Super resolution measurement of collagen fibers in biological samples: Validation of a commercial solution for multiphoton microscopy

Multiphoton microscopy is a powerful, non-invasive technique to image biological specimens. One current limitation of multiphoton microscopy is resolution as many of the biological molecules and structures investigated by research groups are similar in size or smaller than the diffraction limit. To date, the combination of multiphoton and super-resolution imaging has proved technically challenging for biology focused laboratories to implement. Here we validate that the commercial super-resolution Airyscan detector from ZEISS, which is based on image scanning microscopy, can be integrated under warranty with a pulsed multi-photon laser to enable multiphoton microscopy with super-resolution. We demonstrate its biological application in two different imaging modalities, second harmonic generation (SHG) and two-photon excited fluorescence (TPEF), to measure the fibre thicknesses of collagen and elastin molecules surpassing the diffraction limit by a factor of 1.7±0.3x and 1.4±0.3x respectively, in human heart and lung tissues, and 3-dimensional in vitro models. We show that enhanced resolution and signal-to-noise of SHG using the Airyscan compared to traditional GaAs detectors allows for automated and precise measurement of collagen fibres using texture analysis in biological tissues.

Advanced Methodology and Preliminary Measurements of Molecular and Mechanical Properties of Heart Valves under Dynamic Strain

Mammalian heart valves are soft tissue assemblies with multi-scale material properties. This is because they are constructs comprising both muscle and non-contractile extracellular matrix proteins (such as collagens and proteoglycans) and transition regions where one form of tissue structure becomes another, significantly different form. The leaflets of the mitral and tricuspid valves are connected to chordae tendinae which, in turn, bind through papillary muscles to the cardiac wall of the ventricle. The transition regions between these tissue subsets are complex and diffuse. Their material composition and mechanical properties have not been previously described with both micro and nanoscopic data recorded simultaneously, as reported here. Annotating the mechanical characteristics of these tissue transitions will be of great value in developing novel implants, improving the state of the surgical simulators and advancing robot-assisted surgery. We present here developments in multi-scale methodology that produce data that can relate mechanical properties to molecular structure using scanning X-ray diffraction. We correlate these data to corresponding tissue level (macro and microscopic) stress and strain, with particular emphasis on the transition regions and present analyses to indicate points of possible failure in these tissues.

Myocardial Dysfunction after Severe Food Restriction Is Linked to Changes in the Calcium-Handling Properties in Rats

Severe food restriction (FR) impairs cardiac performance, although the causative mechanisms remain elusive. Since proteins associated with calcium handling may contribute to cardiac dysfunction, this study aimed to evaluate whether severe FR results in alterations in the expression and activity of Ca²⁺-handling proteins that contribute to impaired myocardial performance. Male 60-day-old Wistar–Kyoto rats were fed a control or restricted diet (50% reduction in the food consumed by the control group) for 90 days. Body weight, body fat pads, adiposity index, as well as the weights of the soleus muscle and lung, were obtained. Cardiac remodeling was assessed by morphological measures. The myocardial contractile performance was analyzed in isolated papillary muscles during the administration of extracellular Ca²⁺ and in the absence or presence of a sarcoplasmic reticulum Ca²⁺-ATPase (SERCA2a) specific blocker. The expression of Ca²⁺-handling regulatory proteins was analyzed via Western Blot. Severe FR resulted in a 50% decrease in body weight and adiposity measures. Cardiac morphometry was substantially altered, as heart weights were nearly twofold lower in FR rats. Papillary muscles isolated from FR hearts displayed mechanical dysfunction, including decreased developed tension and reduced contractility and relaxation. The administration of a SERCA2a blocker led to further decrements in contractile function in FR hearts, suggesting impaired SERCA2a activity. Moreover, the FR rats presented a lower expression of L-type Ca²⁺ channels. Therefore, myocardial dysfunction induced by severe food restriction is associated with changes in the calcium-handling properties in rats.

nNOS splice variants differentially regulate myofilament function but are dispensable for intracellular calcium and force transients in cardiac papillary muscles

Cardiac muscle expresses three neuronal nitric oxide synthase (nNOS) splice variants: nNOSα, nNOSμ and nNOSβ. The functions of these nNOS splice variants in cardiac muscle, particularly myofilament-associated nNOSβ are unclear. To decipher cardiac nNOS splice variant function we investigated myofilament function and intracellular calcium and force transients in demembranated and intact papillary muscles from two lines of nNOS knockout mice. The first line (KN1) lacks nNOSα and nNOSμ. The second line (KN2) lacks active nNOSα, nNOSμ and nNOSβ. Demembranated KN1 papillary muscles exhibited reduced myofilament ATPase activity (-35%) and specific force (-10%) relative to controls. Demembranated KN2 muscles exhibited a smaller decrease in myofilament ATPase activity (-21%), but a greater reduction in specific force (-26%) relative to controls. Myofilament calcium sensitivity in demembranated KN1 and KN2 papillary muscles was similar to controls. Thus, papillary muscle-expressed nNOS splice variants are necessary for control levels of myofilament ATPase activity and force generation, but dispensable for myofilament calcium sensitivity. The greater reduction in myofilament ATPase relative to specific force in KN1, but not KN2 muscle, reduced the energy cost of muscle contraction, suggesting that nNOSβ increased the energetic efficiency of contraction in the absence of nNOSμ and nNOSα. Analyses of intact KN1 and KN2 papillary muscles showed that both intracellular calcium transients and their evoked force transients were similar to controls at stimulation frequencies between 1 and 3 Hz. Therefore, nNOS was dispensable for baseline excitation-contraction coupling. In summary, these data suggest that nNOS splice variants differentially regulate myofilament function, but not baseline calcium handling in papillary muscles. More importantly, they suggest that nNOSβ is a novel modulator of myofilament function, and ultimately the energetic efficiency of cardiac papillary muscle contraction.

Role of the C-terminus mobile domain of cardiac troponin I in the regulation of thin filament activation in skinned papillary muscle strips

The C-terminus mobile domain of cTnI (cTnI-MD) is a highly conserved region which stabilizes the actin-cTnI interaction during the diastole. Upon Ca²⁺-binding to cTnC, cTnI-MD participates in a regulatory switching that involves cTnI to switch from interacting with actin toward interacting with the Ca²⁺-regulatory domain of cTnC. Despite many studies targeting the cTnI-MD, the role of this region in the length-dependent activation of cardiac contractility is yet to be determined. The present study investigated the functional consequences of losing the entire cTnI-MD in cTnI(1-167) truncation mutant, as it was exchanged for endogenous cTnI in skinned rat papillary muscle fibers. The influence of cTnI-MD truncation on the extent of the N-domain of cTnC hydrophobic cleft opening and the steady-state force as a function of sarcomere length (SL), cross-bridge state, and [Ca²⁺] was assessed using the simultaneous in situ time-resolved FRET and force measurements at short (1.8 μm) and long (2.2 μm) SLs. Our results show the significant role of cTnI-MD in the length dependent thin filament activation and the coupling between thin and thick filament regulations affected by SL. Our results also suggest that cTnI-MD transmits the effects of SL change to the core of troponin complex.

In Vivo Genome Editing Restores Dystrophin Expression and Cardiac Function in Dystrophic Mice

RATIONALE

Duchenne muscular dystrophy is a severe inherited form of muscular dystrophy caused by mutations in the reading frame of the dystrophin gene disrupting its protein expression. Dystrophic cardiomyopathy is a leading cause of death in Duchenne muscular dystrophy patients, and currently no effective treatment exists to halt its progression. Recent advancement in genome editing technologies offers a promising therapeutic approach in restoring dystrophin protein expression. However, the impact of this approach on Duchenne muscular dystrophy cardiac function has yet to be evaluated. Therefore, we assessed the therapeutic efficacy of CRISPR (clustered regularly interspaced short palindromic repeats)-mediated genome editing on dystrophin expression and cardiac function in mdx/Utr +/- mice after a single systemic delivery of recombinant adeno-associated virus.

OBJECTIVE

To examine the efficiency and physiological impact of CRISPR-mediated genome editing on cardiac dystrophin expression and function in dystrophic mice.

METHODS AND RESULTS

Here, we packaged SaCas9 (clustered regularly interspaced short palindromic repeat-associated 9 from Staphylococcus aureus) and guide RNA constructs into an adeno-associated virus vector and systemically delivered them to mdx/Utr +/- neonates. We showed that CRIPSR-mediated genome editing efficiently excised the mutant exon 23 in dystrophic mice, and immunofluorescence data supported the restoration of dystrophin protein expression in dystrophic cardiac muscles to a level approaching 40%. Moreover, there was a noted restoration in the architecture of cardiac muscle fibers and a reduction in the extent of fibrosis in dystrophin-deficient hearts. The contractility of cardiac papillary muscles was also restored in CRISPR-edited cardiac muscles compared with untreated controls. Furthermore, our targeted deep sequencing results confirmed that our adeno-associated virus-CRISPR/Cas9 strategy was very efficient in deleting the ≈23 kb of intervening genomic sequences.

CONCLUSIONS

This study provides evidence for using CRISPR-based genome editing as a potential therapeutic approach for restoring dystrophic cardiomyopathy structurally and functionally.

Atorvastatin reduces β-Adrenergic dysfunction in rats with diabetic cardiomyopathy

Background

In the diabetic heart the β-adrenergic response is altered partly by down-regulation of the β1-adrenoceptor, reducing its positive inotropic effect and up-regulation of the β3-adrenoceptor, increasing its negative inotropic effect. Statins have clinical benefits on morbidity and mortality in diabetic patients which are attributed to their “pleiotropic” effects. The objective of our study was to investigate the role of statin treatment on β-adrenergic dysfunction in diabetic rat cardiomyocytes.

Methods

β-adrenergic responses were investigated in vivo (echocardiography) and ex vivo (left ventricular papillary muscles) in healthy and streptozotocin-induced diabetic rats, who were pre-treated or not by oral atorvastatin over 15 days (50 mg.kg^-1.day^-1). Micro-array analysis and immunoblotting were performed in left ventricular homogenates. Data are presented as mean percentage of baseline ± SD.

Results

Atorvastatin restored the impaired positive inotropic effect of β-adrenergic stimulation in diabetic hearts compared with healthy hearts both in vivo and ex vivo but did not suppress the diastolic dysfunction of diabetes. Atorvastatin changed the RNA expression of 9 genes in the β-adrenergic pathway and corrected the protein expression of β1-adrenoceptor and β1/β3-adrenoceptor ratio, and multidrug resistance protein 4 (MRP4). Nitric oxide synthase (NOS) inhibition abolished the beneficial effects of atorvastatin on the β-adrenoceptor response.

Conclusions

Atorvastatin restored the positive inotropic effect of the β-adrenoceptor stimulation in diabetic cardiomyopathy. This effect is mediated by multiple modifications in expression of proteins in the β-adrenergic signaling pathway, particularly through the NOS pathway.

Sex Differences in the Response of Rat Heart Ventricle to Calcium

Calcium (Ca2+ ) is a key mediator of myocardial function. Calcium regulates contraction, and disruption of myocellular Ca2+ handling plays a role in cardiac pathologies such as arrhythmias and heart failure. This investigation examines sex differences in sensitivity of the contractile proteins to Ca2+ and myofibrillar Ca2+ delivery in the ventricular myocardium. Sensitivity of contractile proteins to Ca2+ was measured in weight-matched male and female Sprague-Dawley rats using the skinned ventricular papillary muscle fiber and Ca2+ -stimulated Mg2+ -dependent adenosine triphosphatase (ATPase) activity methodologies. Calcium delivery was examined by measuring the contractile response to a range of extracellular Ca2+ concentrations in isolated ventricular myocytes, papillary muscle, and the isolated perfused whole heart. Findings from studies in the whole heart suggest that at a fixed preload, the male left ventricle generates more pressure than a female ventricle over a range of extracellular Ca2+ concentrations. In contrast, results from myocyte and papillary muscle studies suggest that females require less extracellular Ca2+ to elicit a similar contractile response. Results obtained from the 2 methods used to determine sex differences in Ca2+ sensitivity were equivocal. Further studies are required to elucidate sex differences in myocardial Ca2+ handling and the reasons for disparate results in different heart muscle preparations. The results of these studies will lead to the design of sex-optimized therapeutic interventions for cardiac disease.

Intracellular Zn(2+) Increase in Cardiomyocytes Induces both Electrical and Mechanical Dysfunction in Heart via Endogenous Generation of Reactive Nitrogen Species

Oxidants increase intracellular free Zn(2+) concentration ([Zn(2+)]i) in ventricular myocytes, which contributes to oxidant-induced alterations in excitation-contraction coupling (ECC). However, it is not clear whether increased [Zn(2+)]i in cardiomyocytes via increased reactive nitrogen species (RNS) has a role on heart function under pathological conditions, such as hyperglycemia. In this study, first we aimed to investigate the role of increased [Zn(2+)]i under in vitro condition in the development of both electrical and mechanical dysfunction of isolated papillary muscle strips from rat heart via exposed samples to a Zn(2+)-ionophore (Zn-pyrithione; 1 μM) for 20 min. Under simultaneous measurement of intracellular action potential and contractile activity in these preparations, Zn-pyrithione exposure caused marked prolongation in action potential repolarization phase and slowdown in both contraction and relaxation rates of twitch activity. Second, in order to demonstrate an association between increased [Zn(2+)]i and increased RNS, we monitored intracellular [Zn(2+)]i under an acute exposure of nitric oxide (NO) donor sodium nitroprusside, SNP, in freshly isolated quiescent cardiomyocytes loaded with FluoZin-3. Resting level of free Zn(2+) is significantly higher in cardiomyocytes under hyperglycemic condition compared to those of the controls, which seems to be associated with increased level of RNS production in hyperglycemic cardiomyocytes. Western blot analysis showed that Zn-pyrithione exposure induced a marked decrease in the activity of protein phosphatase 1 and 2A, member of macromolecular protein complex of cardiac ryanodine receptors, RyR2, besides significant increase in the phosphorylation level of extracellular signal-regulated kinase1/2 as a concentration-dependent manner. Overall, the present data demonstrated that there is a cross-relationship between increased RNS production and increased [Zn(2+)]i level in cardiomyocytes under pathological conditions such as hyperglycemia.

The Actions of Lyophilized Apple Peel on the Electrical Activity and Organization of the Ventricular Syncytium of the Hearts of Diabetic Rats

This study was designed to examine the effects of lyophilized red delicious apple peel (RDP) on the action potentials (APs) and the input resistance-threshold current relationship. The experiments were performed on isolated papillary heart muscles from healthy male rats, healthy male rats treated with RDP, diabetic male rats, and diabetic male rats treated with RDP. The preparation was superfused with oxygenated Tyrode's solution at 37°C. The stimulation and the recording of the APs, the input resistance, and the threshold current were made using conventional electrophysiological methods. The RDP presented no significant effect in normal rats. Equivalent doses in diabetic rats reduced the APD and ARP. The relationship between input resistance and threshold current established an inverse correlation. The results indicate the following: (1) The functional structure of the cardiac ventricular syncytium in healthy rats is heterogeneous, in terms of input resistance and threshold current. Diabetes further accentuates the heterogeneity. (2) As a consequence, conduction block occurs and increases the possibility of reentrant arrhythmias. (3) These modifications in the ventricular syncytium, coupled with the increase in the ARP, are the adequate substrate so that, with diabetes, the heart becomes more arrhythmogenic. (4) RDP decreases the APD, the ARP, and most syncytium irregularity caused by diabetes.

Cardiac Dysfunction Induced by Obesity Is Not Related to β-Adrenergic System Impairment at the Receptor-Signalling Pathway

Obesity has been shown to impair myocardial performance. Some factors have been suggested as responsible for possible cardiac abnormalities in models of obesity, among them beta-adrenergic (βA) system, an important mechanism of regulation of myocardial contraction and relaxation. The objective of present study was to evaluate the involvement of βA system components in myocardial dysfunction induced by obesity. Thirty-day-old male Wistar rats were distributed in control (C, n = 25) and obese (Ob, n = 25) groups. The C group was fed a standard diet and Ob group was fed four unsaturated high-fat diets for 15 weeks. Cardiac function was evaluated by isolated papillary muscle preparation and βA system evaluated by using cumulative concentrations of isoproterenol and Western blot. After 15 weeks, the Ob rats developed higher adiposity index than C rats and several comorbidities; however, were not associated with changes in systolic blood pressure. Obesity caused structural changes and the myocardial responsiveness to post-rest contraction stimulus and increased extracellular calcium (Ca²⁺) was compromised. There were no changes in cardiac function between groups after βA stimulation. The obesity was not accompanied by changes in protein expression of G protein subunit alpha (Gsα) and βA receptors (β₁AR and β₂AR). In conclusion, the myocardial dysfunction caused by unsaturated high-fat diet-induced obesity, after 15 weeks, is not related to βAR system impairment at the receptor-signalling pathway.

Contractility Measurements on Isolated Papillary Muscles for the Investigation of Cardiac Inotropy in Mice

Papillary muscle isolated from adult mouse hearts can be used to study cardiac contractility during different physiological/pathological conditions. The contractile characteristics can be evaluated independently of external influences such as vascular tonus or neurohumoral status. It depicts a scientific approach between single cell measurements with isolated cardiac myocytes and in vivo studies like echocardiography. Thus, papillary muscle preparations serve as an excellent model to study cardiac physiology/pathophysiology and can be used for investigations like the modulation by pharmacological agents or the exploration of transgenic animal models. Here, we describe a method of isolating the murine left anterior papillary muscle to investigate cardiac contractility in an organ bath setup. In contrast to a muscle strip preparation isolated from the ventricular wall, the papillary muscle can be prepared in toto without damaging the muscle tissue severely. The organ bath setup consists of several temperature-controlled, gassed and electrode-equipped organ bath chambers. The isolated papillary muscle is fixed in the organ bath chamber and electrically stimulated. The evoked twitch force is recorded using a pressure transducer and parameters such as twitch force amplitude and twitch kinetics are analyzed. Different experimental protocols can be performed to investigate the calcium- and frequency-dependent contractility as well as dose-response curves of contractile agents such as catecholamines or other pharmaceuticals. Additionally, pathologic conditions like acute ischemia can be simulated.

Architectural correlates of myocardial conduction: changes to the topography of cellular coupling, intracellular conductance, and action potential propagation with hypertrophy in Guinea-pig ventricular myocardium

BACKGROUND

We tested the hypothesis that alterations to action potential conduction velocity (CV) and conduction anisotropy in left ventricular hypertrophy are associated with topographical changes to gap-junction coupling and intracellular conductance by measuring these variables in the same preparations.

METHODS AND RESULTS

Left ventricular papillary muscles were excised from aortic-banded or sham-operated guinea-pig hearts. With intracellular stimulating and recording microelectrodes, CV was measured in 3 dimensions with simultaneous conductance mapping with subthreshold stimuli and correlated with quantitative histomorphometry of myocardial architecture and connexin 43 distribution. In hypertrophied myocardium, CV in the longitudinal axis was smaller and transverse velocity was greater compared with control; associated with similar differences of intracellular conductance, consistent with more cell contacts per cell (5.7 ± 0.2 versus 8.1 ± 0.5; control versus hypertrophy), and more intercalated disks mediating side-to-side coupling (8.2 ± 0.2 versus 10.2 ± 0.4 per cell). Intercalated disk morphology and connexin 43 immunolabelling were not different in hypertrophy. Hypertrophied preparations showed local submillimeter (≈250 μm) regions with slow conduction and low intracellular conductance, which, although not affecting CV on the millimeter scale, were consistent with discontinuities from increased microscopical connective tissue content.

CONCLUSIONS

With myocardial hypertrophy, altered longitudinal and transverse CV, and greater nonuniformity of CV anisotropy correspond to changes of intracellular conductance. These are associated with alteration of myocardial architecture, specifically the topography of cell-cell coupling and gap-junction connectivity.

Physiological contractility of cardiomyocytes in the wall of mouse and rat azygos vein

We recently demonstrated the abundant presence of cardiomyocytes in the wall of thoracic veins of adult mouse and rat. The highly differentiated morphology and myofilament protein contents of the venous cardiomyocytes suggested contractile functions. Here we further investigated the contractility of mouse and rat azygos venous rings compared with that of atrial strips and ventricular papillary muscle. 5-Bromo-4-chloro-indolyl-galactopyranoside (X-gal) staining of transgenic mouse vessels expressing lacZ under a cloned cardiac troponin T promoter demonstrated that the venous cardiomyocytes are discontinuous from atrial myocardium and aligned in the wall of thoracic veins perpendicular to the vessel axis. Histological sections displayed sarcomeric striations in the venous cardiomyocytes, which indicate an encirclement orientation of myofibrils in the vessel wall. Mechanical studies found that the rings of mouse and rat azygos vein produce strong cardiac type twitch contractions when stimulated with electrical pacing in contrast to the weak and slow smooth muscle contractions induced using 90 mM KCl. The twitch contraction and relaxation of mouse azygos veins further exhibited a cardiac type of β-adrenergic responses. Quantitative comparison showed that the contractions of venous cardiomyocytes are slightly slower than those of atrium muscle but significantly faster than those of ventricular papillary muscle. These novel findings indicate that the cardiomyocytes abundant in the wall of rodent thoracic veins possess fully differentiated cardiac muscle phenotype despite their anatomical and functional segregations from the heart.

Effect of polydatin on action potential in ventricular papillary muscle of rat and the underlying ionic mechanism

It is proved that polydatin has cardioprotection against ischemia-induced arrhythmia, but the electrophysiological mechanism is not clear. The aim of the present study was to investigate the effect of polydatin on action potential (AP) in ventricular papillary muscle and the underlying ionic mechanism in rat using intracellular recording and whole-cell patch clamp techniques. The results showed: (1) In normal papillary muscles, polydatin (50 and 100 µmol/L) shortened duration of 50% repolarization (APD(50)) and duration of 90% repolarization (APD(90)) in a concentration-dependent manner (P<0.01). But polydatin had no effects on resting potential (RP), overshoot (OS), amplitude of action potential (APA) and maximal rate of depolarization in phase 0 (V(max)) in normal papillary muscles (P>0.05). (2) In partially depolarized papillary muscles, polydatin (50 µmol/L) not only shortened APD(50) and APD(90) (P<0.05), but also decreased OS, APA and V(max) (P<0.05). (3) After pretreatment with glibenclamide (10 µmol/L), an ATP-sensitive K(+) channel blocker, the electrophysiological effect of polydatin (50 µmol/L) was partially inhibited. (4) Pretreatment with N(G)-nitro-L-arginine methyl ester (L-NAME, 1 mmol/L), a nitric oxide (NO) synthase inhibitor, failed to abolish the effect of polydatin (50 µmol/L) on AP. (5) Polydatin (25, 50, 75 and 100 µmol/L) decreased L-type Ca(2+) current in ventricular myocytes in a concentration-dependent manner (P<0.05). (6) Polydatin (50 µmol/L) increased ATP-sensitive K(+) current in ventricular myocytes (P<0.05). The results suggest that polydatin can shorten the repolarization of AP in normal papillary muscle and inhibit AP in partially depolarized papillary muscle, which might be related to the blocking of L-type Ca(2+) channel and the opening of ATP-sensitive K(+) channel.

Mechanisms underlying hypothermia-induced cardiac contractile dysfunction

Rewarming patients after profound hypothermia may result in acute heart failure and high mortality (50-80%). However, the underlying pathophysiological mechanisms are largely unknown. We characterized cardiac contractile function in the temperature range of 15-30 degrees C by measuring the intracellular Ca(2+) concentration ([Ca(2+)](i)) and twitch force in intact left ventricular rat papillary muscles. Muscle preparations were loaded with fura-2 AM and electrically stimulated during cooling at 15 degrees C for 1.5 h before being rewarmed to the baseline temperature of 30 degrees C. After hypothermia/rewarming, peak twitch force decreased by 30-40%, but [Ca(2+)](i) was not significantly altered. In addition, we assessed the maximal Ca(2+)-activated force (F(max)) and Ca(2+) sensitivity of force in skinned papillary muscle fibers. F(max) was decreased by approximately 30%, whereas the pCa required for 50% of F(max) was reduced by approximately 0.14. In rewarmed papillary muscle, both total cardiac troponin I (cTnI) phosphorylation and PKA-mediated cTnI phosphorylation at Ser23/24 were significantly increased compared with controls. We conclude that after hypothermia/rewarming, myocardial contractility is significantly reduced, as evidenced by reduced twitch force and F(max). The reduced myocardial contractility is attributed to decreased Ca(2+) sensitivity of force rather than [Ca(2+)](i) itself, resulting from increased cTnI phosphorylation.

Hypertrophic gene expression induced by chronic stretch of excised mouse heart muscle

Altered mechanical stress and strain in cardiac myocytes induce modifications in gene expression that affects cardiac remodeling and myocyte contractile function. To study the mechanisms of mechanotransduction in cardiomyocytes, probing alterations in mechanics and gene expression has been an effective strategy. However, previous studies are self-limited due to the general use of isolated neonatal rodent myocytes or intact animals. The main goal of this study was to develop a novel tissue culture chamber system for mouse myocardium that facilitates loading of cardiac tissue, while measuring tissue stress and deformation within a physiological environment. Intact mouse right ventricular papillary muscles were cultured in controlled conditions with superfusate at 95% O2/ 5% CO2, and 34 degrees C, such that cell to extracellular matrix adhesions as well as cell to cell adhesions were undisturbed and both passive and active mechanical properties were maintained without significant changes. The system was able to measure the induction of hypertrophic markers (BNP, ANP) in tissue after 2 hrs and 5 hrs of stretch. ANP induction was highly correlated with the diastolic load of the muscle but not with developed systolic load. Load induced ANP expression was blunted in muscles from muscle-LIM protein knockout mice, in which defective mechanotransduction pathways have been predicted.

Dissociation of FKBP12.6 from ryanodine receptor type 2 is regulated by cyclic ADP-ribose but not beta-adrenergic stimulation in mouse cardiomyocytes

AIMS

Beta-adrenergic augmentation of Ca(2+) sparks and cardiac contractility has been functionally linked to phosphorylation-dependent dissociation of FK506 binding protein 12.6 (FKBP12.6) regulatory proteins from ryanodine receptors subtype 2 (RYR2). We used FKBP12.6 null mice to test the extent to which the dissociation of FKBP12.6 affects Ca(2+) sparks and mediates the inotropic action of isoproterenol (ISO), and to investigate the underlying mechanisms of cyclic ADP-ribose (cADPR) regulation of Ca(2+) sparks.

METHODS AND RESULTS

Ca(2+) sparks and contractility were measured in cardiomyocytes and papillary muscle segments from FKBP12.6 null mice, and western blot analysis was carried out on sarcoplasmic reticulum microsomes prepared from mouse heart. Exposure to ISO resulted in a three- and two-fold increase in Ca(2+) spark frequency in wild-type (WT) and FKBP12.6 knockout (KO) myocytes, respectively, and Ca(2+) spark kinetics were also significantly altered in both types of cells. The effects of ISO on Ca(2+) spark properties in KO cells were inhibited by pre-treatment with thapsigargin or phospholamban inhibitory antibody, 2D12. Moreover, twitch force magnitude and the rate of force development were not significantly different in papillary muscles from WT and KO mice. Unlike beta-adrenergic stimulation, cADPR stimulation increased Ca(2+) spark frequency (2.8-fold) and altered spark kinetics only in WT but not in KO mice. The effect of cADPR on spark properties was not entirely blocked by pre-treatment with thapsigargin or 2D12. In voltage-clamped cells, cADPR increased the peak Ca(2+) of the spark without altering the decay time. We also noticed that basal Ca(2+) spark properties in KO mice were markedly altered compared with those in WT mice.

CONCLUSION

Our data demonstrate that dissociation of FKBP12.6 from the RYR2 complex does not play a significant role in beta-adrenergic-stimulated Ca(2+) release in heart cells, whereas this mechanism does underlie the action of cADPR.

Peroxisome proliferator-activated receptors gamma and alpha agonists stimulate cardiac glucose uptake via activation of AMP-activated protein kinase

Myocardial energy and glucose homeostasis are crucial for normal cardiac structure and function. Peroxisome proliferator-activated receptors (PPARs) play an important role in controlling transcriptional expression of key enzymes that are involved in glucose metabolism, and they have been demonstrated to significantly reduce tissue injury in cardiovascular diseases. Adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a sensor that maintains intracellular energy homeostasis and mediates a number of physiological signals. It has been reported that AMPK promotes glucose uptake. We hypothesize that PPAR gamma and alpha agonists may play a role in the regulation of glucose metabolism through AMPK. We tested this hypothesis by using isolated papillary muscles of rat hearts treated with PPAR gamma and alpha agonists, troglitazone and GW7647, respectively. Our results demonstrated that both troglitazone and GW7647 significantly stimulated 2-deoxyglucose uptake of cardiac muscles. Interestingly, both agonists stimulated phosphorylation of AMPK and its downstream protein target acetyl-CoA carboxylase. Endothelial nitric oxide synthase (eNOS) was also activated by both agonists. In addition, AMPK activator 5-amino-4-imidazole-1-beta-D-carboxamide ribofuranoside increased glucose uptake, while AMPK inhibitor compound C and NOS inhibitor, N(omega)-nitro-L-arginine, significantly blocked troglitazone- and GW7647-stimulated glucose uptake in cardiac muscles. There was also a reduction of glucose uptake with a marked decrease in AMPK and eNOS phosphorylation. In conclusion, both PPAR gamma and alpha activation play a role in the regulation of glucose uptake in cardiac muscles and this regulation is mediated by the AMPK and eNOS signaling pathways.

Effect of chronic intermittent hypobaric hypoxia on α(1)-adrenergic receptor of myocardium participates in the cardioprotection

The purpose of the present study was to investigate the effect of chronic intermittent hypobaric hypoxia (CIHH) on α(1)-adrenergic receptors and the role of alpha(1)-adrenergic receptors in the protection of CIHH against ischemic injury of myocardium. Sixty-six adult male Sprague-Dawley rats were randomly divided into four groups: control group (Con), 14-day CIHH treatment group (CIHH14), 28-day CIHH treatment group (CIHH28) and 42-day CIHH treatment group (CIHH42). CIHH rats were exposed to hypoxia mimicking 5 000 m altitude (p(B)=404 mmHg, p(O(2))=84 mmHg) in a hypobaric chamber, 6 h daily for 14, 28 and 42 d, respectively. Control animals lived in the same environment as CIHH animals except hypoxia exposure. After anesthesia with sodium pentobarbital (3.0-3.5 mL/kg body weight, i.p.), papillary muscle was taken from the right ventricle of rat and perfused with modified Tyrode's solution continuously, at constant temperature (37 °C) and perfusion speed (12 mL/min). Muscle contraction was evoked by electric stimuli. Different concentrations (1x10(-7), 1x10(-6) and 1x10(-5) mol/L) of phenylephrine (PE), an alpha(1)-adrenergic receptor agonist, were applied cumulatively to investigate the effect of PE on the mechanic contraction of right ventricular papillary muscles of rats in Con, CIHH14, CIHH28 and CIHH42 groups. Also, prazosin (1x10(-6) mol/L), an α(1)-adrenergic receptor antagonist, was used to investigate the role of α(1)-adrenergic receptor in the protective effect of CIHH on papillary muscle. The results showed: (1) PE increased the maximal isometric tension (P(max)) and maximal velocity of tension development (P(dT/dt)) of muscle contraction in a dose-dependent manner (P<0.05), and the increase of the muscle contraction was much greater in CIHH28 and CIHH42 rats than that in Con rats (P<0.05). Under 1x10(-5) mol/L of PE, the increases of P(max) and P(dT/dt) over the baseline were 51.2% and 44.5% in CIHH28 group, 48.6% and 44.5% in CIHH42 group, and 28.7% and 24.5% in Con group, respectively; (2) The contraction of papillary muscle decreased during simulated ischemia, but the decrease was slighter in CIHH rats than that in Con rats (P<0.05). The decreases in P(max) and P(dT/dt) were 59.6% and 53.6% in CIHH28 group, 60.4% and 49.9% in CIHH42 group, and 74.4% and 64.7% in Con group, respectively; (3) The protective effect of CIHH on ischemic papillary muscle was abolished by prazosin (1x10(-6) mol/L). The results of the present study suggest that CIHH increases the activity of α(1)-adrenergic receptor, which is possibly one of the mechanisms for the cardioprotection of CIHH.

Experimental hyperlipidaemia does not prevent preconditioning and it reduces ischemia-induced apoptosis

BACKGROUND

Although ischemic preconditioning (PC) is known to confer cardioprotection in healthy subjects, it is unclear whether this phenomenon exists in the presence of hyperlipidaemia. The goal of this study was to determine whether the cardioprotective effect of PC is affected by hyperlipidaemia in a guinea pig model.

METHODS

We investigated the influence of preconditioning in normo- and hyperlipidaemic animals on papillary muscle contractility and myocardial damage as expressed by the percentage of apoptotic cells. Guinea pigs were fed a normal diet or a hyperlipidaemic diet for 5 weeks. Experiments were performed on papillary muscles subjected to experimental ischemia-reperfusion with or without prior PC.

RESULTS

The dietary treatment resulted in significant changes in lipid parameters, which had not affected the functionality of the right ventricle papillary muscle, both at basal conditions and in response to ischemia-reperfusion injury. However, it was found that the hyperlipidaemic diet had an effect on ischemia-induced apoptosis. Papillary muscles of hyperlipidaemic animals with higher HDL plasma concentrations were less susceptible to ischemia-reperfusion injury.

CONCLUSIONS

This study demonstrates that hyperlipidaemia does not alter the benefits of ischemic preconditioning such as a reduction of apoptosis and preservation of myocardial contractility. Additionally, it has been shown that plasma HDL may protect cardiomyocytes against ischemia-induced apoptosis.

Nitric oxide and prostaglandins – important players in endothelin-1 induced myocardial distensibility

This study investigated whether endothelin (ET)-1-induced increase in myocardial distensibility is preserved in heart failure (HF) and whether it is modulated by nitric oxide (NO) and prostaglandins. New Zealand white rabbits were treated with doxorubicin (1 mg/kg, intravenously twice a week for 8 weeks, DOX-HF group) or saline (control group). Effects of ET-1 (0.1, 1, 10 nM) were tested in papillary muscles from the DOX-HF group and a control group in the presence of: i) intact endocardial endothelium (EE); ii) damaged EE; iii) NG -nitro-L-arginine (L-NNA; NO synthase inhibitor), and iv) indomethacin (INDO; cyclooxygenase inhibitor). In the presence of an intact EE, ET-1 promoted concentration-dependent positive inotropic and lusitropic effects that were maintained after damaging the EE, in the presence of L-NNA or INDO and in the DOX-HF Group. ET-1 reduced resting tension at the end of the isometric twitch (increased diastolic distensibility) by 3.2±1.3 %, 6.0±1.6 % and 8.8±2.7 % (at 0.1, 1 and 10 nM, respectively), in muscles with intact EE, effect that was completely abolished after damaging EE, in the presence of L-NNA or INDO or in the DOX-HF Group. This study demonstrated that the increase in myocardial distensibility induced by ET-1 is absent in HF and is dependent of NO and prostaglandin release.

Phosphodiesterase 5A Inhibition Induces Na + /H + Exchanger Blockade and Protection Against Myocardial Infarction

Acute phosphodiesterase 5A inhibition by sildenafil or EMD360527/5 promoted profound inhibition of the cardiac Na(+)/H(+) exchanger (NHE-1), detected by the almost null intracellular pH recovery from an acute acid load (ammonium prepulse) in isolated papillary muscles from Wistar rats. Inhibition of phosphoglycerate kinase-1 (KT5823) restored normal NHE-1 activity, suggesting a causal link between phosphoglycerate kinase-1 increase and NHE-1 inhibition. We then tested whether the beneficial effects of NHE-1 inhibitors against the deleterious postmyocardial infarction (MI) remodeling can be detected after sildenafil-mediated NHE-1 inhibition. MI was induced by left anterior descending coronary artery ligation in Wistar rats, which were randomized to placebo or sildenafil (100 mg kg(-1) day(-1)) for 6 weeks. Sildenafil significantly increased left ventricular phosphoglycerate kinase-1 activity in the post-MI group without affecting its expression. MI increased heart weight/body weight ratio, left ventricular myocyte cross-sectional area, interstitial fibrosis, and brain natriuretic peptide and NHE-1 expression. Sildenafil blunted these effects. Neither a significant change in infarct size nor a change in arterial or left ventricular systolic pressure was detected after sildenafil. MI decreased fractional shortening and the ratio of the maximum rate of rise of LVP divided by the pressure at the moment such maximum occurs, effects that were prevented by sildenafil. Intracellular pH recovery after an acid load was faster in papillary muscles from post-MI hearts (versus sham), whereas sildenafil significantly inhibited NHE-1 activity in both post-MI and sildenafil-treated sham groups. We conclude that increased phosphoglycerate kinase-1 activity after acute phosphodiesterase 5A inhibition blunts NHE-1 activity and protects the heart against post-MI remodeling and dysfunction.

Electrophysiological effects of hydrogen sulfide on guinea pig papillary muscles in vitro

The cardiac electrophysiological effects of hydrogen sulfide (H2S) were examined in guinea pig papillary muscles in vitro using intracellular microelectrode technique. The results obtained were as follows: (1) the duration of action potential (APD) in the normal papillary muscles was decreased by NaHS (H(2)S donor, 50, 100, 200 micromol/L) in a concentration-dependent manner; (2) in partially depolarized papillary muscles, 100 micromol/L NaHS not only reduced APD, but also decreased the amplitude of action potential (APA), overshoot (OS) and maximal velocity of depolarization at phase 0 (V(max)); (3) pretreatment with ATP-sensitive K(+) (K(ATP)) channel blocker glibenclamide (20 micromol/L) partially blocked the effects of NaHS (100 micromol/L); (4) pretreatment with L-type Ca(2+) channel agonist Bay K8644 (0.5 micromol/L) also partially blocked the effects of NaHS (100 micromol/L); (5) pretreatment with Ca(2+)-free Krebs-Henseleit solution containing glibenclamide (20 micromol/L) completely blocked the effects of NaHS (100 micromol/L); (6) APD in the normal papillary muscles was increased by DL-propargylglycine (PPG, an inhibitor of cystathionine gamma-lyase, 200 micromol/L). All these results suggest that the electrophysiological effects of H(2)S on papillary muscles in our study are due to an increase in potassium efflux through the opening of K(ATP) channels and a decrease in calcium influx. Endogenous H(2)S may act as an important regulator in electrophysiological characters in papillary muscles.

Superior logistic model for decay of Ca2+ transient and isometric relaxation force curve in rabbit and mouse papillary muscles

A decrease in myocardial intracellular calcium concentration ([Ca(2+)](i)) precedes relaxation, and a monoexponential function is typically used for fitting the decay of the Ca(2+) transient. However, a logistic function has been shown to be a better fit for the relaxation force curve, compared to the conventional monoexponential function. In the present study, we compared the logistic and monoexponential functions for fitting the [Ca(2+)](i) declines, which were measured using the aequorin method, and isometric relaxation force curves at 4 different onsets: the minimum time-derivative of [Ca(2+)](i) (d[Ca(2+)](i)/dt (min)) and force (dF/dt(min)), and the 10%, 20% and 30% lower [Ca(2+)](i) levels and forces over the data-sampling period in 7 isolated rabbit right ventricular and 15 isolated mouse left ventricular papillary muscles. Logistic functions were significantly superior for fitting the [Ca(2+)] (i) declines and relaxation force curves, compared to monoexponential functions. Changes in the normalized logistic [Ca(2+)] (i) decline and relaxation force time constants at the delayed onsets relative to their 100% values at d[Ca(2+)] (i)/dt(min) and dF/dt(min) were significantly smaller than the changes in the normalized monoexponential time constants. The ratio of the logistic relaxation force time constant relative to the logistic [Ca(2+)](i) decline time constant was significantly smaller in mouse than in rabbit. We conclude that the logistic function more reliably characterizes the [Ca(2+)](i) decline and relaxation force curve at any onset, irrespective of animal species. Simultaneous analyses using the logistic model for decay of the Ca(2+) transient and myocardial lusitropism might be a useful strategy for analysis of species-specific myocardial calcium handling.

Endothelium-derived nitric oxide mediates the antiadrenergic effect of human vasostatin-1 in rat ventricular myocardium

Vasostatins (VSs) are vasoactive peptides derived from chromogranin A (CgA), a protein contained in secretory granules of chromaffin and other cells. The negative inotropic effect and the reduction of isoproterenol (Iso)-dependent inotropism induced by VSs in the heart suggest that they have an antiadrenergic function. However, further investigation of the mechanisms of action of VSs is needed. The aim of the present study was to define the signaling pathways activated by VS-1 in mammalian ventricular myocardium and cultured endothelial cells that lead to the modulation of cardiac contractility. Ca(2+) and nitric oxide (NO) fluorometric confocal imaging was used to study the effects induced by recombinant human VS-1 [STA-CgA-(1-76)] on contractile force, L-type Ca(2+) current, and Ca(2+) transients under basal conditions and after beta-adrenergic stimulation in rat papillary muscles and ventricular cells and the effects on intracellular Ca(2+) concentration and NO production in cultured bovine aortic endothelial (BAE-1) cells. VS-1 had no effect on basal contractility of papillary muscle, but the effect of Iso stimulation was reduced by 27%. Removal of endocardial endothelium and inhibition of NO synthesis and phosphatidylinositol 3-kinase (PI3K) activity abolished the antiadrenergic effect of VS-1 on papillary muscle. In cardiomyocytes, 10 nM VS-1 was ineffective on basal and Iso (1 microM)-stimulated L-type Ca(2+) current and Ca(2+) transients. In BAE-1 cells, VS-1 induced a Ca(2+)-independent increase in NO production that was blocked by the PI3K inhibitor wortmannin. Our results suggest that the antiadrenergic effect of VS-1 is mainly due to a PI3K-dependent NO release by endothelial cells, rather than a direct action on cardiomyocytes.

Assessment of the Effects of L- and N-Type Ca2+ Channel Blocking Drugs Using Canine Blood-Perfused Papillary Muscle Preparations

It is important to accurately and conveniently assess the effects of L- and N-type Ca(2+) channel blocking drugs, which are commonly used for treatment of hypertension, but no method is available to simultaneously assess the effects of them in the same preparation. We have therefore designed an ex vivo method to measure the changes in contractile response of anterior papillary muscle of right ventricle and myocardial interstitial norepinephrine (NE) level using canine blood-perfused papillary muscle preparations. Papillary muscle-developed tension (PMDT) induced by an electronic stimulator was measured with force transducer. Myocardial interstitial NE effluent was collected by microdialysis fiber, which was implanted at the base of the papillary muscle, and measured with high performance liquid chromatography. Cilnidipine, a typical L- and N-type Ca(2+) channel blocker, was used to prove the efficiency of this method. First, to assess the effects of drugs on L-type Ca(2+) channel, the changes in basal PMDT were measured. Cilnidipine and nicardipine, a selective L-type Ca(2+) channel blocker, but not omega-conotoxin GVIA (omega-CTX), a selective N-type Ca(2+) channel blocking peptide, decreased basal PMDT dose-dependently. Second, to assess the effects of drugs on N-type Ca(2+) channel, the changes in PMDT and myocardial interstitial NE level by intracardiac sympathetic ganglion stimulation were measured. Cilnidipine and omega-CTX, but not nicardipine, dose-dependently reduced sympathomimetic increases in PMDT and myocardial interstitial NE level. These results indicate that our method is efficient to assess the effects of various L- and N-type Ca(2+) channel blocking drugs in the same papillary muscle preparation.

[Involvement of nitric oxide in negative inotropic effect of interferon-alpha in rat cardiac muscle]

OBJECTIVE

To investigate the effects and underlying mechanisms of interferon-alpha (IFN-alpha) on the isolated Langendorff perfused rat hearts and the isolated papillary muscles.

METHODS

The left ventricular developed pressure (LVDP), maximal rise/fall rate of left ventricular pressure (+/-dP/dt(max)), left ventricular end-diastolic pressure (LVEDP), heart rate (HR) and coronary flow (CF) were recorded in isolated Langendorff perfused rat hearts. The average contractile force was measured in the isolated papillary muscles of rat right ventricle.

RESULT

IFN-alpha (10 - 10,000 U/ml) induced a concentration-dependent decrease of LVDP and +/-dP/dt(max), and increase of LVEDP and CF in the isolated perfused rat heart (P < 0.05), and decrease of the average contractile force of the papillary muscle (P <0.05). Pretreatment with L-NAME (10(-4) mol/L), an inhibitor of nitric oxide synthase, attenuated the effect of IFN-alpha in the isolated rat hearts and the isolated papillary muscles (P <0.05). Isoproterenol (ISO, 10(-9) - 10(-6)mol/L) increased the contractile force of the rat papillary muscles in a concentration-dependent manner. Perfusion for 10 min with IFN-alpha at 1,000 U/ml attenuated the enhancing effect of ISO. Pretreatment with L-NAME reduced the effects of IFN-alpha on the isolated papillary muscles.

CONCLUSION

IFN-alpha may induce a negative inotropic effect in normal and beta-adrenergic activated cardiac muscles and this effect at least partly be mediated by nitric oxide.

Altered tension cost in (TG(mREN-2)27) rats overexpressing the mouse renin gene

The present study aimed to characterize cardiac hypertrophy induced by activation of the renin-angiotensin system in terms of functional alterations on the level of the contractile proteins, employing transgenic rats harboring the mouse renin gene (TGR(mREN2)27). Ca2+-dependent tension and myosin ATPase activity were measured in skinned fiber preparations obtained from TGR(mREN2)27 and from age-matched Sprague-Dawley rats (SPDR). Western blots for troponin I (TnI) and troponin T (TnT) were performed and the phosphorylation status of TnI were evaluated in myocardial preparations. TnT and myosin heavy chain (MHC) isoforms were analyzed by RT-PCR. The pCa/tension relationship was shifted to the right in TGR(mREN2)27 compared to SPDR as indicated by increased Ca2+-concentrations required for half maximal activation of tension (SPDR 5.80, 95% confidence limits 5.77-5.82 vs. TGR(mREN2)27 5.69, 95% confidence limits 5.67-5.72, pCa units), while maximal developed tension was unaltered. Even more pronounced was the shift in the relationship between pCa and myosin-ATPase (SPDR 6.01, 95% confidence limits 5.99-6.03 vs. TGR(mREN2)27 5.77, 95% confidence limits 5.73-5.79, pCa units). The maximal myosin-ATPase activity was reduced in TGR(mREN2)27 compared to SPDR, respectively (211.0 +/- 28.77 micromol ADP/s vs. 271.6 +/- 43.66 micromol ADP/s, P < 0.05). Tension cost (ATPase activity/tension) was significantly reduced in TGR(mREN2)27. The beta-MHC expression was significantly increased in TGR(mREN2)27. There was no isoform shift for TnT (protein and mRNA), as well as TnI, and no alteration of the phosphorylation of TnI in TGR(mREN2)27 compared to SPRD. The present study demonstrates that cardiac hypertrophy, induced by an activation of the renin-angiotensin system, leads to adapting alterations on the level of the contractile filaments, which reduce tension cost.

A role for the alpha1-adrenergic signalling pathway in the response of papillary muscles from type 2 diabetic hearts to anoxia-reoxygenation

Hearts from animal models of type 2 diabetes present with abnormal contractility patterns. The role of altered alpha(1)-adrenergic signalling in this is not understood. In this study we report overexpression and altered regulation of alpha(1)-adrenergic receptors in two models of type 2 diabetic rat hearts. In combination with reduced contractile performance, papillary muscles from these hearts presented with an enhanced ability to react to alpha(1)-adrenergic stimulation. Concurrently, these muscles were protected against anoxia/reoxygenation induced damage. This protection could be abolished by pretreatment with the alpha(1)-adrenergic antagonist, prazosin. Overexpression of alpha(1)-adrenergic receptors may therefore be a two-edged sword: supplying a contractile reserve that can protect against anoxia/reoxygenation induced effects on inotropic ability on the one hand but also predisposing the hearts to elevated induction of intracellular Ca(2+) release and possible arrhythmic effects on the other hand.

Mg2+ activates the ryanodine receptor type 2 (RyR2) at intermediate Ca2+ concentrations

To clarify whether activity of the ryanodine receptor type 2 (RyR2) is reduced in the sarcoplasmic reticulum (SR) of cardiac muscle, as is the case with the ryanodine receptor type 1 (RyR1), Ca(2+)-dependent [(3)H]ryanodine binding, a biochemical measure of Ca(2+)-induced Ca(2+) release (CICR), was determined using SR vesicle fractions isolated from rabbit and rat cardiac muscles. In the absence of an adenine nucleotide or caffeine, the rat SR showed a complicated Ca(2+) dependence, instead of the well-documented biphasic dependence of the rabbit SR. In the rat SR, [(3)H]ryanodine binding initially increased as [Ca(2+)] increased, with a plateau in the range of 10-100 microM Ca(2+), and thereafter further increased to an apparent peak around 1 mM Ca(2+), followed by a decrease. In the presence of these modulators, this complicated dependence prevailed, irrespective of the source. Addition of 0.3-1 mM Mg(2+) unexpectedly increased the binding two- to threefold and enhanced the affinity for [(3)H]ryanodine at 10-100 microM Ca(2+), resulting in the well-known biphasic dependence. In other words, the partial suppression of RyR2 is relieved by Mg(2+). Ca(2+) could be a substitute for Mg(2+). Mg(2+) also amplifies the responses of RyR2 to inhibitory and stimulatory modulators. This stimulating effect of Mg(2+) on RyR2 is entirely new, and is referred to as the third effect, in addition to the well-known dual inhibitory effects. This effect is critical to describe the role of RyR2 in excitation-contraction coupling of cardiac muscle, in view of the intracellular Mg(2+) concentration.

Biophysical Characterization of the New Human Ether-A-Go-Go-Related Gene Channel Opener NS3623 [N-(4-Bromo-2-(1H-tetrazol-5-yl)-phenyl)-N′-(3′-trifluoromethylphenyl)urea]

Within the field of new antiarrhythmic compounds, the interesting idea of activating human ether-a-go-go-related gene (HERG1) potassium channels has recently been introduced. Potentially, drugs that increase HERG1 channel activity will augment the repolarizing current of the cardiac myocytes and stabilize the diastolic interval. This may make the myocardium more resistant to events that cause arrhythmias. We here present the compound N-(4-bromo-2-(1H-tetrazol-5-yl)-phenyl)-N'-(3'-trifluoromethylphenyl)urea (NS3623), which has the ability to activate HERG1 channels expressed in Xenopus laevis oocytes with an EC50 value of 79.4 microM. Exposure of HERG1 channels to NS3623 affects the voltage-dependent release from inactivation, resulting in a half-inactivation voltage that is rightward-shifted by 17.7 mV. Moreover, the compound affects the time constant of inactivation, leading to a slower onset of inactivation of the macroscopic HERG1 currents. We also characterized the ability of NS3623 to increase the activity of different mutated HERG1 channels. The mutants S620T and S631A are severely compromised in their ability to inactivate. Application of NS3623 to any of these two mutants did not result in increased HERG1 current. In contrast, application of NS3623 to the mutant F656M increased HERG1 current to a larger extent than what was observed with wild-type HERG1 channels. Because the amino acid F656 is essential for high-affinity inhibition of HERG1 channels, it is concluded that NS3623 has a dual mode of action, being both an activator and an inhibitor of HERG1 channels. Finally, we show that NS3623 has the ability to shorten action potential durations in guinea pig papillary muscle.

Role of endocannabinoids in the pathogenesis of cirrhotic cardiomyopathy in bile duct-ligated rats

Cardiac contractility in cirrhosis is normal at baseline but hyporesponsive to stimuli, a phenomenon known as 'cirrhotic cardiomyopathy'. The pathogenesis remains unclear. Endocannabinoids are vasoactive, but have not previously been examined in the cirrhotic heart. We therefore aimed to systematically clarify a possible role of endocannabinoids in the pathogenesis of cirrhotic cardiomyopathy. Cirrhosis was induced in Sprague-Dawley rats by bile duct ligation; controls underwent a sham operation. At 4 weeks after operation, isolated left ventricular papillary muscle contractility was studied. Dose-response curve for a beta-adrenergic agonist isoproterenol was constructed in the presence and absence of a CB-1 antagonist AM251 (1 microM). Cirrhotic muscles had a blunted response to isoproterenol, which was completely restored by AM251. Dose-response curves to anandamide, and CB-1 and CB-2 protein and mRNA expression in Western blot and reverse transcriptase-polymerase chain reaction experiments were not significantly different between cirrhotic and sham muscles. Force-frequency relationship studies were performed in cirrhotic and normal muscles. At higher frequencies, anandamide reuptake blockers (VDM11 and AM404) significantly enhanced muscle relaxation in cirrhotic muscles, but not in controls. This effect was completely blocked by AM251 and pertussis toxin, whereas tetrodotoxin partially reversed it. Taken together, these results indicate a pathogenic role for increased local (neuronal) production of endocannabinoids, mediated by a G(i)-protein-dependent CB-1-responsive pathway in cirrhotic cardiomyopathy. The increased tachycardia-stress-induced release of endocannabinoids may help explain why contractility is normal at baseline but attenuated with stress.

pH-dependent modulation of intracellular free magnesium ions with ion-selective electrodes in papillary muscle of guinea pig

A change in pH can alter the intracellular concentration of electrolytes such as intracellular Ca²⁺ and Na⁺ ([Na⁺]_i) that are important for the cardiac function. For the determination of the role of pH in the cardiac magnesium homeostasis, the intracellular Mg²⁺ concentration ([Mg²⁺]_i), membrane potential and contraction in the papillary muscle of guinea pigs using ion-selective electrodes changing extracellular pH ([pH]_o) or intracellular pH ([pH]_i) were measured in this study. A high CO₂-induced low [pH]_o causes a significant increase in the [Mg²⁺]_i and [Na⁺]_i, which was accompanied by a decrease in the membrane potential and twitch force. The high [pH]_o had the opposite effect. These effects were reversible in both the beating and quiescent muscles. The low [pH]_o-induced increase in [Mg²⁺]_i occurred in the absence of [Mg²⁺]_o. The [Mg²⁺]_i was increased by the low [pH]_i induced by propionate. The [Mg²⁺]_i was increased by the low [pH]_i induced by NH₄Cl-prepulse and decreased by the recovery of [pH]_i induced by the removal of NH₄Cl. These results suggest that the pH can modulate [Mg²⁺]_i with a reverse relationship in heart, probably by affecting the intracellular Mg²⁺ homeostasis, but not by Mg²⁺ transport across the sarcolemma.

Defect in oxidative phosphorylation in LV papillary muscle mitochondria of patients undergoing mitral valve replacement

Mitochondria play a pivotal role in cellular metabolism, especially in energy production. Myocardial function depends on adenosine triphosphate (ATP) supplied by oxidation of several substrates. In the adult heart, this energy is obtained primarily from fatty acid oxidation through oxidative phosphorylation (OXPHOS). With this in view, we studied OXPHOS, Total-ATPase and cytochrome content in the mitochondria of the left ventricular (LV) papillary muscles in excised mitral valves of patients who underwent mitral valve replacement (MVR). The mitochondrial OXPHOS, cytochrome content and ATPase activity were studied in 70 patients (ranging from 22 to 40 years) operated on for mitral valve disease. Control study includes 25 normal mitral valves removed at necropsy from patients who died of extracardiac causes. In the presence of glutamate and succinate as substrates, the rate of mitochondrial oxygen consumption was significantly lower in LV papillary muscles of pathological mitral valves (P<0.001) by using with and without addition of ADP. The ADP/O ratio indices for glutamate and succinate were not significantly affected. Using glutamate as substrate, respiratory control index was significantly raised (P<0.05) as compared with control. A significant reduction of total cytochrome content and ATPase activity (P<0.001) was noted in LV papillary muscles of patients operated for mitral valve disease. Our results showed that OXPHOS, cytochromes 'a', 'b', 'c+c(1)' and ATP activity are significantly impaired in LV papillary muscles in patients with pathological mitral valve. Cardiac mitochondrial oxygen consumption is a very valuable tool to investigate the regulation of cardiac mitochondrial energy metabolism. There is increasing evidence that mitochondrial diseases, such as mitochondrial cardiomyopathy, valvular disease and some myopathies, can be responsive to treatment with metabolic intermediates such as coenzyme Q(10), thiamine, prednisone, and vitamin therapy.

Nifedipine enhances cGMP production through the activation of soluble guanylyl cyclase in rat ventricular papillary muscle

It is known that nifedipine, an L-type calcium channel blocker, increases cGMP production, which partially contributes to the relaxation of vascular smooth muscle. The aim of our investigation was to clarify whether or not nifedipine regulates cGMP production, which has a physiological role in cardiac muscle. To measure contractile responses and tissue cGMP levels, left ventricular papillary muscles prepared from male Wistar rats (350-400 g) were mounted in the isolated organ chamber under isometric conditions and electrically paced by means of platinum punctate electrodes (1 Hz, 1 ms duration). In papillary muscle preparation, the negative inotropic effect induced by nifedipine (30 to 300 nM) was significantly inhibited in the presence of ODQ(1H-[1,2,4]oxidazolo[4,3-a]quinoxaline1-one; 10 microM), a soluble guanylyl cyclase inhibitor. Furthermore, nifedipine (100 nM) strongly increased the tissue cGMP level, which was significantly decreased in the presence of ODQ. On the other hand,N(G)-monomethyl-(L)-arginine (100 microM), a nitric oxide synthase inhibitor, did not inhibit either the negative inotropic effect or cGMP production induced by nifedipine. These results indicate that in rat left ventricular papillary muscle, nifedipine augments its negative inotropic effect at least partly through direct activation of cardiac soluble guanylyl cyclase but not nitric oxide synthase.

Effects of Dofetilide and EGIS-7229, an Antiarrhythmic Agent Possessing Class III, IV, and IB Activities, on Myocardial Refractoriness in Hyperkalemia, Hypokalemia, and During β-Adrenergic Activation in the Rabbit Papillary Muscle In Vitro

Lengthening of the effective refractory period (ERP) by EGIS-7229, a class III/Ib/IV drug, and by dofetilide, a selective I(Kr) blocker, was compared in normokalemia (NK), hypokalemia (LK), and hyperkalemia (HK) in right ventricular papillary muscles of rabbits paced at 0.5, 1, and 2 Hz, in vitro, and also during beta-adrenergic activation. In NK, EGIS-7229 (3 and 10 microM) and dofetilide (30 and 100 nM) similarly lengthened ERP in a steeply reverse frequency-dependent manner. The two compounds produced smaller ERP prolongations at 0.5 Hz in HK and LK, so rate-dependence of ERP changes decreased. EGIS-7229 lengthened ERP more at 2 Hz than at 0.5 Hz at 10 microM in LK, that is, the effect of EGIS-7229 turned into positive frequency-dependence from 3 to 10 microM. Furthermore, EGIS-7229 lengthened ERP at 10 microM more than dofetilide at 100 nM at 2 Hz stimulation rate (P<0.05). Isoproterenol (30 nM) eliminated the effect of dofetilide on ERP, while EGIS-7229 prolonged ERP during beta-adrenergic activation. In conclusion, efficacy of EGIS-7229 was superior to that of dofetilide in LK and during beta-adrenergic stimulation, suggesting improved antiarrhythmic action for EGIS-7229 under certain conditions in the patient.

[Angiotensin II-induced endothelin-1 release in cardiac myocytes]

Many of the effects thought to be due to angiotensin II (Ang II) are due to the release/formation of endothelin (ET). We tested whether Ang II elicits its positive inotropic effect (PIE) by the action of endogenous ET-1 and the role played by the reactive oxygen species (ROS) in this mechanism. Experiments were performed in cat isolated ventricular myocytes in which sarcomere shortening (SS) was measured to asses contractility after pharmacological interventions and the effect of Ang II on inotropism were analyzed. Ang II 1 nM increased SS by 31.8 +/- 3.8% (p < 0.05). This PIE was cancelled by AT1 receptor blockade, by ET-1 receptors blockade, by Na+/H+ exchanger (NHE) inhibition, by reverse mode Na+/Ca2+ exchanger (NCX) blockade or by ROS scavenging. Ang II 100 nM increases SS by 70.5 +/- 7.6% (p < 0.05). This PIE was completely abolished by AT1 receptors blockade and were partially bocked by ET-1 receptors blockade, by NHE inhibition, by reverse mode NCX blockade or by ROS scavenging. Ang II increased preproET-1 mRNA, effect that was blunted by AT1 receptors blockade. We conclude that Ang II induces (through its AT1 receptor) release/formation of ET-1, which acting in autocrine fashion on ET receptors of the isolated myocytes increases inotropism through NHE stimulation and NCX reverse mode activation. The participation of ROS is involved is this chain of events.

alpha1-Adrenoceptor-mediated Negative Inotropic Effect Caused by Intracellular Ionic Activities in Guinea-pig Papillary Muscle

In guinea-pig papillary muscle, phenylephrine (PE), an agonist of alpha1-adrenoceptor (alpha1-AR), led to a transient negative inotropic effect (NIE) and a subsequent sustained positive inotropic effect (PIE). To clarify the ionic mechanisms underlying the NIE, we measured the [Na+]i or [pH]i by ion-selective microelectrodes. PE produced a decrease in the intracellular Na+ concentration ([Na+]i) and an increase in intracellular pH ([pH]i). During the phase of NIE, PE produced only a (-) change of [Na+]i (Delta[Na+]i). With a decrease in extracellular Na+ or an increase in extracellular Ca2+, the PE-induced NIE was attenuated and PE produced (+)Delta[Na+]i. The PE-induced NIE and (-)Delta[Na+]i were definitely strengthened by lowering the bath temperature or increasing the stimulation frequency. 2-(2,6-di-methoxyphenoxyethyl)amino-methyl-1,4-benzidioxane HCl, an antagonist of alpha1A-AR, completely abolished the PE-induced NIE and (-)Delta[Na+]i. Phorbol 12,13-dibutyrate, an activator of protein kinase C (PKC), decreased the baseline [Na+]i and twitch force and increased the baseline [pH]i in mimicry of PE. Pretreatment with 1-5(isoquinolinesulphonyl)-2-methylpiperazine, an inhibitor of PKC, abolished the PE-induced NIE and (-)Delta[Na+]i. During pretreatment with benzamil, an inhibitor of Na+/Ca2+ exchange, we found that the PE-induced NIE and (-)Delta[Na+]i were reversibly abolished. Our results indicate the PE-induced NIE may be elicited upon the activation of Na+/Ca2+ exchange which can be attributed to the (-)Delta[Na+]i. (-)Delta[Na+]i is mediated through a PKC-dependent pathway via an activation of alpha1A-AR subtype and its effect could be strengthened remarkably at high [Na+]i and [Ca2+]i values.

Involvement of the Na+-independent Cl-/HCO3- exchange (AE) isoform in the compensation of myocardial Na+/H+ isoform 1 hyperactivity in spontaneously hypertensive rats

Enhanced activity of Na+/H+ isoform 1 (NHE-1) and the Na+-independent Cl-/HCO3- exchange (AE) is a feature of the hypertrophied myocardium in spontaneously hypertensive rats (SHR). The present study explored the possibility that sustained intracellular acidosis due to increased myocardial acid loading through AE causes NHE-1 enhancement. To this aim, SHR were treated for 2 weeks with a rabbit polyclonal antibody against an AE3 isoform that was recently developed and proven to have inhibitory effects on myocardial AE activity. We then compared the AE activity in the left ventricle papillary muscles isolated from untreated SHR with antiAE3-treated SHR; AE activity was measured in terms of the rate of intracellular pH recovery after an intracellular alkali load was introduced. AE activity was diminished by approximately 70% in SHR treated with the antiAE3 antibody, suggesting that the AE3 isoform is a major carrier of acid-equivalent influx in the hypertrophied myocardium. However, the antibody treatment failed to normalize NHE-1 activity that remained elevated in the myocardium of normotensive rats. The data therefore rule out the possibility that NHE-1 hyperactivity in hypertensive myocardium was due to sustained intracellular acidosis induced by increased AE activity that characterizes SHR myocardial tissue.

Selective androgen receptor modulator treatment improves muscle strength and body composition and prevents bone loss in orchidectomized rats

The partial agonist activity of a selective androgen receptor modulator (SARM) in the prostate was demonstrated in orchidectomized rats. In the current study, we characterized the full agonist activity of S-3-(4-acetylamino-phenoxy)-2-hydroxy-2-methyl-N-(4-nitro-3-trifluoromethyl-phenyl)-propionamide (a structurally related SARM referred to in other publications and hereafter as S-4) in skeletal muscle, bone, and pituitary of castrated male rats. Twelve weeks after castration, animals were treated with S-4 (3 or 10 mg/kg), dihydrotestosterone (DHT) (3 mg/kg), or vehicle for 8 wk. S-4 (3 and 10 mg/kg) restored soleus muscle mass and strength and levator ani muscle mass to that seen in intact animals. Similar changes were also observed in DHT-treated (3 mg/kg) animals. Compared with the anabolic effects observed in muscle, DHT (3 mg/kg) stimulated prostate and seminal vesicle weights more than 2-fold greater than that observed in intact controls, whereas S-4 (3 mg/kg) returned these androgenic organs to only 16 and 17%, respectively, of the control levels. S-4 (3 and 10 mg/kg) and DHT (3 mg/kg) restored castration-induced loss in lean body mass. Furthermore, S-4 treatment caused a significantly larger increase in total body bone mineral density than DHT. S-4 (3 and 10 mg/kg) also demonstrated agonist activity in the pituitary and significantly decreased plasma LH and FSH levels in castrated animals in a dose-dependent manner. In summary, the strong anabolic effects of S-4 in skeletal muscle, bone, and pituitary were achieved with minimal pharmacologic effect in the prostate. The tissue-selective pharmacologic activity of SARMs provides obvious advantages over steroidal androgen therapy and demonstrates the promising therapeutic utility that this new class of drugs may hold.

Contribution of endogenous opioids and nitric oxide to papillary muscle contractile impairment in cholestatic rats

Attenuated responsiveness to adrenoceptor stimulation has been proposed as an important factor underlying cardiovascular complications of cholestasis. We examined isolated papillary muscle responsiveness to alpha (phenylephrine) and beta-adrenoceptor (isoproterenol) agonists in 7-day bile duct-ligated rats. We investigated the role of nitric oxide (NO) and endogenous opioids in papillary muscle hyporesponsiveness to isoproterenol stimulation. In order to evaluate the effect of NO and endogenous opioids, animals were treated with chronic subcutaneous injections of N(omega)-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg/day) or naltrexone (20 mg/kg/day), or isolated papillary muscles were exposed acutely to the same drugs (10(-4) and 10(-6) M, respectively) in an organ bath. The basal contractile force of papillary muscle, +dT/dtmax and -dT/dtmax, was significantly decreased in bile duct-ligated rats compared to sham-operated ones (P<0.05, for each value). The concentration-response curve for phenylephrine and isoproterenol demonstrated a reduced maximum effect in bile duct-ligated rats compared to the sham-operated group (P<0.01 and 0.05, respectively). Basal contractile abnormalities of bile duct-ligated rats were corrected by L-NAME or naltrexone treatment, either acute or chronic. While chronic L-NAME treatment resulted in a left-ward shift (P<0.05), it had no effect on the maximum effect in bile duct-ligated rats. Acute L-NAME treatment did not influence isoproterenol responsiveness. Acute and chronic naltrexone treatment resulted in partial and complete correction of the hyporesponsiveness of bile duct-ligated rats, respectively (P<0.05). This investigation demonstrates that the papillary muscles of 7-day bile duct ligated-rats have an impaired basal contractility and hyporesponsiveness to both alpha and beta-adrenoceptor stimulation. It also provides evidence for the involvement of increased opioidergic tone and NO overproduction in cholestasis-induced cardiac impairment.

Role of calmodulin methionine residues in mediating productive association with cardiac ryanodine receptors

Calmodulin (CaM) binds to the cardiac ryanodine receptor Ca2+ release channel (RyR2) with high affinity and may act as a regulatory channel subunit. Here we determine the role of CaM Met residues in the productive association of CaM with RyR2, as assessed via determinations of [3H]ryanodine and [35S]CaM binding to cardiac muscle sarcoplasmic reticulum (SR) vesicles. Oxidation of all nine CaM Met residues abolished the productive association of CaM with RyR2. Substitution of the COOH-terminal Mets of CaM with Leu decreased the extent of CaM inhibition of cardiac SR (CSR) vesicle [3H]ryanodine binding. In contrast, replacing the NH2-terminal Met of CaM with Leu increased the concentration of CaM required to inhibit CSR [3H]ryanodine binding but did not alter the extent of inhibition. Site-specific substitution of individual CaM Met residues with Gln demonstrated that Met124 was required for both high-affinity CaM binding to RyR2 and for maximal CaM inhibition. These results thus identify a Met residue critical for the productive association of CaM with RyR2 channels.

Increased cross-bridge cycling rate in stunned myocardium

Decreased Ca2+ responsiveness of the myofilaments underlies myocardial stunning. Given that cross-bridge cycling is a major determinant of myofilament behavior, we quantified cross-bridge cycling rate in stunned myocardium. After stabilization, rat hearts were subjected to 20 min of no-flow global ischemia and 30 min of reperfusion at 37 degrees C. Control hearts were perfused continuously at 37 degrees C for 60 min. Trabeculae were dissected and chemically skinned with 1% Triton X-100. The muscles were then activated with solutions of varied Ca2+ concentration ([Ca2+]). Force-[Ca2+] relations, rate of force redevelopment after release (k(tr)), muscle stiffness (k(m)), and myofilament ATP consumption were determined. Maximal Ca2+-activated force (Fmax) was depressed in stunned myocardium (49 +/- 5 vs. 82 +/- 5 mN/mm2, P < 0.01). Western immunoblotting showed degradation of troponin I in stunned myocardium. The k(tr) at Fmax was significantly increased in stunned muscles (19.82 +/- 2.74 vs. 13.19 +/- 0.96 s(-1), 22 degrees C, P < 0.01; 7.49 +/- 0.52 vs. 5.81 +/- 0.54 s(-1), 10 degrees C, P < 0.05). The ratio of k(m) measured at 100 Hz over that at 1 Hz, during Fmax, is lower in stunned muscles (8.22 +/- 1.56 vs. 12.94 +/- 0.71, P < 0.05). In comparison with k(m) at rigor, k(m) at Fmax is significantly lower in the stunned group (78.82 +/- 6.11 vs. 93.27 +/- 3.03%, P < 0.05). Myofilament ATP consumption at Fmax did not change in stunned muscles (5,901 +/- 952 vs. 5,596 +/- 972 pmol x microl(-1) x min(-1), P = 0.49). These results show that cross-bridge cycling is increased in stunned myocardium. Such increases are likely the result of increased transition rate from force-generating states to non-force-generating states. Thus stunned myocardium still maintains ATP consumption in spite of lower force development, rationalizing the long-standing paradox of decreased force but unchanged oxygen consumption in the postischemic heart.

Reduction of perifusate magnesium alters inotropic response of papillary muscle to ion channel modulators

Magnesium has a significant role in the regulation of ion transport. Marginal deficiency of Mg can therefore affect myocardial excitability and contractility. This study was taken up with the objective of examining the inotropic response of the myocardium to variation in extracellular [Mg]o and identifying the ion channels and pumps mediating the inotropic changes. Electrically stimulated rat papillary muscle was used as the experimental model and mechanical changes were recorded using a physiograph. Channel specific antagonists were used to identify the channels mediating the functional changes. Diastolic Ca2+ levels were determined in isolated myocytes by the ratiometric method using the fluorescent indicator Fura2-AM. A negative association was observed between the level of [Mg]o and force of contraction, with a peak at 0.48 mM Mg. The force of contraction in Mg deficient medium (0.48 mM) was 158% of control (1.2 mM Mg) (p < 0.001). Inotropic response to the L-type channel antagonist (verapamil-1 microm) and NaK ATPase inhibitor (Ouabain-0.3 mM) was augmented in Mg deficiency (p < 0.005), indicating activation of the channel and the pump. The response to T-type channel inhibitor (NiCl2-40 microM) was attenuated in Mg deficiency (p < 0.05). The response to the sarcoplasmic reticular Ca pump inhibitor (caffeine-10 mM) and the SR Ca2+ release channel inhibitor (ryanodine-1 microM) were not significantly affected by Mg deficiency. Diastolic level of Ca2+ increased with a decrease in Mg (p < 0.05). The observations of the study lead to the conclusion that the positive inotropic response in Mg deficiency is mediated by an increase in basal Ca2+ combined with Ca-induced-Ca release consequent to Ca2+ influx through L-type Ca channel. Variation in sensitivity to Ca channel blockers and NaK ATPase inhibitor in Mg deficiency can have pharmacological implications.

gp91phox-containing NAD(P)H oxidase mediates attenuation of nitric oxide-dependent control of myocardial oxygen consumption by ANG II

We have previously reported that ANG II stimulation increased superoxide anion (O2−) through the activation of NAD(P)H oxidase and inhibited nitric oxide (NO)-dependent control of myocardial oxygen consumption (MV̇o2) by scavenging NO. Our objective was to investigate the role of NAD(P)H oxidase, especially the gp91phox subunit, in the NO-dependent control of MV̇o2. MV̇o2 in mice with defects in the expression of gp91phox [gp91phox(−/−)] was measured with a Clark-type oxygen electrode. Baseline MV̇o2 was not significantly different between wild-type (WT) and gp91phox(−/−) mice. Stimulation of NO production by bradykinin (BK) induced significant decreases in MV̇o2 in WT mice. BK-induced reduction in MV̇o2 was enhanced in gp91phox(−/−) mice. BK-induced reduction in MV̇o2 in WT mice was attenuated by 10−8 mol/l ANG II, which was restored by coincubation with Tiron or apocynin. In contrast to WT mice, BK-induced reduction in MV̇o2 in gp91phox(−/−) mice was not altered by ANG II. There was a decrease in lucigenin (5 × 10−6 mol/l)-detectable O2− in gp91phox(−/−) mice compared with WT mice. ANG II resulted in significant increases in O2− production in WT mice, which was inhibited by coincubation with Tiron or apocynin. However, ANG II had no effect on O2− production in gp91phox(−/−) mice. Histological examination showed that the development of abscesses and/or the invasion of inflammatory cells occurred in lungs and livers but not in hearts and kidneys from gp91phox(−/−) mice. These results indicate that the gp91phox subunit of NAD(P)H oxidase mediates O2− production through the activation of NAD(P)H oxidase and attenuation of NO-dependent control of MV̇o2 by ANG II.