Cardiac expression of ryanodine receptor subtype 3; a strategic component in the intracellular Ca2+ release system of Purkinje fibers in large mammalian heart

BACKGROUND

Three distinct Ca²⁺ release channels were identified in dog P-cells: the ryanodine receptor subtype 2 (RyR2) was detected throughout the cell, while the ryanodine receptor subtype 3 (RyR3) and inositol phosphate sensitive Ca²⁺ release channel (InsP3R) were found in the cell periphery. How each of these channels contributes to the Ca²⁺ cycling of P-cells is unclear. Recent modeling of Ca²⁺ mobilization in P-cells suggested that Ca²⁺ sensitivity of Ca²⁺induced Ca²⁺release (CICR) was larger at the P-cell periphery. Our study examined whether this numerically predicted region of Ca²⁺ release exists in live P-cells. We compared the regional Ca²⁺ dynamics with the arrangement of intracellular Ca²⁺ release (CR) channels.

METHODS

Gene expression of CR channels was measured by qPCR in Purkinje fibers and myocardium of adult Yucatan pig hearts. We characterized the CR channels protein expression in isolated P-cells by immuno-fluorescence, laser scanning confocal microscopy, and 3D reconstruction. The spontaneous Ca²⁺ activity and electrically-evoked Ca²⁺ mobilization were imaged by 2D spinning disk confocal microscopy. Functional regions of P-cell were differentiated by the characteristics of local Ca²⁺ events. We used the Ca²⁺ propagation velocities as indicators of channel Ca²⁺ sensitivity.

RESULTS

RyR2 gene expression was identical in Purkinje fibers and myocardium (6 hearts) while RyR3 and InsP₃R gene expressions were, respectively, 100 and 16 times larger in the Purkinje fibers. Specific fluorescent immuno-staining of Ca²⁺ release channels revealed an intermediate layer of RyR3 expression between a near-membrane InsP3R-region and a central RyR2-region. We found that cell periphery produced two distinct forms of spontaneous Ca²⁺-transients: (1) large asymmetrical Ca²⁺ sparks under the membrane, and (2) typical Ca²⁺-wavelets propagating exclusively around the core of the cell. Larger cell-wide Ca²⁺ waves (CWWs) appeared occasionally traveling in the longitudinal direction through the core of Pcells. Large sparks arose in a micrometric space overlapping the InsP3R expression. The InsP3R antagonists 2-aminoethoxydiphenyl borate (2-APB; 3μM) and xestospongin C (XeC; 50μM) dramatically reduced their frequency. The Ca²⁺ wavelets propagated in a 5-10μm thick layered space which matched the intermediate zone of RyR3 expression. The wavelet incidence was unchanged by 2-APB or XeC, but was reduced by 60% in presence of the RyR3 antagonist dantrolene (10μM). The velocity of wavelets was two times larger (86±16μm/s; n=14) compared to CWWs' (46±10μm/s; n=11; P<0.05). Electric stimulation triggered a uniform and large elevation of Ca²⁺ concentration under the membrane which preceded the propagation of Ca²⁺ into the interior of the cell. Elevated Ca_i propagated at 150μm/s (147±34μm/s; n=5) through the region equivalent to the zone of RyR3 expression. This velocity dropped by 50% (75±24μm/s; n=5) in the central region wherein predominant RyR2 expression was detected.

CONCLUSION

We identified two layers of distinct Ca²⁺ release channels in the periphery of Pcell: an outer layer of InsP₃Rs under the membrane and an inner layer of RyR3s. The propagation of Ca²⁺ events in these layers revealed that Ca²⁺ sensitivity of Ca²⁺ release was larger in the RyR3 layer compared to that of other sub-cellular regions. We propose that RyR3 expression in P-cells plays a role in the stability of electric function of Purkinje fibers.

What is a Ca(2+) wave? Is it like an Electrical Wave?

Arrhythmia subcellular mechanisms are constantly being explored. Recent knowledge has shown that travelling Ca(2+) waves in cardiac cells are critical for delayed afterdepolarisations and in some cases, early afterdepolarisations. In this review, we comment on the properties of cardiac Ca(2+) waves and abnormal Ca(2+) releases in terms of properties used to describe electrical waves; propagation, excitability and refractoriness.

Modeling our understanding of the His-Purkinje system

The His-Purkinje System (HPS) is responsible for the rapid electric conduction in the ventricles. It relays electrical impulses from the atrioventricular node to the muscle cells and, thus, coordinates the contraction of ventricles in order to ensure proper cardiac pump function. The HPS has been implicated in the genesis of ventricular tachycardia and fibrillation as a source of ectopic beats, as well as forming distinct portions of reentry circuitry. Despite its importance, it remains much less well characterized, structurally and functionally, than the myocardium. Notably, important differences exist with regard to cell structure and electrophysiology, including ion channels, intracellular calcium handling, and gap junctions. Very few computational models address the HPS, and the majority of organ level modeling studies omit it. This review will provide an overview of our current knowledge of structure and function (including electrophysiology) of the HPS. We will review the most recent advances in modeling of the system from the single cell to the organ level, with considerations for relevant interspecies distinctions.

Small caliber arterial endothelial cells calcium signals elicited by PAR2 are preserved from endothelial dysfunction

Endothelial cell (EC)-dependent vasodilation by proteinase-activated receptor 2 (PAR2) is preserved in small caliber arteries in disease states where vasodilation by muscarinic receptors is decreased. In this study, we identified and characterized the PAR2-mediated intracellular calcium (Ca²⁺)-release mechanisms in EC from small caliber arteries in healthy and diseased states. Mesenteric arterial EC were isolated from PAR2 wild-type (WT) and null mice, after saline (controls) or angiotensin II (AngII) infusion, for imaging intracellular calcium and characterizing the calcium-release system by immunofluorescence. EC Ca²⁺ signals comprised two forms of Ca²⁺-release events that had distinct spatial-temporal properties and occurred near either the plasmalemma (peripheral) or center of EC. In healthy EC, PAR2-dependent increases in the densities and firing rates of both forms of Ca²⁺-release were abolished by inositol 1,4,5- trisphosphate receptor (IP₃R) inhibitor, but partially reduced by transient potential vanilloid channels inhibitor ruthenium red (RR). Acetylcholine (ACh)-induced less overall Ca²⁺-release than PAR2 activation, but enhanced selectively the incidence of central events. PAR2-dependent Ca²⁺-activity, inhibitors sensitivities, IP₃R, small- and intermediate-conductance Ca²⁺-activated potassium channels expressions were unchanged in EC from AngII WT. However, the same cells exhibited decreases in ACh-induced Ca²⁺-release, RR sensitivity, and endothelial nitric oxide synthase expression, indicating AngII-induced dysfunction was differentiated by receptor, Ca²⁺-release, and downstream targets of EC activation. We conclude that PAR2 and muscarinic receptors selectively elicit two elementary Ca²⁺ signals in single EC. PAR2-selective IP₃R-dependent peripheral Ca²⁺-release mechanisms are identical between healthy and diseased states. Further study of PAR2-selective Ca²⁺-release for eliciting pathological and/or normal EC functions is warranted.

Release kinetics of circulating cardiac myosin binding protein-C following cardiac injury

Diagnosis of myocardial infarction (MI) is based on ST-segment elevation on electrocardiographic evaluation and/or elevated plasma cardiac troponin (cTn) levels. However, troponins lack the sensitivity required to detect the onset of MI at its earliest stages. Therefore, to confirm its viability as an ultra-early biomarker of MI, this study investigates the release kinetics of cardiac myosin binding protein-C (cMyBP-C) in a porcine model of MI and in two human cohorts. Release kinetics of cMyBP-C were determined in a porcine model of MI (n = 6, pigs, either sex) by measuring plasma cMyBP-C level serially from 30 min to 14 days after coronary occlusion, with use of a custom-made immunoassay. cMyBP-C plasma levels were increased from baseline (76 ± 68 ng/l) at 3 h (767 ± 211 ng/l) and peaked at 6 h (2,418 ± 780 ng/l) after coronary ligation. Plasma cTnI, cTnT, and myosin light chain-3 levels were all increased 6 h after ligation. In a cohort of patients (n = 12) with hypertrophic obstructive cardiomyopathy undergoing transcoronary ablation of septal hypertrophy, cMyBP-C was significantly increased from baseline (49 ± 23 ng/l) in a time-dependent manner, peaking at 4 h (560 ± 273 ng/l). In a cohort of patients with non-ST segment elevation MI (n = 176) from the SYNERGY trial, cMyBP-C serum levels were significantly higher (7,615 ± 4,514 ng/l) than those in a control cohort (416 ± 104 ng/l; n = 153). cMyBP-C is released in the blood rapidly after cardiac damage and therefore has the potential to positively mark the onset of MI.

Evoked centripetal Ca(2+) mobilization in cardiac Purkinje cells: insight from a model of three Ca(2+) release regions

Despite strong suspicion that abnormal Ca(2+) handling in Purkinje cells (P-cells) is implicated in life-threatening forms of ventricular tachycardias, the mechanism underlying the Ca(2+) cycling of these cells under normal conditions is still unclear. There is mounting evidence that P-cells have a unique Ca(2+) handling system. Notably complex spontaneous Ca(2+) activity was previously recorded in canine P-cells and was explained by a mechanistic hypothesis involving a triple layered system of Ca(2+) release channels. Here we examined the validity of this hypothesis for the electrically evoked Ca(2+) transient which was shown, in the dog and rabbit, to occur progressively from the periphery to the interior of the cell. To do so, the hypothesis was incorporated in a model of intracellular Ca(2+) dynamics which was then used to reproduce numerically the Ca(2+) activity of P-cells under stimulated conditions. The modelling was thus performed through a 2D computational array that encompassed three distinct Ca(2+) release nodes arranged, respectively, into three consecutive adjacent regions. A system of partial differential equations (PDEs) expressed numerically the principal cellular functions that modulate the local cytosolic Ca(2+) concentration (Cai). The apparent node-to-node progression of elevated Cai was obtained by combining Ca(2+) diffusion and 'Ca(2+)-induced Ca(2+) release'. To provide the modelling with a reliable experimental reference, we first re-examined the Ca(2+) mobilization in swine stimulated P-cells by 2D confocal microscopy. As reported earlier for the dog and rabbit, a centripetal Ca(2+) transient was readily visible in 22 stimulated P-cells from six adult Yucatan swine hearts (pacing rate: 0.1 Hz; pulse duration: 25 ms, pulse amplitude: 10% above threshold; 1 mm Ca(2+); 35°C; pH 7.3). An accurate replication of the observed centripetal Ca(2+) propagation was generated by the model for four representative cell examples and confirmed by statistical comparisons of simulations against cell data. Selective inactivation of Ca(2+) release regions of the computational array showed that an intermediate layer of Ca(2+) release nodes with an ~30-40% lower Ca(2+) activation threshold was required to reproduce the phenomenon. Our computational analysis was therefore fully consistent with the activation of a triple layered system of Ca(2+) release channels as a mechanism of centripetal Ca(2+) signalling in P-cells. Moreover, the model clearly indicated that the intermediate Ca(2+) release layer with increased sensitivity for Ca(2+) plays an important role in the specific intracellular Ca(2+) mobilization of Purkinje fibres and could therefore be a relevant determinant of cardiac conduction.

Role of reactive oxygen species and Ca(2+) dissociation from the myofilaments in determination of Ca(2+) wave propagation in rat cardiac muscle

Ca(2+) waves are initiated not only by Ca(2+) leak from the sarcoplasmic reticulum (SR), but also by Ca(2+) dissociation from the myofilaments in the myocardium with nonuniform contraction. We investigated whether contractile properties and the production of reactive oxygen species (ROS) affect Ca(2+) wave propagation. Trabeculae were obtained from 76 rat hearts. Force was measured with a strain gauge, sarcomere length with a laser diffraction technique, and [Ca(2+)](i) with fura-2 and a CCD camera (24°C, 2.0mmol/L [Ca(2+)](o)). ROS production was estimated from 2',7'-dichlorofluorescein (DCF) fluorescence. Trabeculae were regionally exposed to a jet of solution containing 1) 10mmol/L Ca(2+) to initiate Ca(2+) waves by SR Ca(2+) leak due to Ca(2+) overload within the jet-exposed region, and 2) 0.2mmol/L Ca(2+) or 5mmol/L caffeine to initiate such waves by Ca(2+) dissociation from the myofilaments due to nonuniform contraction. Ca(2+) waves were induced by stimulus trains for 7.5s. Ten-percent muscle stretch increased DCF fluorescence and accelerated Ca(2+) waves initiated due to both Ca(2+) overload and nonuniform contraction. Preincubation with 3μmol/L diphenyleneiodonium or 10μmol/L colchicine suppressed the increase in DCF fluorescence but suppressed acceleration of Ca(2+) waves initiated only due to Ca(2+) overload. Irrespective of preincubation with colchicine, reduction of force after the addition of 10μmol/L blebbistatin did not decelerate Ca(2+) waves initiated due to Ca(2+) overload, while it did decelerate waves initiated due to nonuniform contraction. These results suggest that Ca(2+) wave propagation is modulated by ROS production through an intact microtubule network only during stretch and may be additionally modulated by Ca(2+) dissociated from the myofilaments in the case of nonuniform contraction.

Effect of nonuniform muscle contraction on sustainability and frequency of triggered arrhythmias in rat cardiac muscle

BACKGROUND

Arrhythmias are benign or lethal, depending on their sustainability and frequency. To determine why lethal arrhythmias are prone to occur in diseased hearts, usually characterized by nonuniform muscle contraction, we investigated the effect of nonuniformity on sustainability and frequency of triggered arrhythmias.

METHODS AND RESULTS

Force, membrane potential, and intracellular Ca(2+) concentration ([Ca(2+)](i)) were measured in 51 rat ventricular trabeculae. Nonuniform contraction was produced by exposing a restricted region of muscle to a jet of 20 mmol/L 2,3-butanedione monoxime (BDM) or 20 mumol/L blebbistatin. Sustained arrhythmias (>10 seconds) could be induced by stimulus trains for 7.5 seconds only with the BDM or blebbistatin jet (100 nmol/L isoproterenol, 1.0 mmol/L [Ca(2+)](o), 24 degrees C). During sustained arrhythmias, Ca(2+) surges preceded synchronous increases in [Ca(2+)](i), whereas the stoppage of the BDM jet made the Ca(2+) surges unclear and arrested sustained arrhythmias (n=6). With 200 nmol/L isoproterenol, 2.5 mmol/L [Ca(2+)](o), and the BDM jet, lengthening or shortening of the muscle during sustained arrhythmias accelerated or decelerated their cycle in both the absence (n=10) and presence (n=10) of 100 mumol/L streptomycin, a stretch-activated channel blocker, respectively. The maximum rate of force relaxation correlated inversely with the change in cycle lengths (n=14; P<0.01). Sustained arrhythmias with the BDM jet were significantly accelerated by 30 mumol/L SCH00013, a Ca(2+) sensitizer of myofilaments (n=10).

CONCLUSIONS

These results suggest that nonuniformity of muscle contraction is an important determinant of the sustainability and frequency of triggered arrhythmias caused by the surge of Ca(2+) dissociated from myofilaments in cardiac muscle.

Function of Ca(2+) release channels in Purkinje cells that survive in the infarcted canine heart: a mechanism for triggered Purkinje ectopy

BACKGROUND

Triggered Purkinje ectopy can lead to the initiation of serious ventricular arrhythmias in post-myocardial infarction patients. In the canine model, Purkinje cells from the subendocardial border of the healing infarcted heart can initiate ventricular arrhythmias. Intracellular Ca(2+) abnormalities underlie these arrhythmias, yet the subcellular reasons for these abnormalities remain unknown.

METHODS AND RESULTS

Using 2D confocal microscopy, we directly quantify and compare typical spontaneous Ca(2+) events in specific subcellular regions of normal Purkinje cells with those Purkinje cells from the subendocardium of the 48-hour infarcted canine heart (IZPCs). The Ca(2+) event rate was higher in the subsarcolemmal region of IZPCs when compared with normal Purkinje cells; IZPC amplitudes were higher, yet the spatial extents of these events were similar. The amplitude of caffeine-releasable Ca(2+) in either the subsarcolemmal or core regions of IZPCs did not differ from normal Purkinje cells, suggesting that Ca(2+) overload was not related to the frequency change. In permeabilized Purkinje cells from both groups, the event rate was related to free [Ca(2+)] in both subsarcolemmal and core, but in IZPCs, this event rate was significantly increased at each free Ca(2+), suggesting an enhanced sensitivity to Ca(2+) release. Furthermore, decays of wide long lasting Ca(2+) release events in IZPC's core were significantly accelerated compared with those in normal Purkinje cells. JTV519 (K201) suppressed IZPC cell wide Ca(2+) waves as well as normalized the enhanced event rate and its response to free Ca(2+).

CONCLUSIONS

Increased spontaneous Ca(2+) release events in IZPCs are due to uniform regionally increased Ca(2+) release channel sensitivity to Ca(2+) without a change in sarcoplasmic reticulum content. In addition, Ca(2+) reuptake in IZPCs is accelerated. These properties would lower the threshold of Ca(2+) release channels, setting the stage for the highly frequent arrhythmogenic cell wide Ca(2+) waves observed in IZPCs.

Cellular and molecular determinants of altered Ca2+ handling in the failing rabbit heart: primary defects in SR Ca2+ uptake and release mechanisms

Myocytes from the failing myocardium exhibit depressed and prolonged intracellular Ca(2+) concentration ([Ca(2+)](i)) transients that are, in part, responsible for contractile dysfunction and unstable repolarization. To better understand the molecular basis of the aberrant Ca(2+) handling in heart failure (HF), we studied the rabbit pacing tachycardia HF model. Induction of HF was associated with action potential (AP) duration prolongation that was especially pronounced at low stimulation frequencies. L-type calcium channel current (I(Ca,L)) density (-0.964 +/- 0.172 vs. -0.745 +/- 0.128 pA/pF at +10 mV) and Na(+)/Ca(2+) exchanger (NCX) currents (2.1 +/- 0.8 vs. 2.3 +/- 0.8 pA/pF at +30 mV) were not different in myocytes from control and failing hearts. The amplitude of peak [Ca(2+)](i) was depressed (at +10 mV, 0.72 +/- 0.07 and 0.56 +/- 0.04 microM in normal and failing hearts, respectively; P < 0.05), with slowed rates of decay and reduced Ca(2+) spark amplitudes (P < 0.0001) in myocytes isolated from failing vs. control hearts. Inhibition of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)2a revealed a greater reliance on NCX to remove cytosolic Ca(2+) in myocytes isolated from failing vs. control hearts (P < 0.05). mRNA levels of the alpha(1C)-subunit, ryanodine receptor (RyR), and NCX were unchanged from controls, while SERCA2a and phospholamban (PLB) were significantly downregulated in failing vs. control hearts (P < 0.05). alpha(1C) protein levels were unchanged, RyR, SERCA2a, and PLB were significantly downregulated (P < 0.05), while NCX protein was significantly upregulated (P < 0.05). These results support a prominent role for the sarcoplasmic reticulum (SR) in the pathogenesis of HF, in which abnormal SR Ca(2+) uptake and release synergistically contribute to the depressed [Ca(2+)](i) and the altered AP profile phenotype.

Young MLP deficient mice show diastolic dysfunction before the onset of dilated cardiomyopathy

Targeted deletion of cytoskeletal muscle LIM protein (MLP) in mice consistently leads to dilated cardiomyopathy (DCM) after one or more months. However, next to nothing is known at present about the mechanisms of this process. We investigated whether diastolic performance including passive mechanics and systolic behavior are altered in 2-week-old MLP knockout (MLPKO) mice, in which heart size, fractional shortening and ejection fraction are still normal. Right ventricular trabeculae were isolated from 2-week-old MLPKO and wildtype mice and placed in an apparatus that allowed force measurements and sarcomere length measurements using laser diffraction. During a twitch from the unloaded state at 1 Hz, MLPKO muscles relengthened to slack length more slowly than controls, although the corresponding force relaxation time was unchanged. Active developed stress at a diastolic sarcomere length of 2.00 microm was preserved in MLPKO trabeculae over a wide range of pacing frequencies. Force relaxation under the same conditions was consistently prolonged compared with wildtype controls, whereas time to peak and maximum rate of force generation were not significantly altered. Ca2+ content of the sarcoplasmic reticulum (SR) and the quantities of Ca2+ handling proteins were similar in both genotypes. In summary, young MLPKO mice revealed substantial alterations in passive myocardial properties and relaxation time, but not in most systolic characteristics. These results indicate that the progression to heart failure in the MLPKO model may be driven by diastolic myocardial dysfunction and abnormal passive properties rather than systolic dysfunction.

Spatial Nonuniformity of Contraction Causes Arrhythmogenic Ca2+Waves in Rat Cardiac Muscle

Landesberg and Sideman's four state model of the cardiac cross-bridge (XB) hypothesizes a feedback of force development to Ca(2+) binding by troponin C (TnC). We have further modeled this behavior and observed that the force (F)-Ca(2+) relationship as well as the F-sarcomere length (SL) relationship and the time course of F and Ca(2+) transients in cardiac muscle can be reproduced faithfully by a single effect of F on deformation of the TnC-Ca complex and, thereby, on the dissociation rate of Ca(2+). Furthermore, this feedback predicts that rapid decline of F in the activated sarcomere causes release of Ca(2+) from TnC-Ca(2+), which is sufficient to initiate arrhythmogenic Ca(2+) release from the sarcoplasmic reticulum (SR). This work investigated the initiation of Ca(2+) waves underlying triggered propagated contractions (TPCs) in rat cardiac trabeculae under conditions that simulate functional nonuniformity caused by mechanical or ischemic local damage of the myocardium. A mechanical discontinuity along the trabeculae was created by exposing the preparation to a small constant flow jet of solution that reduces excitation-contraction coupling in myocytes within that segment. Force was measured, and SL as well as [Ca(2+)](i) were measured regionally. When the jet contained caffeine, 2,3-butanedione monoxime or low-[Ca(2+)], muscle-twitch F decreased and the sarcomeres in the exposed segment were stretched by shortening the normal regions outside the jet. During relaxation, the sarcomeres in the exposed segment shortened rapidly. Short trains of stimulation at 2.5 Hz reproducibly caused Ca(2+) waves to rise from the borders exposed to the jet. Ca(2+) waves started during F relaxation of the last stimulated twitch and propagated into segments both inside and outside of the jet. Arrhythmias, in the form of nondriven rhythmic activity, were triggered when the amplitude of the Ca(2+) wave increased by raising [Ca(2+)](o). The arrhythmias disappeared when the muscle uniformity was restored by turning the jet off. These results show that nonuniform contraction can cause Ca(2+) waves underlying TPCs, and suggest that Ca(2+) dissociated from myofilaments plays an important role in the initiation of arrhythmogenic Ca(2+) waves.

Arrhythmogenic Ca2+ release from cardiac myofilaments

We investigated the initiation of Ca(2+)waves underlying triggered propagated contractions (TPCs) occurring in rat cardiac trabeculae under conditions that simulate the functional non-uniformity caused by mechanical or ischemic local damage of the myocardium. A mechanical discontinuity along the trabeculae was created by exposing the preparation to a small constant flow jet of solution with a composition that reduces excitation-contraction coupling in myocytes within that segment. Force was measured and sarcomere length as well as [Ca(2+)](i) were measured regionally. When the jet-contained Caffeine, BDM or Low-[Ca(2+)], muscle-twitch force decreased and the sarcomeres in the exposed segment were stretched by shortening of the normal regions outside the jet. During relaxation the sarcomeres in the exposed segment shortened rapidly. Short trains of stimulation at 2.5 Hz reproducibly caused Ca(2+)-waves to rise from the borders exposed to the jet. Ca(2+)-waves started during force relaxation of the last stimulated twitch and propagated into segments both inside and outside of the jet. Arrhythmias, in the form of non-driven rhythmic activity, were triggered when the amplitude of the Ca(2+)-wave increased by raising [Ca(2+)](o). The arrhythmias disappeared when the muscle uniformity was restored by turning the jet off. We have used the four state model of the cardiac cross bridge (Xb) with feedback of force development to Ca(2+) binding by Troponin-C (TnC) and observed that the force-Ca(2+) relationship as well as the force-sarcomere length relationship and the time course of the force and Ca(2+) transients in cardiac muscle can be reproduced faithfully by a single effect of force on deformation of the TnC.Ca complex and thereby on the dissociation rate of Ca(2+). Importantly, this feedback predicts that rapid decline of force in the activated sarcomere causes release of Ca(2+) from TnC.Ca(2+),which is sufficient to initiate arrhythmogenic Ca(2+) release from the sarcoplasmic reticulum. These results show that non-uniform contraction can cause Ca(2+)-waves underlying TPCs, and suggest that Ca(2+) dissociated from myofilaments plays an important role in the initiation of arrhythmogenic Ca(2+)-waves.

Spontaneous diastolic contractions and phosphorylation of the cardiac ryanodine receptor at serine-2808 in congestive heart failure in rat

OBJECTIVE

The role of phosphorylation of the ryanodine receptor at serine-2808 (RyRS2808) in congestive heart failure (CHF) is controversial, and effects of RyRS2808 phosphorylation on contraction are unclear. It has been reported that diastolic sarcomere length (SL) fluctuations accompany propagating contractile waves due to propagating SR Ca2+ release in trabeculae from rats with CHF. Here, we studied the influence of RyR destabilization by FK506 and isoproterenol on twitch force (Ftw) and SL fluctuations in right ventricular (RV) trabeculae. We measured phosphorylation of RyRS2808 in rats with myocardial infarction (MI) with or without beta-blockade and in rats during isoproterenol stimulation in order to assess the role of RyRS2808 phosphorylation in SL fluctuations in failing hearts.

METHODS

Five groups of male Lewis Brown-Norway rats were studied 3 months after MI: i) Sham; ii) MI with CHF (cMI); iii) MI without CHF; iv) metoprolol-treated MI, with and without CHF. The root mean square (RMSSL) of SL fluctuations in RV trabeculae was calculated.

RESULTS

RMSSL increased strongly both following a short train of stimuli at 2.5 Hz and following catecholamine activation in trabeculae from MI with CHF, resulting in a decrease in Ftw in proportion to RMSSL. RyRS2808 phosphorylation was increased significantly in the left ventricle (LV; approximately 58%, P<0.05) but not in the RV (n.s.) in MI rats with CHF. FK506 tripled high frequency stimulation-induced RMSSL in nonfailing trabecula but did not further enhance RMSSL in failing trabecula. Isoproterenol increased RMSSL in nonfailing trabeculae only modestly despite a substantial increase in RyRS2808 phosphorylation in the RV (approximately 60%, P<0.05). Isoproterenol induced SL fluctuation without an increase in RV-RyRS2808 phosphorylation in failing trabeculae. Chronic beta-blockade decreased high frequency and catecholamine stimulation-induced RMSSL while RyRS2808 phosphorylation in the RV was indistinguishable from that in cMI.

CONCLUSIONS

Acute RyRS2808 phosphorylation by itself does not cause spontaneous contractile waves owing to RyR2 destabilization. Spontaneous contractile waves in CHF are not caused by RyRS2808 phosphorylation alone, suggesting that factors other than RyRS2808 phosphorylation affect RyR function.

Spatial Nonuniformity of Excitation–Contraction Coupling Causes Arrhythmogenic Ca 2+ Waves in Rat Cardiac Muscle

Ca2+ waves underlying triggered propagated contractions (TPCs) are initiated in damaged regions in cardiac muscle and cause arrhythmias. We studied Ca2+ waves underlying TPCs in rat cardiac trabeculae under experimental conditions that simulate the functional nonuniformity caused by local mechanical or ischemic local damage of myocardium. A mechanical discontinuity along the trabeculae was created by exposing the preparation to a small jet of solution with a composition that reduces excitation-contraction coupling (ECC) in myocytes within that segment. The jet solution contained either caffeine (5 mmol/L), 2,3-butanedione monoxime (BDM; 20 mmol/L), or low Ca2+ concentration ([Ca2+]; 0.2 mmol/L). Force was measured with a silicon strain gauge and sarcomere length with laser diffraction techniques in 15 trabeculae. Simultaneously, [Ca2+]i was measured locally using epifluorescence of Fura-2. The jet of solution was applied perpendicularly to a small muscle region (200 to 300 microm) at constant flow. When the jet contained caffeine, BDM, or low [Ca2+], during the stimulated twitch, muscle-twitch force decreased and the sarcomeres in the exposed segment were stretched by shortening normal regions outside the jet. Typical protocols for TPC induction (7.5 s-2.5 Hz stimulus trains at 23 degrees C; [Ca2+]o=2.0 mmol/L) reproducibly generated Ca2+ waves that arose from the border between shortening and stretched regions. Such Ca2+ waves started during force-relaxation of the last stimulated twitch of the train and propagated (0.2 to 2.8 mm/sec) into segments both inside and outside of the jet. Arrhythmias, in the form of nondriven rhythmic activity, were induced when the amplitude of the Ca2+-wave was increased by raising [Ca2+]o. Arrhythmias disappeared rapidly when uniformity of ECC throughout the muscle was restored by turning the jet off. These results show, for the first time, that nonuniform ECC can cause Ca2+ waves underlying TPCs and suggest that Ca2+ dissociated from myofilaments plays an important role in the initiation of Ca2+ waves.

Ca2+ sparks and waves in canine purkinje cells: a triple layered system of Ca2+ activation

We have investigated the subcellular spontaneous Ca2+ events in canine Purkinje cells using laser scanning confocal microscopy. Three types of Ca2+ transient were found: (1) nonpropagating Ca2+ transients that originate directly under the sarcolemma and lead to (2) small Ca2+ wavelets in a region limited to 6-microm depth under the sarcolemma causing (3) large Ca2+ waves that travel throughout the cell (CWWs). Immunocytochemical studies revealed 3 layers of Ca2+ channels: (1) channels associated with type 1 IP3 receptors (IP3R1) and type 3 ryanodine receptors (RyR3) are prominent directly under the sarcolemma; (2) type 2 ryanodine receptors (RyR2s) are present throughout the cell but virtually absent in a layer between 2 and 4 microm below the sarcolemma (Sub-SL); (3) type 3 ryanodine receptors (RyR3) is the dominant Ca2+ release channel in the Sub-SL. Simulations of both nonpropagating and propagating transients show that the generators of Ca2+ wavelets differ from those of the CWWs with the threshold of the former being less than that of the latter. Thus, Purkinje cells contain a functional and structural Ca2+ system responsible for the mechanism that translates Ca2+ release occurring directly under the sarcolemma into rapid Ca2+ release in the Sub-SL, which then initiates large-amplitude long lasting Ca2+ releases underlying CWWs. The sequence of spontaneous diastolic Ca2+ transients that starts directly under the sarcolemma and leads to Ca2+ wavelets and CWWs is important because CWWs have been shown to cause nondriven electrical activity.

Effect of stimulation rate, sarcomere length and Ca(2+) on force generation by mouse cardiac muscle

The relations between stress, stimulation rate and sarcomere length (SL) were investigated in 24 cardiac trabeculae isolated from right ventricles of mice (CF-1 males, 25-30 g) and superfused with Hepes solution ([Ca(2+)](o) = 1 mM, pH 7.4, 25 degrees C). Stress and SL were measured by a strain gauge transducer and laser diffraction technique, respectively. Stress versus stimulation frequency formed a biphasic relation (25 degrees C, [Ca(2+)](o) = 2 mM) with a minimum at 0.7-1 Hz (~15 mN mm(-2)), a 150 % decrease from 0.1 to 1 Hz (descending limb) and a 75 % increase from 1 to 5 Hz (ascending limb). Ryanodine (0.1 microM) inhibited specifically the descending limb, while nifedipine (0.1 microM) affected specifically the ascending limb. This result suggests two separate sources of Ca(2+) for stress development: (1) net Ca(2+) influx during action potentials (AP); and (2) Ca(2+) entry into the cytosol from the extracellular space during diastolic intervals; Ca(2+) from both (1) and (2) is sequestered by the SR between beats. Raising the temperature to 37 degrees C lowered the stress-frequency relation (SFR) by approximately 0-15 mN mm(-2) at each frequency. Because the amount of Ca(2+) carried by I(Ca,L) showed a approximately 3-fold increase under the same conditions, we conclude that reduced Ca(2+) loading of the SR was probably responsible for this temperature effect. A simple model of Ca(2+) fluxes addressed the mechanisms underlying the SFR. Simulation of the effect of inorganic phosphates (P(i)) on force production was incorporated into the model. The results suggested that O(2) diffusion limits force production at stimulation rates >3 Hz. The stress-SL relations from slack length (approximately 1.75 microm) to 2.25 microm showed that the passive stress-SL curve of mouse cardiac trabeculae is exponential with a steep increase at SL >2.1 microm. Active stress (at 1 Hz) increased with SL, following a curved relation with convexity toward the abscissa at [Ca(2+)] = 2 mM. At [Ca(2+)] from 4 to 12 mM, the stress-SL curves superimposed and the relation became linear, which revealed a saturation step in the activation of force production. EC coupling in mouse cardiac muscle is similar to that observed previously in the rat, although important differences exist in the Ca(2+) dependence of force development. These results may suggest a lower capacity of the SR for buffering Ca(2+), which makes the generation of force in mouse cardiac ventricle more dependent on Ca(2+) entering during action potentials, particularly at high heart rate.

Ca(2+)-dependence of passive properties of cardiac sarcomeres

Rat cardiac trabeculae constitute a well-known experimental model in studies of cardiac contraction at the sarcomere level. Continuous measurement of length of the sarcomeres (SL) by laser diffraction technique permits one to monitor the active shortening of the contractile units during generation of force. When the preparation is stimulated repetitively (0.5 Hz) by electrical pulses, active shortenings are separated by periods corresponding approximately to the diastolic interval in the heart and wherein normally no major contractile event would have been expected. In contrast to this expectation, studies conducted with high-resolution (2-4 nm) SL measurements technique revealed that sarcomeres continuously lengthened (by 10-60 nm) from the end of twitch relaxation to the next stimulation. Such lengthening resulted from an internal expansion of the sarcomere and not from stretch exerted by extra-sarcomeric sources. We further characterized diastolic changes by measuring sarcomere stiffness (Sarc-Stiff) estimated from the response to short bursts (30 ms) of sinusoidal perturbations (frequency: 500 Hz) at 5 moments of the resting interval separating twitches. Sarc-Stiff increased continuously by approximately 30% during the diastolic interval (29 degrees C, pH: 7.4, [Ca2+]o = 1 mM). We then investigated during the same period the intracellular dynamics of Ca2+, as a major determinant of sarcomere motions in muscle. Intracellular free-Ca2+ concentration ([Ca2+]i) was measured continuously in trabeculae microinjected with the fluorescent Ca(2+)-probe Fura-2 and stimulated at 0.5 Hz. It appeared that the Ca(2+)-transient, which drives the twitch, did not end with the apparent relaxation of the force. Instead, [Ca2+]i kept decreasing in an exponential manner throughout the diastolic interval. At [Ca2+]o = 1 mM, [Ca2+]i decreased from 230 to 90 nM with a time constant of approximately 250 ms. The similarity in time courses of Ca(2+)-decline and of Sarc-Stiff increase suggested that properties of resting sarcomeres were related to [Ca2+]i in the sub-micromolar range. In order to examine this possibility, Sarc-Stiff was measured in chemically skinned trabeculae, i.e. in a preparation allowing control of [Ca2+] surrounding the sarcomeres. Sarc-Stiff was measured at different [Ca2+] from 1 to 450 nM. We found that 1) below 70 nM, Sarc-Stiff was independent on [Ca2+], 2) between 70 and 200 nM, i.e., approximately the range wherein [Ca2+]i decreased during diastole in intact muscle, Sarc-Stiff decreased by approximately 50% with increase of [Ca2+] and 3) above 200 nM, Sarc-Stiff increased steeply with increase of [Ca2+] as was expected from Ca(2+)-dependent attachment of cross-bridges between actin and myosin. The data fitted accurately to the sum of 2 sigmoid functions: 1) at [Ca2+] < 200 nM, Sarc-Stiff decreased with increase of [Ca2+] with a Hill coefficient (nH) = -2.6 and [Ca2+] at half maximal activation (EC50) = 0.16 +/- 0.013 microM; 2) at [Ca2+] > 200 nM, Sarc-Stiff increased with [Ca2+] (nH: 2.1; EC50:3.4 +/- 0.3 microM) consistent with Ca(2+)-dependent attachment of cross-bridges. It was possible to reproduce the diastolic variation of Sarc-Stiff observed in intact muscle by using the time course of [Ca2+]i in the Sarc-Stiff--[Ca2+] relationship determined from skinned trabeculae. We conclude that physical properties of the sarcomeres are inversely related to Ca2+ below 200 nM, i.e., in a range of concentrations where the myocytes operate during diastole while the influence of cross-bridges is negligible.

Ca(2+)-dependence of diastolic properties of cardiac sarcomeres: involvement of titin

The stiffness of the sarcomeres was studied during the diastolic interval of 18 stimulated (0.5 Hz) cardiac trabeculae of rat (pH 7.4; temperature = 25 degrees C). Sarcomere length (SL) and force (F) were measured using, respectively, laser diffraction techniques (resolution: 4 nm) and a silicon strain gauge (resolution: 0.63 microN). Sinusoidal perturbations (frequency = 500 Hz) were imposed to the length of the preparation. The stiffness was evaluated from the corresponding F and SL sinusoids by analysis of both signals together either in the time domain or in the frequency domain. A short burst (duration = 30 ms) of sinusoidal perturbations was repeated at 5 predetermined times during diastole providing 5 measurements of stiffness during the time interval separating two twitches. These measurements revealed that stiffness increases by approximately 30% during diastole, while a simultaneous expansion of the sarcomeres (amplitude = 10-60 nm) was detected. Measurements of the fluorescence of fura-2 under the same conditions revealed a continuous exponential decline of [Ca2+]i from 210 to 90 nM (constant of time approximately 300 ms) during diastole. In order to test the possibility that the increase of sarcomere stiffness and the decline of [Ca2+]i were coupled during diastole of intact trabeculae, we studied the effect of different free Ca(2+)-concentrations ([Ca2+]) between 1 and 430 nM on sarcomere stiffness in rat cardiac trabeculae skinned by saponin (n = 17). Stiffness was studied using 500 Hz sinusoidal perturbations of muscle length (ML). We found that, below 70 nM, the stiffness was independent of [Ca2+]; between 70 and 200 nM, the stiffness declined with increase of [Ca2+]; above 200 nM, the stiffness increased steeply with [Ca2+]. The data fitted accurately to the sum of two sigmoids (Hill functions): (1) at [Ca2+] < 200 nM the stiffness decreased with [Ca2+] (EC50 = 160 +/- 13 nM; n = -2.6 +/- 0.7) and (2) at [Ca2+] > 200 nM, stiffness increased with [Ca2+] (EC50 = 3.4 +/- 0.3 microM; n = 2.1 +/- 0.2) due to attachment of cross-bridges. From these results, it was possible to reproduce accurately the time course of diastolic stiffness observed in intact trabeculae and to predict the effect on stiffness of a spontaneous elevation of the diastolic [Ca2+]. Identical stiffness measurements were performed in 4 skinned preparations exposed to a cloned fragment of titin (Ti I-II) which has been shown to exhibit a strong interaction with F-actin in vitro. It was anticipated that Ti I-II would compete with endogenous titin for the same binding site on actin in the I-band. Below 200 nM, Ti I-II (2 microM) eliminated the Ca(2+)-dependence of stiffness. These results are consistent with the hypothesis that the Ca(2+)-sensitivity of the sarcomeres at [Ca2+] < 200 nM, i.e. where the myocytes in intact muscle operate during diastole, involves an association between titin molecules and the thin filament.

Diastolic viscoelastic properties of rat cardiac muscle; involvement of Ca2+

Diastolic cardiac sarcomere stiffness, sarcomere length changes, and calcium concentration [Ca2+]i were investigated in 18 trabeculae, dissected from the right ventricle of rat heart. [Ca2+]i declined following a mono-exponential diastolic time course with a time constant of 210-350 ms. During diastole, ([Ca2+]o = 1 mM); sarcomere length (SL) increases (amplitude: 5-65 nm; time constant: 600 ms). Eighty percent of muscles showed discrete spontaneous motion of sarcomeres near the end of diastole; this phenomenon occurred earlier at higher [Ca2+]o. The stiffness modulus of the sarcomere (MOD) increased by 30% during diastole (n = 158; p < 0.05), while the phase difference, phi, between force and SL decreased by 13% (n = 158; p < 0.05). The increase of MOD and the decrease of phi reversed when spontaneous activation occurred. These results show that the mechanical diastolic properties of the cardiac sarcomere are time dependent. The time dependence of the diastolic properties can be faithfully reproduced by a simple linear four element viscoelastic model. The diastolic changes of MOD and of phi could be reproduced by assuming an exponential change of the elastic and viscous coefficients of the model over time with a time constant similar to the time constant of change of [Ca2+]i. We suggest that the simplest combination of structural counterparts of the model in the sarcomere consists of titin bound to both actin and myosin in the myofibril, while the sarcomere is in parallel with another purely elastic element. We propose that the Ca(2+)-dependence of diastolic stiffness might be the result of an inverse relation between [Ca2+]i and the affinity of titin for actin.

Dynamics of viscoelastic properties of rat cardiac sarcomeres during the diastolic interval: involvement of Ca2+

1. Cardiac sarcomere stiffness was investigated during diastole in eighteen trabeculae dissected from the right ventricle of rat heart. The trabeculae were stimulated at 0.5 Hz, in a modified Krebs-Henseleit solution (pH, 7.4; 25 degrees C). Sarcomere length (SL) was measured using high resolution (+/-2 nm) laser diffraction techniques. Force (F) was measured with a silicon strain gauge. 2. SL increased exponentially (amplitude, 25 +/- 9 nm; n = 15) throughout diastole. This increase occurred even at slack SL, showing that this phenomenon was due to an internal expansion. The majority of the muscles showed discrete spontaneous fluctuations of SL (amplitude < 20 nm) starting approximately 1 s after the end of the twitch. 3. The intracellular free Ca2+ concentration ([Ca2+]i) was measured from the fluorescence of microinjected fura-2 salt in seven trabeculae under the same experimental conditions. [Ca2+]i continuously declined (from 240 to 90 nM) during diastole following a monoexponential time course (time constant, 210-325 ms). 4. The stiffness of the sarcomere was evaluated at 10, 30, 50, 70 and 90% of diastole using bursts (30 ms) of 500 Hz sinusoidal perturbations of muscle length (amplitude of SL oscillations < 30 nm). At 1 nM external Ca2+ concentration ([Ca2+]o), the average stiffness modulus (Mod) increased from 9.3 +/- 0.6 to 12 +/- 0.6 nN mm-2 micron-1 (n = 18; P < 0.05), while the average phase shift (phi) between F and SL signals decreased from 84 +/- 3 to 73 +/- 4 deg (n = 18; P < 0.05) between 10 and 90% during diastole. The increase in Mod and the decrease in phi reversed when spontaneous activity occurred. When [Ca2+]o was raised to 2 mM, the stiffness time course reversed approximately 450 ms earlier, simultaneously with the occurrence of spontaneous activity. 5. Our results show that diastole is only an apparent steady state and suggest that the structural system responsible for the viscoelastic properties of the sarcomere is regulated by [Ca2+]i in the submicromolar range. Different possible origins of the dynamic changes in viscoelasticity during diastole are discussed.