthenium Red and procaine reduced the [Ca2+](i)rise only partially. Diltiazem (L-type Ca2+ channel antagonist) blocked the prodding-induced [Ca2+](i)rise completely, and reduced the stretch-induced [Ca2+](i)rise by about 50%. The stretch-induced [Ca2+](i)rise was unaffected by U73122, an inhibitor of PLC activity. Stretch did not cause cellular alkalinization. In conclusion, in cardiomyocytes, PLC and [Ca2+](i)levels are involved in the stretch-induced signal transduction, whereas PLD plays apparently no role. The stretch-induced rise in [Ca2+](i)in cardiomyocytes is most probably caused by [Ca2+](i)influx through L-type Ca2+ channels and stretch-activated channels, leading to Ca2+-induced Ca2+ -release from the SR via the ryanodine receptor.

Species- and temperature-dependency of the decrease in myofilament Ca2+ sensitivity induced by beta-adrenergic stimulation

Although beta-adrenergic stimulation has been shown in many studies to decrease myofilament Ca2+ sensitivity in various types of cardiac muscle such as rat and rabbit ventricles, other studies disagree with this conclusion. In the present study, we aimed to explain these contradictory findings. We examined the effect of beta-adrenoceptor stimulation on Ca2+ sensitivity using guinea pig and rat ventricles. We performed the experiment at two different temperatures and compared the results. In guinea pig ventricles, isoproterenol and forskolin did not alter the relationship between [Ca2+]i and muscle force during the relaxation phase of tetanic contraction at either 24 degrees C or 30 degrees C. In rat ventricles, in contrast, isoproterenol shifted the [Ca2+]i-force curve to the right at 24 degrees C, but not at 30 degrees C. In guinea pig ventricles permeabilized by alpha-toxin, in which the cAMP/PK-A system is intact, the addition of cAMP did not decrease Ca2+ sensitivity. These results suggest that there are species- and temperature-dependent differences in the regulation of myofilament Ca2+ sensitivity by beta-adrenergic stimulation.

Inositol polyphosphates regulate Ca2+ efflux in a cardiac membrane subtype distinct from junctional sarcoplasmic reticulum

The membrane location and mechanism of inositol 1,3,4,5-tetrakisphosphate (InsP4)-regulated Ca2+ uptake in cardiac membrane vesicles was investigated. In canine and rat membranes separated by sucrose density gradient centrifugation, InsP4-regulated Ca2+ uptake was slightly more enriched in low density than in higher density membranes. Membranes supporting InsP4-regulated Ca2+ uptake were correspondingly enriched in type 1 InsP3 receptors. Junctional sarcoplasmic reticulum (J-SR), enriched in sarcoplasmic reticulum Ca2+ ATPase (SERCA2a) and ryanodine receptors, separated predominantly with higher density membranes. In membranes supporting InsP4-regulated Ca2+ uptake, Ca2+ uptake was facilitated by a high Ca2+ affinity carrier that was insensitive to thapsigargin. Ca2+ uptake in J-SR was mediated by thapsigargin-sensitive SERCA2a. Net Ca accumulation was enhanced by oxalate in both SR subtypes. Although Ca2+-carrier-mediated Ca2+ uptake was ATP independent, ATP indirectly regulated net Ca2+ accumulation by modifying Ca2+ efflux via a Ca2+ channel with properties of type 1 InsP3 receptors. In the presence of < or = 0.1 mM ATP, InsP4 enhanced Ca2+ accumulation whereas InsP4 inhibited Ca2+ uptake at higher ATP concentrations. In the presence of 0.15 mM ATP, InsP4 stimulated Ca2+ efflux from vesicles preloaded with Ca. Several other InsP4 isomers and 1,3,4-InsP3 also stimulated Ca2+ efflux but with slightly less potency than 1,3,4,5-InsP4. Ruthenium red enhanced net Ca accumulation by the Ca2+ carrier and reduced the potency of ATP, InsP4, and InsP3 to stimulate Ca2+ efflux in vesicles. In summary, this investigation shows that a Ca2+ carrier facilitates Ca loading in a sarcoplasmic reticulum subtype distinct from J-SR. InsP4 and InsP3 are proposed to regulate Ca2+ efflux in low density SR by acting on an ATP-modulated Ca2+ channel with properties of type 1 InsP3 receptors.

Functional InsP3 receptors that may modulate excitation-contraction coupling in the heart

The roles of the Ca2+-mobilising messenger inositol 1,4,5-trisphosphate (InsP3) in heart are unclear, although many hormones activate InsP3 production in cardiomyocytes and some of their inotropic, chronotropic and arrhythmogenic effects may be due to Ca2+ release mediated by InsP3 receptors (InsP3Rs) [1-3]. In the present study, we examined the expression and subcellular localisation of InsP3R isoforms, and investigated their potential role in modulating excitation-contraction coupling (EC coupling). Western, PCR and InsP3-binding analysis indicated that both atrial and ventricular myocytes expressed mainly type II InsP3Rs, with approximately sixfold higher levels of InsP3Rs in atrial cells. Co-immunostaining of atrial myocytes with antibodies against type II ryanodine receptors (RyRs) and type II InsP3Rs revealed that the latter were arranged in the subsarcolemmal space where they largely co-localised with the junctional RyRs. Stimulation of quiescent or electrically paced atrial myocytes with a membrane-permeant InsP3 ester, which enters cells and directly activates InsP3Rs, caused the appearance of spontaneous Ca2+-release events. In addition, in paced cells, the InsP3 ester evoked an increase in the amplitudes of action potential-evoked Ca2+ transients. These data indicate that atrial cardiomyocytes express functional InsP3Rs, and that these channels could modulate EC coupling.

Cardiac excitation-contraction coupling in the portal hypertensive rat

Basal contractility and responses to beta-adrenoceptor activation are compromised in hearts from rats with chronic portal vein stenosis. Here we report the effect of partial ligation of the portal vein on myocardial G protein expression, beta-adrenoceptor-G protein coupling, and excitation-contraction coupling (ECC). Contractility (dT/dt) was reduced 30-50% in right and left ventricles, but the rate of relaxation (-dT/dt) was unaffected. Isoproterenol-induced positive inotropism was diminished, but there was no difference in ED(50). The concentration-dependent increase in -dT/dt was unaffected. G(s)alpha and G(i)alpha expression, cholera toxin- and pertussis toxin-induced ADP-ribosylation, and formation of the agonist-receptor-G(s) complex were unaffected by portal vein stenosis. Of the components of ECC examined, the caffeine-sensitive sarcoplasmic reticulum Ca(2+) pool was reduced 35%, although the Ca(2+) uptake and release processes were unchanged; the apparent density of L-type Ca(2+) channels decreased 60% with no change in affinity; the dihydropyridine Ca(2+) channel agonist BAY K 8644 produced relative changes in dT/dt that were similar in both groups, suggesting normal function in the remaining Ca(2+) channels; and Na(+)/Ca(2+) exchange was reduced 50% in the portal vein stenosis group. These data suggest that the effect of portal vein stenosis on the myocardium is the result of alterations to ECC.

Xestospongin C, a selective and membrane-permeable inhibitor of IP(3) receptor, attenuates the positive inotropic effect of alpha-adrenergic stimulation in guinea-pig papillary muscle

We evaluated the role of the inositol 1,4,5-triphosphate (IP(3)) receptor-mediated Ca(2+) release on the positive inotropic effects of alpha-adrenergic stimulation using a novel, potent, selective membrane-permeable blocker of IP(3) receptor, xestospongin C. Guinea-pig papillary muscle permeabilized with saponin exhibited spontaneous oscillatory contractions in solution buffered with pCa(2+) 6.5 by a low concentration of EGTA. The oscillatory activity was increased by adding 100 microM IP(3) and abolished by 1 microM ryanodine or 30 microM cyclopiazonic acid. Xestospongin C (3 microM) inhibited the IP(3)-induced increase in the oscillatory contractions without affecting basal oscillations. In intact papillary muscle, xestospongin C (3 microM) inhibited the positive inotropic effects of phenylephrine, resulting in a rightward and downward shift of the concentration-response curve for phenylephrine. On the contrary, xestospongin C did not affect the concentration-response curve for phenylephrine obtained in the presence of ryanodine (1 microM). On the other hand, xestospongin C affected neither basal contractions nor the positive inotropic effects of a high extracellular Ca(2+) concentration (3.2 mM) or that of isoprenaline (1 and 10 nM). These results suggest that the IP(3)-mediated increase in Ca(2+) release is involved in the positive inotropic effects of alpha-adrenergic stimulation in the guinea-pig cardiac muscle.

Cardiovascular α1-adrenoceptor subtypes: functions and signaling

α1-Adrenoceptors (α1AR) are G protein-coupled receptors and include α1A, α1B, and α1D subtypes corresponding to cloned α1a, α1b, and α1d, respectively. α1AR mediate several cardiovascular actions of sympathomimetic amines such as vasoconstriction and cardiac inotropy, hypertrophy, metabolism, and remodeling. α1AR subtypes are products of separate genes and differ in structure, G protein-coupling, tissue distribution, signaling, regulation, and functions. Both α1AAR and α1BAR mediate positive inotropic responses. On the other hand, cardiac hypertrophy is primarily mediated by α1AAR. The only demonstrated major function of α1DAR is vasoconstriction. α1AR are coupled to phospholipase C, phospholipase D, and phospholipase A2; they increase intracellular Ca2+ and myofibrillar sensitivity to Ca2+ and cause translocation of specific phosphokinase C isoforms to the particulate fraction. Cardiac hypertrophic responses to α1AR agonists might involve activation of phosphokinase C and mitogen-activated protein kinase via Gq. α1AR subtypes might interact with each other and with other receptors and signaling mechanisms.Key words: cardiac hypertrophy, inotropic responses, central α1-adrenoreceptors, arrythmias.

Vasopressin-evoked [Ca2+]i responses in neonatal rat cardiomyocytes

The presence of arginine vasopressin (AVP) V1 receptors on neonatal rat cardiomyocytes (NRCs) linked to processes capable of elevating intracellular free calcium ([Ca2+]i) is now firmly established. This study examined the sources and signaling involved in [Ca2+]i elevations evoked by AVP in NRCs. AVP promoted increases in both [Ca2+]i and 1,4,5-inositoltrisphosphate (IP3) levels in NRCs. The degree of [Ca2+]i elevation was less than that of angiotensin II, but greater than that of endothelin-1. Extracellular Mg2+ depletion led to diminution of the maximal [Ca2+]i response, with a rightward shift in the concentration-response curves to AVP. The phospholipase C inhibitors, D-609, NCDC, or U73122, and the IP3 receptor blocker, heparin, abolished the [Ca2+]i response to AVP. Neither cyclooxygenase inhibition with indomethacin nor PKC inhibition with staurosporine had any effect. Neither ryanodine nor caffeine, which deplete sarcoplasmic reticulum (SR) Ca2+ stores, nor ruthenium red, which inhibits both SR and mitochondrial Ca2+ stores, affected [Ca2+]i responses to AVP. The SR Ca2+ pump inhibitor, cyclopiazonic acid, abolished, and removal of extracellular Ca2+ attenuated, the response to AVP. These data indicate that activation of cardiac V1 receptors by AVP results in mobilization of Ca2+ from a distinct, non-SR, nonmitochondrial, intracellular Ca2+ pool that is Ca2+ pump replenished and IP3 sensitive. This process occurs secondary to phospholipase C (PLC)-mediated generation of IP3, requires the presence of Mg2+ and extracellular Ca2+, and occurs in a manner independent of PKC and cyclooxygenase activation. Such mechanisms of Ca2+ mobilization might indicate a distinct role for AVP in cardiac physiology and disease.

Adenosine A2a-receptor activation enhances cardiomyocyte shortening via Ca2+-independent and -dependent mechanisms

Adenosine A2a receptor (A2aR) stimulation enhances the shortening of ventricular myocytes. Whether the A2aR-mediated increase in myocyte contractility is associated with alterations in the amplitude of intracellular Ca2+ transients was investigated in isolated, contracting rat ventricular myocytes using the Ca2+-sensitive fluorescent dye fura 2-AM. In the presence of intact inhibitory G protein pathways, 10(-4) M 2-p-(2-carboxyethyl)phenethyl-amino-5'-N-ethylcarboxamidoadenosine (CGS-21680), an A2aR agonist, insignificantly increased Ca2+ transients by 8 +/- 5%, whereas myocyte shortening increased by 54 +/- 1%. In contrast, 2 x 10(-7) M isoproterenol, a beta-adrenergic receptor agonist, increased Ca2+ transients by 104 +/- 15% and increased myocyte shortening by 61 +/- 6%. When A2aR were stimulated in myocytes that had the antiadrenergic actions of adenosine (Ado) abolished by either treatment with pertussis toxin (PTx) or the presence of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an adenosine A1-receptor antagonist, the maximum increases in Ca2+ transients were similarly nominal (with PTx: 10(-4) M CGS-21680, 14 +/- 6% and 10(-4) M Ado, 15 +/- 4%; without PTx: 10(-5) M Ado + 2 x 10(-7) M DPCPX, 19 +/- 1%). These results indicate that compared with beta-adrenergic stimulation, which markedly increases myocyte Ca2+ transients and shortening, A2aR-mediated increases in myocyte shortening are accompanied by only modest increases in Ca2+ transients. These observations suggest that the A2aR-induced contractile effects are mediated predominantly by Ca2+-independent inotropic mechanisms.

The role of calcium in the response of cardiac muscle to stretch

This review focuses on the complex interactions between two major regulators of cardiac function; Ca2+ and stretch. Initial consideration is given to the effect of stretch on myocardial contractility and details the rapid and slow increases in contractility. These are shown to be related to two diverse changes in Ca2+ handling (enhanced myofilament Ca2+ sensitivity and increased intracellular Ca2+ transient, respectively). Interaction between stretch and Ca2+ is also demonstrated with respect to the rhythm of cardiac contraction. Stretch has been shown to alter action potential configuration, generate stretch-activated arrhythmias, and increase the rate of beating of the sino-atrial node. A variety of Ca(2+)-dependent mechanisms including attenuation of Ca2+ extrusion via Na+/Ca2+ exchange, Ca2+ entry through stretch-activated channels (SACs) and mobilisation of intracellular Ca2+ stores have been proposed to account for the effect of stretch on rhythm. Finally, the interaction between stretch and Ca2+ in the secretion of natriuretic peptides and onset of hypertrophy is discussed. Evidence is presented that Ca2+ (entering through L-type Ca2+ channels or SACs, or released from sarcoplasmic reticular stores) influences secretion of both atrial and B-type natriuretic peptide; there is data to support both positive and negative modulation by Ca2+. Ca2+ also appears to be important in the pathway that leads to expression of precursors of hypertrophic protein synthesis. In conclusion, two of the major regulators of cardiac muscle function, Ca2+ and stretch, interact to produce effects on the heart; in general these effects appear to be additive.

Effects of sympathetic nerve stimulation on membrane potential, [Ca2+]i, and force in the toad sinus venosus

The effects of sympathetic nerve stimulation on beat rate, force, intracellular Ca2+ concentration ([Ca2+]i) measured using fura 2, and membrane potential were recorded from the spontaneously beating toad sinus venosus. Short trains of stimuli evoked an increase in the beat rate and force. During this tachycardia the amplitude of pacemaker action potentials was not changed, but there was an increase in the basal level of [Ca2+]i with little change in peak [Ca2+]i measured during each action potential. Depletion of intracellular Ca2+ stores with caffeine (3 mM) abolished all responses to sympathetic nerve stimulation. The effects of caffeine were fully reversible. Caffeine (3 mM), in the presence of the Ca2+-ATPase inhibitor thapsigargin (30 microM), abolished irreversibly the chronotropic and inotropic responses evoked by sympathetic nerve stimulation. Ryanodine (10 microM) attenuated, but did not abolish, these responses. These results suggest that, in the toad sinus venosus, increases in force and beat rate evoked by sympathetic nerve stimulation result from the release of Ca2+ from intracellular Ca2+ stores.

Highly selective sigma receptor ligands elevate inositol 1,4,5-trisphosphate production in rat cardiac myocytes

Exposure of cardiac myocytes from adult rat ventricles to the highly selective, high affinity sigma receptor ligands 1S,2 R-cis-N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)-cycloh exylamine (BD-737) (0.1-100 nM) and N-[2-(3,4-dichlorophenyl)ethyl]-N,N',N'-trimethylethylenediamine (BD-1047) (0.01-10 nM), caused potentiation of electrically-evoked amplitudes of contraction and Ca2+ transients, while exposure to 100 nM BD-1047 caused attenuation of these amplitudes. In addition, BD-737 (1-100 nM) and BD-1047 (10-100 nM) caused an increase in the incidence of spontaneous twitches. These effects were inhibited when the incubation with BD-737 was done in the presence of the phospholipase C inhibitor, neomycin, or after pre-incubation with thapsigargin or caffeine which deplete the sarcoplasmic reticulum Ca2+ stores. Inositol 1,4,5-trisphosphate (IP3) production in cardiac myocytes was determined by the IP3 binding protein assay. Both substances caused an increase in the intracellular concentration of IP3. BD-737 caused a rapid transient increase to 3.2-fold in 1 min and stabilization at 2.1-fold of control thereafter. BD-1047 caused a gradual increase reaching 4.4-fold after 5 min. The results suggest that the effects of these sigma receptor ligands on contractility and spontaneous contractions are mediated by activation of phospholipase C and elevation of intracellular IP3 level.

alpha-Adrenergic effects on Na+-K+ pump current in guinea-pig ventricular myocytes

1. The whole-cell patch clamp was employed to study Na+-K+ pump current (Ip) in acutely isolated myocytes. alpha-Adrenergic receptors were activated with noradrenaline (NA) after blocking beta-adrenergic receptors with propranolol. Ip was measured as the current blocked by strophanthidin (Str). 2. Activation of alpha-receptors by NA increased Ip in a concentration-dependent manner. The K0.5 depended on intracellular calcium ([Ca2+]i), however maximal stimulation did not. At 15 nM [Ca2+]i the K0.5 was 219 nM NA whereas at 1.4 microM [Ca2+]i it was 3 nM. 3. The voltage dependence of Ip was not shifted by NA at either high or low [Ca2+]i. At each voltage, maximal stimulation of Ip was 14-15 %. 4. Staurosporine (St), an inhibitor of protein kinase C (PKC), eliminated the alpha-receptor-mediated stimulation of Ip at either high or low[Ca2+]i. 5. The stimulation of Ip was independent of changes in intracellular sodium or external potassium concentrations, and did not reflect a change in affinity for Str. 6. Phenylephrine, methoxamine and metaraminol, three selective alpha1-adrenergic agonists, stimulate Ip in a similar manner to NA. Stimulation of Ip by NA was eliminated by prazosin, an alpha1-antagonist, but was unaffected by yohimbine, an alpha2-antagonist. 7. We conclude noradrenaline activates ventricular alpha1-receptors, which are specifically coupled via PKC to increase Na+-K+ pump current. The sensitivity of the coupling is [Ca2+]i dependent, however the maximal increase in pump current is [Ca2+]i and voltage independent.

Effects of cardiac versus circulatory angiotensin-converting enzyme inhibition on left ventricular diastolic function and coronary blood flow in hypertrophic obstructive cardiomyopathy

BACKGROUND

Left ventricular (LV) diastolic function and coronary flow are impaired in hypertrophic obstructive cardiomyopathy (HOCM). This study was designed to evaluate the impact of cardiac and circulatory ACE inhibition on such derangements.

METHODS AND RESULTS

Twenty patients with HOCM underwent cardiac ACE inhibition with intracoronary (IC) enalaprilat (0.05 mg/min infused into the left anterior descending coronary artery for 15 minutes) followed by circulatory ACE inhibition with 25 mg sublingual (SL) captopril. Contrast ventriculography, pressure, and coronary flow measurements were performed at baseline, after IC enalaprilat infusion, and 45 minutes after SL captopril. Heart rate was not affected by the respective interventions (75+/-11 versus 76+/-13 versus 75+/-10 bpm; P=NS), whereas mean aortic pressure dropped slightly after IC enalaprilat and significantly after SL captopril (90+/-8 versus 85+/-10 versus 74+/-9 mm Hg; P<.05). Compared with baseline, IC enalaprilat resulted in a decrease in LV end-diastolic pressure (17.6+/-5.9 versus 14.4+/-4.9 mm Hg; P<.05), time constant of isovolumic LV pressure relaxation (tauG) (69+/-9 versus 52+/-10 ms; P<.05), and outflow gradient (45.2+/-6.9 versus 24.4+/-3.7 mm Hg; P<.05) and in an increase in coronary blood flow (107+/-10 versus 127+/-12 mL/min; P<.05) and coronary flow reserve (2.2+/-0.4 versus 2.6+/-0.3; P<.05). After SL captopril, tauG was prolonged (60+/-13 ms; P<.05 versus IC enalaprilat), and LV outflow gradient, coronary blood flow, and coronary flow reserve values returned to baseline (45.5+/-5.3 mm Hg, 107+/-12 mL/min, and 2.2+/-0.5, respectively; P=NS versus baseline).

CONCLUSIONS

Activation of the cardiac renin-angiotensin system contributes to LV diastolic dysfunction as well as to the decreased coronary blood flow and coronary flow reserve in HOCM. Cardiac ACE inhibition restores and circulatory ACE inhibition aggravates the above derangements.

Tumour necrosis factor-alpha, but not septic plasma depresses cardiac myofilament contraction

PURPOSE

In sepsis, myocardial depression may be caused by mediators released as part of the inflammatory reaction, lumour necrosis factor alpha (TNF alpha) is one mediator that may contribute to this depression. In the present study, we contrasted the effects of TNF alpha and septic plasma fraction (SP) obtained from an E. coli model on contractile tension in intact and skinned canine ventricular trabecular (VT) preparations. The objectives were to determine whether SP or TNF alpha could impair contractile tension at the level of the myofilaments, and to determine the extent to which TNF alpha may account for myocardial depression found in E. coli sepsis.

METHODS

Measurements of isometric tensions were made after TNF alpha and SP (10,000 to 30,000 MW fraction) were added to respective intact or skinned canine VT preparations. In the skinned preparation, trabeculae were chemically skinned with Triton X-100.

RESULTS

Septic plasma caused a decrease in contraction in the intact preparation compared with preseptic plasma (50 +/- 7 vs 33 +/- 7%, P < 0.05), but had no effect in the skinned preparation. On the other hand, TNF alpha (30 ng.ml-1) caused an approximately 50% reduction in tension (29 +/- 2 mg vs 16 +/- 5 mg) in the skinned preparation (P < 0.05), but had no effect in the intact preparation.

CONCLUSION

These results suggest that TNF alpha and SP act through different mechanisms. While SP requires an intact membrane, TNF alpha impairs function by a direct effect on the myofilaments.

Heterogeneity and underlying mechanism for inotropic action of endothelin-1 in rat ventricular myocytes

1. To clarify the mechanisms underlying the positive inotropic action of endothelin-1 (ET-1), we investigated the effect of ET-1 on twitch cell shortening and the Ca2+ transient in rat isolated ventricular myocytes loaded with a fluorescent Ca2+ indicator indo-1. 2. There was a cell-to-cell heterogeneity in response to ET-1. ET-1 (100 nM) increased twitch cell shortening in only 6 of 14 cells (44%) and the increase in twitch cell shortening was always accompanied by an increase in the amplitude of the Ca2+ transient. 3. The ET(A)- and ET(B)-receptors antagonist TAK-044 (100 nM) almost reversed both the ET-1-induced increases in twitch cell shortening and in the Ca2+ transient. In the ET-1 non-responding cells, the amplitude of the Ca2+ transient never increased. 4. Intracellular pH slightly increased (approximately 0.08 unit) after 30 min perfusion of ET-1 in rat ventricular myocytes. However, ET-1 did not change the myofilament responsiveness to Ca2+, which was assessed by (1) the relationship between the Ca2+ transient amplitude and twitch cell shortening, and by (2) the Ca2+ transient-cell shortening phase plane diagram during negative staircase. 5. We concluded that there was a cell-to-cell heterogeneity in the positive inotropic effect of ET-1, and that the ET-receptor-mediated positive inotropic effect was mainly due to an increase in the Ca2+ transient amplitude rather than to an increase in myofilament responsiveness to Ca2+.

Possible mechanism of the negative inotropic effect of alpha1-adrenoceptor agonists in rat isolated left atria after exposure to free radicals

1. This study was designed to investigate the mechanism(s) of the negative inotropic effects of alpha1-adrenoceptor agonists observed in rat isolated left atria after exposure to free radicals. 2. Ouabain and calphostin C were used in contraction experiments to block the sodium pump and protein kinase C. Methoxamine-induced phospholipase C and Na+/K+ ATPase activities were measured. 3. Methoxamine (300 microM) increased contractile force by 1.6 +/- 0.2 mN in control atria but decreased contractile force in electrolysis-treated atria by 2.0 +/- 0.1 mN (P < 0.05), as determined 10 min after methoxamine addition. In contrast, the positive inotropic effects of endothelin-1 (30 nM) and isoprenaline (10 microM) were reduced from 2.6 +/- 0.3 to 1.3 +/- 0.1 mN and from 2.6 +/- 0.3 to 1.7 +/- 0.2 mN, respectively, by electrolysis treatment (P < 0.05), but not converted into a negative inotropic action. 4. In an inositol phosphate assay we observed that the stimulation of phospholipase C by methoxamine was attenuated by electrolysis when the (electrolyzed) medium from the organ bath was used, but the phospholipase C responses were restored by the use of fresh medium. However, fresh medium did not counteract the negative inotropic effect of methoxamine. Accordingly, the negative inotropic effect of methoxamine is not directly related to the impaired phospholipase C responses seen in atria subjected to electrolysis. 5. Ouabain (10 microM) and the protein kinase C inhibitor calphostin C (50 nM), completely prevented the negative inotropic effect of 300 microM methoxamine in electrolysis-treated atria. 6. Measurement of the Na+/K+ ATPase activity, revealed that in control atria, alpha1-adrenoceptor stimulation with 300 microM methoxamine, decreased the Na+/K+ ATPase activity by 14.4 +/- 7.7%. In contrast, methoxamine increased the Na+/K+ ATPase activity by 48.8 +/- 8.9% (P < 0.05) in electrolysis-treated atria. Interestingly, this increase in Na+/K+ ATPase activity was completely counteracted by calphostin C (1.4 +/- 0.1% over basal). 7. These results indicate that the negative inotropic effects of alpha1-adrenoceptor agonists, observed in rat isolated left atria exposed to free radicals, are likely to be caused by protein kinase C-mediated phosphorylation and subsequent activation of the Na+/K+ ATPase.

Different mechanisms for [Ca2+]i oscillations induced by carbachol and high concentrations of [Ca2+]o in the rat ventricular myocyte

1. The purpose of the present study was to explore the different mechanisms of [Ca2+]i oscillations induced by high concentrations of either carbachol (CCh) or extracellular Ca2+ ([Ca2+]o). First, we compared the oscillations induced by CCh at concentrations of 100-300 micromol/L and [Ca2+]o (5 mmol/L) in the single rat ventricular myocyte. Second, we studied CCh- and [Ca2+]o-induced [Ca2+]i oscillations following either interference with the production of inositol trisphosphate (IP3), reductions in cytosolic Ca2+ ([Ca2+]i), inhibition of Ca2+ influx and Na+-Ca2+ exchange or depletion of Ca2+ from its intracellular store. 2. The [Ca2+]i oscillations induced by CCh were frequent and were superimposed on [Ca2+]i transients in electrically stimulated cells, whereas those induced by high [Ca2+]o were occasional and occurred in quiescent cells and between [Ca2+]i transients in electrically stimulated cells. In both cases, [Ca2+]i oscillations were preceded by an increase in resting levels of [Ca2+]i. 3. Carbachol-induced [Ca2+]i oscillations were accompanied by an increase in amplitude and prolongation of the time of decline to 80% of the peak of the [Ca2+]i transient, while high [Ca2+]o-induced [Ca2+]i oscillations were the opposite. 4. A reduction of [Ca2+]o to 0.1 mmol/L and treatment with Ni2+ or ryanodine or 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid AM (BAPTA-AM) abolished the [Ca2+]i oscillations induced by both CCh and high [Ca2+]o. 5. The calcium channel blockers verapamil and nifedipine and inhibitors of phospholipase C (neomycin and U-73122) abolished the [Ca2+]i oscillations induced by CCh; Li+ accelerated the onset of the [Ca2+]i oscillations induced by CCh. 6. These observations suggest that the mechanisms responsible for the [Ca2+]i oscillations induced by CCh and high [Ca2+]o are different from each other. Other than an increase in extracellular Ca2+ influx as a mechanism common for both CCh- and high [Ca2+]o-induced [Ca2+]i oscillations, the CCh-induced [Ca2+]i oscillations involve influx of Ca2+ via L-type Ca2+ channels, Na+-Ca2+ exchange, mobilization of intracellular Ca2+ and IP3 production.

Effects of sympathetic nerve stimulation on membrane potential, [Ca2+]i and force in the arrested sinus venosus of the toad, Bufo marinus

1. The effects of sympathetic nerve stimulation on membrane potential and on the intracellular concentration of calcium ions, [Ca2+]i, were recorded concurrently from the sinus venosus of the toad, Bufo marinus, in preparations where beating had been abolished by adding an organic calcium antagonist to the physiological saline. In a separate set of experiments the effects of sympathetic nerve stimulation on force production were examined. 2. Stimulation of the sympathetic nerves caused a membrane depolarization and a simultaneous increase in [Ca2+]i. Both responses were reduced by dihydroergotamine (20 microM). 3. The membrane depolarization and increase in [Ca2+]i evoked by sympathetic nerve stimulation were abolished by ryanodine (10 microM), or caffeine (3 mM). The effects of caffeine, but not those of ryanodine, were fully reversible. 4. Although the Ca(2+)-ATPase inhibitor thapsigargin (30 microM) itself had little effect on the responses to sympathetic nerve stimulation, in its presence caffeine (3 mM) irreversibly abolished the responses. 5. In the presence of nifedipine (10 microM), sympathetic nerve stimulation caused contractions of the sinus venosus. These responses were abolished by either ryanodine (10 microM) or caffeine (3 mM). 6. The results suggest that neuronally released transmitter activates a complex biochemical pathway which triggers the release of Ca2+ from internal stores.

Differential Effects of Protein Kinase C Activators and Inhibitors on alpha- and beta-Adrenoceptor-mediated Positive Inotropic Effect in Isolated Rabbit Papillary Muscle

BACKGROUND: A number of novel agents that activate or inhibit protein kinase C (PKC) in vitro have been developed to evaluate the physiologic role of PKC in regulation of cellular function. However, most of the PKC inhibitors also affect the protein kinase A, and the effects of these agents in intact myocardium remain still controversial. The present study was carried out to examine the effects of these agents on the positive inotropic effect (PIE) medicated by alpha- and beta-adrenoceptors in isolated rabbit papillary muscle. METHODS AND RESULTS: A potent PKC activator, phorbol 12, 13-dibutyrate (PDBu) at 10 and 30 nM, induced a significant PIE. PDBu at 3 nM and higher inhibited the alpha-mediated PIE and abolished it at 100 nM without affecting the beta-mediated PIE. Phorbol 12-myrisate 13-acetate (PMA) and 1-oleyl-2-acetyl-sn-glycerol (OAG) elicited a similar selective inhibitory action on the alpha-mediated PIE. The PIE of PDBu was abolished by chelerythrine and partially inhibited by staurosporine, but H-7 or calphostin-C did not affect the PIE. These PKC inhibitors consistently inhibited the alpha-mediated PIE by 20-30% at concentrations that they did not affect the beta-mediated PIE. None of the PKC inhibitors influence the PDBu-induced inhibitory action on the alpha-mediated PIE, an indication that they failed to reach the site of the inhibitory action of PDBu. CONCLUSION: Selective modulation by the PKC activators and inhibitors of the alpha-mediated PIE with little effect on the beta-mediated PIE implies that the activation of PKC has a physiological relevance to the alpha-mediated PIE. However, the externally administered PKC activators do not mimic the effect of diacylglycerol that is generated endogenously by alpha-stimulation. By contrast, externally applied PKC inhibitors selectively antagonize the alpha-adrenoreceptor-mediated PIE in rabbit ventricular myocardium.

Positive inotropic effect of angiotensin II. Increases in intracellular Ca2+ or changes in myofilament Ca2+ responsiveness?

Although it is well known that Angiotensin II (Ang II) has a direct positive inotropic effect in several species, the mechanisms of this action are still poorly understood. The aim of this review is to analyze the possible subcellular mechanisms underlying Ang II-induced positive inotropic action. The binding of Ang II to its receptor triggers a complex signal transduction cascade that stimulates the intracellular formation of two second messengers, inositol 1,4,5-triphosphate (IP3), and 1,2, diacylglycerol (DAG). IP3 triggers the release of Ca2+ from intracellular stores in several cell types and has been shown to increase myofilament Ca2+ sensitivity. DAG activates protein kinase C (PKC), an enzyme that catalyzes the phosphorylation of different cellular proteins, including several proteins of the myofibrils. Distinct ionic transporters, like the Na+/H+ antiporter and the Na(+)-independent Cl-/HCO3- exchanger, implicated in the regulation of intracellular pH, and the Na+/Ca2+ exchanger which contribute to the intracellular Ca2+ homeostasis, have been shown to be activated by a PKC-dependent mechanism. Thus, either one of the Ang II-induced second messengers, that is, IP3 and DAG, has the potential to affect myocardial contractility by modifying either intracellular Ca2+, myofilament Ca2+ responsiveness, or both. As described herein, the available data do not allow a definitive single model to explain the mechanism of the Ang II-induced positive inotropic effect. Moreover, it is possible that the final action of Ang II on myocardial inotropism is the end product of a complex interaction of several of the mechanisms triggered by the hormone.

Diastolic properties in canine hypertensive left ventricular hypertrophy: effects of angiotensin converting enzyme inhibition and angiotensin II type-1 receptor blockade

OBJECTIVE

Angiotensin II has been suggested to be involved in the pathogenesis of diastolic dysfunction in left ventricular hypertrophy (LVH). The purpose of this study was to asses the effects of enalaprilat and L-158,809, an angiotensin II type-1 receptor antagonist, on LV diastolic function in 16 normal control dogs and 20 LVH dogs with perinephritic hypertension.

METHODS

LV hemodynamics was studied before and after intravenous injection of enalaprilat (0.25 mg/kg) or L-158,809 (0.3 mg/kg). The hemodynamic data were analyzed in relation to the changes in myocardial blood flow (measured by radioactive microspheres) and in the circulating angiotensin II and norepinephrine levels.

RESULTS AND CONCLUSIONS

At baseline, significant increases were observed for LV/body weight ratio as well as LV systolic and end-diastolic pressure in the LVH dogs (all P < 0.01 vs. the control group). In addition, LV relaxation time constant was prolonged and the chamber and myocardial stiffness constants were increased (P < 0.01) in the LVH dogs, suggesting an impairment of LV diastolic function. Administration of enalaprilat or L-158,809 improved LV stiffness constants in the LVH dogs (P < 0.05). The diastolic LV pressure-diameter relation shifted downwards in the LVH dogs whereas diastolic distensibility was not altered in the control dogs. Although the circulating angiotensin II levels were significantly decreased by enalaprilat in the LVH dogs, they did not correlate with the changes in the stiffness constants. Furthermore, the alterations of LV diastolic properties in the LVH group could not be attributed to myocardial perfusion, which was rather decreased by administration of enalaprilat and L-158,809. These results suggest that angiotensin II, particularly at the local level, is involved in the pathogenesis of diastolic dysfunction in pressure-overload LVH. The data also support the concept that ACE inhibitors and angiotensin II receptor blockers are potentially beneficial in the treatment of the hypertrophied heart.

Inositol-1,4,5-trisphosphate mass content in isolated perfused rat heart during alpha-1-adrenoceptor stimulation

Inositol-1,4,5-trisphosphate (IP3) has been proposed to be a second messenger in response to alpha-1-adrenoceptor stimulation also in myocardial cells. We studied the effect of alpha-1-adrenoceptor stimulation (5 x 10(-5) mol/l phenylephrine or 5 x 10(-5) mol/l noradrenaline both in the presence of 10(-6) mol/l timolol) on IP3 mass content in isolated perfused rat hearts. IP3 content was determined by a specific receptor-binding assay-kit (TRK 1000, Amersham) after validating the method. For comparison also the effect of muscarinic stimulation (10(-4) mol/l carbachol in the presence of 10(-6) mol/l timolol) on IP3 content was measured in corresponding preparations. A basal IP3 level of about 75 pmol/mg protein was found. There were no prominent effects of alpha-1-adrenoceptor stimulation on total IP3 content in isolated perfused rat hearts. Phenylephrine gave a statistically significant increase of about 40% at 1/4 min and a statistically significant decrease of about 25% at 4 min after start of exposure. Noradrenaline, however, gave no statistically significant change of IP3 at the time-points studied. Muscarinic stimulation caused a slight, statistically insignificant, increase of IP3 at 1/4 min. The results are compatible with an assumption that agonist stimulation evokes a localized increase of IP3 which may be masked by a relatively high total IP3 mass content. The IP3 peak after phenylephrine coincided with the early positive inotropic phase of the response reported earlier in perfused rat hearts for alpha-1-adrenoceptor stimulation by phenylephrine. Although this might be compatible with a role for IP3 in this early and transient phase, a mediator function of IP3 in the inotropic response is not established.

Catecholamines and cardiac growth

The present knowledge concerning the alpha- and beta-adrenergic systems in the regulation of cardiac growth and gene expression is reviewed. To investigate the mechanism by which cAMP regulates the expression of cardiac genes we have used cultured myocytes derived from fetal rat hearts. We have shown previously that the addition of Br cAMP to the culture medium produced an increase in alpha-myosin heavy chain (alpha-MHC) mRNA level, in its rate of transcription as well as in the amount of V1 isomyosin. To characterize the promoter element(s) involved in cAMP responsive regulation of alpha-MHC expression we performed transient transfection analysis with a series of alpha-MHC gene promoter-CAT constructs. We have identified a 13 bp E-box/M-CAT hybrid motif (EM element) which conferred a basal muscle specific and cAMP inducible expression of the alpha-MHC gene. Using mobility shift assay we have documented that one of the EM element binding protein is TEF-1. Moreover, by incubating cardiac nuclear extracts with the catalytic subunit of PK-A we have found that factor(s) binding to the EM element is a substrate for cAMP dependent phosphorylation.

Purinergic stimulation of rat cardiomyocytes induces tyrosine phosphorylation and membrane association of phospholipase C gamma: a major mechanism for InsP3 generation

Phospholipase C gamma (PLC gamma) expression and activation by a purinergic agonist were investigated in adult rat cardiomyocytes. PLC gamma is expressed in isolated cardiomyocytes. Stimulation of cells with extracellular ATP induces a rapid increase in membrane-associated PLC gamma immunoreactivity most probably due to redistribution of the lipase from the cytosol to the membrane. The purine triggers a significant phosphorylation on tyrosine residues of a cytosolic pool of PLC gamma with a time course that correlates with that of translocation. Extracellular ATP also increases intracellular Ins(1,4,5)P3 content. All these events (translocation and phosphorylation of PLC gamma, InsP3 formation) are blocked by genistein, a tyrosine kinase inhibitor. The purinergic effect on both PLC gamma translocation and phosphorylation are Ca-sensitive. We thus propose that the purinergic stimulation activates a non-receptor tyrosine kinase that phosphorylates PLC gamma in the presence of an increased Ca level and induces PLC gamma redistribution to the membrane. There, PLC gamma becomes activated leading to the hydrolysis of phosphatidylinositol diphosphate and in turn Ins(1,4,5)P3 formation. This cascade of events may play a significant role in the induction of arrhythmogenesis by purinergic agonists.