Impact of stretch on sarcomere length variability in isolated fully relaxed rat cardiac myocytes.

The contractility of cardiac muscle is greatly affected by preload via the Frank-Starling mechanism (FSM). It is based on preload-dependent activation of sarcomeres-the elementary contractile units in muscle cells. Recent findings show a natural variability in sarcomere length (SL) in resting cardiomyocytes that, moreover, is altered in an actively contracting myocyte. SL variability may contribute to the FSM, but it remains unresolved whether the change in the SL variability is regulated by activation process per se or simply by changes in cell stretch, i.e., average SL. To separate the roles of activation and SL, we characterized SL variability in isolated, fully relaxed rat ventricular cardiomyocytes (n = 12) subjected to a longitudinal stretch with the carbon fiber (CF) technique. Each cell was tested in three states: without CF attachment (control, no preload), with CF attachment without stretch, and with CF attachment and ~ 10% stretch of initial SL. The cells were imaged by transmitted light microscopy to retrieve and analyze individual SL and SL variability off-line by multiple quantitative measures such as coefficient of variation or median absolute deviation. We found that CF attachment without stretch did not affect the extent of SL variability nor average SL. In stretched myocytes, the averaged SL significantly increased, while the SL variability remained unchanged. This result clearly indicates that the non-uniformity of individual SL is not sensitive to the average SL itself in fully relaxed myocytes. We conclude that SL variability per se does not contribute to the FSM in the heart.

Impact of stretch on sarcomere length variability in isolated fully relaxed rat cardiac myocytes.

The contractility of cardiac muscle is greatly affected by preload via the Frank-Starling Mechanism (FSM). It is based on the preload-dependent activation of sarcomeres - the elementary contractile units in muscle cells. Recent findings show a natural variability in sarcomere length (SL) in resting cardiomyocytes that, moreover, is altered in an actively contracting myocyte. SL variability may contribute to the FSM but it remains unresolved whether the change in the SL variability is regulated by activation process per se or simply by changes in cell stretch, i.e. average SL. To separate the roles of activation and SL, we characterized SL variability in isolated fully relaxed rat ventricular cardiomyocytes ( n = 12) subjected to a longitudinal stretch with the carbon fiber (CF) technique. Each cell was tested in three states: without CF attachment (control, no preload), with CF attachment without stretch, and with CF attachment and ~ 10% stretch of initial SL. The cells were imaged by transmitted light microscopy to retrieve and analyze individual SL and SL variability off-line by multiple quantitative measures like coefficient of variation or median absolute deviation. We found that CF attachment without stretch did not affect the extent of SL variability and averaged SL. In stretched myocytes, the averaged SL significantly increased while the SL variability remained unchanged. This result clearly indicates that the non-uniformity of individual SL is not sensitive to the average SL itself in fully relaxed myocytes. We conclude that SL variability per se does not contribute to the FSM in the heart.

Differences in Effects of Length-Dependent Regulation of Force and Ca2+ Transient in the Myocardial Trabeculae of the Rat Right Atrium and Ventricle.

The comparative differences in the fundamental mechanisms of contractility regulation and calcium handling of atrial and ventricular myocardium remain poorly studied. An isometric force-length protocol was performed for the entire range of preloads in isolated rat right atrial (RA) and ventricular (RV) trabeculae with simultaneous measurements of force (Frank-Starling mechanism) and Ca2+ transients (CaT). Differences were found between length-dependent effects in RA and RV muscles: (a) the RA muscles were stiffer, faster, and presented with weaker active force than the RV muscles throughout the preload range; (b) the active/passive force-length relationships were almost linear for the RA and RV muscles; (c) the value of the relative length-dependent growth of passive/active mechanical tension did not differ between the RA and RV muscles; (d) the time-to-peak and amplitude of CaT did not differ between the RA and RV muscles; (e) the CaT decay phase was essentially monotonic and almost independent of preload in the RA muscles, but not in the RV muscles. Higher peak tension, prolonged isometric twitch, and CaT in the RV muscle may be the result of higher Ca2+ buffering by myofilaments. The molecular mechanisms that constitute the Frank-Starling mechanism are common in the rat RA and RV myocardium.

Cardiomyocyte sarcomere length variability: Membrane fluorescence versus second harmonic generation myosin imaging.

Sarcomere length (SL) and its variation along the myofibril strongly regulate integrated coordinated myocyte contraction. It is therefore important to obtain individual SL properties. Optical imaging by confocal fluorescence (for example, using ANEPPS) or transmitted light microscopy is often used for this purpose. However, this allows for the visualization of structures related to Z-disks only. In contrast, second-harmonic generation (SHG) microscopy visualizes A-band sarcomeric structures directly. Here, we compared averaged SL and its variability in isolated relaxed rat cardiomyocytes by imaging with ANEPPS and SHG. We found that SL variability, evaluated by several absolute and relative measures, is two times smaller using SHG vs. ANEPPS, while both optical methods give the same average (median) SL. We conclude that optical methods with similar optical spatial resolution provide valid estimations of average SL, but the use of SHG microscopy for visualization of sarcomeric A-bands may be the "gold standard" for evaluation of SL variability due to the absence of optical interference between the sarcomere center and non-sarcomeric structures. This contrasts with sarcomere edges where t-tubules may not consistently colocalize to Z-disks. The use of SHG microscopy instead of fluorescent imaging can be a prospective tool to map sarcomere variability both in vitro and in vivo conditions and to reveal its role in the functional behavior of living myocardium.

Contractile Behavior of Right Atrial Myocardium of Healthy Rats and Rats with the Experimental Model of Pulmonary Hypertension.

There is a lack of data about the contractile behavior of the right atrial myocardium in chronic pulmonary heart disease. We thoroughly characterized the contractility and Ca transient of isolated right atrial strips of healthy rats (CONT) and rats with the experimental model of monocrotaline-induced pulmonary hypertension (MCT) in steady state at different preloads (isometric force-length), during slow force response to stretch (SFR), and during post-rest potentiation after a period of absence of electrical stimulation (PRP). The preload-dependent changes in the isometric twitch and Ca transient did not differ between CONT and MCT rats while the kinetics of the twitch and Ca transient were noticeably slowed down in the MCT rats. The magnitude of SFR was significantly elevated in the MCT right atrial strips and this was accompanied by the significantly higher elevation of the Ca transient relative amplitude at the end of SFR. The slow changes in the contractility and Ca transient in the PRP protocol did not differ between CONT and MCT. In conclusion, the alterations in the contractility and Ca transient of the right atrial myocardium of monocrotaline-treated rats with pulmonary hypertension mostly concern the elevation in SFR. We hypothesize that this positive inotropic effect in the atrial myocardium may (partly) compensate the systolic deficiency of the right ventricular failing myocardium.

Changes in the Cardiotoxic Effects of Lead Intoxication in Rats Induced by Muscular Exercise.

Exposure to lead is associated with an increased risk of cardiovascular diseases. Outbred white male rats were injected with lead acetate intraperitoneally three times a week and/or were forced to run at a speed of 25 m/min for 10 min 5 days a week. We performed noninvasive recording of arterial pressure, electrocardiogram and breathing parameters, and assessed some biochemical characteristics. Electrophoresis in polyacrylamide gel was used to determine the ratio of myosin heavy chains. An in vitro motility assay was employed to measure the sliding velocity of regulated thin filaments on myosin. Isolated multicellular preparations of the right ventricle myocardium were used to study contractility in isometric and physiological modes of contraction. Exercise under lead intoxication normalized the level of calcium and activity of the angiotensin-converting enzyme in the blood serum, normalized the isoelectric line voltage and T-wave amplitude on the electrocardiogram, increased the level of creatine kinase-MB and reduced the inspiratory rate. Additionally, the maximum sliding velocity and the myosin heavy chain ratio were partly normalized. The effect of exercise under lead intoxication on myocardial contractility was found to be variable. In toto, muscular loading was found to attenuate the effects of lead intoxication, as judged by the indicators of the cardiovascular system.

Contractile State Dependent Sarcomere Length Variability in Isolated Guinea-Pig Cardiomyocytes.

Cardiomyocytes contract keeping their sarcomere length (SL) close to optimal values for force generation. Transmural heterogeneity in SL across the ventricular wall coordinates the contractility of the whole-ventricle. SL heterogeneity (variability) exists not only at the tissue (macroscale) level, but also presents at the level of a single cardiomyocyte (microscale level). However, transmural differences in intracellular SL variability and its possible dependence on the state of contraction (e.g. end-diastole or end-systole) have not been previously reported. In the present study, we studied three aspects of sarcomere-to-sarcomere variability in intact cardiomyocytes isolated from the left ventricle of healthy guinea-pig: 1) transmural differences in SL distribution between subepi- (EPI) and subendocardial (ENDO) cardiomyocytes; 2) the dependence of intracellular variability in SL upon the state of contraction; 3) local differences in SL variability, comparing SL distributions between central and peripheral regions within the cardiomyocyte. To characterize the intracellular variability of SL, we used different normality tests for the assessment of SL distributions, as well as nonparametric coefficients to quantify the variability. We found that individual SL values in the end-systolic state of contraction followed a normal distribution to a lesser extent as compared to the end-diastolic state of contraction (∼1.3-fold and ∼1.6-fold in ENDO and EPI, respectively). The relative and absolute coefficients of sarcomere-to-sarcomere variability in end-systolic SL were significantly greater (∼1.3-fold) as compared to end-diastolic SL. This was independent of both the transmural region across the left ventricle and the intracellular region within the cardiomyocyte. We conclude that the intracellular variability in SL, which exists in normal intact guinea-pig cardiomyocytes, is affected by the contractile state of the myocyte. This phenomenon may play a role in inter-sarcomere communication in the beating heart.

Omecamtiv mecarbil attenuates length-tension relationship in healthy rat myocardium and preserves it in monocrotaline-induced pulmonary heart failure.

The cardiac-specific myosin activator, omecamtiv mecarbil (OM), is an effective inotrope for treating heart failure but its effects on active force and Ca2+ kinetics in healthy and diseased myocardium remain poorly studied. We tested the effect of two concentrations of OM (0.2 and 1 µmol/L in saline) on isometric contraction and Ca-transient (CaT) in right ventricular trabeculae of healthy rats (CONT, n = 8) and rats with monocrotaline-induced pulmonary heart failure (MCT, n = 8). The contractions were obtained under preload of 75%-100% of optimal length (tension-length relationship). The 0.2 µmol/L OM did not affect the diastolic level, amplitude, or kinetics of isometric contraction and CaT, irrespective of the group of rats or preload. The 1 µmol/L OM significantly suppressed active tension-length relationships in CONT but not in MCT, while leading in both groups to a significantly prolonged relaxation. CaT time-to-peak was unaffected in CONT and MCT, but CaT decay was slightly accelerated in its early phase and considerably prolonged in its late phase to a similar extent in both groups. We conclude that the substantial prolongation of CaT decay is due to enhanced Ca2+ utilisation by troponin C mediated by the direct effect of OM on the cooperative activation of myofilaments. The lack of beneficial effect of OM in the healthy rat myocardium may be due to a relatively high level of activating Ca2+ in cells with normal Ca2+ handling, whereas the preservation of the tension-length relationship in the failing heart may relate to the diminished Ca2+ levels of sarcoplasmic reticulum.

Cardioinotropic Effects in Subchronic Intoxication of Rats with Lead and/or Cadmium Oxide Nanoparticles.

Subchronic intoxication was induced in outbred male rats by repeated intraperitoneal injections with lead oxide (PbO) and/or cadmium oxide (CdO) nanoparticles (NPs) 3 times a week during 6 weeks for the purpose of examining its effects on the contractile characteristics of isolated right ventricle trabeculae and papillary muscles in isometric and afterload contractions. Isolated and combined intoxication with these NPs was observed to reduce the mechanical work produced by both types of myocardial preparation. Using the in vitro motility assay, we showed that the sliding velocity of regulated thin filaments drops under both isolated and combined intoxication with CdO-NP and PbO-NP. These results correlate with a shift in the expression of myosin heavy chain (MHC) isoforms towards slowly cycling ß-MHC. The type of CdO-NP + PbO-NP combined cardiotoxicity depends on the effect of the toxic impact, the extent of this effect, the ratio of toxicant doses, and the degree of stretching of cardiomyocytes and muscle type studied. Some indices of combined Pb-NP and CdO-NP cardiotoxicity and general toxicity (genotoxicity included) became fully or partly normalized if intoxication developed against background administration of a bioprotective complex.

The role of pacing rate in the modulation of mechano-induced immediate and delayed changes in the force and Ca-transient of cardiac muscle.

Myocardial function is tuned by dynamic changes in length and load via mechano-calcium feedback. This regulation may be significantly affected by heart rhythm. We evaluated the mechano-induced modulation of contractility and Ca-transient (CaT) in the rat myocardium subjected to twitch-by-twitch shortening-re-lengthening (↓-↑) trains of different lengths (N = 1 … 720 cycles) at low (1 Hz) and near-physiological (3.5 Hz) pacing rates. Force/CaT characteristics were evaluated in the first post-train isometric twitch (immediate effect) and during slow changes (delayed maximal elevation/decrease) and compared with those of the pre-train twitch. The immediate inotropic effect was positive for N = 30 … 720 and negative for N = 1 … 20, while the delayed effect was always positive. The immediate and delayed inotropic effects were significantly higher at 3.5-Hz vs 1-Hz (P < 0.05). The prominent inotropism was accompanied by much smaller changes in the CaT diastolic level/amplitude. The shortening-re-lengthening train induced oscillations of the slow change in force at 3.5-Hz (always) and at 1-Hz (∼50% of muscles), which were dependent of the train length and independent of the pacing rate. We suggest that twitch-by-twitch shortening-re-lengthening of cardiac muscle decreases Ca2+ buffering by troponin C and elevates Ca2+ loading of the sarcoplasmic reticulum (SR); the latter cumulatively depends on the train length. A high pacing rate intensifies the cumulative transient shift in the SR Ca2+ loading, augmenting the post-train inotropic response and prolonging its recovery to the pre-train level. The pacing-dependent mechano-induced inotropic effects remain to be elucidated in the myocardium with impaired Ca handling.

Force-velocity characteristics of isolated myocardium preparations from rats exposed to subchronic intoxication with lead and cadmium acting separately or in combination.

This investigation continues our study of the effects of Pb-Cd poisoning on the heart, extending the enquiry from isometric to auxotonic contractions, thereby examining the effect on the ability of myocardial tissues to perform mechanical work. Different shifts were revealed in myocardial force-velocity relations following subchronic exposure of rats to lead acetate and cadmium chloride acting separately, in combination, or in combination with a bioprotective complex (BPC). The experiments were conducted on isolated preparations of trabecules and papillary muscles of the right ventricle in physiological loading conditions and on isolated heart muscle contractile proteins examined by the in vitro motility assay. The results of the latter correlate with the shifts in the ratio of cardiac myosin isoforms. The amount of work performed by the myocardium was calculated on the basis of the tension-shortening loop area and was found to be similar in the preparations from all experimental groups. This fact presumably reflects adaptive capacity of the myocardial function even when contractility is damaged due to the metallic intoxication of a moderate severity. Some characteristics of rat myocardium altered by the impact of lead-cadmium intoxication became fully or partly normalized if intoxication developed against background administration of a bioprotective complex (BPC). Together with previously reported results obtained in the isometric mode of contractility, all these results strengthen the scientific foundations of risk assessment and risk management projects in the occupational and environmental conditions characterized by human exposure to lead and/or cadmium.

The use of Ca-transient to evaluate Ca2+ utilization by myofilaments in living cardiac muscle.

The kinetics of Ca2+ interaction with myofilaments is an important determinant of the preload-dependent effects on myocardial contractility (the Frank-Starling Mechanism). However, the direct evaluation of this interaction in intact tissue is limited. To overcome this issue, the method of difference curve was proposed, which implements the subtraction of the referent Ca-transient (measured in non-stretched muscle) from the Ca-transients measured at different preloads. This method was tested on the cardiac trabeculae of healthy (CONT) and monocrotaline-treated rats (MCT), subjected to force-length protocol with simultaneous measurement of isometric twitch and Ca-transient. The difference curve had two components, C2 and C3, which are distinct in their directions and, as hypothesized, may reflect mainly the kinetics of Ca2+ utilization by and release from myofilaments, respectively. Both the components were quantitatively evaluated by their amplitude, integral magnitude and time-to-peak. The C3 component in either CONT or MCT was significantly higher in its amplitude/integral magnitude vs the C2 component, at any preload (P < .05). The time-to-peak value was preload-dependent only for the C3 component. There were tight relationships between the above characteristics of C2/C3 components and the characteristics of isometric tension (peak value, time-to-peak and the maximal rates of rise/decline) in CONT and MCT muscles. The C3 component was highly consistent with tension relaxation (Ca2+ release from myofilaments), but the C2 component was partially consistent with tension development (Ca2+ utilization by myofilaments). The novel method of the analysis of Ca-transients can be utilized for indirect evaluation of Ca2+ interaction with myofilaments in healthy and diseased myocardium.

Changes in rat myocardium contractility under subchronic intoxication with lead and cadmium salts administered alone or in combination.

Subchronic intoxications induced in male rats by repeated intraperitoneal injections of lead acetate and cadmium chloride, administered either alone or in combination, are shown to affect the biochemical, cytological and morphometric parameters of blood, liver, heart and kidneys. The single twitch parameters of myocardial trabecular and papillary muscle preparations were measured in the isometric regime to identify changes in the heterometric (length-force) and chronoinotropic (frequency-force) contractility regulation systems. Differences in the responses of these systems in trabecules and papillary muscles to the above intoxications are shown. A number of myocardium mechanical characteristics changing in rats under the effect of a combined lead-cadmium intoxication and increased proportion of α-myosin heavy chains were observed to normalize fully or partially if such intoxication was induced against background administration of a proposed bioprotective complex. Based on the experimental results and literature data, some assumptions are suggested concerning the mechanisms of the cardiotoxic effects produced by lead and cadmium.

Further analysis of rat myocardium contractility changes associated with a subchronic lead intoxication.

A moderate subchronic lead intoxication was observed in male rats after repeated intraperitoneal injections of lead acetate. Right ventricular trabeculae and papillary muscles were isolated for in vitro studying of the contraction-relaxation cycle under isotonic and physiological loading. The contractile function of the myocardium was also assessed by measuring the velocity of thin filament movement over myosin. Lead intoxication led in papillary muscles to a decrease in the maximal rate of isotonic shortening for all afterloads and a decrease in the thin filament sliding velocity. Papillary muscles from lead-exposed rats displayed marked changes in most of the main characteristics of afterload contraction-relaxation cycles, but in trabeculae these changes were less pronounced. The reported changes were attenuated to some extent in rats treated with a Ca-containing bioprotector. The amount of work produced by both types of heart muscle preparations was not changed by lead. Only in papillary muscles the load-dependent relaxation index was significantly increased in the lead-treated groups. Thus subchronic lead intoxication affects the peak rate of force development and relaxation properties of cardiac muscle contracting in isotonic/physiological regimes rather than the total amount of mechanical work, which may reflect adaptive changes in the myocardial function under decreased contractility.

Length-Dependent Activation of Contractility and Ca-Transient Kinetics in Auxotonically Contracting Isolated Rat Ventricular Cardiomyocytes.

Length-dependent activation (LDA) of contraction is an important mechanism of proper myocardial function that is often blunted in diseases accompanied by deficient contractility and impaired calcium homeostasis. We evaluated how the extent of LDA is related to the decreased force in healthy rat myocardium under negative inotropic conditions that affect the calcium cycle. The length-dependent effects on auxotonic twitch and Ca-transient were compared in isolated rat ventricular cardiomyocytes at room temperature ("25C") and near-physiological temperature ("35C") in normal Tyrode and at 25°C with thapsigargin-depleted sarcoplasmic reticulum ("25C + Thap"). At the slack length, a similar negative inotropy in "35C" and "25C + Thap" was accompanied by totally different changes in Ca-transient amplitude, time-to-peak, and time-to-decline from peak to 50% amplitude. End-systolic/end-diastolic tension-sarcomere length relationships were obtained for each individual cell, and the ratio of their slopes, the dimensionless Frank-Starling Gain index, was 2.32 ± 0.16, 1.78 ± 0.09, and 1.37 ± 0.06 in "25C," "35C" and "25C + Thap," respectively (mean ± S.E.M.). Ca-transient diastolic level and amplitude did not differ between "25C" and "35C" at any SL, but in "35C" it developed and declined significantly faster. In contrast, thapsigargin-induced depletion of SERCA2a significantly attenuated and retarded Ca-transient. The relative amount of Ca2+ utilized by troponin C, evaluated by the integral magnitude of a short-lived component of Ca-transient decline ("bump"), increased by ~25% per each 0.05 µm increase in SL in all groups. The kinetics of the Ca-TnC dissociation, evaluated by the bump time-to-peak, was significantly faster in "35C" and slower in "25C + Thap" vs. "25C" (respectively, 63.7 ± 5.3 and 253.6 ± 8.3% of the value in "25C," mean ± S.E.M.). In conclusion, a similar inotropic effect can be observed in rat ventricular myocardium under totally different kinetics of free cytosolic calcium. The extent of LDA is not determined by actual peak systolic tension but is regulated by the level of peak systolic calcium and the kinetics of Ca-transient decline which, in turn, are governed by Ca-TnC dissociation and Ca2+ reuptake by the sarcoplasmic reticulum. Altogether, these findings constitute new evidence about the role of the length-dependent modulation of Ca2+ homeostasis in the mechanisms of calcium regulation of contraction and mechano-calcium feedback in the myocardium.

The lack of slow force response in failing rat myocardium: role of stretch-induced modulation of Ca-TnC kinetics.

The slow force response (SFR) to stretch is an important adaptive mechanism of the heart. The SFR may result in ~ 20-30% extra force but it is substantially attenuated in heart failure. We investigated the relation of SFR magnitude with Ca2+ transient decay in healthy (CONT) and monocrotaline-treated rats with heart failure (MCT). Right ventricular trabeculae were stretched from 85 to 95% of optimal length and held stretched for 10 min at 30 °C and 1 Hz. Isometric twitches and Ca2+ transients were collected on 2, 4, 6, 8, 10 min after stretch. The changes in peak tension and Ca2+ transient decay characteristics during SFR were evaluated as a percentage of the value measured immediately after stretch. The amount of Ca2+ utilized by TnC was indirectly evaluated using the methods of Ca2+ transient "bump" and "difference curve." The muscles of CONT rats produced positive SFR and they showed prominent functional relation between SFR magnitude and the magnitude (amplitude, integral intensity) of Ca2+ transient "bump" and "difference curve." The myocardium of MCT rats showed negative SFR to stretch (force decreased in time) which was not correlated well with the characteristics of Ca2+ transient decay, evaluated by the methods of "bump" and "difference curve." We conclude that the intracellular mechanisms of Ca2+ balancing during stretch-induced slow adaptation of myocardial contractility are disrupted in failing rat myocardium. The potential significance of our findings is that the deficiency of slow force response in diseased myocardium may be diminished under augmented kinetics of Ca-TnC interaction.

Effects of subchronic lead intoxication of rats on the myocardium contractility.

Outbred male rats were repeatedly injected IP with sub-lethal doses of lead acetate 3 times a week during 5 weeks. They developed an explicit, even if moderate, lead intoxication characterized by typical hematological and some other features. The next day after the last injection the heart of each animal was excised, and the trabecules and papillary muscles from the right ventricle were used for modeling in vitro isometric (with varying starting length of the preparation) regimes of the contraction-relaxation cycle with different preloads. Several well-established parameters of this model were found changed compared with the preparations taken from the hearts of healthy control rats. Background in vivo calcium treatment attenuated both systemic and cardiotoxic effects of lead to an extent. We show for the first time that subchronic intoxication with lead caused myocardial preparations in a wide range of lengths to respond by a decrease in the time and speed parameters of the isometric contraction while maintaining its amplitude and by a decrease in the passive stiffness of trabecules. The responses of the various heart structures are outlined, and the isomyosin ratio is shown to have shifted towards the slow isoform. Mechanistic and toxicological inferences from the results are discussed.

Inhibition of AT1 receptors by losartan affects myocardial slow force response in healthy but not in monocrotaline-treated young rats.

The slow force response (SFR) of a cardiac muscle to a sudden stretch is thought to be important in the regulatory adaptation of myocardial contraction. Autocrine-paracrine regulation pathways which involve angiotensin II are participating in this mechanism. On the other hand, renin-angiotensin-aldosterone system (RAS) is altered in hypertrophic or failing myocardium. We compared the effects of sudden stretch to SFR as well as to twitch and Ca2+ transient characteristics in rat myocardium with monocrotaline-induced heart failure with those in normal rat myocardium without and with inhibition of angiotensin II type-1 (AT1) receptors. Our findings indicate that the myocardium of rats with monocrotaline-induced right ventricular failure is deficient with activation of local RAS and therefore expresses blunted SFR, very similar to the depression of SFR observed in normal myocardium under inhibition of AT1 receptors. The "failing" myocardium does not further respond to the "putative" inhibition of AT1 receptors by losartan. In conclusion, SFR is related to autocrine-paracrine regulation of myocardial contraction in normal rat myocardium and that the involvement of RAS into stretch-induced modulation of contractility may be significantly altered in failing heart.

Load-dependent effects of apelin on murine cardiomyocytes.

The apelin peptide is described as one of the most potent inotropic agents, produced endogenously in a wide range of cells, including cardiomyocytes. Despite positive effects on cardiac contractility in multicellular preparations, as well as indications of cardio-protective actions in several diseases, its effects and mechanisms of action at the cellular level are incompletely understood. Here, we report apelin effects on dynamic mechanical characteristics of single ventricular cardiomyocytes, isolated from mouse models (control, apelin-deficient [Apelin-KO], apelin-receptor KO mouse [APJ-KO]), and rat. Dynamic changes in maximal velocity of cell shortening and relaxation were monitored. In addition, more traditional indicators of inotropic effects, such as maximum shortening (in mechanically unloaded cells) or peak force development (in auxotonic contracting cells, preloaded using the carbon fibre technique) were studied. The key finding is that, using Apelin-KO cardiomyocytes exposed to different preloads with the 2-carbon fibre technique, we observe a lowering of the slope of the end-diastolic stress-length relation in response to 10 nM apelin, an effect that is preload-dependent. This suggests a positive lusitropic effect of apelin, which could explain earlier counter-intuitive findings on an apelin-induced increase in contractility occurring without matching rise in oxygen consumption.

The length-dependent activation of contraction is equally impaired in impuberal male and female rats in monocrotaline-induced right ventricular failure.

The length-dependent activation of contraction is attenuated in the failing myocardium of adult male rats. This pathological change is not seen in adult female rats, possibly because of a protective effect of sex hormones. The present study evaluated length-dependent changes in isometric twitch, Ca(2+) transient (CaT) and action potential (AP) in the right ventricular myocardium of impuberal healthy male and female rats (control) and in rats treated with a single injection of 50 mg/kg monocrotaline (MCT). Compared with sex-matched control rats, MCT-treated male and female rats exhibited increased right ventricular weight (134% and 142% of control, respectively), decreased left ventricular weight (72% and 79%), twitch attenuation (48.8 ± 2.7% and 57.5 ± 1.2%) and prolongation (125 ± 3% and 127 ± 2%), CaT attenuation (37.8 ± 0.4% and 39.1 ± 1.1%) and prolongation (114 ± 1% and 116 ± 1%) and AP prolongation at 90% repolarization (195 ± 2% and 203 ± 1%). The MCT-treated male rats exhibited a 50% lower integral magnitude and an approximately 25% larger time-to-peak 'bump' compared with control male rats. These parameters in MCT-treated female rats tended to show similar changes to those seen in the control female rats, with no significant difference between the two groups. In all groups, integral magnitude and time-to-peak 'bump' increased with length. In conclusion, the length-dependent activation of contraction was equally blunted in the failing right ventricular myocardium of impuberal male and female rats. This was related to changes in CaT and AP, which were similar between male and female rats. Therefore, puberty is necessary for manifestation of the protective effects of sex hormones on this remodelling.