Can activation account for 80% of skeletal muscle energy use during isometric contraction?
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Animals↗
Biomedical subjects
D S Loiselle
Publications and source records attributed to D S Loiselle.
Effects of BDM, [Ca2+]o, and temperature on the dynamic stiffness of quiescent cardiac trabeculae from rat.
Studies of the passive mechanical properties of cardiac tissue have traditionally been conducted at subphysiological temperatures and various concentrations of extracellular Ca(2+) ([Ca(2+)](o)). More recently, the negative inotropic agent 2,3-butanedione monoxime (BDM) has been used. However, there remains a lack of data regarding the influence of temperature, Ca(2+), and BDM on the passive mechanical properties of cardiac tissue. We have used the dynamic stiffness technique, a sensitive measurement of cross-bridge activity, in which minute (approximately 0.2% of muscle length) sinusoidal perturbations are applied at various frequencies (0.2-100 Hz) to quiescent, viable right ventricular rat trabeculae at two temperatures (20 degrees C and 26 degrees C) and at two [Ca(2+)](o) (0.5 and 1.25 mM) in the presence and absence of BDM (20 mM). The stiffness spectra (amplitude and phase) were sensitive to temperature and [Ca(2+)](o) in the absence of BDM but insensitive in the presence of BDM. From the index of cross-bridge cycling (the ratio of high- to low-frequency stiffness amplitude), we infer that BDM inhibits a small degree of spontaneous sarcomere activity, thereby allowing the true passive properties of trabeculae to be determined. In the absence of BDM, the extent of spontaneous sarcomere activity decreases with increasing temperature. We caution that the measured mechanical properties of passive cardiac tissue are critically dependent on the experimental conditions under which they are measured. Experiments must be performed at sufficiently high temperatures (>25 degrees C) to ensure a low resting concentration of intracellular Ca(2+) or in the presence of an inhibitor of cross-bridge cycling.
Animals↗
Strain softening behaviour in nonviable rat right-ventricular trabeculae, in the presence and the absence of butanedione monoxime.
Strain softening is commonly reported during mechanical testing of passive whole hearts. It is typically manifested as a stiffer force-extension relationship in the first deformation cycle relative to subsequent cycles and is distinguished from viscoelasticity by a lack of recovery of stiffness, even after several hours of rest. The cause of this behaviour is presently unknown. In order to investigate its origins, we have subjected trabeculae to physiologically realistic extensions (5-15% of muscle length at 26 degrees C and 0.5 mm Ca(2+)), while measuring passive force and dynamic stiffness. While we did not observe strain softening in viable trabeculae, we found that it was readily apparent in nonviable (electrically inexcitable) trabeculae undergoing the same extensions. This result was obtained in both the presence and absence of 2,3-butanedione monoxime (BDM). Furthermore, BDM had no effect on the passive compliance of viable specimens, while its presence partly inhibited, but could not prevent, stiffening of nonviable specimens. Loss of viability was accompanied by a uniform increase of dynamic stiffness over all frequencies examined (0.2-100 Hz). The presence of strain softening during length extensions of nonviable tissue resulted in a comparable uniform decrease of dynamic stiffness. It is therefore concluded that strain softening is neither intrinsic to viable rat right ventricular trabeculae nor influenced by BDM but, rather, reflects irreversible damage of tissue in partial, or full, rigor.
Animals↗
Strain softening is not present during axial extensions of rat intact right ventricular trabeculae in the presence or absence of 2,3-butanedione monoxime.
Recent studies of passive myocardial mechanics have shown that strain softening behavior is present during both inflation of isolated whole rat hearts and shearing of tissue blocks taken from the left ventricular free wall in pigs. Strain softening is typically manifested by a stiffer force-extension relation in the first deformation cycle relative to subsequent cycles and is distinguished from viscoelasticity by a lack of recovery of stiffness, even after several hours of rest. The causes of this behaviour are unknown. We investigated whether strain softening is observed in uniaxial extensions of intact, viable, rat right ventricular (RV) cardiac trabeculae. Stretch and release cycles of 5%, 10%, and 15% muscle length were applied at a constant velocity at 26 degrees C. Muscles were tested in random order in the presence and absence of 50 mM 2,3-butanedione monoxime (BDM). Whereas strain softening was displayed by nonviable trabeculae, it was not observed in viable preparations undergoing physiologically relevant extensions whether in the presence or absence of BDM. BDM also had no effect on passive compliance. There was a reversible increase of muscle compliance between the first and subsequent cycles, with recovery after 30 s of rest, independent of the presence of BDM. We conclude that strain softening is neither intrinsic to viable rat RV trabeculae nor influenced by BDM and that passive trabeculae compliance is not altered by the addition of BDM.
Animals↗
Restoration of osmotically inhibited twitch force in rat cardiac trabeculae: role of Na+-H+ exchange.
1. When rat cardiac muscle is subjected to an increase of osmolality, its peak twitch force is immediately inhibited. Subsequently, over a period of several minutes, twitch force undergoes restoration, the extent of which is determined by the osmolality. The aim of the present study was to determine the factors that contribute to this restorative phenomenon. 2. Trabeculae were isolated from the right ventricles of rat hearts and mounted in an organ bath at 37 degrees C. The osmolality of the bathing solution was increased by 100 mOsmol (to 400 mOsmol) by the addition of various proportions of NaCl and sucrose while recording twitch force production. The role of Na+-H+ exchange in restoring twitch force was examined by use of the specific inhibitor cariporide (HOE 642). The role of Na+-Ca2+ exchange was examined by reducing [Ca2+]o (from 2 mmol/L to 0.5 mmol/L) or by substituting LiCl for NaCl. 3. Cariporide (25 micro mol/L) completely abolished twitch force restoration, thereby implicating a central role for the Na+-H+ exchanger. At constant [Na+]o, the extent of restoration was [Ca2+]o dependent, suggesting an independent contribution by the Na+-Ca2+ exchanger. This suggestion was supported by the finding that Li+, which substitutes for Na+ on the Na+-H+ exchanger, but not on the Na+-Ca2+ exchanger, also reduced the extent of restoration of hyperosmotically inhibited twitch force. 4. We conclude that the immediate inhibition of peak twitch force of rat cardiac muscle by hyperosmotic solutions reflects, in part, elevation of [H+]i, subsequent to reduction of cell volume. Hyperosmotic activation of Na+-H+ exchange then progressively relieves the inhibitory effect of protons on force development. The accompanying increase in [Na+]i in turn enhances Ca2+ influx on the Na+-Ca2+ exchanger, with the result that twitch force undergoes further restoration.
Animals↗
Metabolic consequences of a species difference in Gibbs free energy of Na+/Ca2+ exchange: rat versus guinea pig.
The Gibbs free energy of the sarcolemmal Na+/Ca2+ exchanger (DeltaG(Na/Ca)) determines its net Ca2+ flux. We tested the hypothesis that a difference of diastolic DeltaG(Na/Ca) exists between rat and guinea pig myocardium. We measured the suprabasal rate of oxygen consumption (VO2) of arrested Langendorff-perfused hearts of both species, manipulating DeltaG(Na/Ca) by reduction of extracellular Na+ concentration, [Na+](o). Hill equations fitted to the resulting VO2-[Na+](o) relationships yielded Michaelis constant (K(m)) values of 67 and 25 mM for rat and guinea pig, respectively. We developed and tested a simple thermodynamic model that attributes this difference of K(m) values to a 7.84 kJ/mol difference of DeltaG(Na/Ca). The model predicts that reversal of Na+/Ca2+ exchange, leading to diastolic Ca2+ influx, should occur at a value of [Na+](o) about three times higher in rat myocardium. We verified this quantitative prediction using fura 2 fluorescence to index intracellular Ca2+ concentration in isolated ventricular trabeculae at 37 degrees C. The postulated difference in free energy of Na+/Ca2+ exchange explains a number of reported disparities of Ca2+ handling at rest between rat and guinea pig myocardia.
Animals↗
Cardiac basal metabolism.
We endeavor to show that the metabolism of the nonbeating heart can vary over an extreme range: from values approximating those measured in the beating heart to values of only a small fraction of normal--perhaps mimicking the situation of nonflow arrest during cardiac bypass surgery. We discuss some of the technical issues that make it difficult to establish the magnitude of basal metabolism in vivo. We consider some of the likely contributors to its magnitude and point out that the biochemical reasons for a sizable fraction of the heart's basal ATP usage remain unresolved. We consider many of the physiological factors that can alter the basal metabolic rate, stressing the importance of substrate supply. We point out that the protective effect of hypothermia may be less than is commonly assumed in the literature and suggest that hypoxia and ischemia may be able to regulate basal metabolic rate, thus making an important contribution to the phenomenon of cardiac hibernation.
Adenosine Triphosphate↗
3-Dimensional configuration of perimysial collagen fibres in rat cardiac muscle at resting and extended sarcomere lengths.
1. We have used fluorescence confocal laser scanning microscopy to attain the three-dimensional (3-D) microstructure of perimysial collagen fibres over the range of sarcomere lengths (1.9-2.3 micrometers) in which passive force of cardiac muscle increases steeply. 2. A uniaxial muscle preparation (right ventricular trabecula of rat) was used so that the 3-D collagen configuration could be readily related to sarcomere length. Transmission electron microscopy showed that these preparations were structurally homologous to ventricular wall muscle. 3. Trabeculae were mounted on the stage of an inverted microscope and fixed at various sarcomere lengths. After a trabecula was stained with the fluorophore Sirius Red F3BA and embedded in resin, sequential optical sectioning enabled 3-D reconstruction of its perimysial collagen fibres. The area fraction of these fibres, determined from the cross-sections of seven trabeculae, was 10.5 +/- 3.9 % (means +/- s.d.). 4. The reconstructed 3-D images show that perimysial collagen fibres are wavy (as distinct from coiled) cords which straighten considerably as the sarcomere length is increased from 1.85 +/- 0.06 micrometer (near-resting length) to 2.3 +/- 0.04 micrometer (means +/- s.d., n = 4). These observations are consistent with the notion that the straightening of these fibres is responsible for limiting extension of the cardiac sarcomere to a length of approximately 2.3 micrometers.
Analysis of Variance↗
Mechanisms of force inhibition by halothane and isoflurane in intact rat cardiac muscle.
1. We investigated the mechanisms underlying the negative inotropic effect of the volatile anaesthetics halothane and isoflurane using twenty-two intact, right ventricular trabeculae of rat. [Ca2+]1 was measured qualitatively using either fluo-3 or fura-2, loaded into the cytosol via the acetoxymethyl (AM) ester form. Diastolic sarcomere length was adjusted to 2.1-2.2 micrograms and experiments were performed at 21-23 degrees C. 2. Halothane (0.25-3%) and isoflurane (0.48-4%) produced dose-dependent decreases in the amplitudes of the intracellular Ca2+ transients and twitch force. When the fluorescent Ca2+ indicator signals were corrected for changes in autofluorescence, neither volatile anaesthetic significantly changed diastolic [Ca2+]. 3. The ability of halothane and isoflurane to induce Ca2+ release from the sarcoplasmic reticulum of quiescent trabeculae was examined. When the superfusate was Ca2+ ad Na+ free (thereby preventing Na(+)-Ca2+ exchange and Ca2+ influx), 2% halothane, but not 4% isoflurane, evoked a transient increase in [Ca2+]i. 4. Halothane and isoflurane produced reversible, dose-dependent changes in cellular autofluorescence, the pattern of which was consistent with an increase in concentration of the reduced forms of nicotinamide adenine nucleotides and flavoproteins. This observation supports the putative inhibitory action of volatile anaesthetics at the site of Complex I of the mitochondrial electron transport chain. 5. Addition of the fatty acid hexanoate, a substrate that can be metabolized in the face of Complex I inhibition, did not appreciably attenuate the anaesthetic-induced negative inotropy; however, it greatly diminished autofluorescence changes. 6. To determine whether direct actions of the volatile anaesthetics on the contractile system contributed to the negative inotropy, external [Ca2+] was varied to modulate the amplitude of the Ca2+ transient. In the presence of 2% halothane or 4% isoflurane, restoration of the peak Ca2+ transient to control levels did not restore peak force. Moreover, halothane (1%) and isoflurane (16%) each reduced maximal Ca2(+)-activated force (attained using ryanodine tetani and a high external [Ca2+]) by around 15%. 7. We conclude that the negative inotropic actions of halothane and isoflurane on intact cardiac muscle reflect both reduced availability of Ca2+ and decreased responsiveness of the contractile system to Ca2+. The inhibitory action of the volatile anaesthetics on mitochondrial function does not contribute significantly to the negative inotropy but may lead to changes in cellular autofluorescence and misinterpretation of fluorescent Ca2+ indicator signals.
Anesthetics, General↗
Role of extracellular [Ca2+] in fatigue of isolated mammalian skeletal muscle.
The possible role of altered extracellular Ca2+ concentration ([Ca2+]o) in skeletal muscle fatigue was tested on isolated slow-twitch soleus and fast-twitch extensor digitorum longus muscles of the mouse. The following findings were made. 1) A change from the control solution (1.3 mM [Ca2+]o) to 10 mM [Ca2+]o, or to nominally Ca2+-free solutions, had little effect on tetanic force in nonfatigued muscle. 2) Almost complete restoration of tetanic force was induced by 10 mM [Ca2+]o in severely K+-depressed muscle (extracellular K+ concentration of 10-12 mM). This effect was attributed to a 5-mV reversal of the K+-induced depolarization and subsequent restoration of ability to generate action potentials (inferred by using the twitch force-stimulation strength relationship). 3) Tetanic force depressed by lowered extracellular Na+ concentration (40 mM) was further reduced with 10 mM [Ca2+]o. 4) Tetanic force loss at elevated extracellular K+ concentration (8 mM) and lowered extracellular Na+ concentration (100 mM) was partially reversed with 10 mM [Ca2+]o or markedly exacerbated with low [Ca2+]o. 5) Fatigue induced by using repeated tetani in soleus was attenuated at 10 mM [Ca2+]o (due to increased resting and evoked forces) and exacerbated at low [Ca2+]o. These combined results suggest, first, that raised [Ca2+]o protects against fatigue rather than inducing it and, second, that a considerable depletion of [Ca2+]o in the transverse tubules may contribute to fatigue.
Action Potentials↗
Different effects of raised [K+]o on membrane potential and contraction in mouse fast- and slow-twitch muscle.
Increasing extracellular K+ concentration ([K+]o) from 4 to 7-14 mM reduced both tetanic force and resting membrane potential (Em) in isolated slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles of the mouse. The tetanic force-[K+]o relationships showed a greater force loss over 8-11 mM [K+]o in soleus than EDL, mainly because the Em was 2-3 mV less negative at each [K+]o in soleus. The tetanic force-resting Em relationships show that force was reduced in two phases: phase 1 (Em < -60 mV), a 20% force decline in which the relationships superimposed in soleus and EDL, and phase 2 (Em -60 to -55 mV), a marked force decline that was steeper in EDL than soleus. Additionally in phase 2, longer stimulation pulses restored tetanic force; the twitch force-stimulation strength relationship was shifted toward higher voltages; caffeine, a myoplasmic Ca2+ concentration elevator, increased maximum force; and twitch force fell abruptly. We suggest that 1) the K(+)-depressed force is due to reduced Ca2+ release resulting from an altered action potential profile (phase 1) and inexcitable fibers due to an increased action potential threshold (phase 2), and 2) K+ contributes to fatigue in both fast- and slow-twitch muscle when it causes depolarization to about -60 mV.
Animals↗
The effect of hyperosmolality on the rate of heat production of quiescent trabeculae isolated from the rat heart.
We have measured the rate of heat production of isolated, quiescent, right ventricular trabeculae of the rat under isosmotic and hyperosmotic conditions, using a microcalorimetric technique. In parallel experiments, we measured force production and intracellular calcium concentration ([Ca2+]i). The rate of resting heat production under isosmotic conditions (mean +/- SEM, n = 32) was 100 +/- 7 mW (g dry wt)-1; it increased sigmoidally with osmolality, reaching a peak that was about four times the isosmotic value at about twice normal osmotic pressure. The hyperosmotic thermal response was: (a) abolished by anoxia, (b) attenuated by procaine, (c) insensitive to verapamil, ouabain, and external calcium concentration, and (d) absent in chemically skinned trabeculae bathed in low-Ca2+ "relaxing solution." Active force production was inhibited at all osmolalities above isosmotic. Passive (tonic) force increased to, at most, 15% of the peak active force developed under isosmotic conditions while [Ca2+]i increased, at most, 30% above its isosmotic value. We infer that hyperosmotic stimulation of resting cardiac heat production reflects, in large part, greatly increased activity of the sarcoplasmic reticular Ca2+ ATPase in the face of increased efflux via a procaine-inhibitable Ca(2+)-release channel.
Animals↗
Oxygen exchange in the isolated, arrested guinea pig heart: theoretical and experimental observations.
A model of oxygen transport in perfused myocardial tissue is presented. Steady-state conditions are assumed in order to mimic the metabolic rate of the arrested heart. The model incorporates Michaelis-Menten dependence of mitochondrial oxygen consumption, oxymyoglobin saturation and oxyhemoglobin saturation on oxygen partial pressure (PO2). The transport equations model both the advective supply of oxygen via the coronary circulation and the diffusive exchange of oxygen between tissues and environment across the epicardial and endocardial surfaces. The left ventricle is approximated by an axisymmetric prolate spheroid and the transport equations solved numerically using finite element techniques. Solution yields the PO2 profile across the heart wall. Integration of this profile yields the simulated rate of metabolic oxygen uptake determined according to the Fick principle. Correction for the diffusive flux of oxygen across the surfaces yields the simulated true metabolic rate of oxygen consumption. Simulated values of oxygen uptake are compared with those measured experimentally according to the Fick principle, using saline-perfused, Langendorff-circulated, K(+)-arrested, guinea pig hearts. Four perfusion variables were manipulated: arterial PO2, environmental PO2, coronary flow and perfusion pressure. In each case agreement between simulated and experimentally determined rates of oxygen consumption gives confidence that the model adequately describes the advective and diffusive transport of oxygen in the isolated, arrested, saline-perfused heart.
Animals↗
Effect of hyperosmotic perfusion on rate of oxygen consumption of isolated guinea pig and rat hearts during cardioplegia.
OBJECTIVE: The aim was to determine the metabolic consequence of increasing the osmolality of a crystalloid cardioplegic solution during periods of cardiac arrest. METHODS: Isolated hearts of guinea pig and rat were Langendorff perfused with Krebs-Henseleit solution at 37 degrees C and arrested by an increase in KCl. The rate of oxygen consumption was measured under standard isosmotic conditions and with the osmolality of the perfusate increased by addition of sucrose. RESULTS: Increased osmolality stimulated the rate of myocardial oxygen consumption in a dose dependent manner. At optimal dose (about twice normal osmolality), the oxygen consumption of the arrested heart approximated that of the beating, non-working heart measured prior to arrest. Potentiation of cardiac resting metabolism was greater in the rat than in the guinea pig, whether expressed in absolute terms or relative to the metabolism of the beating heart. Metabolic potentiation was accompanied by an increase of passive or diastolic left ventricular pressure in the rat but not in the guinea pig. The metabolic response was unaffected by coronary vasodilation (adenosine) and by inhibition of Ca2+ channels (verapamil); it was moderately diminished by perfusion with Ca(2+)-free solution. Procaine inhibited the hyperosomotic potentiation of oxygen consumption in a dose dependent manner. CONCLUSIONS: From the absence of passive force development in the guinea pig heart, it appears that the hyperosmotic stimulation of cardiac resting metabolism primarily reflects increased activity of the sarcoplasmic reticular Ca(2+)-ATPase subsequent to release of Ca2+ through a procaine inhibitable channel. Blunting of both the metabolic and mechanical responses in the guinea pig vis-a-vis the rat heart is attributed to the greater capability of the former to buffer myoplasmic Ca2+ via the energetically neutral Na(+)-Ca2+ exchange mechanism.
Adenosine↗
Energetic effects of caffeine in face of retarded Na+/Ca2+ exchange in isolated, arrested guinea pig hearts.
The effects of caffeine, a widely used pharmacological tool for releasing Ca2+ from the sarcoplasmic reticulum (SR), on the resting rate of oxygen consumption and left ventricular diastolic pressure development of isolated, KCl-arrested, guinea pig hearts was examined. Caffeine (10 mmol/l) had no effect on either the rate of oxygen consumption or left ventricular pressure development. However, when Ca2+ extrusion via the Na+/Ca2+ exchanger was retarded, whether by reducing the external Na+ concentration ([Na+]o) from 143 to 57 mmol/l or through further depolarizing the membrane by increasing external K+ concentration ([K+]o) from 20 to 40 mmol/l, the subsequent introduction of caffeine evoked a pronounced increase in the rate of oxygen consumption. This was accompanied by a small contracture in the low [Na+]o condition only. In the absence of external Ca2+ the stimulatory effects of caffeine on cardiac energetics in either the low [Na+]o or 40 mmol/l [K+]o condition was totally prevented. It is concluded that Na+/Ca2+ exchange plays a major role in dictating the energetic response of the cardiac cell to pharmacological activation of the SR Ca2+ release channel by caffeine.
Analysis of Variance↗
Calculation of oxygen diffusion across the surface of isolated perfused hearts.
Although exact mathematical descriptions of oxygen diffusion into unperfused isolated organs are known, no analytic solution is available for perfused organs. Here, we derive an equation for oxygen diffusion across the epicardial surface of perfused hearts. Our oxygen transport model incorporates oxygen delivery from the capillary perfusate, oxygen consumption, diffusion among adjacent capillary exchange regions, and diffusion across the epicardial surface. The wall of the heart is modeled by a slab of tissue. The derived equation fits experimental data on the diffusive oxygen flux across the surface of isolated saline-perfused guinea pig hearts, obtained by measuring oxygen uptake from the perfusate at various oxygen tensions in the surroundings of the isolated heart. The model predicts that in isolated arrested guinea pig hearts suspended in air, 25% of the oxygen taken up from the saline perfusate (PO2 approximately 680 mmHg) diffuses out of the heart across the surface. In beating isolated guinea pig hearts, 5% of the oxygen taken up from the perfusate diffuses across the surface.
Animals↗
Dependence of muscle fatigue on stimulation protocol: effect of hypocaloric diet.
This study tested the hypothesis that the decreased resistance of skeletal muscles to fatigue after a period of inadequate nutrition is a consequence of the type of fatigue test employed. Rats (n = 8) were fed hypocalorically for 10 days, resulting in a 33.5 +/- 3.0% (SE) reduction in body weight. The fatigue resistance of muscles was assessed with use of isolated soleus muscles at a temperature of 25 degrees C. Fatigue resistance was determined from the decline in isometric force during a series of tetanic contractions. Two contrasting contraction protocols were devised to test the susceptibility of muscles to fatigue. One protocol was designed to require high rates of energy supply and hence force a dependence on glycolytic energy supply pathways. The second protocol required much lower rates of energy supply that could be entirely met by oxidative energy supply processes. During the high energy demand (glycolytic) protocol, the force produced by muscles from a control group of rats decreased to 78.7 +/- 0.8% (n = 12 muscles) of the initial value. Hypocaloric feeding significantly reduced the resistance of muscles to fatigue: force declined to 61.3 +/- 3.5% (n = 16 muscles) during the contraction protocol. In contrast, when fatigue was quantified using the low energy demand (oxidative) contraction protocol, the effect of nutritional status on fatigue resistance was entirely abolished. This result supports the hypothesis that resistance to fatigue is critically dependent on the fatigue test employed.
Animals↗
Exchange of oxygen across the epicardial surface distorts estimates of myocardial oxygen consumption.
The rate of oxygen consumption of isolated, Langendorff-circulated, saline-perfused hearts of guinea pigs, rats, and rabbits was measured using the classical Fick Principle method. The heart was suspended in a glass chamber the oxygen partial pressure, PO2, of which could be varied. The measured rate of oxygen consumption was found to vary inversely with the ambient (heart chamber) PO2. This result prevailed whether the chamber was filled with air, saline, or oil, and whether the pericardium was present or the heart was wrapped in Saran. The effect varied inversely with heart size both within and across species. It is concluded that the epicardial surface is permeable to oxygen which will diffuse either into or out of the heart as the PO2 gradient dictates. In either case the classically measured rate of oxygen consumption will be in error. The error can be large in studies of cardiac basal metabolism. A simple model is developed to describe the observed rate of oxygen consumption as classically measured. The measured rate is partitioned into two components: the true rate of oxygen consumption of the heart, and the rate of loss of oxygen by diffusive exchange across the epicardial surface. The latter component is proportional to the gradient of oxygen partial pressure from myocardium to environment and to the diffusive oxygen conductance of myocardial tissue. Application of the model allows the true rate of oxygen consumption of the heart to be recovered from measured values which may be considerably in error.
Animals↗