Rabbit Papillary Muscle Research Articles

Cardiac electrophysiological effects of ibuprofen in dog and rabbit ventricular preparations: possible implication to enhanced proarrhythmic risk.

Ibuprofen is a widely used nonsteroidal anti-inflammatory drug, which has recently been associated with increased cardiovascular risk, but its electrophysiological effects have not yet been properly studied in isolated cardiac preparations. We studied the effects of ibuprofen on action potential characteristics and several transmembrane ionic currents using the conventional microelectrode technique and the whole-cell configuration of the patch-clamp technique on cardiac preparations and enzymatically isolated ventricular myocytes. In dog (200 µM; n= 6) and rabbit (100 µM; n= 7) papillary muscles, ibuprofen moderately but significantly prolonged repolarization at 1Hz stimulation frequency. In dog Purkinje fibers, repolarization was abbreviated and maximal rate of depolarization was depressed in a frequency-dependent manner. Levofloxacin (40 µM) alone did not alter repolarization, but augmented the ibuprofen-evoked repolarization lengthening in rabbit preparations (n= 7). In dog myocytes, ibuprofen (250 µM) did not significantly influence IK1, but decreased the amplitude of Ito and IKr potassium currents by 28.2% (60mV) and 15.2% (20mV), respectively. Ibuprofen also depressed INaL and ICa currents by 19.9% and 16.4%, respectively. We conclude that ibuprofen seems to be free from effects on action potential parameters at lower concentrations. However, at higher concentrations it may alter repolarization reserve, contributing to the observed proarrhythmic risk in patients.

Stretch-induced compliance: a novel adaptive biological mechanism following acute cardiac load.

The heart is constantly challenged with acute bouts of stretching or overload. Systolic adaptations to these challenges are known but adaptations in diastolic stiffness remain unknown. We evaluated adaptations in myocardial stiffness due to acute stretching and characterized the underlying mechanisms. Left ventricles (LVs) of intact rat hearts, rabbit papillary muscles and myocardial strips from cardiac surgery patients were stretched. After stretching, there was a sustained >40% decrease in end-diastolic pressure (EDP) or passive tension (PT) for 15 min in all species and experimental preparations. Stretching by volume loading in volunteers and cardiac surgery patients resulted in E/E' and EDP decreases, respectively, after sustained stretching. Stretched samples had increased myocardial cGMP levels, increased phosphorylated vasodilator-stimulated phosphoprotein phosphorylation, as well as, increased titin phosphorylation, which was reduced by prior protein kinase G (PKG) inhibition (PKGi). Skinned cardiomyocytes from stretched and non-stretched myocardia were studied. Skinned cardiomyocytes from stretched hearts showed decreased PT, which was abrogated by protein phosphatase incubation; whereas those from non-stretched hearts decreased PT after PKG incubation. Pharmacological studies assessed the role of nitric oxide (NO) and natriuretic peptides (NPs). PT decay after stretching was significantly reduced by combined NP antagonism, NO synthase inhibition and NO scavenging, or by PKGi. Response to stretching was remarkably reduced in a rat model of LV hypertrophy, which also failed to increase titin phosphorylation. We describe and translate to human physiology a novel adaptive mechanism, partly mediated by titin phosphorylation through cGMP-PKG signalling, whereby myocardial compliance increases in response to acute stretching. This mechanism may not function in the hypertrophic heart.

An historical perspective of the discovery of titin filaments -Part 2.

In 2017, a Special Issue of Biophysical Reviews was devoted to "Titin and Its Binding Partners. The issue contained a review: "An historical perspective of the discovery of titin filaments" by dos Remedios and Gilmour that was intended to be a history of the discovery of the giant protein titin, previously named connectin. The review took readers back to the earliest discovery of the so-called third filament component of skeletal and cardiac muscle sarcomeres and ended in 1969. Recently, my colleague Shin'ichi Ishiwata gently reminded me of two papers published in 1990 and 1993 that were unwittingly omitted from the original historical perspective. In the first paper (J Cell Biol 110:53-62, 1990), Funatsu et al. examined the elastic filaments in skeletal muscle using a combination of light and electron microscopy, but they also measured resting as well as passive stiffness mechanical measurements to establish that connectin (titin) is responsible for both stiffness and fiber tension. In the second paper (J Cell Biol 120:711-724, 1993), Funatsu et al. used permeabilised cardiac muscle myocytes (from rabbit papillary muscles) and focussed on filament ultrastructure using either freeze-substitution or deep-etched replica methods to visualise connectin/titin filaments in fibers with and without actin and myosin filaments.

Abstract 210: Titin Phosphorylation by Protein Kinase G as a Novel Mechanism of Diastolic Adaptation to Acute Hemodynamic Overload

Titin is the main determinant of myocardial passive tension and its distensibility is increased via phosphorylation by protein kinase G (PKG), activated by nitric oxide (NO) and natriuretic peptides (NP) upon acute overload. We hypothesized whether myocardial stretch led to decreased stiffness, optimizing diastolic filling along with the usual increase in contractility. Intact rat Langendorff hearts, strips dissected from the LV or right atrium of cardiac surgery patients and rabbit papillary muscles were acutely stretched for 15min. Passive tension (PT) was measured in skinned cardiomyocytes extracted from the LV of control and stretched rat hearts for sarcomere lengths (SL) 1.8-2.3μm before and after incubation with PKG. Rabbit muscles were incubated with a PKG inhibitor or, simultaneously, a NO synthase inhibitor, a NO scavenger, a NP receptor A antagonist. All-total titin phosphorylation was stained with Pro-Q Diamond and indexed to total-protein signals using SYPRO Ruby. Values are given as mean±SEM and statistical significance was set to p<0.05. After acute stretch there was a progressive decrease in passive tension/diastolic pressure over 15min: 27±8% and 27±6% in human atrium and ventricular muscles, respectively, and 43±2% in rabbit papillary muscles and isolated hearts. This decrease in myocardial stiffness was significantly blunted by PKG inhibition (40%) and NO/NP pathway inhibition (29%). PT of cardiomyocytes was significantly lower (≈60%) in the previously stretched group for all SL. A similar effect, only significant in the control group, was observed after incubation with PKG. Titin phosphorylation increased markedly 15 minutes after acute myocardial stretch in human (atrial: 11±1% vs 41±8%; LV: 27±8% vs 71±21%) and rabbit (13±2% vs 23±3%) myocardium. The progressive decrease in myocardial stiffness after acute hemodynamic overload is preserved at the myofilamental level and seems to depend on PKG activity, representing a potential therapeutic target in patients with pathologically rigid myocardium. Moreover, blocking PKG activation seems to attenuate this adaptive diastolic response. Therefore, titin phosphorylation by this kinase is probably involved in this new myocardial response to stretch in both animals and humans.

Differential effects of thioridazine enantiomers on action potential duration in rabbit papillary muscle

The antipsychotic drug thioridazine has potential for treatment of multidrug-resistant microbes including tuberculosis but also causes cardiotoxic QT interval prolongation. Both thioridazine enantiomers have potent antimicrobial effects, but the neuroleptic effect primarily resides with (+)-thioridazine. In this study we for the first time investigate the cardiotoxicity of the isolated thioridazine enantiomers and show their effects on ventricular repolarization. The effects of (+)-thioridazine, (−)-thioridazine, and racemate on the rabbit ventricular action potential duration (APD) were investigated in a randomized controlled blinded experiment. Action potentials were measured in papillary muscles isolated from 21 female rabbits, and the drug effect on 90% APD in comparison with control (ΔΔ-APD90) was evaluated. Increasing concentrations of (+)-thioridazine and the racemate caused significant dose-dependent ΔΔ-APD90 prolongation, while (−)-thioridazine did not. At 0.5 and 2Hz pacing, (+)-thioridazine caused 19.5% and 20.1% ΔΔ-APD90 prolongation, the racemate caused 8.0% and 12.9%, and (−)-thioridazine caused 1.5% and 1.1%. The effect of (−)-thioridazine on APD90 was significantly less than that of the other drugs at both pacing rates (P<0.01 in all cases), and there was no significant difference between (−)-thioridazine and control. The results of this study indicate that the APD prolonging effect of thioridazine is primarily due to the (+)-thioridazine enantiomer. If these results are valid in humans, (−)-thioridazine may be a safer drug for treatment of multidrug-resistant tuberculosis and other microbes.

The systolic response to myocardial stretch in the normal and ischemic myocardium: the role of PKG and related pathways

Purpose: Myocardial ischemia is frequently associated with acute hemodynamic overload. The effects of ischemia on the Frank-Starling Mechanism (FSM) and the Slow Force Response (SFR) are largely unknown. We aimed to characterize the systolic response to acute stretch and its modulation by PKG and related pathways. Methods: Rabbit papillary muscles (0.2 Hz, 30°C) were acutely stretched from 92% to 100% Lmax (muscle length at which maximal force is developed) in a modified Krebs-Ringer solution under normal conditions (A) in the absence (n=9) and in the presence of (B) Rp-8-Br-PET-cGMPS (an inhibitor of PKG, 10-6M, n=7). The other protocols were performed in ischemic conditions (C) in the absence (n=7) and in the presence of (D) 8-Bromo-cGMP (an agonist of PKG, 10-5M, n=7), (E) S-nitroso-N-acetyl-penicillamine (a NO donor, 10-5M, n=9), (F) BNP (B-type natriuretic peptide, 10-6M, n=9) and (G) sildenafil (a PDE5 inhibitor, 10-6M, n=7). Immediate and delayed contractile responses to muscle stretch were evaluated. Results are presented as mean±standard error of mean (P < 0.05). Results: Under normal conditions, acute myocardial stretch elicited an immediate increase in contractility (FSM) of 35.6±5.3% and a further increase over the next 15 minutes (SFR) of 25.2±4.3%. PKG inhibition was associated with a significant attenuation of FSM (13.7%±6.0, P=0.016) and SFR (11.5%±5.9, P=0.016). Ischemia induced a significant decrease of the FSM (20.0%±4.2, P=0.044). After the FSM, ischemic muscles exhibited a progressive decrease of contractility over the following 15 minutes (-59.1%±4.9). The NO donor and the BNP addition did not significantly modify the response under ischemic conditions. FSM was further inhibited, under ischemia, by the addition of 8-Bromo-cGMP (-2.8%±7.9, P=0.026) and sildenafil (0.1%±7.8, P=0.045). While 8-Bromo-cGMP did not influence the progressive force decline, sildenafil significantly attenuated the contractility decrease after acute stretch (-8.6%±17.6, P=0.007) thereby compensating the FSM abolishment (active tension variation vs pre-stretch: -2.4±20.9 vs -50.5±6.7 in the ischemic control group, P=0.049). Conclusions: Systolic response to acute myocardial stretch is dependent on PKG activity. This response is markedly impaired in the ischemic myocardium. We also demonstrated that, contrarily to the activation of NO and BNP pathways, sildenafil exerts a previously unknown protective effect on the contractile function following stretch in the ischemic myocardium highlighting its potential therapeutic role in this setting.

The effects of angiotensin II signaling pathway in the systolic response to acute stretch in the normal and ischemic myocardium

Acute myocardial stretch elicits a biphasic increase in contractility: an immediate increase, known as Frank–Starling mechanism (FSM), followed by a progressive increase, regarded as slow force response (SFR). In this study, we characterized the contractile response to acute stretch from 92 to 100% Lmax in rabbit papillary muscles (n=86) under normoxic and ischemic conditions, and its modulation by angiotensin II signaling pathway. Under normoxia, the FSM was independent of Na+/H+-exchanger, reverse mode of Na+/Ca2+-exchanger (r-NCX), AT1 receptor, AT2 receptor and PKC. Regarding the SFR, it was mediated by AT1 receptor activation and its downstream effectors PKC, Na+/H+-exchanger and r-NCX. Ischemia negatively impacted on the FSM and abolished the SFR, with the muscles exhibiting a time-dependent decline in contractility. Under ischemic conditions, FSM was not influenced by AT1 and AT2 receptors or PKC activation. AT1 receptor antagonism rescued the progressive deterioration in contractility, an effect partially dependent on AT2 receptor activation.

On the Modeling of Extension-Torsion Experimental Data for Transversely Isotropic Biological Soft Tissues

For the problem of torsion superimposed on extension of incompressible nonlinearly elastic transversely isotropic circular cylinders, a simple asymptotic analysis is carried out on using a small parameter that reflects the moderate twisting of slender cylinders, which corresponds to a typical testing regime for biological soft tissue. The analysis is carried out for a subclass of strain-energy densities that reflect transversely isotropic material response. On using a four-parameter polynomial expression for the strain-energy density in terms of certain classical invariants, this analysis is shown to be in excellent agreement with experimental data obtained by other authors for rabbit papillary muscles. An explicit condition on the strain-energy density is obtained that determines whether the stretched cylinder tends to elongate or shorten on twisting. For the special case of pure torsion where no extension is allowed, this condition determines whether the classical or reverse Poynting effect occurs. For the rabbit papillary muscles, the theoretical results predict and the experimental results confirm that a reverse Poynting-type effect occurs where the stretched rabbit muscle tends to shorten on twisting.

Enhancement of amrinone-induced positive inotropy in rabbit papillary muscles with depressed contractile function: effects on cyclic nucleotide levels and phosphodiesterase isoenzymes.

The inotropic activity of amrinone and its effects on cyclic nucleotide levels in rabbit papillary muscles with normal and depressed contractile function have been compared. The effects of amrinone on the cyclic (c) AMP hydrolytic activity of cyclic nucleotide phosphodiesterase (PDE) isoenzymes were also examined. Amrinone (2.4 x 10(-4) - 1.2 x 10(-3) M) produced a relatively weak (maximal increase 11%) positive inotropic effect in papillary muscles stimulated at the near optimal stimulation frequency of 1 Hz. In contrast, large positive inotropic responses (maximal 138-200%) were obtained with amrinone in papillary muscles in which contractile force had been depressed by: (a) lowering stimulation frequency to 0.4 Hz, (b) reducing extracellular Ca2+ concentration from 2.5 x 10(-3) M to 6.3 x 10(-4) M, (c) prior addition of sodium pentobarbitone (6.5 x 10(-4) M). The EC50 values for amrinone under conditions (a), (b), and (c) were 3.0 x 10(-3), 2.6 x 10(-3), and 2.8 x 10(-3) M, respectively. Force-frequency curves in rabbit papillary muscles were compared at normal (2.5 x 10(-3) M) and low (6.3 x 10(-4) M) extracellular Ca2+ concentration. Contractions at low frequencies of stimulation (less than 0.4 Hz) were less sensitive to removal of extracellular Ca2+ than higher stimulation rates indicating that in the former situation, recycling of intracellular Ca2+ is more important for maintaining contractile force.(ABSTRACT TRUNCATED AT 250 WORDS)

The Inotropic Effect of 4−Aminopyridine and pH Changes in Rabbit Papillary Muscle

The effects of 4-aminopyridine (4AP), and pH changes have been examined on tension responses and cyclic nucleotide levels in rabbit isolated right ventricular papillary muscles. 4AP augmented papillary muscle contractions in a concentration-dependent manner. However, this positive inotropic action was largely due to an increase in extracellular pH produced by 4AP, rather than an intrinsic activity of the drug. Increases in extracellular pH (from 5 to 9) produced graded and reproducible increases in contractile force which were not blocked by propranolol (1 x 10(-7) M) but were inhibited by verapamil in a concentration-dependent manner. The positive inotropic effects of Ca2+ were enhanced and depressed by alkaline and acidic pH, respectively. Neither 4AP nor alkaline pH significantly changed cyclic AMP concentration in rabbit papillary muscles. The cyclic GMP content, however, was increased by 4AP only and this effect was blocked by atropine. The results suggest that the positive inotropic effect associated with a rise in pH from neutrality may be due to facilitation of translocation of membrane Ca2+ and/or to increase the release of Ca2+ from sources within the cell. They also illustrate that a major component of the inotropic effect of 4AP is a result of an increase in extracellular pH.

Inhibitory effects of dauricine on early afterdepolarizations and L-type calcium current

We have previously reported that dauricine exerted antiarrhythmic effects on various experimental arrhythmias. To further clarify its mechanism, the effects of dauricine on action potential duration (APD), early afterdepolarizations (EADs), triangulation, which is defined as the repolarization time from APD at 30% level (APD30) to APD at 90% level (APD90), and L-type calcium current (I(Ca-L)) were studied using standard microelectrode techniques on rabbit papillary muscles and whole-cell patch clamp techniques on single myocytes isolated from rabbits by enzymatic digestion, respectively. Cardiac hypertrophy was induced by coarctating the abdominal aorta of rabbits. The results showed that in papillary muscles of hypertrophied rabbits, 1 micromol/L dofetilide, a selective IKr blocker, prolonged APD50 and APD90 and induced EADs (4/6, p < 0.01) with hypokalemia ([K+]o = 2.7 mmol/L). Dauricine inhibited EADs (p < 0.01) and shortened the prolonged APD (p < 0.01). In single myocytes, dauricine also inhibited EADs induced by dofetilide, hypokalemia, and hypomagnesaemia. Dauricine decreased the triangulation and reduced the peak amplitude of I(Ca-L) at all potentials tested. Dauricine shifted the steady-state activation curves to the right and steady-state inactivation curves to the left and prolonged the tau value of the recovery curve. These results suggest that dauricine inhibits EADs and this effect may be associated with its blockade of I(Ca-L).

Effects of the NO donor sodium nitroprusside on oxygen consumption and energetics in rabbit myocardium

Nitric oxide (NO) has influence on various cellular functions. Little is known of the influence of NO on myocardial energetics. In the present study oxygen consumption and mechanical parameters of isometrically contracting rabbit papillary muscles (1 Hz stimulation frequency) were investigated at varying interventions while maintaining physiological conditions (37°C; 2.5 mM Ca2+) to study the effects of NO on energetics. The NO donor sodium nitroprusside (SNP) showed a negative inotropic effect. SNP decreased the maximal force in normal rabbit muscle strips by 30%, the force time integral (FTI) by 40% and the relaxation time by 20%. In addition the oxygen consumption decreased by 60%, a notably disproportional decrease compared to the mechanical parameters. Consequently, the economy as a ratio of FTI and oxygen consumption is significantly increased by SNP. In contrast the negative inotropic effect due to a reduction in extracellular Calcium (Ca2+) from 2.5 to 1.25 mM reduced FTI and oxygen consumption proportionally by 40% and did not change economy. The effect of NO on force and oxygen consumption could be reproduced by the application of the cyclic guanosine monophosphate (cGMP) analogue 8-bromo-cGMP. In summary, NO increased the economy of isometrically contracting papillary muscles. The improvement in contraction economy under NO seems to be mediated by cGMP as the secondary messenger and maybe due to alterations of the crossbridge cycle.

Urotensin II-induced increase in myocardial distensibility is modulated by angiotensin II and endothelin-1

Endogenous regulators, such as angiotensin-II (AngII), endothelin-1 (ET-1) and urotensin-II (U-II) are released from various cell types and their plasma levels are elevated in several cardiovascular diseases. The present study evaluated a potential crosstalk between these systems by investigating if the myocardial effects of U-II are modulated by AngII or ET-1. Effects of U-II (10(-8), 10(-7), 10(-6) M) were tested in rabbit papillary muscles in the absence and in the presence of losartan (selective AT(1) receptor antagonist), PD-145065 (nonselective ET-1 receptors antagonist), losartan plus PD-145065, AngII or ET-1. U-II promoted concentration-dependent negative inotropic and lusitropic effects that were abolished in all experimental conditions. Also, U-II increased resting muscle length up to 1.008+/-0.002 L/L(max). Correcting it to its initial value resulted in a 19.5+/-3.5 % decrease of resting tension, indicating increased muscle distensibility. This effect on muscle length was completely abolished in the presence of losartan and significantly attenuated by PD-145065 or losartan plus PD-145065. This effect was increased in the presence of AngII, resulting in a 27.5+/-3.9 % decrease of resting tension, but was unaffected by the presence of ET-1. This study demonstrated an interaction of the U-II system with the AngII and ET-1 systems in terms of regulation of systolic and diastolic function.

Effects of adrenomedullin on systolic and diastolic myocardial function

Adrenomedullin (AM) effects were studied in rabbit papillary muscles by adding increasing concentrations (10 −10 to 10 −6 M) either alone or after pre-treatment with l-NNA, indomethacin, AM22–52 (AM receptor antagonist), CGRP(8–37) (CGRP receptors antagonist), KT5720 (PKA inhibitor), as well as after endocardial endothelium (EE) removal. Passive length–tension relations were constructed before and after a single concentration of AM (10 −6 M). AM concentration-dependently induced negative inotropic and lusitropic effects, and increased resting muscle length (RL). At 10 −6 M, AT, d T/d t max and d T/d t min decreased 20.9 ± 4.9%, 18.3 ± 7.3% and 16.7 ± 7.8%, respectively, and RL increased to 1.010 ± 0.004 L/ L max. Correcting RL to its initial value resulted in a 26.6 ± 6.4% decrease of resting tension, indicating decreased muscle stiffness, also patent in the down and rightward shift of the passive length–tension relation. The negative inotropic effect of AM was dependent on its receptor, CGRP receptor, PKA, the EE and NO, while the effects of AM on myocardial stiffness were abolished by EE damage and NO inhibition. This latter effect represents a novel mechanism of acute neurohumoral modulation of diastolic function, suggesting that AM is an important regulator of cardiac filling.

Abstract 1499: Angiotensin And Endothelin Systems - Important Players In Urotensin II Induced Myocardial Distensibility

Objective: Urotensin II (U-II) is a vasoactive peptide with important actions in the cardiovascular system. It was recently shown that it acutely decreases myocardial stiffness. Although angiotensin II (AngII) and endothelin-1 (ET-1) also modulate myocardial diastolic properties, their interaction with U-II at this level has not yet been investigated. Methods: Effects of increasing concentrations of U-II (10 – 8–10 – 6M) were studied in rabbit papillary muscles immersed in a modified Krebs solution (0.6Hz;1.8mM Ca2+;35°C) in the absence (n=12) and in presence of losartan, a selective competitive antagonist of AngII receptor type 1 (10 – 6M;n=8) or PD-145065, a nonselective endothelin receptor antagonist (10 – 6M;n=6). Calculated parameters: passive tension (PT), active tension (AT), maximum velocity of tension rise and decline (dT/dtmax and dT/dtmin, respectively) and muscle length. Results presented as mean±SEM (p&lt;0.05). Results: U-II induced a concentration dependent negative inotropic and lusitropic effects, decreasing at 10 – 6M 15.8±5.6%AT, 13.5±5.4%dT/dtmax and 18.1±4.5%%dT/dtmin. This effect was inhibited by losartan or PD-145065. Additionally, U-II induced a concentration dependent increase in passive muscle length of 1.0081±0.002L/ Lmax. Restoring muscle length to Lmax decreased 19.5±3.5%PT, indicating increased myocardial distensibility. In the presence of PD-145065, the maximal concentration of U-II only increased muscle length to 1.004±0.002%L/Lmax, which corresponded to a decrease of PT of just 11.6±2.7%. With losartan these effects of U-II were abolished. Conclusion: The acute increase in myocardial distensibility induced by U-II is attenuated by ET-1 receptor blockade and completely abolished in the presence of an AT1 antagonist. This interaction between U-II, AngII and ET-1 is a novel finding with potential pathophysiologic and therapeutic implications in heart failure that deserves further investigation.

Ca2+-stimulated Basal Adenylyl Cyclase Activity Localization in Membrane Lipid Microdomains of Cardiac Sinoatrial Nodal Pacemaker Cells

Spontaneous, rhythmic subsarcolemmal local Ca(2+) releases driven by cAMP-mediated, protein kinase A (PKA)-dependent phosphorylation are crucial for normal pacemaker function of sinoatrial nodal cells (SANC). Because local Ca(2+) releases occur beneath the cell surface membrane, near to where adenylyl cyclases (ACs) reside, we hypothesized that the dual Ca(2+) and cAMP/PKA regulatory components of automaticity are coupled via Ca(2+) activation of AC activity within membrane microdomains. Here we show by quantitative reverse transcriptase PCR that SANC express Ca(2+)-activated AC isoforms 1 and 8, in addition to AC type 2, 5, and 6 transcripts. Immunolabeling of cell fractions, isolated by sucrose gradient ultracentrifugation, confirmed that ACs localize to membrane lipid microdomains. AC activity within these lipid microdomains is activated by Ca(2+) over the entire physiological Ca(2+) range. In intact SANC, the high basal AC activity produces a high level of cAMP that is further elevated by phosphodiesterase inhibition. cAMP and cAMP-mediated PKA-dependent activation of ion channels and Ca(2+) cycling proteins drive sarcoplasmic reticulum Ca(2+) releases, which, in turn, activate ACs. This feed forward "fail safe" system, kept in check by a high basal phosphodiesterase activity, is central to the generation of normal rhythmic, spontaneous action potentials by pacemaker cells.

Cardiotin localization in mitochondria of cardiomyocytes in vivo and in vitro and its down-regulation during dedifferentiation

Cardiotin expression is observed in adult cardiac tissue. In the present study, we provide evidence for the specific localization of cardiotin in cardiac mitochondria and for its down-regulation during adaptive remodeling (dedifferentiation) of cardiomyocytes. Immunocytochemistry was used to study cardiotin localization in adult rabbit papillary muscle, in late-stage embryonic rabbit left ventricular tissue, and in left ventricle samples of rabbits suffering from pressure and volume overload. Western blot analysis of cardiotin was performed in purified pig heart mitochondrial fractions. Cardiotin expression was monitored in vitro in isolated adult rat and rabbit left ventricular cardiomyocytes. Western blot analysis revealed the presence of cardiotin in the mitochondrial fractions of pig heart. Immunoelectron microscopy confirmed the presence of cardiotin in cardiac mitochondria of normal adult rabbits both in vivo and in vitro. Quantification of the localization of immunogold particles suggests an association of cardiotin with the mitochondrial inner membrane. Cardiotin expression is initiated in late-stage embryonic rabbit heart, whereas in adult ventricular tissue cardiotin clearly stained longitudinal arrays of mitochondria. Pressure- and volume-overloaded myocardium showed a reduction in cardiotin expression in dispersed local myocardial areas. Cell cultures of adult cardiomyocytes showed a gradual loss in cardiotin expression in parallel with a sarcomeric remodeling. Our results demonstrate the specific localization of cardiotin in adult cardiomyocyte mitochondria and propose its use as an early marker for cardiomyocyte adaptive remodeling and dedifferentiation.

Drug development for treatment of cardiac arrhythmias: targeting the gap junctions

reentrant excitation ([12][1]) is likely the most important mechanism for serious life-threatening arrhythmias. Traditionally, a major focus of antiarrhythmic drug development has been to explore the actions of drugs designed to block conduction in reentrant pathways(s). During the latter part of

The Development of Subsarcolemmal Sodium Gradients in Ischaemic Rabbit Papillary Muscle Measured by X-Ray Microanalysis

The Development of Subsarcolemmal Sodium Gradients in Ischaemic Rabbit Papillary Muscle Measured by X-Ray Microanalysis

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.