The effects of changes in muscle length during diastole on the calcium transient in ferret ventricular muscle

Piezo buffers mechanical stress via modulation of intracellular Ca2+ handling in the Drosophila heart

Throughout its lifetime the heart is buffeted continuously by dynamic mechanical forces resulting from contraction of the heart muscle itself and fluctuations in haemodynamic load and pressure. These forces are in flux on a beat-by-beat basis, resulting from changes in posture, physical activity or emotional state, and over longer timescales due to altered physiology (e.g. pregnancy) or as a consequence of ageing or disease (e.g. hypertension). It has been known for over a century of the heart's ability to sense differences in haemodynamic load and adjust contractile force accordingly (Frank, Z. biology, 1895, 32, 370-447; Anrep, J. Physiol., 1912, 45 (5), 307-317; Patterson and Starling, J. Physiol., 1914, 48 (5), 357-79; Starling, The law of the heart (Linacre Lecture, given at Cambridge, 1915), 1918). These adaptive behaviours are important for cardiovascular homeostasis, but the mechanism(s) underpinning them are incompletely understood. Here we present evidence that the mechanically-activated ion channel, Piezo, is an important component of the Drosophila heart's ability to adapt to mechanical force. We find Piezo is a sarcoplasmic reticulum (SR)-resident channel and is part of a mechanism that regulates Ca2+ handling in cardiomyocytes in response to mechanical stress. Our data support a simple model in which Drosophila Piezo transduces mechanical force such as stretch into a Ca2+ signal, originating from the SR, that modulates cardiomyocyte contraction. We show that Piezo mutant hearts fail to buffer mechanical stress, have altered Ca2+ handling, become prone to arrhythmias and undergo pathological remodelling.

Modulations of Cardiac Functions and Pathogenesis by Reactive Oxygen Species and Natural Antioxidants

Homeostasis in the level of reactive oxygen species (ROS) in cardiac myocytes plays a critical role in regulating their physiological functions. Disturbance of balance between generation and removal of ROS is a major cause of cardiac myocyte remodeling, dysfunction, and failure. Cardiac myocytes possess several ROS-producing pathways, such as mitochondrial electron transport chain, NADPH oxidases, and nitric oxide synthases, and have endogenous antioxidation mechanisms. Cardiac Ca²⁺-signaling toolkit proteins, as well as mitochondrial functions, are largely modulated by ROS under physiological and pathological conditions, thereby producing alterations in contraction, membrane conductivity, cell metabolism and cell growth and death. Mechanical stresses under hypertension, post-myocardial infarction, heart failure, and valve diseases are the main causes for stress-induced cardiac remodeling and functional failure, which are associated with ROS-induced pathogenesis. Experimental evidence demonstrates that many cardioprotective natural antioxidants, enriched in foods or herbs, exert beneficial effects on cardiac functions (Ca²⁺ signal, contractility and rhythm), myocytes remodeling, inflammation and death in pathological hearts. The review may provide knowledge and insight into the modulation of cardiac pathogenesis by ROS and natural antioxidants.

Stretch modulation of cardiac contractility: importance of myocyte calcium during the slow force response

The mechanical response of the heart to myocardial stretch has been understood since the work of muscle physiologists more than 100 years ago, whereby an increase in ventricular chamber filling during diastole increases the subsequent force of contraction. The stretch-induced increase in contraction is biphasic. There is an abrupt increase in the force that coincides with the stretch (the rapid response), which is then followed by a slower response that develops over several minutes (the slow force response, or SFR). The SFR is associated with a progressive increase in the magnitude of the Ca²⁺ transient, the event that initiates myocyte cross-bridge cycling and force development. However, the mechanisms underlying the stretch-dependent increase in the Ca²⁺ transient are still debated. This review outlines recent literature on the SFR and summarizes the different stretch-activated Ca²⁺ entry pathways. The SFR might result from a combination of several different cellular mechanisms initiated in response to activation of different cellular stretch sensors.

Regulation of connexin 43 by interleukin 1β in adult rat cardiac fibroblasts and effects in an adult rat cardiac myocyte: fibroblast co-culture model

Connexin 43 expression (Cx43) is increased in cardiac fibroblasts (CFs) following myocardial infarction. Here, potential mediators responsible for increasing Cx43 expression and effects of differential CF phenotype on cardiac myocyte (CM) function were investigated. Stimulating adult rat CFs with proinflammatory mediators revealed that interleukin 1β (IL-1β) significantly enhanced Cx43 levels through the IL-1β pathway. Additionally, IL-1β reduced mRNA levels of the myofibroblast (MF) markers: (i) connective tissue growth factor (CTGF) and (ii) α smooth muscle actin (αSMA), compared to control CFs. A co-culture adult rat CM:CF model was utilised to examine cell-to-cell interactions. Transfer of calcein from CMs to underlying CFs suggested functional gap junction formation. Functional analysis revealed contraction duration (CD) of CMs was shortened in co-culture with CFs, while treatment of CFs with IL-1β reduced this mechanical effect of co-culture. No effect on action potential rise time or duration of CMs cultured with control or IL-1β-treated CFs was observed. These data demonstrate that stimulating CFs with IL-1β increases Cx43 and reduces MF marker expression, suggesting altered cell phenotype. These changes may underlie the reduced mechanical effects of IL-1β treated CFs on CD of co-cultured CMs and therefore have an implication for our understanding of heterocellular interactions in cardiac disease.

The slow force response to stretch: Controversy and contradictions

When exposed to an abrupt stretch, cardiac muscle exhibits biphasic active force enhancement. The initial, instantaneous, force enhancement is well explained by the Frank-Starling mechanism. However, the cellular mechanisms associated with the second, slower phase remain contentious. This review explores hypotheses regarding this "slow force response" with the intention of clarifying some apparent contradictions in the literature. The review is partitioned into three sections. The first section considers pathways that modify the intracellular calcium handling to address the role of the sarcoplasmic reticulum in the mechanism underlying the slow force response. The second section focuses on extracellular calcium fluxes and explores the identity and contribution of the stretch-activated, non-specific, cation channels as well as signalling cascades associated with G-protein coupled receptors. The final section introduces promising candidates for the mechanosensor(s) responsible for detecting the stretch perturbation.

Regulation of cardiac Ca2+ and ion channels by shear mechanotransduction

Cardiac contraction is controlled by a Ca²⁺ signaling sequence that includes L-type Ca²⁺ current-gated opening of Ca²⁺ release channels (ryanodine receptors) in the sarcoplasmic reticulum (SR). Local Ca²⁺ signaling in the atrium differs from that in the ventricle because atrial myocytes lack transverse tubules and have more abundant corbular SR. Myocardium is subjected to a variety of forces with each contraction, such as stretch, shear stress, and afterload, and adapts to those mechanical stresses. These mechanical stimuli increase in heart failure, hypertension, and valvular heart diseases that are clinically implicated in atrial fibrillation and stroke. In the present review, we describe distinct responses of atrial and ventricular myocytes to shear stress and compare them with other mechanical responses in the context of local and global Ca²⁺ signaling and ion channel regulation. Recent evidence suggests that shear mechanotransduction in cardiac myocytes involves activation of gap junction hemichannels, purinergic signaling, and generation of mitochondrial reactive oxygen species. Significant alterations in Ca²⁺ signaling and ionic currents by shear stress may be implicated in the pathogenesis of cardiac arrhythmia and failure.

Mechanosensitivity of microdomain calcium signalling in the heart

In cardiac myocytes, calcium (Ca²⁺) signalling is tightly controlled in dedicated microdomains. At the dyad, i.e. the narrow cleft between t-tubules and junctional sarcoplasmic reticulum (SR), many signalling pathways combine to control Ca²⁺-induced Ca²⁺ release during contraction. Local Ca²⁺ gradients also exist in regions where SR and mitochondria are in close contact to regulate energetic demands. Loss of microdomain structures, or dysregulation of local Ca²⁺ fluxes in cardiac disease, is often associated with oxidative stress, contractile dysfunction and arrhythmias. Ca²⁺ signalling at these microdomains is highly mechanosensitive. Recent work has demonstrated that increasing mechanical load triggers rapid local Ca²⁺ releases that are not reflected by changes in global Ca²⁺. Key mechanisms involve rapid mechanotransduction with reactive oxygen species or nitric oxide as primary signalling molecules targeting SR or mitochondria microdomains depending on the nature of the mechanical stimulus. This review summarizes the most recent insights in rapid Ca²⁺ microdomain mechanosensitivity and re-evaluates its (patho)physiological significance in the context of historical data on the macroscopic role of Ca²⁺ in acute force adaptation and mechanically-induced arrhythmias. We distinguish between preload and afterload mediated effects on local Ca²⁺ release, and highlight differences between atrial and ventricular myocytes. Finally, we provide an outlook for further investigation in chronic models of abnormal mechanics (eg post-myocardial infarction, atrial fibrillation), to identify the clinical significance of disturbed Ca²⁺ mechanosensitivity for arrhythmogenesis.

New Insights in Cardiac Calcium Handling and Excitation-Contraction Coupling

Excitation-contraction (EC) coupling denotes the conversion of electric stimulus in mechanic output in contractile cells. Several studies have demonstrated that calcium (Ca²⁺) plays a pivotal role in this process. Here we present a comprehensive and updated description of the main systems involved in cardiac Ca²⁺ handling that ensure a functional EC coupling and their pathological alterations, mainly related to heart failure.

Effect of SR load and pH regulatory mechanisms on stretch-dependent Ca(2+) entry during the slow force response

When cardiac muscle is stretched, there is an initial inotropic response that coincides with the stretch followed by a slower increase in twitch force that develops over several minutes (the "slow force response", or SFR). Unlike the initial response to stretch, the SFR is produced by an increase in Ca(2+) transient amplitude, but the cellular mechanisms that give rise to the increased transients are still debated. We have examined the relationship between the SFR, intracellular [Ca(2+)] and the inotropic state of right ventricular trabeculae from rat hearts at 37°C. The magnitude of the SFR varied with [Ca(2+)]o and stimulation frequency, so that the SFR was greatest for conditions associated with a reduced SR Ca(2+) content. The SFR was not blocked by the AT1 receptor blocker losartan, but was reduced by SN-6, an inhibitor of reverse mode Na(+)/Ca(2+)-exchange (NCX). The Na(+)/H(+)-exchange (NHE) inhibitor HOE642 had no effect in HCO3(-)-buffered solutions, but blocked 50% of the SFR in HCO3(-)-free solution. Inhibition of HCO3(-) transport by DIDS increased the SFR and made it sensitive to HOE642. The addition of cross-bridge cycle inhibitors (20mM BDM or 20μM blebbistatin) to the superfusate reduced the SFR as monitored by changes in Ca(2+). In HCO3(-)-free conditions, the SFR was associated with a slow acidification that was inhibited by BDM, and by stopping electrical stimulation. These results can be explained by stretch increasing metabolic demand and stimulating Na(+) entry via both NHE and the Na(+)/HCO3(-) transporters. This mechanism provides a novel link between inotropic state and stretch, as well as a way for the cell to compensate for increased acid load. The feedback mechanism between force and Ca(2+) transient amplitude that we describe is also limited by the degree of SR Ca(2+) load.

Matching ATP supply and demand in mammalian heart: in vivo, in vitro, and in silico perspectives

Although the heart rapidly adapts cardiac output to match the body's circulatory demands, the regulatory mechanisms ensuring that sufficient ATP is available to perform the required cardiac work are not completely understood. Two mechanisms have been suggested to serve as key regulators: (1) ADP and Pi concentrations--ATP utilization/hydrolysis in the cytosol increases ADP and Pi fluxes to mitochondria and hence the amount of available substrates for ATP production increases; and (2) Ca2+ concentration--ATP utilization/hydrolysis is coupled to changes in free cytosolic calcium and mitochondrial calcium, the latter controlling Ca2+-dependent activation of mitochondrial enzymes taking part in ATP production. Here we discuss the evolving perspectives of each of the putative regulatory mechanisms and the precise molecular targets (dehydrogenase enzymes, ATP synthase) based on existing experimental and theoretical evidence. The data synthesis can generate novel hypotheses and experimental designs to solve the ongoing enigma of energy supply-demand matching in the heart.

The role of Ca(2+) signaling in the coordination of mitochondrial ATP production with cardiac work

The heart is capable of balancing the rate of mitochondrial ATP production with utilization continuously over a wide range of activity. This results in a constant phosphorylation potential despite a large change in metabolite turnover. The molecular mechanisms responsible for generating this energy homeostasis are poorly understood. The best candidate for a cytosolic signaling molecule reflecting ATP hydrolysis is Ca(2+). Since Ca(2+) initiates and powers muscle contraction as well as serves as the primary substrate for SERCA, Ca(2+) is an ideal feed-forward signal for priming ATP production. With the sarcoplasmic reticulum to cytosolic Ca(2+) gradient near equilibrium with the free energy of ATP, cytosolic Ca(2+) release is exquisitely sensitive to the cellular energy state providing a feedback signal. Thus, Ca(2+) can serve as a feed-forward and feedback regulator of ATP production. Consistent with this notion is the correlation of cytosolic and mitochondrial Ca(2+) with work in numerous preparations as well as the localization of mitochondria near Ca(2+) release sites. How cytosolic Ca(2+) signaling might regulate oxidative phosphorylation is a focus of this review. The relevant Ca(2+) sensitive sites include several dehydrogenases and substrate transporters together with a post-translational modification of F1-FO-ATPase and cytochrome oxidase. Thus, Ca(2+) apparently activates both the generation of the mitochondrial membrane potential as well as utilization to produce ATP. This balanced activation extends the energy homeostasis observed in the cytosol into the mitochondria matrix in the never resting heart.

Stretch and quick release of rat cardiac trabeculae accelerates Ca2+ waves and triggered propagated contractions

Rapid shortening of active cardiac muscle [quick release (QR)] dissociates Ca2+ from myofilaments. We studied, using muscle stretches and QR, whether Ca2+ dissociation affects triggered propagated contractions (TPCs) and Ca2+ waves. The intracellular Ca2+ concentration was measured by a SIT camera in right ventricular trabeculae dissected from rat hearts loaded with fura 2 salt, force was measured by a silicon strain gauge, and sarcomere length was measured by laser diffraction while a servomotor controlled muscle length. TPCs (n = 27) were induced at 28 degrees C by stimulus trains (7.5 s at 2.65 +/- 0.13 Hz) at an extracellular Ca2+ concentration ([Ca2+]o) = 2.0 mM or with 10 microM Gd3+ at [Ca2+]o = 5.2 +/- 0.73 mM. QR during twitch relaxation after a 10% stretch for 100-200 ms reduced both the time between the last stimulus and the peak TPC (PeakTPC) and the time between the last stimulus and peak Ca2+ wave (PeakCW) and increased PeakTPC and PeakCW (n = 13) as well as the propagation velocity (Vprop; n = 8). Active force during stretch also increased Vprop (r = 0.84, n = 12, P < 0.01), but Gd3+ had no effect (n = 5). These results suggest that Ca2+ dissociation by QR during relaxation accelerates the initiation and propagation of Ca2+ waves.

Effects of volatile anesthetics on stiffness of mammalian ventricular muscle

To assess the effects of halothane, isoflurane, and sevoflurane on cross bridges in intact cardiac muscle, electrically stimulated (0.25 Hz, 25 degrees C) right ventricular ferret papillary muscles (n = 14) were subjected to sinusoidal load oscillations (37-182 Hz, 0.2-0.5 mN peak to peak) at the instantaneous self-resonant frequency of the muscle-lever system. At resonance, stiffness is proportional to m * omega(2) (where m is equivalent moving mass and omega is angular frequency). Dynamic stiffness was derived by relating total stiffness to values of passive stiffness at each length during shortening and lengthening. Shortening amplitude and dynamic stiffness were decreased by halothane > isoflurane > or = sevoflurane. At equal peak shortening, dynamic stiffness was higher in halothane or isoflurane in high extracellular Ca(2+) concentration than in control. Halothane and isoflurane increased passive stiffness. The decrease in dynamic stiffness and shortening results in part from direct effects of volatile anesthetics at the level of cross bridges. The increase in passive stiffness caused by halothane and isoflurane may reflect an effect on weakly bound cross bridges and/or an effect on passive elastic elements.

Heart rate modulates the slow enhancement of contraction due to sudden left ventricular dilation

In isovolumic blood-perfused dog hearts, left ventricular developed pressure (DP) was recorded while a sudden ventricular dilation was promoted at three heart rate (HR) levels: low (L: 52 +/- 1.7 beats/min), intermediate (M: 82 +/- 2.2 beats/min), and high (H: 117 +/- 3.5 beats/min). DP increased instantaneously with chamber expansion (Delta(1)DP), and another continuous increase occurred for several minutes (Delta(2)DP). HR elevation did not alter Delta(1)DP (32.8 +/- 1.6, 33.6 +/- 1.5, and 34.3 +/- 1.2 mmHg for L, M, and H, respectively), even though it intensified Delta(2)DP (17.3 +/- 0.9, 20.7 +/- 1.0, and 26.8 +/- 1.2 mmHg for L, M, and H, respectively), meaning that the treppe phenomenon enhances the length dependence of the contraction component related to changes in intracellular Ca(2+) concentration. Frequency increments reduced the half time of the slow response (82 +/- 3.6, 67 +/- 2.6, and 53 +/- 2.0 s for L, M, and H, respectively), while the number of beats included in half time increased (72 +/- 2.9, 95 +/- 2.9, and 111 +/- 3.2 beats for L, M, and H, respectively). HR modulation of the slow response suggests that L-type Ca(2+) channel currents and/or the Na(+)/Ca(2+) exchanger plays a relevant role in the stretch-triggered Ca(2+) gain when HR increases in the canine heart.

Cytochalasin D reduces Ca2+ sensitivity and maximum tension via interactions with myofilaments in skinned rat cardiac myocytes

The F-actin disrupter cytochalasin D depresses cardiac contractility, an effect previously ascribed to the interaction of cytochalasin D with cytoskeletal actin. We have investigated the possibility that this negative inotropic effect is due to the interaction of cytochalasin D with sarcomeric actin of the thin filament. Confocal images of Triton X-100-skinned myocytes incubated with a fluorescent conjugate of cytochalasin D revealed a longitudinally striated pattern of binding, consistent with a myofibrillar rather than cytoskeletal structure.Tension-pCa relationships were determined at sarcomere lengths (SLs) of 2.0 and 2.3 [mu]m following 2 min incubation with 1 [mu]M cytochalasin D. Cytochalasin D significantly reduced the pCa for half-maximal activation (pCa50) at both SLs. The shift in pCa50 was significantly greater at a SL of 2.3 [mu]m compared with that at a SL of 2.0 [mu]m. Cytochalasin D had no effect on the Hill co-efficient at either SL. Cytochalasin D significantly reduced the maximum tension at both SLs. We suggest that the length-dependent decrease in myofilament Ca2+ sensitivity in response to cytochalasin D is due to a decrease in the affinity of troponin C for Ca2+. Cytochalasin D has been used for many years as the agent of choice for disruption of cytoskeletal actin. However, we have demonstrated for the first time an interaction of cytochalasin D with sarcomeric actin of the thin filament, which can account for the effects of cytochalasin D on cardiac contractility.

Intracellular acidosis modulates the stretch-induced changes in E-C coupling of the rat atrium

By inducing a small reduction of the intracellular pH (0.18 units) with 20 mmol L-1 propionate we demonstrated that acidification changed the responses of isolated rat atria to stretch. Stretch (increase of the intra-atrial pressure) in normal pH increased the Ca2+ transients' amplitude (Indo-1 fluorescence) from 0.26 +/- 0.09 in 1 mmHg to 0.36 +/- 0.13 in 4 mmHg (P < 0.05, n=6), without affecting the diastolic [Ca2+]i level (n.s. n=6). The changes in Ca2+ balance during stretch were accompanied by a biphasic increase in the contraction force. Five minutes of continuous stretch increased the action potential duration (APD90%, P < 0.01, n=13) and decreased the APD15% (P < 0.001, n=13). During acidosis, the stretch-induced increase of the Ca2+ transient amplitude (0.4 +/- 0. 13 vs. 0.3 +/- 0.08, P < 0.05, n=6) was accompanied by the increase of the diastolic [Ca2+]i (1.16 +/- 0.07, P < 0.05, n=6) compared with non-acidotic control (1.06 +/- 0.06, n=6). Acidic intracellular pH also inhibited the stretch-induced changes in the action potentials (n=10) and slowed down the development of the contractile force during stretch. The results showed that acidosis modulates the mechanotransduction. It does this by interfering with the intracellular Ca2+ balance, inhibiting the Ca2+ extrusion mechanisms and reducing the Ca2+-buffering power of the cells. The physiological and pathological processes associated with stretch are therefore modulated by intracellular pH owing to its concerted effects on intracellular Ca2+ handling caused by a competitive inhibition of various Ca2+-binding molecules.

Mechanisms underlying the increase in force and Ca(2+) transient that follow stretch of cardiac muscle: a possible explanation of the Anrep effect

Myocardial stretch produces an increase in developed force (DF) that occurs in two phases: the first (rapidly occurring) is generally attributed to an increase in myofilament calcium responsiveness and the second (gradually developing) to an increase in [Ca(2+)](i). Rat ventricular trabeculae were stretched from approximately 88% to approximately 98% of L(max), and the second force phase was analyzed. Intracellular pH, [Na(+)](i), and Ca(2+) transients were measured by epifluorescence with BCECF-AM, SBFI-AM, and fura-2, respectively. After stretch, DF increased by 1.94+/-0.2 g/mm(2) (P<0.01, n = 4), with the second phase accounting for 28+/-2% of the total increase (P<0.001, n = 4). During this phase, SBFI(340/380) ratio increased from 0.73+/-0.01 to 0.76+/-0.01 (P<0.05, n = 5) with an estimated [Na(+)](i) rise of approximately 6 mmol/L. [Ca(2+)](i) transient, expressed as fura-2(340/380) ratio, increased by 9.2+/-3.6% (P<0.05, n = 5). The increase in [Na(+)](i) was blocked by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA). The second phase in force and the increases in [Na(+)](i) and [Ca(2+)](i) transient were blunted by AT(1) or ET(A) blockade. Our data indicate that the second force phase and the increase in [Ca(2+)](i) transient after stretch result from activation of the Na(+)/H(+) exchanger (NHE) increasing [Na(+)](i) and leading to a secondary increase in [Ca(2+)](i) transient. This reflects an autocrine-paracrine mechanism whereby stretch triggers the release of angiotensin II, which in turn releases endothelin and activates the NHE through ET(A) receptors.

Role of the sarcoplasmic reticulum in the modulation of rat cardiac action potential by stretch

We have investigated the role of sarcoplasmic reticulum (SR) in the modulation on rat action potentials by stretch. The action potentials were recorded intracellularly from rat atrial myocytes in an isolated atrial preparation with small, physiological stretch produced by pressure (1-3 mmHg) inside the atria. The SR function was inhibited by pharmacological interventions, either with ryanodine (100 nmol L-1), thapsigargin (10 nmol L-1) or caffeine (1 mmol L-1). The duration of action potentials was increased by stretch from 1 to 3 mmHg. The repolarization indices APD30% (P < 0.05), APD60% (P < 0.01), and APD90% (P < 0.01) were all increased significantly (n=10). Ryanodine, thapsigargin, and caffeine inhibited this prolongation, or even reversed the effect with repolarization indices APD30% (P < 0.05) and APD60% (P < 0.05) which decreased in stretch with thapsigargin treatment. As a conclusion, we suggest that the SR and the intracellular calcium balance play an important role in the modulation of the shape of the rat atrial action potential during stretch.

The role of calcium in the response of cardiac muscle to stretch

This review focuses on the complex interactions between two major regulators of cardiac function; Ca2+ and stretch. Initial consideration is given to the effect of stretch on myocardial contractility and details the rapid and slow increases in contractility. These are shown to be related to two diverse changes in Ca2+ handling (enhanced myofilament Ca2+ sensitivity and increased intracellular Ca2+ transient, respectively). Interaction between stretch and Ca2+ is also demonstrated with respect to the rhythm of cardiac contraction. Stretch has been shown to alter action potential configuration, generate stretch-activated arrhythmias, and increase the rate of beating of the sino-atrial node. A variety of Ca(2+)-dependent mechanisms including attenuation of Ca2+ extrusion via Na+/Ca2+ exchange, Ca2+ entry through stretch-activated channels (SACs) and mobilisation of intracellular Ca2+ stores have been proposed to account for the effect of stretch on rhythm. Finally, the interaction between stretch and Ca2+ in the secretion of natriuretic peptides and onset of hypertrophy is discussed. Evidence is presented that Ca2+ (entering through L-type Ca2+ channels or SACs, or released from sarcoplasmic reticular stores) influences secretion of both atrial and B-type natriuretic peptide; there is data to support both positive and negative modulation by Ca2+. Ca2+ also appears to be important in the pathway that leads to expression of precursors of hypertrophic protein synthesis. In conclusion, two of the major regulators of cardiac muscle function, Ca2+ and stretch, interact to produce effects on the heart; in general these effects appear to be additive.

Cellular mechanisms for the slow phase of the Frank-Starling response

Following a step increase in sarcomere length, isometric cardiac muscle tension increases instantaneously by the Frank-Starling mechanism. In isolated papillary muscle and myocytes, there is an additional significant rise in developed tension over the following 15 min due to an unknown mechanism. This slow change in tension could not be explained by mechanical heterogeneity of the muscle preparations or by an increase in myofilament sensitivity to Ca2+. The slow change in tension was not dependent on sarcoplasmic reticulum Ca2+ loading assessed with rapid cooling contractures, and was not significantly altered by sarcoplasmic reticulum Ca2+ depletion (ryanodine) or inhibition of sarcoplasmic reticulum Ca2+ reuptake (cyclopiazonic acid). We used the Luo-Rudy ionic model of the ventricular myocyte together with a model of the length-dependent myofilament activation by Ca2+ to examine the effects of step changes in the parameters of sarcolemmal ion fluxes as possible mechanisms for the slow change in stress. The slow increase in tension was simulated by step changes in the Na+-K+ pump or Na+ leak currents, suggesting that the slow change in stress may be caused by length induced changes in Na+ fluxes. The model also predicted a slow increase in the magnitude of the initial repolarization during phase 1 of the action potential. The combination of experimental and computational models used in this investigation represents a valuable technique in elucidating the cellular mechanisms of fundamental processes in cardiac excitation-contraction coupling.

Mechanisms of stretch-induced changes in [Ca2+]i in rat atrial myocytes: role of increased troponin C affinity and stretch-activated ion channels

To study the effects of stretch on the function of rat left atrium, we recorded contraction force, calcium transients, and intracellular action potentials (APs) during stretch manipulations. The stretch of the atrium was controlled by intra-atrial pressure. The Frank-Starling behavior of the atrium was manifested as a biphasic increase of the contraction force after increasing the stretch level. The development of the contraction force after step increase of the stretch (intra-atrial pressure from 1 to 3 mm Hg) was accompanied by the increase in the amplitude of the calcium transients (P<0.05, n=4) and decrease in the time constant of the Ca2+ transient decay. The APs of the individual myocytes were also affected by stretch; the duration of the AP was decreased at positive voltages (AP duration at 15% repolarization level, P<0.001; n=13) and increased at negative voltages (AP duration at 90% repolarization level, P<0. 01; n=13). To study the mechanisms causing these changes we developed a mathematical model describing [Ca2+]i and electrical behavior of single rat atrial myocytes. Stretch was simulated in the model by increasing the troponin (TnC) sensitivity and/or applying a stretch-activated (SA) calcium influx. We mimicked the Ca2+ influx by introducing a nonselective cationic conductance, the SA channels, into the membrane. Neither of the 2 plausible mechanosensors (TnC or SA channels) alone could produce similar changes in the Ca2+ transients or APs as seen in the experiments. The model simulated the effects of stretch seen in experiments best when both the TnC affinity and the SA conductance activation were applied simultaneously. The SA channel activation led to gradual augmentation of Ca2+ transients, which modulated the APs through increased Na+/Ca2+-exchanger inward current. The role of TnC affinity change was to modulate the Ca2+ transients, stabilize the diastolic [Ca2+]i, and presumably to produce the immediate increase of the contraction force after stretch seen in experiments. Furthermore, we found that the same mechanism that caused the normal physiological responses to stretch could also generate arrhythmogenic afterpotentials at high stretch levels in the model.

Strong binding of myosin modulates length-dependent Ca2+ activation of rat ventricular myocytes

Reductions in sarcomere length (SL) and concomitant increases in interfilament lattice spacing have been shown to decrease the Ca2+ sensitivity of tension in myocardium. We tested the idea that increased lattice spacing influences the SL dependence of isometric tension by reducing the probability of strong interactions of myosin crossbridges with actin, thereby decreasing cooperative activation of the thin filament. Single ventricular myocytes were isolated by enzymatic digestion of rat hearts and were subsequently rapidly skinned. Maximal tension and Ca2+ sensitivity of tension (ie, pCa50) were measured in the absence and presence of N-ethylmaleimide-modified myosin subfragment 1 (NEM-S1) at both short and long SLs. NEM-S1, a strong-binding non-tension-generating derivative of the myosin head, was applied to single skinned myocytes to cooperatively promote strong binding of endogenous myosin crossbridges. Compared with control myocytes at SL of approximately 1.90 microm, application of NEM-S1 markedly increased submaximal Ca2+-activated tensions and thereby increased Ca2+ sensitivity; ie, pCa50 increased from 5.40+/-0.02 to 5.52+/-0.02 pCa units in the presence of NEM-S1. Furthermore, NEM-S1 treatment reversibly eliminated the SL dependence of the Ca2+ sensitivity of tension, in that the DeltapCa50 between short and long lengths was 0. 02+/-0.01 pCa units in the presence of NEM-S1 compared with a DeltapCa50 of 0.10+/-0.01 pCa units in control myocytes. From these results we conclude that the decrease in the Ca2+ sensitivity of tension at short SL results predominantly from decreased cooperative activation of the thin filament due to reductions in the number of strong-binding crossbridges.

Mechanisms of length history-dependent tension in an ionic model of the cardiac myocyte

The ionic model of the ventricular myocyte developed by Luo and Rudy (Circ. Res. 74: 1071-1096, 1994) was used to investigate potential mechanisms of the slow changes in stress (SCS) that follow step changes in muscle length. A step change in myofilament sensitivity alone caused an immediate increase in active tension, but no SCS. The effects of additional step changes in the parameters of sarcolemmal ion fluxes were examined for each ion flux in the model. Changes in the coefficients of Ca2+ or K+ channels did not produce SCS. SCS was produced by step changes in parameters of the Na(+)-K+ pump or the Na+ leak current. This simulated mechanism was mediated through a slow increase in intracellular Na+ concentration and a resulting increase in systolic Ca2+ entry through the Na+/Ca2+ exchanger. The model reproduced the effects of several experimental interventions such as sarcoplasmic reticulum Ca2+ depletion, "diastolic" length changes, and changes in extracellular Ca2+. Thus SCS in cardiac muscle may be caused by length-induced changes in sarcolemmal Na+ fluxes.

Effect of ventricular stretch on contractile strength, calcium transient, and cAMP in intact canine hearts

Isovolumic contractions were imposed by intraventricular balloon in 39 isolated, blood-perfused canine hearts to investigate the effects of myocardial stretch on contractile force. After stabilization at 37 degrees C, left ventricular volume was increased so that end-diastolic pressure increased from 0 to 5 mmHg. After the immediate increase in developed pressure [DP; from 37 +/- 14 to 82 +/- 22 mmHg (means +/- SD)], there was a slow secondary rise in DP (97 +/- 27 mmHg) that peaked at 3 min. However, DP subsequently decreased over the next 7 min back to the initial value (84 +/- 25 mmHg). Light emission from microinjected aequorin (n = 10 hearts) showed that changes in intracellular calcium [3 min: 124 +/- 15% (P < 0.01); 10 min: 99 +/- 18% of baseline] paralleled DP changes. Increases in myocardial adenosine 3',5'-cyclic monophosphate (cAMP) content (n = 12) accompanied the secondary rise in DP. In contrast, the gradual elevation of DP after the stretch was not exerted during continuous beta-adrenergic stimulation by isoproterenol. Thus, in contrast to isolated muscle, stretch only transiently increases intracellular calcium and contractile strength in intact hearts. The findings of changes in cAMP and abolition of the phenomena by beta-stimulation suggest that a primary stretch-mediated influence on cAMP metabolism may underlie these phenomena.

Changes in force and cytosolic Ca2+ concentration after length changes in isolated rat ventricular trabeculae

1. Changes in cytosolic [Ca2+] ([Ca2+]i) were measured in isolated rat trabeculae that had been micro-injected with fura-2 salt, in order to investigate the mechanism by which twitch force changes following an alteration of muscle length. 2. A step increase in length of the muscle produced a rapid potentiation of twitch force but not of the Ca2+ transient. The rapid rise of force was unaffected by inhibiting the sarcoplasmic reticulum (SR) with ryanodine and cyclopiazonic acid. 3. The force-[Ca2+]i relationship of the myofibrils in situ, determined from twitches and tetanic contractions in SR-inhibited muscles, showed that the rapid rise of force was due primarily to an increase in myofibrillar Ca2+ sensitivity, with a contribution from an increase in the maximum force production of the myofibrils. 4. After stretch of the muscle there was a further, slow increase of twitch force which was due entirely to a slow increase of the Ca2+ transient, since there was no change in the myofibrillar force-[Ca2+]i relationship. SR inhibition slowed down, but did not alter the magnitude of, the slow force response. 5. During the slow rise of force there was no slow increase of diastolic [Ca2+]i, whether or not the SR was inhibited. The same was true in unstimulated muscles. 6. We conclude that the rapid increase in twitch force after muscle stretch is due to the length-dependent properties of the myofibrils. The slow force increase is not explained by length dependence of the myofibrils or the SR, or by a rise in diastolic [Ca2+]i. Evidence from tetani suggests the slow force responses result from increased Ca2+ loading of the cell during the action potential.

Sarcomere length dependence of the rate of tension redevelopment and submaximal tension in rat and rabbit skinned skeletal muscle fibres

1. We examined the hypothesis that in skeletal muscle the steep relationship between twitch tension and sarcomere length (SL) within the range 2.30 to 1.85 microns involves SL-dependent alterations in the rate of tension development. 2. In skinned preparations of both rat slow-twitch and rabbit fast-twitch skeletal muscle fibres the rate of tension redevelopment (ktr) at 15 degrees C was reduced at short SL (approximately 2.00 microns) compared with a longer SL (approximately 2.30 microns). In submaximally activated fibres, the decrease in ktr over this range of lengths was greater in fast-twitch fibres (38% reduction) than in slow-twitch fibres (14% reduction). 3. Ca2+ sensitivity of tension, as assessed as the pCa (-log[Ca2+]) for half-maximal activation, or pCa50, decreased to a greater extent in rabbit fast-twitch skeletal muscle fibres than in slow-twitch fibres from both rabbit and rat when SL was reduced from approximately 2.30 to approximately 1.85 microns. The delta pCa50 over this SL range was 0.24 +/- 0.07 pCa units in fast-twitch fibres from rabbit psoas muscle. The delta pCa50 for slow-twitch fibres from rabbit and rat soleus muscle was 0.08 +/- 0.02 and 0.10 +/- 0.04 pCa units, respectively. 4. Osmotic compression of both slow-twitch and fast-twitch fibres at a SL of 2.00 microns increased ktr to values similar to those obtained at a SL of 2.30 microns in the absence of dextran. This result indicates that the slower rate of tension redevelopment at short SL is due in large part to the increase in interfilament lattice spacing associated with shorter SL. 5. Taken together, these results suggest that length dependence of twitch tension is, in part, due to length dependence of isometric cross-bridge interaction kinetics, an effect that is mediated by length-dependent changes in interfilament lattice spacing.

Rapid shortening during relaxation increases activation and improves systolic performance

BACKGROUND

Previous studies in cardiac muscle and isolated heart preparations generally have attributed positive effects of ejection to greater length-dependent activation. However, there have been some reports of an ejection-related increase in contractile function that is independent of end-diastolic volume (EDV) history. The present study was designed to more fully characterize the mechanoenergetic results of the latter effect in the intact ventricle.

METHODS AND RESULTS

A servomotor was used to initiate left ventricular volume reduction (VR) at end systole, with EDV kept constant. Seven isolated, red blood cell-perfused rabbit hearts were studied at constant EDV during isovolumic contraction, slow VR (5.0 +/- 0.9 EDV/s), and rapid VR (26.8 +/- 5.1 EDV/s). Compared with isovolumic beats, VR caused an enhancement in contractility. This effect was greater for rapid VR and required > 50 beats to attain steady state. Rapid VR increased developed pressure by 15% (92.2 +/- 23.7 [mean +/- SD] versus 105.9 +/- 27.6 mm Hg), maximum dP/dt by 17% (1223 +/- 401 versus 1435 +/- 505 mm Hg.s-1), and Emax (slope of the end-systolic pressure-volume relation) by 13% (69.4 +/- 19.9 versus 78.6 +/- 23.0 mm Hg/mL) (all P < .01). Left ventricular oxygen consumption (VO2) was unchanged with slow VR and decreased by 8% with rapid VR (0.0744 +/- 0.0194 versus 0.0683 +/- 0.0141 mL O2.beat-1.100 g-1; P < .05). In separate hearts (n = 8), costs (basal metabolism and excitation-contraction coupling) were estimated by use of 2,3-butanedione monoxime. Compared with control, rapid VR was associated with a 26% increase in nonmechanical VO2 (0.0248 +/- 0.0021 versus 0.0312 +/- 0.0022 mL O2.beat-1.100 g-1; P < .01), consistent with an increase in calcium cycled per beat.

CONCLUSIONS

Ejection after end systole has a positive effect on ventricular performance that cannot be ascribed to length-dependent activation and is likely related to an increase in calcium available for activation. Similarly, an increase in nonmechanical VO2 associated with ejection suggests a positive interaction between myofilament shortening and activator calcium cycling.

Effect of gadolinium on stretch-induced changes in contraction and intracellularly recorded action- and afterpotentials of rat isolated atrium

1. Atrial arrhythmias, like atrial fibrillation and extrasystoles, are common in clinical situations when atrial pressure is increased. Although cardiac mechanoelectrical feedback has been under intensive study for many years, the mechanisms of stretch-induced arrhythmias are not known in detail. This is partly due to methodological difficulties in recording intracellular voltage during stretch stimulation. In this study we investigated the effects of gadolinium (Gd3+), a blocker of stretch-activated (SA) channels, on stretch-induced changes in rat atrial action potentials and contraction force. 2. By intracellular voltage recordings from rat isolated atria we studied the effects of Gd3+ (80 microM) on stretch-induced changes in action potentials. The stretch was induced by increasing pressure inside the atrium (1 mmHg to 7 mmHg). An elastic electrode holder that moved along the atrial tissue was used in the recordings. Thus the mechanical artifacts were eliminated and the cell-electrode contact was made more stable. To examine the influence of Gd3+ on atrial contraction we stretched the atria at different diastolic pressure levels (1 to 7 mmHg) with Gd3+ application of (80 microM) or diltiazem (5.0 microM). Contraction force was monitored by recording the pressure changes generated by the atrial contractions. 3. Our results show that: (1) atrial stretch induces delayed afterdepolarizations (DADs), increase in action potential amplitude and increase in relative conduction speed; (ii) Gd3+ blocks stretch-induced DADs and action potential changes; (iii) Gd3+ inhibits pressure-stimulated increase in the atrial contraction force, while similar inhibition is not observed with diltiazem, a blocker of L-type calcium channels. 4. This study suggests that Gd3+ inhibits stretch-induced changes in cell electrophysiology and contraction in the rat atrial cells and that the effects of gadolinium are due to rather specific block of stretch-activated ion channels with only a small effect on voltage-activated calcium channels.

Osmotic compression of single cardiac myocytes eliminates the reduction in Ca2+ sensitivity of tension at short sarcomere length

According to the Frank-Starling relation, cardiac output varies as a function of end-diastolic volume of the ventricle. The cellular basis of the relation is thought to involve length-dependent variations in Ca2+ sensitivity of tension; ie, as sarcomere length is increased in cardiac muscle, Ca2+ sensitivity of tension also increases. One possible explanation for this effect is that the decrease in myocyte diameter as muscle length is increased reduces the lateral spacing between thick and thin filaments, thereby increasing the likelihood of cross-bridge interaction with actin. To examine this idea, we measured the effects of osmotic compression of single skinned cardiac myocytes on Ca2+ sensitivity of tension. Single myocytes from rat enzymatically digested ventricles were attached to a force transducer and piezoelectric translator, and tension-pCa relations were subsequently characterized at short sarcomere length (SL), at the same short SL in the presence of 2.5% dextran, and at long SL. The pCa (-log[Ca2+]) for half-maximal tension (ie, pCa50) increased from 5.54 +/- 0.09 to 5.65 +/- 0.10 (n = 7, mean +/- SD, P < .001) as SL was increased from approximately 1.85 to approximately 2.25 microns. Osmotic compression of myocytes at short length also increased Ca2+ sensitivity of tension, shifting tension-pCa relations to [Ca2+] levels similar to those observed at long length (pCa50, 5.68 +/- 0.11). These results support the idea that the length dependence of Ca2+ sensitivity of tension in cardiac muscle arises in large part from the changes in interfilament lattice spacing that accompany changes in SL.

Regional differences in rest decay and recoveries of contraction and the calcium transient in rabbit ventricular muscle

The rates of rest decay (for rest periods of between 0.5 min and 10 min) and recovery from the rested state (following 10 min of rest) of cell shortening and the amplitude of the intracellular calcium transient were compared in epicardial and endocardial ventricular myocytes isolated from rabbit hearts. The object of these experiments was to determine whether reported transmural differences in action potential duration, myosin type expression and metabolic enzyme content are able to influence the control of contraction. Cells isolated from these two regions of the ventricular wall displayed almost identical twitch shortening and calcium transient characteristics during steady-state electrical stimulation at 0.5 Hz. Despite this, rest decay of cell shortening was faster and recovery from the rested state slower in endocardial cells than in epicardial cells. Neither of these differences could be explained in terms of changes of calcium transient amplitude or time course. We tried to mimic the effect of prolonged rest by application of caffeine to empty the sarcoplasmic reticulum of calcium. The regional differences in recovery of contraction from the rested state were not reproduced in the recovery of contraction after caffeine application, suggesting that the effect is produced by something other than refilling of the sarcoplasmic reticulum. It is suggested that changes in factors that affect myofilament calcium sensitivity produce the regional differences in rest decay and post-rest recovery of contraction.

Length dependence of Ca2+ sensitivity of tension in mouse cardiac myocytes expressing skeletal troponin C

1. Beat-to-beat performance of myocardium is highly dependent on sarcomere length. The physiological basis for this effect is not well understood but presumably includes alterations in the extent of overlap between thick and thin filaments. Sarcomere length dependence of activation also appears to be involved since length-tension relationships in cardiac muscle are usually steeper than those in skeletal muscle. 2. An explanation recently proposed to account for the difference between length-tension relationships is that the cardiac isoform of troponin C (cTnC) has intrinsic properties that confer greater length-dependent changes in the Ca2+ sensitivity of tension than does skeletal troponin C (sTnC), presumably due to greater length-dependent changes in the Ca(2+)-binding affinity of cTnC. To test this hypothesis, transgenic mice were developed in which fast sTnC was expressed ectopically in the heart. This allowed a comparison of the length dependence of the Ca2+ sensitivity of tension between myocytes having thin filaments that contained either endogenous cTnC or primarily sTnC. 3. In myocytes from both transgenic and normal mice, the Ca2+ sensitivity of tension increased similarly when mean sarcomere length was increased from approximately 1.83 to 2.23 microns. In both cases, the mid-point (pCa50) of the tension-pCa (i.e. -log[Ca2+]) relationship shifted 0.12 +/- 0.01 pCa units (mean +/- S.E.M.) in the direction of lower Ca2+. 4. We conclude that the Ca2+ sensitivity of tension in myocytes changes as a function of sarcomere length but is independent of the isoform of troponin C present in the thin filaments.

The effects of mechanical loading and changes of length on single guinea-pig ventricular myocytes

1. The effects of mechanical loading and changes of length on the contraction of single guinea-pig ventricular myocytes has been investigated. 2. Cell shortening was monitored during isotonic contractions (in which the cell shortened freely) and after attaching carbon fibres of known compliance to the ends of the cell, so that the cell contracted auxotonically (the cell both shortened and developed force). 3. Mechanically loading the cells decreased the amount of shortening during a contraction and abbreviated the contraction. There were, however, no consistent changes in the action potential or the [Ca2+]i transient (measured with the fluorescent dye fura-2). 4. Increasing stimulation rate increased the size of the contraction and the [Ca2+]i transient in both isotonic and auxotonic conditions. The increase in the size of the contraction induced by an increase in stimulation rate was greater in auxotonic conditions but the increase in the size of the [Ca2+]i transient was not. 5. When cells were stretched, there was a step increase in the size of the contraction and a prolongation of its time course. However, neither the size nor the time course of the accompanying [Ca2+]i transient was significantly altered by this intervention. 6. When a stretch was maintained, a further, slow increase in the size of the contraction occurred during the following 3-11 min, in about half the cells studied. The probability of this slow response occurring was increased if the initial degree of activation of the cell was decreased. 7. These data suggest that the mechanisms underlying the responses to mechanical loading and changes of length are the same in both multicellular and single cell preparations of cardiac muscle.

Intracellular calcium and mechanical function in isolated perfused hearts from rats and guinea pigs

We tested the hypothesis that the variable functional properties of rat versus guinea pig hearts are due to differences in intracellular Ca2+ handling. Hearts isolated from rats and guinea pigs were perfused with physiological saline, and isovolumic left ventricular (LV) pressure as well as coronary perfusion pressure were recorded simultaneously with Ca2+ transients from aequorin-loaded cells. Guinea pig hearts developed 47% less LV pressure than rat hearts, and the time to peak pressure was prolonged by 71% at similar heart rates. Diastolic and systolic levels of myoplasmic Ca2+ were approximately the same in both species at normal external Ca2+ concentration (1 mM); however, at low Ca2+ concentration (0.5 mM), guinea pig hearts maintained a higher level of myoplasmic Ca2+ than rat hearts, and the relative depression of LV systolic pressure was less. Guinea pig hearts also exhibited higher resistance to the negative inotropic effect of caffeine and did not respond to increments in perfusion pressure with increases in LV-developed pressure and systolic Ca2+ levels as did rat hearts. These contrasting findings with regard to intracellular Ca2+ handling may be attributed to a different organization of the ionic transport system with higher dependence of rat cardiomyocytes on normal function of the sarcoplasmic reticulum.

Nonlinearity of the left ventricular end-systolic wall stress-velocity of fiber shortening relation in young pigs: a potential pitfall in its use as a single-beat index of contractility

OBJECTIVES

We sought to evaluate in the young heart the primary assumptions on which the current use of the mean "velocity of fiber shortening corrected for heart rate" as a noninvasive index of contractility are based.

BACKGROUND

End-systolic wall stress-velocity of fiber shortening relation has been applied as a single-beat, load-independent index of contractility in children. This use is based on poorly validated assumptions of linearity, parallel shifts with changing contractile state and inotropic sensitivity of the end-systolic wall stress-velocity of fiber shortening relation.

METHODS

In eight anesthetized young piglets, 5F mciromanometric catheters were placed in the ascending aorta and balloon occlusion catheters in the descending aorta. End-systolic wall stress and velocity of fiber shortening were calculated from aortic pressure and M-mode echocardiography under six conditions: in three contractile states 1) baseline, 2) increased contractility during dobutamine infusion (10 micrograms/kg per min), and 3) decreased contractility after propranolol injection (1 mg/kg), each at two afterload states (normal and increased load by partial aortic occlusion).

RESULTS

Dobutamine increased and propranolol decreased afterload-matched velocity of fiber shortening corrected for heart rate significantly to 140% and 77% of baseline, respectively. However, the slope of end-systolic wall stress-velocity of fiber shortening relation was much greater (251% of baseline) during dobutamine infusion, which also significantly decreased wall stress, and was much less (27% of baseline) after propranolol injection, which increased wall stress.

CONCLUSIONS

The velocity of fiber shortening corrected for heart rate did change predictably with changes in contractility and as such can be used noninvasively in the temporal evaluation of individual patients undergoing therapeutic interventions or to define the natural history of a disease process. However, the relation on which it is based is not defined by parallel straight lines across contractile states, so that abnormal single point measurements may reflect only the nonlinearity of the relation rather than abnormalities in contractility. Thus, we recommend that the end-systolic wall stress-velocity of fiber shortening relation should not be used as a single-beat index of contractility.

Endocardial versus epicardial differences of intracellular free calcium under normal and ischemic conditions in perfused rat hearts

Transmural heterogeneity of myocardial metabolism and function are present in the left ventricle under normal and ischemic conditions. To determine if endocardial versus epicardial differences of [Ca2+]i are also present, perfused rat heart studies using indo-1 fluorescence as an index of [Ca2+]i were performed in the left ventricular epicardium and endocardium. Hearts were studied during control conditions and low-flow ischemia. Results demonstrated the following: 1) At a pacing rate of 1.5 Hz, endocardial levels of diastolic and systolic [Ca2+]i (470 +/- 40 and 1,240 +/- 170 nM) were higher than epicardial levels (290 +/- 30 and 920 +/- 150 nM). 2) At a more physiological pacing rate of 5 Hz, endocardial levels of diastolic and systolic [Ca2+]i (680 +/- 50 and 1,230 +/- 70 nM) were also higher than epicardial levels (390 +/- 20 and 950 +/- 60 nM. 3) During low-flow ischemia, endocardial levels of diastolic [Ca2+]i rose to a greater degree (from 680 +/- 50 to 1,050 +/- 70 nM at 10% of control coronary flow) compared with epicardial levels (from 390 +/- 20 to 580 +/- 40 nM at 10% of control flow), suggesting that the endocardium is more susceptible to low-flow ischemia. 4) The amplitude of the [Ca2+]i transient was the same at the endocardium (540 +/- 50 nM) and epicardium (560 +/- 50 nM) and did not change during low-flow ischemia, despite marked contractile dysfunction.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ regulation of mechanical properties of striated muscle. Mechanistic studies using extraction and replacement of regulatory proteins

Extraction of regulatory proteins from thick and thin filaments of vertebrate striated muscle has proven to be an important approach in elucidating roles of these proteins in regulating contraction and in probing specific mechanisms of activation. For some proteins, such as LC2 and C protein, extraction has been fundamental in demonstrating the importance of these proteins in modulating contraction and the kinetics of cross-bridge interaction. For other proteins, such as TnC and troponin, extraction has provided significant insight into the importance of thin-filament intermolecular cooperativity in modulating Ca2+ sensitivity of the contractile process. A combination of extraction and readdition has provided a means of introducing mutated or derivatized proteins into fibers to accomplish a variety of experimental objectives. The use of this approach is likely to grow with the need to test the functional consequences of site-specific mutations as part of studies directed to mechanisms of regulation or altered regulation in heart and skeletal muscles under normal and pathophysiological conditions. Such studies are likely to include extraction in combination with other probes of function such as flash photolysis of reaction substrates or products within the cross-bridge interaction cycle. Although extraction is a powerful approach and is likely to be extended to proteins not discussed in this review, an essential element of experimental design in studies such as these is that appropriate control experiments be done to verify that observed effects of the extraction protocol are specifically attributable to the protein that is removed.

Effect of perfusion pressure on force of contraction in thin papillary muscles and trabeculae from rat heart

1. Increased coronary perfusion leads to increased myocardial contraction and oxygen consumption (Gregg's phenomenon) even when oxygen supply is presumably sufficient. Previous studies concerned whole hearts, however, in which local hypoxia may play a role. We developed techniques for internal perfusion of thin papillary muscles from rat heart. The influence of perfusion pressure on muscle contraction was studied. We investigated whether Gregg's phenomenon is due to (a) hypoxia, (b) stretch of the muscle fibres, or (c) increased contractility. 2. The effectiveness of the perfusion technique was demonstrated in four ways: (a) the diameter of the capillaries increased with perfusion pressure; (b) 14 +/- 4% (mean +/- S.D., n = 11) increase in muscle diameter was observed on a change of perfusion pressure from 0 to 50 cmH2O; (c) addition of India ink to the perfusate caused rapid staining of the entire muscle; (d) during internal perfusion and external superfusion peak force was mainly determined by the [Ca2+] in the internal perfusate. 3. An increase of perfusion pressure from 0 to 70 cmH2O induced 74 +/- 20% (mean +/- S.D., n = 11) increase in peak force of contraction. In the absence of internal perfusion peak force was not affected by approximately 50% reduction of the PO2 in the bathing solution (from 700 to 350 mmHg). Hence, oxygen supply was not a limiting factor, i.e. the effect of internal perfusion on force was not related to hypoxia. 4. Segment length was measured with markers attached to the surface of the muscle. Perfusion-induced changes in segment length were negligible (-0.2 +/- 1.5%, n = 11). Force-length relationships at different perfusion pressures show that the perfusion-induced increase in force was generally larger than the maximum increase in force that could be induced by stretch. Furthermore, the time course of stretch and perfusion effects on force was different. We conclude that Gregg's phenomenon is not related to changes in fibre length, i.e. the hypothesis of pressure-induced stretch ('garden hose' effect) does not apply to papillary muscles. 5. The pressure-induced changes in the force-length relationship were similar to the changes obtained with interventions that increase contractility, such as increased [Ca2+]. 6. Since hypoxia and length effects were not involved, and the effect of perfusion pressure was similar to that of inotropic interventions, we conclude that Gregg's phenomenon is a change in contractility. Possible explanations include changes in the ionic composition or volume of the interstitium, and inotropic factors produced by the endothelium or intramyocardial neurons.

Substitution of cardiac troponin C into rabbit muscle does not alter the length dependence of Ca2+ sensitivity of tension

1. The isometric length-tension relationship for cardiac muscle is generally steeper than for skeletal muscle in the physiological range of sarcomere lengths. Recent studies suggest that cardiac troponin C (cTnC) may have intrinsic properties that confer greater length-dependent changes in Ca2+ sensitivity of tension than for skeletal troponin C (sTnC). We tested this hypothesis by characterizing tension-pCa (pCa is -log[Ca2+]) relationships in rabbit skinned psoas muscle fibres at mean sarcomere lengths of 2.32 and 1.87 microns both before and after partial replacement of endogenous sTnC with cTnC. 2. In untreated control fibres, the mid-point (pCa50) of the tension-pCa relationship shifted to lower pCa by 0.15 +/- 0.02 pCa units, i.e. became less sensitive to Ca2+, when sarcomere length was reduced, and the relationship became steeper. 3. Partial extraction of endogenous sTnC and reconstitution with cTnC resulted in no change in the length-dependent shift of pCa50 when reconstitution with cTnC was more than 95% complete; however, when reconstitution was less than 95% complete, there were significant increases in the length-dependent shift in pCa50. 4. An increase in the length-dependent shift of pCa50 was also observed in fibres from which sTnC was partially extracted, but no cTnC was subsequently re-added. 5. We conclude that differences in type of TnC alone are not sufficient to explain differences between skeletal and cardiac muscles in the length dependence of Ca2+ sensitivity of tension.

Regional changes in ventricular excitability during load manipulation of the in situ pig heart

1. The effect of load manipulation on myocardial excitability was studied in the anaesthetized, in situ pig heart. 2. A 33% increase in systolic left ventricular pressure achieved by aortic clamping reduced the mean effective refractory period by 11 ms (7.6%, P less than 0.01); whereas a 15% reduction in ventricular pressure achieved by intravenous infusion of sodium nitroprusside increased the mean effective refractory period by 4 ms (3.2%, P less than 0.05). 3. Changes in action potential duration, measured to 70% repolarization, roughly paralleled those of the effective refractory period. 4. The changes in effective refractory period were inhomogeneous, with a greater change occurring at the apex compared to the base in response to an increase in load, i.e. there was an increase in regional dispersion of refractoriness across the left ventricle. 5. Since inhomogeneity of repolarization and refractoriness is known to be potentially arrhythmogenic, these findings suggest that mechanical factors may contribute directly to the arrhythmias commonly seen clinically in high load states such as congestive cardiac failure and may also have consequences for the treatment of such arrhythmias.