A positive inotropic effect of ATP in the human cardiac atrium

Further investigations on the influence of protein phosphatases on the signaling of muscarinic receptors in the atria of mouse hearts

The vagal regulation of cardiac function involves acetylcholine (ACh) receptor activation followed by negative chronotropic and negative as well as positive inotropic effects. The resulting signaling pathways may include Gi/o protein-coupled reduction in adenylyl cyclase (AC) activity, direct Gi/o protein-coupled activation of ACh-activated potassium current (IKACh), inhibition of L-type calcium ion channels, and/or the activation of protein phosphatases. Here, we studied the role of the protein phosphatases 1 (PP1) and 2A (PP2A) for muscarinic receptor signaling in isolated atrial preparations of transgenic mice with cardiomyocyte-specific overexpression of either the catalytic subunit of PP2A (PP2A-TG) or the inhibitor-2 (I2) of PP1 (I2-TG) or in double transgenic mice overexpressing both PP2A and I2 (DT). In mouse left atrial preparations, carbachol (CCh), cumulatively applied (1 nM-10 µM), exerted at low concentrations a negative inotropic effect followed by a positive inotropic effect at higher concentrations. This biphasic effect was noted with CCh alone as well as when CCh was added after β-adrenergic pre-stimulation with isoprenaline (1 µM). Whereas the response to stimulation of β-adrenoceptors or adenosine receptors (used as controls) was changed in PP2A-TG, the response to CCh was unaffected in atrial preparations from all transgenic models studied here. Therefore, the present data tentatively indicate that neither PP2A nor PP1, but possibly other protein phosphatases, is involved in the muscarinic receptor-induced inotropic and chronotropic effects in the mouse heart.

Adenosine Triphosphate Stress Myocardial Strain in Ischemic Heart Disease: An Animal Study with Histological Validation

RATIONALE AND OBJECTIVES

It is still challenging for cardiac magnetic resonance (CMR) to detect ischemic heart disease (IHD) without the use of gadolinium contrast. We aimed to evaluate the potential value of adenosine triphosphate (ATP) stress myocardial strain derived from feature tracking (FT) as a novel method for detecting IHD in a swine model.

MATERIALS AND METHODS

CMR cines, myocardial perfusion imaging at rest and during ATP stress, and late gadolinium enhancement were obtained in both control and IHD swine. Normal, remote, ischemic, and infarcted myocardium were analyzed. The diagnostic accuracy of myocardial strain for infarction and ischemia was assessed using coronary angiography and pathology as reference.

RESULTS

Eleven IHD swine and five healthy control swine were enrolled in this study. Strain parameters, even at rest, were associated with myocardial ischemia and infarction(all p < 0.05). The area under receiver operating characteristic curve (AUC) values of all strain parameters for detecting infarcted myocardium exceeded 0.900 (all p < 0.05). The AUC values for detecting ischemic myocardium were as follows: 0.906 and 0.847 for stress and rest radial strain, 0.763 and 0.716 for stress and rest circumferential strain, 0.758 and 0.663 for stress and rest longitudinal strain (all p < 0.001). Heat maps demonstrated that all strain parameters showed mild to moderate correlations with the stress myocardial blood flow and myocardial perfusion reserve (all p < 0.05).

CONCLUSION

CMR-FT-derived ATP stress myocardial strain shows promise as a noninvasive method for detecting myocardial ischemia and infarction in an IHD swine model, with rest strain parameters offering potential as a needle-free diagnostic option.

The role of PP5 and PP2C in cardiac health and disease

Protein phosphatases are important, for example, as functional antagonists of β-adrenergic stimulation of the mammalian heart. While β-adrenergic stimulations increase the phosphorylation state of regulatory proteins and therefore force of contraction in the heart, these phosphorylations are reversed and thus force is reduced by the activity of protein phosphatases. In this context the role of PP5 and PP2C is starting to unravel. They do not belong to the same family of phosphatases with regard to sequence homology, many similarities with regard to location, activation by lipids and putative substrates have been worked out over the years. We also suggest which pathways for regulation of PP5 and/or PP2C described in other tissues and not yet in the heart might be useful to look for in cardiac tissue. Both phosphatases might play a role in signal transduction of sarcolemmal receptors in the heart. Expression of PP5 and PP2C can be increased by extracellular stimuli in the heart. Because PP5 is overexpressed in failing animal and human hearts, and because overexpression of PP5 or PP2C leads to cardiac hypertrophy and KO of PP5 leads to cardiac hypotrophy, one might argue for a role of PP5 and PP2C in heart failure. Because PP5 and PP2C can reduce, at least in vitro, the phosphorylation state of proteins thought to be relevant for cardiac arrhythmias, a role of these phosphatases for cardiac arrhythmias is also probable. Thus, PP5 and PP2C might be druggable targets to treat important cardiac diseases like heart failure, cardiac hypertrophy and cardiac arrhythmias.

P2 Receptors in Cardiac Myocyte Pathophysiology and Mechanotransduction

ATP is a major energy source in the mammalian cells, but it is an extracellular chemical messenger acting on P2 purinergic receptors. A line of evidence has shown that ATP is released from many different types of cells including neurons, endothelial cells, and muscle cells. In this review, we described the distribution of P2 receptor subtypes in the cardiac cells and their physiological and pathological roles in the heart. So far, the effects of external application of ATP or its analogues, and those of UTP on cardiac contractility and rhythm have been reported. In addition, specific genetic alterations and pharmacological agonists and antagonists have been adopted to discover specific roles of P2 receptor subtypes including P2X4-, P2X7-, P2Y2- and P2Y6-receptors in cardiac cells under physiological and pathological conditions. Accumulated data suggest that P2X4 receptors may play a beneficial role in cardiac muscle function, and that P2Y2- and P2Y6-receptors can induce cardiac fibrosis. Recent evidence further demonstrates P2Y1 receptor and P2X4 receptor as important mechanical signaling molecules to alter membrane potential and Ca²⁺ signaling in atrial myocytes and their uneven expression profile between right and left atrium.

Distinct shear-induced Ca2+ signaling in the left and right atrial myocytes: Role of P2 receptor context

Atrial myocytes are continuously exposed to shear stress during cardiac cycles. Previous reports have shown that shear stress induces two different types of global Ca²⁺ signaling in atrial myocytes-longitudinal Ca²⁺ waves (L-waves) and action potential-involved transverse waves (T-waves), and suggested an underlying role of the autocrine activation of P2 receptors. We explored the correlations between ATP release and Ca²⁺ wave generation in atrial myocytes and investigated why the cells develop two Ca²⁺-wave types during the same shear force. We examined whether ATP release correlates with different shear-stress (~16 dyn/cm²)-mediated Ca²⁺ signaling by simultaneous measurement of local Ca²⁺ and ATP release in individual atrial myocytes using two-dimensional confocal imaging and sniffer patch techniques, respectively. Functional P2X7-receptor-expressing HEK293 cells were established as sniffer cells, which generated currents in real time in response to ATP released from a closely positioned atrial myocyte. Both shear-stress-induced L- and T-waves were preceded by sniffer currents with no difference in the current magnitude. Left atrial (LA) myocytes had two- to three-fold larger sniffer currents than right atrial (RA) cells, as was confirmed by ATP chemiluminescence assay. Shear-stress-induced ATP release was eliminated by connexin (Cx) 43 hemichannel inhibition using La³⁺, Gap19, or knock-down of Cx43 expression. The level of phosphorylated Cx43 at Ser386 (p-Cx43^Ser368), but not total Cx43, was higher in LA versus RA myocytes. Most LA cells (~70%) developed L-waves, whereas most RA myocytes (~80%) presented T-waves. Shear-stress-induced T-waves were completely removed by inhibition of P2X4R, which were most abundant in rat atrial cells. Expression of P2X4R was higher in RA than LA myocytes, whereas expression of P2Y1R, the mediator of L-waves, was higher in LA than RA myocytes. ATP release mainly triggers L-waves in LA myocytes and T-waves in RA myocytes under the same shear force, partly because of the differential expression of P2Y1R and P2X4R between LA and RA myocytes. Higher ATP release in LA myocytes under shear stress may not contribute to determination of the wave pattern.

Mineralocorticoid receptor antagonists lead to increased adenosine bioavailability and modulate contractile cardiac parameters

Activation of mineralocorticoid receptor antagonists (MRAs) is cardioprotective; however, this property is lost upon blockade or inactivation of adenosine (ADO) receptor A_2b. In this study, we investigated whether the effects of MRAs are mediated by an interaction between cardioprotective ADO receptors A₁ and A₃. Spironolactone (SPI) or eplerenone (EPL) increased ADO levels in the plasma of treated animals compared to control animals. SPI or EPL increased the protein and activity levels of ecto-5'-nucleotidase (NT5E), an enzyme that synthesizes ADO, compared to control. The levels of ADO deaminase (ADA), which degrades ADO, were not affected by SPI or EPL; however, the activity of ADA was reduced in SPI-treated rats compared to control. Using an isolated cardiomyocyte model, we found inotropic and chronotropic effects, and increased calcium transient [Ca²⁺]_i in cells treated with ADO receptor A₁ or A₃ antagonists compared to control groups. Upon co-treatment with MRAs, EPL and SPI fully and partially reverted the effects of receptor A₁ or A₃ antagonism, respectively. Collectively, MRAs in vivo lead to increased ADO bioavailability. In vitro, the rapid effects of SPI and EPL are mediated by an interaction between ADO receptors A₁ and A₃.

The Ionotropic P2X4 Receptor has Unique Properties in the Heart by Mediating the Negative Chronotropic Effect of ATP While Increasing the Ventricular Inotropy

Background: Mounting evidence indicate that reducing the sinoatrial node (SAN) activity may be a useful therapeutic strategy to control of heart failure. Purines, like ATP and its metabolite adenosine, consistently reduce the SAN spontaneous activity leading to negative cardiac chronotropy, with variable effects on the force of myocardial contraction (inotropy). Apart from adenosine A₁ receptors, the human SAN expresses high levels of ATP-sensitive ionotropic P2X4 receptors (P2X4R), yet their cardiac role is unexplored. Methods: Here, we investigated the activity of P2 purinoceptors on isolated spontaneously beating atria (chronotropy) and on 2 Hz-paced right ventricular (RV, inotropy) strips from Wistar rats. Results: ATP (pEC ₅₀ = 4.05) and its stable analogue ATPγS (pEC ₅₀ = 4.69) concentration-dependently reduced atrial chronotropy. Inhibition of ATP breakdown into adenosine by NTPDases with POM-1 failed to modify ATP-induced negative chronotropy. The effect of ATP on atrial rate was attenuated by a broad-spectrum P2 antagonist, PPADS, as well as by 5-BDBD, which selectively blocks the P2X4R subtype; however, no effect was observed upon blocking the A₁ receptor with DPCPX. The P2X4R positive allosteric modulator, ivermectin, increased the negative chronotropic response of ATP. Likewise, CTP, a P2X agonist that does not generate adenosine, replicated the P2X4R-mediated negative chronotropism of ATP. Inhibition of the Na⁺/Ca²⁺ exchanger (NCX) with KB-R7943 and ORM-10103, but not blockage of the HCN channel with ZD7288, mimicked the effect of the P2X4R blocker, 5-BDBD. In paced RV strips, ATP caused a mild negative inotropic effect, which magnitude was 2 to 3-fold increased by 5-BDBD and KB-R7943. Immunofluorescence confocal microscopy studies confirm that cardiomyocytes of the rat SAN and RV co-express P2X4R and NCX1 proteins. Conclusions: Data suggest that activation of ATP-sensitive P2X4R slows down heart rate by reducing the SAN activity while increasing the magnitude of ventricular contractions. The mechanism underlying the dual effect of ATP in the heart may involve inhibition of intracellular Ca²⁺-extrusion by bolstering NCX function in the reverse mode. Thus, targeting the P2X4R activation may create novel well-tolerated heart-rate lowering drugs with potential benefits in patients with deteriorated ventricular function.

Evidence for Arrhythmogenic Effects of A2A-Adenosine Receptors

Adenosine can be released from the heart and may stimulate four different cardiac adenosine receptors. A receptor subtype that couples to the generation of cyclic adenosine monophosphate (cAMP) is the A_2A-adenosine receptor (A_2A-AR). To better understand its role in cardiac function, we studied mechanical and electrophysiological effects in transgenic mice that overexpress the human A_2A-AR in cardiomyocytes (A_2A-TG). We used isolated preparations from the left atrium, the right atrium, isolated perfused hearts with surface electrocardiogram (ECG) recording, and surface body ECG recordings of living mice. The hypothesized arrhythmogenic effects of transgenicity per se and A_2A-AR stimulation were studied. We noted an increase in the incidence of supraventricular and ventricular arrhythmias under these conditions in A_2A-TG. Moreover, we noted that the A_2A-AR agonist CGS 21680 exerted positive inotropic effect in isolated human electrically driven (1 Hz) right atrial trabeculae carneae. We conclude that A_2A-ARs are functional not only in A_2A-TG but also in isolated human atrial preparations. A_2A-ARs in A_2A-TG per se and their stimulation can lead to cardiac arrhythmias not only in isolated cardiac preparations from A_2A-TG but also in living A_2A-TG.

On inotropic effects of UTP in the human heart

Uridine 5'-triphosphate (UTP) exerts a positive inotropic effect (PIE) in isolated electrically driven isolated right atrial trabeculae carneae from patients undergoing heart surgery. This review discusses some aspects of the current knowledge on the putative receptor(s) involved and the potential biochemical transduction steps leading to the PIE.

Mechanism underlying the contractile activity of UTP in the mammalian heart

We previously reported that uridine 5'-triphosphate (UTP), a pyrimidine nucleoside triphosphate produced a concentration- and time-dependent increase in the contraction force in isolated right atrial preparations from patients undergoing cardiac bypass surgery due to angina pectoris. The stimulation of the force of contraction was sustained rather than transient. In the present study, we tried to elucidate the underlying receptor and signal transduction for this effect of UTP. Therefore, we measured the effect of UTP on force of contraction, phosphorylation of p38 and ERK1/2, in human atrial preparations, atrial preparations from genetically modified mice, cardiomyocytes from adult mice and cardiomyocytes from neonatal rats. UTP exerted a positive inotropic effect in isolated electrically driven left atrial preparations from wild-type (WT) mice and P2Y₂-, P2Y₄- and P2Y₆-receptor knockout mice. Therefore, we concluded that these P2Y receptors did not mediate the inotropic effects of UTP in atrial preparations from mice. However, UTP (like ATP) increased the phosphorylation states of p38 and ERK1/2 in neonatal rat cardiomyocytes, adult mouse cardiomyocytes and human atrial tissue in vitro. U0126, a MEK 1/2- signal cascade inhibitor, attenuated this phosphorylation and the positive inotropic effects of UTP in murine and human atrial preparations. We suggest that presently unknown receptors mediate the positive inotropic effect of UTP in murine and human atria. We hypothesize that UTP stimulates inotropy via p38 or ERK1/2 phosphorylation. We speculate that UTP may be a valuable target in the development of new drugs aimed at treating human systolic heart failure.

Increased Gi protein signaling potentiates the negative chronotropic effect of adenosine in the SHR right atrium

Hypertension is a risk factor for cardiovascular diseases, which have been associated with dysfunction of sympathetic and purinergic neurotransmission. Therefore, herein, we evaluated whether modifications of adenosine receptor signaling may contribute to the cardiac dysfunction observed in hypertension. Isolated right atria from spontaneously hypertensive (SHR) or normotensive Wistar rats (NWR) were used to investigate the influence of adenosine receptor signaling cascade in the cardiac chronotropism. Our results showed that adenosine, the endogenous agonist of adenosine receptors, and CPA, a selective agonist of A₁ receptor, decreased the atrial chronotropism of NWR and SHR in a concentration- and time-dependent manner, culminating in cardiac arrest (0 bpm). Interestingly, a 3-fold lower concentration of adenosine was required to induce the negative chronotropic effect in SHR atria. Pre-incubation of tissues from both strains with DPCPX, a selective A₁ receptor antagonist, inhibited the negative chronotropic effect of CPA, while simultaneous inhibition of A₂ and A₃ receptors, with ZM241385 and MRS1523, did not change the adenosine chronotropic effects. Moreover, 1 μg/ml pertussis toxin, which inactivates the Gαi protein subunit, reduced by 80% the negative chronotropic effects of adenosine in the NWR atrium, with minor effects in SHR tissue. These data indicate that the negative chronotropic effect of adenosine in right atrium depends exclusively on the activation of A₁ receptors. Moreover, the distinct responsiveness of NWR and SHR atria to pertussis toxin reveals that the enhanced negative chronotropic response of SHR right atrium is probably due to an increased activity of Gαi protein-mediated.

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

Novel protective role of endogenous cardiac myocyte P2X4 receptors in heart failure

BACKGROUND

Heart failure (HF), despite continuing progress, remains a leading cause of mortality and morbidity. P2X4 receptors (P2X4R) have emerged as potentially important molecules in regulating cardiac function and as potential targets for HF therapy. Transgenic P2X4R overexpression can protect against HF, but this does not explain the role of native cardiac P2X4R. Our goal is to define the physiological role of endogenous cardiac myocyte P2X4R under basal conditions and during HF induced by myocardial infarction or pressure overload.

METHODS AND RESULTS

Mice established with conditional cardiac-specific P2X4R knockout were subjected to left anterior descending coronary artery ligation-induced postinfarct or transverse aorta constriction-induced pressure overload HF. Knockout cardiac myocytes did not show P2X4R by immunoblotting or by any response to the P2X4R-specific allosteric enhancer ivermectin. Knockout hearts showed normal basal cardiac function but depressed contractile performance in postinfarct and pressure overload models of HF by in vivo echocardiography and ex vivo isolated working heart parameters. P2X4R coimmunoprecipitated and colocalized with nitric oxide synthase 3 (eNOS) in wild-type cardiac myocytes. Mice with cardiac-specific P2X4R overexpression had increased S-nitrosylation, cyclic GMP, NO formation, and were protected from postinfarct and pressure overload HF. Inhibitor of eNOS, L-N(5)-(1-iminoethyl)ornithine hydrochloride, blocked the salutary effect of cardiac P2X4R overexpression in postinfarct and pressure overload HF as did eNOS knockout.

CONCLUSIONS

This study establishes a new protective role for endogenous cardiac myocyte P2X4R in HF and is the first to demonstrate a physical interaction between the myocyte receptor and eNOS, a mediator of HF protection.

A positive inotropic effect of UTP in the human cardiac atrium

In the cardio-vascular system extracellular UTP can induce receptor-mediated vasoconstriction via smooth muscle cells and vasodilatation via endothelial cells. We evaluated inotropic effects of UTP in preparations from human heart. Contractile effects were studied in atrial preparations from patients undergoing cardiac bypass surgery. For comparison, contractility in isolated spontaneously beating right atrial and paced left atrial preparations from mice was investigated. UTP and UTPγS concentration-dependently exerted a positive inotropic effect with a maximum at 100 µM UTP that amounted to 156% of pre-drug value (n=13) without changing time parameters of contraction. UTP was able to partially attenuate the positive inotropic effect of β-adrenoceptor stimulation. UTP did not change the beating rate in right atrial mouse preparations. The positive inotropic effect of UTP could not be blocked by the P2 purinoceptor antagonists suramin (100 µM and 500 µM), PPADS (50 µM) and reactive blue (100 µM). Likewise inhibitors of PLC activity (U73122) and of adenylyl cyclase activity (SQ22563; 10 µM each) failed to affect the effects of UTP. In summary, we describe a novel positive inotropic effect of UTP on force contraction in the isolated human atrium. We tentatively suggest that UTP might act via P2Y2- or P2Y4-like receptors.

Differential regulation of atrial contraction by P1 and P2 purinoceptors in normotensive and spontaneously hypertensive rats

In the normotensive rat atrium, adenosine-5'-triphosphate and uridine-5'-triphosphate exert a biphasic effect consisting of an initial negative inotropic effect (NIE) followed by a subsequent positive inotropic effect (PIE). We comparatively studied these responses in normotensive Wistar rats (NWRs) and spontaneously hypertensive rats (SHRs). Compared with NWRs, the NIE responses in the atria were lower and the PIE responses were higher in SHRs. The P1 purinoceptor antagonist, D 8-cyclopentyl-1,3-dipropylxanthine, partially blocked the NIE responses of both ATP and UTP and mildly enhanced the PIE responses in both NWRs and SHRs. Furthermore, the P2 purinoceptor blockers suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid tetrasodium salt induced a pronounced block of the PIE responses in both atria types. The PIE responses to ATP were inhibited more efficiently by nifedipine. These responses were depressed by ryanodine and, to a lesser extent, carbonyl cyanide 3-chlorophenylhydrazone in SHR atria compared with NWR atria. The higher responses in SHR rats suggest the existence of an augmented endoplasmic reticulum Ca(2+) store and faster mitochondrial Ca(2+) cycling in SHR atria compared with NWR atria. These data support the hypothesis that a dysfunction of purinergic neurotransmission and enhanced sympathetic activity are contributing factors in the pathogenesis of hypertension.

The therapeutic effect of 2-cyclohexylthio-AMP in heart failure

AIM

: The aim of this study was to investigate the therapeutic effect of 2-cyclohexylthio-adenosine 5'-monophosphate (AMP) in mice with heart failure (HF).

METHODS

: 2-Cyclohexylthio-AMP was dissolved in phosphate-buffered saline and infused in mice with ischemic HF after permanent left coronary [left anterior descending (LAD)] ligation and in calsequestrin (CSQ) mice with HF. Myocardial function ex vivo was determined in the working heart model. Cardiac function in vivo was assessed by echocardiography.

RESULTS

: Injection of 2-cyclohexylthio-AMP induced a dose-dependent increase in +dP/dt, -dP/dt, and left ventricular developed pressure in normal wild-type mice and in CSQ mice with HF using the ex vivo working heart model. Spontaneous heart rate did not change after the injection of 2-cyclohexylthio-AMP. Compared with normal saline-treaded mice, chronic infusion of 2-cyclohexylthio-AMP in mice with ischemic HF after left coronary artery (LAD) ligation and in CSQ mice resulted in improved +dP/dt, -dP/dt, left ventricular developed pressure, and fractional shortening, restored the β-adrenergic response and decreased heart weight/body weight ratios.

CONCLUSIONS

: 2-Cyclohexylthio-AMP improved the cardiac contractile performance and rescued mice from HF. This salutary action may result from the reduction of myocardial hypertrophy and the restoration of the β-adrenergic response in both LAD ligation and CSQ mouse models of HF. The fact that this agent can increase contractile performance without heart rate increase should be desirable in HF therapy.

Inositol 1, 4, 5-trisphosphate receptors and human left ventricular myocytes

BACKGROUND

Little is known about the function of inositol 1,4,5-trisphosphate receptors (IP3Rs) in the adult heart experimentally. Moreover, whether these Ca(2+) release channels are present and play a critical role in human cardiomyocytes remains to be defined. IP3Rs may be activated after Gαq-protein-coupled receptor stimulation, affecting Ca(2+) cycling, enhancing myocyte performance, and potentially favoring an increase in the incidence of arrhythmias.

METHODS AND RESULTS

IP3R function was determined in human left ventricular myocytes, and this analysis was integrated with assays in mouse myocytes to identify the mechanisms by which IP3Rs influence the electric and mechanical properties of the myocardium. We report that IP3Rs are expressed and operative in human left ventricular myocytes. After Gαq-protein-coupled receptor activation, Ca(2+) mobilized from the sarcoplasmic reticulum via IP3Rs contributes to the decrease in resting membrane potential, prolongation of the action potential, and occurrence of early afterdepolarizations. Ca(2+) transient amplitude and cell shortening are enhanced, and extrasystolic and dysregulated Ca(2+) elevations and contractions become apparent. These alterations in the electromechanical behavior of human cardiomyocytes are coupled with increased isometric twitch of the myocardium and arrhythmic events, suggesting that Gαq-protein-coupled receptor activation provides inotropic reserve, which is hampered by electric instability and contractile abnormalities. Additionally, our findings support the notion that increases in Ca(2+) load by IP3Rs promote Ca(2+) extrusion by forward-mode Na(+)/Ca(2+) exchange, an important mechanism of arrhythmic events.

CONCLUSIONS

The Gαq-protein/coupled receptor/IP3R axis modulates the electromechanical properties of the human myocardium and its propensity to develop arrhythmias.

Adenosine-5'-triphosphate (ATP) supplementation improves low peak muscle torque and torque fatigue during repeated high intensity exercise sets

Background

Intracellular concentrations of adenosine-5’-triphosphate (ATP) are many times greater than extracellular concentrations (1–10 mM versus 10–100 nM, respectively) and cellular release of ATP is tightly controlled. Transient rises in extracellular ATP and its metabolite adenosine have important signaling roles; and acting through purinergic receptors, can increase blood flow and oxygenation of tissues; and act as neurotransmitters. Increased blood flow not only increases substrate availability but may also aid in recovery through removal of metabolic waste products allowing muscles to accomplish more work with less fatigue. The objective of the present study was to determine if supplemental ATP would improve muscle torque, power, work, or fatigue during repeated bouts of high intensity resistance exercise.

Methods

Sixteen participants (8 male and 8 female; ages: 21–34 years) were enrolled in a double-blinded, placebo-controlled study using a crossover design. The participants received either supplemental ATP (400 mg/d divided into 2 daily doses) or placebo for 15 d. After an overnight fast, participants underwent strength and fatigue testing, consisting of 3 sets of 50 maximal knee extensions performed on a Biodex® leg dynamometer.

Results

No differences were detected in high peak torque, power, or total work with ATP supplementation; however, low peak torque in set 2 was significantly improved (p < 0.01). Additionally, in set 3, a trend was detected for less torque fatigue with ATP supplementation (p < 0.10).

Conclusions

Supplementation with 400 mg ATP/d for 15 days tended to reduce muscle fatigue and improved a participant’s ability to maintain a higher force output at the end of an exhaustive exercise bout.

Involvement of P2Y receptors in pyridoxal-5'-phosphate-induced cardiac preconditioning

Using an isolated non-working rat heart model, this study investigated the mechanisms of pharmacological pre-conditioning (PC) induced by P2Y receptor stimulation with pyridoxal-5'-phosphate (PLP). After 6-hydroxydopamine pretreatment and a 15-min stabilization period, isolated rat hearts were perfused for 25 min then subjected to 40 min of global ischemia and 30 min of reperfusion (I/R); exposed for 15 min to 0.05 microM PLP bracketed for 25 min with broad-spectrum P2 antagonists (suramin or PPADS) or with more specific P2Y antagonists (AMPalphaS or MRS2578), 1 microM each, followed by a 5-min PLP-free perfusion before I/R; treated during 25 min with either glybenclamide (GLY, 1 microM), 5-hydroxydecanoic acid (5-HD, 100 microM), U73122 (0.5 microM), H89 (1 microM), or KN93 (1 microM), with an infusion starting 5 min before PLP. The main endpoints were the rate-pressure product (RPP), creatine kinase (CK) release and area necrosis. Recovery of RPP, measured 5 min after reperfusion, was rapidly improved by PLP, blocked by the P2 antagonists, and decreased with the different inhibitors. Fifteen minutes after the end of ischemia, CK release reached maximal values in all groups. PLP provided significant protection, whereas the P2 antagonists, 5-HD, a mitochondrial selective K(ATP) antagonist and GLY a non-selective K(ATP) channel blocker, suppressed the protective effect on myocardial injury. The suppression of the cardioprotective effects of PLP by AMPalphaS, the PKA inhibitor (H89), and phospholipase C blocker (U73122) is in agreement with the P2Y11 receptor as a receptor for PLP-induced PC. The suppression of the cardioprotective effects of PLP by MRS2578 and U73122 is in agreement with the P2Y6 receptor as a receptor for PLP-induced PC. Pre-ischemic exposure to nanomolar concentrations of PLP is protective against I/R. P2Y11 and P2Y6 represents the most likely candidate receptors for PLP-induced cardiac PC.

Role of P2X purinergic receptors in the rescue of ischemic heart failure

Evidence is accumulating to support the presence of P2X purinergic receptors in the heart. However, the biological role of this receptor remains to be defined. The objectives here were to determine the role of cardiac P2X receptors in modulating the progression of post-myocardial infarction ischemic heart failure and to investigate the underlying mechanism. The P2X4 receptor (P2X4R) is an important subunit of native cardiac P2X receptors, and the cardiac-specific transgenic overexpression of P2X4R (Tg) was developed as a model. Left anterior descending artery ligation resulted in similar infarct size between Tg and wild-type (WT) mice (P > 0.1). However, Tg mice showed an enhanced cardiac contractile performance at 7 days, 1 mo, and 2 mo after infarction and an increased survival at 1 and 2 mo after infarction (P < 0.01). The enhanced intact heart function was manifested by a greater global left ventricular developed pressure and rate of contraction of left ventricular pressure in vitro and by a significantly increased fractional shortening and systolic thickening in the noninfarcted region in vivo (P < 0.05). The salutary effects on the ischemic heart failure phenotype were seen in both sexes and were not the result of any difference in infarct size in Tg versus WT hearts. An enhanced contractile function of the noninfarcted area in the Tg heart was likely an important rescuing mechanism. The cardiac P2X receptor is a novel target to treat post-myocardial infarction ischemic heart failure.