Sarcolemma, sarcoplasmic reticulum, and sarcomeres as limiting factors in force production in rat heart

Impaired myosin isoform shift and calcium transients contribute to cellular pathogenesis of rat cirrhotic cardiomyopathy

BACKGROUND & AIMS

Cirrhotic cardiomyopathy is a recently recognized entity, but detailed cellular and molecular mechanisms remain unclarified. We aimed to elucidate the role of myosin heavy chain isoform shifts and their relation to calcium transients in the contractile kinetics of cirrhotic rats.

METHODS

Cirrhosis was induced in male Lewis Brown-Norway rats by bile duct ligation (BDL). Myosin heavy chain (MHC) isoform distribution was evaluated by gel electrophoresis. Contractile force, Ca²⁺ transients and cell shortening were studied at varied frequency and extracellular [Ca²⁺ ]. T-tubular integrity was analysed by power spectrum analysis of images of myocytes stained with di-8-ANEPPS.

RESULTS

Compared with sham controls, the phenotypes of cirrhotic rats were as follows: (a) alpha-myosin heavy chain shifted to beta-MHC isoform; (b) mild loss of T-tubular integrity in myocytes; (c) a reduced maximum and rate of rise of the Ca²⁺ transient (max F/F_o ); (d) a reduction in both the rate of rise and fall of contraction; (e) decreased maximal force-generating capacity; (f) loss of the inotropic effect of increased stimulus frequency; (g) unchanged sensitivity of force development to varied extracellular [Ca²⁺ ] and (h) increased spontaneous diastolic sarcomere length fluctuations.

CONCLUSION

Cardiomyocytes and ventricular trabeculae in a cirrhotic rat model showed features of typical heart failure including systolic and diastolic prolongation, impaired force-frequency relation and decreased force-generating capacity. Impaired myosin isoform shift and calcium transients are important contributory mechanisms underlying the pathogenesis of the heart failure phenotype seen in cirrhosis.

Cardiac Disorders and Pathophysiology of Sarcomeric Proteins

The sarcomeric proteins represent the structural building blocks of heart muscle, which are essential for contraction and relaxation. During recent years, it has become evident that posttranslational modifications of sarcomeric proteins, in particular phosphorylation, tune cardiac pump function at rest and during exercise. This delicate, orchestrated interaction is also influenced by mutations, predominantly in sarcomeric proteins, which cause hypertrophic or dilated cardiomyopathy. In this review, we follow a bottom-up approach starting from a description of the basic components of cardiac muscle at the molecular level up to the various forms of cardiac disorders at the organ level. An overview is given of sarcomere changes in acquired and inherited forms of cardiac disease and the underlying disease mechanisms with particular reference to human tissue. A distinction will be made between the primary defect and maladaptive/adaptive secondary changes. Techniques used to unravel functional consequences of disease-induced protein changes are described, and an overview of current and future treatments targeted at sarcomeric proteins is given. The current evidence presented suggests that sarcomeres not only form the basis of cardiac muscle function but also represent a therapeutic target to combat cardiac disease.

The force and stiffness of myosin motors in the isometric twitch of a cardiac trabecula and the effect of the extracellular calcium concentration

KEY POINTS

Fast sarcomere-level mechanics in intact trabeculae, which allows the definition of the mechano-kinetic properties of cardiac myosin in situ, is a fundamental tool not only for understanding the molecular mechanisms of heart performance and regulation, but also for investigating the mechanisms of the cardiomyopathy-causing mutations in the myosin and testing small molecules for therapeutic interventions. The approach has been applied to measure the stiffness and force of the myosin motor and the fraction of motors attached during isometric twitches of electrically paced trabeculae under different extracellular Ca2+ concentrations. Although the average force of the cardiac myosin motor (∼6 pN) is similar to that of the fast myosin isoform of skeletal muscle, the stiffness (1.07 pN nm-1 ) is 2- to 3-fold smaller. The increase in the twitch force developed in the presence of larger extracellular Ca2+ concentrations is fully accounted for by a proportional increase in the number of attached motors.

ABSTRACT

The mechano-kinetic properties of the cardiac myosin were studied in situ, in trabeculae dissected from the right ventricle of the rat heart, by measuring the stiffness of the half-sarcomere both at the twitch force peak (Tp ) of an electrically paced intact trabecula at different extracellular Ca2+ concentrations ([Ca2+ ]o ), and in the same trabecula after skinning and induction of rigor. Taking into account the contribution of filament compliance to half-sarcomere compliance and the lattice geometry, we found that the stiffness of the cardiac myosin motor is 1.07 ± 0.09 pN nm-1 , which is slightly larger than that of the slow myosin isoform of skeletal muscle (0.6-0.8 pN nm-1 ) and 2- to 3-fold smaller than that of the fast skeletal muscle isoform. The increase in Tp from 61 ± 4 kPa to 93 ± 9 kPa, induced by raising [Ca2+ ]o from 1 to 2.5 mm at sarcomere length ∼2.2 μm, is accompanied by an increase of the half-sarcomere stiffness that is explained by an increase of the fraction of actin-attached motors from 0.08 ± 0.01 to 0.12 ± 0.02, proportional to Tp . Consequently, each myosin motor bears an average force of 6.14 ± 0.52 pN independently of Tp and [Ca2+ ]o . The application of fast sarcomere-level mechanics to intact trabeculae to define the mechano-kinetic properties of the cardiac myosin in situ represents a powerful tool for investigating cardiomyopathy-causing mutations in the myosin motor and testing specific therapeutic interventions.

Myosin filament activation in the heart is tuned to the mechanical task

The mammalian heart pumps blood through the vessels, maintaining the dynamic equilibrium in a circulatory system driven by two pumps in series. This vital function is based on the fine-tuning of cardiac performance by the Frank-Starling mechanism that relates the pressure exerted by the contracting ventricle (end systolic pressure) to its volume (end systolic volume). At the level of the sarcomere, the structural unit of the cardiac myocytes, the Frank-Starling mechanism consists of the increase in active force with the increase of sarcomere length (length-dependent activation). We combine sarcomere mechanics and micrometer-nanometer-scale X-ray diffraction from synchrotron light in intact ventricular trabeculae from the rat to measure the axial movement of the myosin motors during the diastole-systole cycle under sarcomere length control. We find that the number of myosin motors leaving the off, ATP hydrolysis-unavailable state characteristic of the diastole is adjusted to the sarcomere length-dependent systolic force. This mechanosensing-based regulation of the thick filament makes the energetic cost of the systole rapidly tuned to the mechanical task, revealing a prime aspect of the Frank-Starling mechanism. The regulation is putatively impaired by cardiomyopathy-causing mutations that affect the intramolecular and intermolecular interactions controlling the off state of the motors.

Hypokalaemia induces Ca²⁺ overload and Ca²⁺ waves in ventricular myocytes by reducing Na⁺,K⁺-ATPase α₂ activity

KEY POINTS

Hypokalaemia is a risk factor for development of ventricular arrhythmias. In rat ventricular myocytes, low extracellular K(+) (corresponding to clinical moderate hypokalaemia) increased Ca(2+) wave probability, Ca(2+) transient amplitude, sarcoplasmic reticulum (SR) Ca(2+) load and induced SR Ca(2+) leak. Low extracellular K(+) reduced Na(+),K(+)-ATPase (NKA) activity and hyperpolarized the resting membrane potential in ventricular myocytes. Both experimental data and modelling indicate that reduced NKA activity and subsequent Na(+) accumulation sensed by the Na(+), Ca(2+) exchanger (NCX) lead to increased Ca(2+) transient amplitude despite concomitant hyperpolarization of the resting membrane potential. Low extracellular K(+) induced Ca(2+) overload by lowering NKA α2 activity. Triggered ventricular arrhythmias in patients with hypokalaemia may therefore be attributed to reduced NCX forward mode activity linked to an effect on the NKA α2 isoform.

ABSTRACT

Hypokalaemia is a risk factor for development of ventricular arrhythmias. The aim of this study was to determine the cellular mechanisms leading to triggering of arrhythmias in ventricular myocytes exposed to low Ko. Low Ko, corresponding to moderate hypokalaemia, increased Ca(2+) transient amplitude, sarcoplasmic reticulum (SR) Ca(2+) load, SR Ca(2+) leak and Ca(2+) wave probability in field stimulated rat ventricular myocytes. The mechanisms leading to Ca(2+) overload were examined. Low Ko reduced Na(+),K(+)-ATPase (NKA) currents, increased cytosolic Na(+) concentration and increased the Na(+) level sensed by the Na(+), Ca(2+) exchanger (NCX). Low Ko also hyperpolarized the resting membrane potential (RMP) without significant alterations in action potential duration. Experiments in voltage clamped and field stimulated ventricular myocytes, along with mathematical modelling, suggested that low Ko increases the Ca(2+) transient amplitude by reducing NKA activity despite hyperpolarization of the RMP. Selective inhibition of the NKA α2 isoform by low dose ouabain abolished the ability of low Ko to reduce NKA currents, to increase Na(+) levels sensed by NCX and to increase the Ca(2+) transient amplitude. We conclude that low Ko, within the range of moderate hypokalaemia, increases Ca(2+) levels in ventricular myocytes by reducing the pumping rate of the NKA α2 isoform with subsequent Na(+) accumulation sensed by the NCX. These data highlight reduced NKA α2 -mediated control of NCX activity as a possible mechanism underlying triggered ventricular arrhythmias in patients with hypokalaemia.

Length-dependent deactivation of ventricular trabeculae in the bivalve, Spisula solidissima

Shortening-deactivation has been identified and characterized in ventricular trabeculae of the bivalve, Spisula solidissima (Heterodonta, Mactridae). This muscle had ultrastructural similarities to vertebrate smooth muscle. Deactivation was defined as the fraction of maximal force lost during a contraction when a muscle is shortened rapidly (by a quick-release, QR) to a known length, relative to a control isometric contraction at that same length. The magnitude of deactivation was dependent on the size of the release and the point at which the release was applied during the cycle of contraction. QR/quick-stretch (QS) perturbations at the same point during the contraction resulted in negligible deactivation. The magnitude of deactivation was independent of shortening rate. Deactivation was attenuated by applying caffeine (100 microM) and blocked with high extracellular Ca(2+) (56 mM). The Ca(2+) ionophore, A23187 (10 microM), augmented deactivation as did the positive inotrope serotonin (100 nM). Treatment with ryanodine (5 microM) had no significant effect on deactivation. These results suggest that a reduction in Ca(2+) at the contractile element and/or sequestration of Ca(2+) may occur during shortening. Deactivation may minimize the magnitude of work done during active shortening of bivalve cardiac muscle, particularly against the low afterload exhibited in the bivalve peripheral circulatory system. Intracellular Ca(2+) fluxes during sudden length perturbations may explain the effect of stretch on action potential duration in the bivalve heart, as shown previously.

Congestive Heart Failure after Myocardial Infarction in the Rat: Cardiac Force and Spontaneous Sarcomere Activity

The causes of reduced cardiac force development in congestive heart failure (CHF) are still uncertain. We explored the subcellular mechanisms leading to decreased force development in trabeculae from rats with a myocardial infarction. We defined CHF according to clinical and pathological criteria and compared properties of trabeculae from animals with CHF (cMI) to those of animals with a myocardial scar but without evidence of CHF (uMI), and sham-operated animals. The new findings of this study on properties of cMI trabeculae are that (1) maximal twitch force following post-extrasystolic potentiation is unchanged; (2) the sensitivity of cMI trabeculae to [Ca(2+)](o) is increased; (3) spontaneous diastolic sarcomere length (SL) fluctuations (SA) are increased in cMI at all levels of SR Ca(2+) loading; and (4) SA is accompanied by a proportional reduction of F(max). The results suggest that the probability of spontaneous diastolic opening of SR Ca(2+) channels is increased in CHF. These data provide the basis for a novel mechanism underlying systolic and diastolic dysfunction as well as arrhythmias in hearts in CHF. If SA proves to be a component of myocardial dysfunction in human CHF, our thinking about therapy of the patient with CHF may be profoundly changed.

Contractile reserve but not tension is reduced in monocrotaline-induced right ventricular hypertrophy

The objective of this study was to evaluate the role of right ventricular hypertrophy on developed tension (Fdev) and contractile reserve of rat papillary muscle by using a model of monocrotaline (Mct)-induced pulmonary hypertension. Calcium handling and the influence of bicarbonate ([Formula: see text]) were also addressed with the use of two different buffers ([Formula: see text] and HEPES). Wistar rats were injected with either Mct (40 mg/kg sc) or vehicle control (Con). Isometrically contracting right ventricular papillary muscles were studied at 80% of the length of maximal developed force. Contractile reserve (1 – Fdev/Fmax) was calculated from Fdev and maximal tension (Fmax). Calcium recirculation was determined with postextrasystolic potentiation. Both groups of muscles were superfused with either [Formula: see text] (Con-B and Mct-B, both n = 6) or HEPES (Con-H and Mct-H, both n = 6) buffer. With hypertrophy, contractions were slower but Fdev was not changed. However, Fmax was decreased ( P < 0.05). With [Formula: see text], Fmax decreased from 23.8 ± 6.5 mN·mm–2 in Con-B, to 13.7 ± 3.3 mN·mm–2 in Mct-B. With HEPES, it decreased from 16.3 ± 3.5 mN·mm–2 ( n = 6, Con-H) to 8.3 ± 1.6 mN·mm–2 (Mct-H). Contractile reserve during hypertrophy was therefore also decreased ( P < 0.05). With [Formula: see text], it decreased from 0.73 ± 0.03 (Con-B) to 0.55 ± 0.04 (Mct-B). With HEPES, it decreased ( P < 0.001) from 0.64 ± 0.07 (Con-H) to 0.19 ± 0.06 (Mct-H). The recirculation fraction decreased ( P < 0.05) from 0.59 ± 0.04 in Con-B to 0.44 ± 0.04 in Mct-B. We conclude that contractile reserve and recirculation fraction are impaired during hypertrophy, with a stronger effect under HEPES than [Formula: see text] superfusion.

Compression of interventricular septum during right ventricular pressure loading

The interventricular septum, which flattens and inverts in conditions such as pulmonary hypertension, is considered by many to be an unstressed membrane, in that its position is assumed to be determined solely by the transseptal pressure gradient. A two-dimensional finite element model was developed to investigate whether compression and bending moments (behavior incompatible with a membrane) exist in the septum during diastole under abnormal loading, i.e., pulmonary artery (PA) constriction. Hemodynamic and echocardiographic data were obtained in six open-chest anesthetized dogs. For both control and PA constriction, the measured left ventricular and right ventricular pressures were applied to a residually stressed mesh. Adjustments were made to the stiffness and end-bending moments until the deformed and loaded residually stressed mesh matched the observed configuration of the septum. During PA constriction, end-bending moments were required to obtain satisfactory matches but not during control. Furthermore, substantial circumferential compressive stresses developed during PA constriction. Such stresses might impede septal blood flow and provoke the unexplained ischemia observed in some conditions characterized by abnormal septal motion.

Effect of misoprostol on myocardial contractility in rats treated with cyclosporin A

The nephrotoxic side effects of the immunosuppressant cyclosporin A in animals and humans are well known. Misoprostol, a prostaglandin E analog, is used clinically in organ-transplant recipients taking cyclosporin A to protect against these side effects. We reported previously that long-term treatment of rats with cyclosporin A causes a diminution in myocardial peak contractile stress. There is an associated spontaneous sarcomere activity and rest depression of force in the absence of a change in myofilaments sensitivity to intracellular Ca2+. Here we investigated the potential protective effects of misoprostol on the myocardium of cyclosporin A-treated rats. Rats were treated with either cyclosporin A, misoprostol, or their combination. Force-[Ca2+]o and -[Sr2+]o, and force-interval relations as well as the sarcomere length were studied in trabeculae isolated from the right ventricles. At suboptimal [Ca2+]o, cyclosporin A shifted the force-[Ca2+]o relation to the left but reduced peak contractile stress by approximately 35% at the highest (optimal) [Ca2+]o. Co-treatment with misoprostol prevented the leftward shift, and treatment with misoprostol alone did not cause a leftward shift. The diminution of peak stress, however, did not recover with misoprostol treatment, and stress was further reduced. Treatment with only misoprostol also reduced stress generated by the muscles more than that by cyclosporin A alone. Intriguingly, activation of the myofilaments by Sr2+ failed to recover peak stress to control levels in any group treated with misoprostol. Unlike cyclosporin A, however, rest potentiation of force was more pronounced, and spontaneous sarcomere activity was absent with misoprostol. No histopathologic changes were observed with cyclosporin A or misoprostol treatment. Misoprostol modifies the cyclosporin A-induced changes in the Ca2+ handling, but further decreases the stress generated by the muscles.

Changes in calcium handling in atrophic heterotopically isotransplanted rat hearts

Atrophy of the rat heart induced by hemodynamic unloading after heterotopic transplantation is associated with impaired relaxation while systolic function remains normal when compared to the heart of the recipient animal. To identify possible underlying mechanisms for the above, we studied some aspects of membrane calcium handling using postextrasystolic potentiation of contractions in the isolated right ventricular papillary muscle and in the left ventricle of the Langendorff-perfused heart. We also compared the alterations of the unloaded heart with those of overloaded hypertrophic hearts of rats with suprarenal aortic constriction. In the atrophic heart the degree of potentiation after one extrasystole, considered to be proportional to the trans-sarcolemmal influx of Ca2+ during an action potential, was increased by 125% when compared with recipient hearts. The rate of decay of potentiation which reflects the fraction of activator Ca2+ recirculating in the cells via the sarcoplasmic reticulum, negatively correlated with the degree of potentiation, although its mean value was not significantly altered. In hypertrophic hearts the decay of potentiation was faster when compared with the hearts of sham-operated animals, indicating a decreased recirculating fraction of Ca2+ The data suggest that the relative importance of trans-sarcolemmal Ca2+ fluxes is increased both in cardiac atrophy and hypertrophy; but their quantitative role in the control of cardiac contraction might differ.

Intracellular calcium transients in potentiated contractions induced by multiple extrasystolic beats in isolated perfused rat hearts

Mechanisms underlying contractile potentiation induced by multiple extrasystolic contractions (ESC) were evaluated with surface fluorometry in isolated perfused rat hearts loaded with Indo-1/AM. After baseline pacing with a 400 ms interval, 1-25 ESC were interposed with a regular 160 ms interval followed by the postextrasystolic beat with a 400 ms interval. With an increase in the ESC number, left ventricular developed pressure and peak positive dP/dt increased in an exponential manner, reaching a plateau, that was the same for 3 extracellular Ca2+ ([Ca2+]o; 0.55 (n = 9), 1.25 (n = 11) and 2.75 mM (n = 7). Increased [Ca2+]o shifted this relationship left and upward, and with 2.75 mM [Ca2+]o developed pressure and dP/dt decreased after the maximum potentiation was obtained. The relationship between the ESC number and the amplitude of the Indo-1 fluorescence (F400/F510; an index of intracellular Ca2+ ([Ca2+]i)) was also exponential and was shifted left and upward by high [Ca2+]o; however, it lacked the declining phase. Thus, the relationship between the amplitude of F400/F510 and developed pressure or dP/dt consisted of a positively linear part until the maximum potentiation was obtained and a negatively linear part with a further increase in the amplitude of F400/F510. This observation suggests that although contractile potentiation is mediated by increased [Ca2+]i transients, the maximum response might be determined by the responsiveness of the sarcomere.

Ca2+ channel antagonists enhance tension in skinned skeletal and heart muscle fibres

Striated muscle fibres, both skeletal and cardiac of different species including human, skinned by freeze-drying, were activated in solutions strongly buffered for Ca2+. The single fibres were immersed in solutions with different [Ca2+]. Sarcomere length was set and controlled by laser diffraction. Fibre type was determined by Sr2+ activation. The relation between the negative logarithm of the Ca2+ concentration and the normalized tension, the Ca2+ sensitivity curve, was investigated. The effect on the contractile machinery of three different Ca2+ channel antagonists (verapamil, diltiazem and nifedipine) in a therapeutic concentration (10(-6) M) was investigated. The possible effects on the Ca2+ sensitivity curve were quantified by: (1) the change in maximal tension developed at pCa2+ = 4.4; (2) the change in pCa2+ value at which 50% of the tension induced at pCa2+ = 4.4; (3) the steepness of the Ca2+ sensitivity curve in this point. The three drugs tested, at a therapeutic concentration of 1 microM, all enhanced maximal induced tension by respectively 25, 20 and 7%. The sarcomere length dependency of the effect proved to be dependent upon the drug, but also slightly on fibre type (skeletal or cardiac), or on species. It is concluded that the drug influences the cooperativity of the two different types of binding sites on troponin-C (low- and high-affinity sites). Tension enhancement was due to increased stiffness of the actin-myosin interaction site.

Determinants of velocity of sarcomere shortening in mammalian myocardium

Maximal unloaded velocity of shortening of cardiac muscle (Vo) depends on the level of activation of the contractile filaments. We have tested the hypothesis that this dependence may be caused by viscous resistance of the muscle to length changes. Twitch force (Fo) and sarcomere shortening were studied in trabeculae dissected from the right ventricle of rat myocardium, superfused with modified Krebs-Henseleit solution at 25 degrees C. Sarcomere length (SL) was measured by laser diffraction techniques; force was measured by a silicon strain gauge; velocity of sarcomere shortening was measured using the "isovelocity release" technique. Vo and Fo at slack SL were a sigmoidal function of [Ca2+]o, but Vo was more sensitive to [Ca2+]o (Km: 0.44 +/- 0.04 mM) than isometric twitch force (Km: 0.68 +/- 0.03 mM). At [Ca2+]o = 1.5 mM, Vo was independent of SL above 1.9 microns, but depended on SL at lower [Ca2+]o and always depended on SL < 1.9 microns. A constant relation was observed between Vo and Fo, irrespective whether Fo was altered by variation of [Ca2+]o or SL above slack length. Visco-elastic properties of unstimulated muscles were studied at SL = 2.0 microns by small linear length changes at varied velocities up to 40 microns/s. The force response to stretch, after correction for the contribution of static parallel elasticity, consisted of an exponential increase of force (tau = 4 ms) and an exponential decline after the stretch. This response would be expected from an arrangement of a viscous element in series with an elastic element. Viscous force increased in proportion to stretch velocity by 0.2-0.5% of Fo/micron/s up to 15 microns/s, while the calculated stiffness of the elastic component was 25-45 N.mm-3, suggesting that the most likely structural candidate for this visco-elastic element is titin. Dynamic stiffness at 500 Hz was proportional to instantaneous force during shortening and was 12% of stiffness at maximal twitch force when shortening occurred at Vo. This suggests that the number of active force generators, even at maximal activation, is strongly reduced during shortening at Vo. The observed relation between Vo and Fo could be explained by a model in which shortening velocity of the cardiac sarcomere depends on the level of activation and hence on the number of cross bridges supporting the viscous load.

An internal viscous element limits unloaded velocity of sarcomere shortening in rat myocardium

1. Peak twitch force (F0) and sarcomere length (SL) were measured in trabeculae that had been dissected from the right ventricle of rat heart and that were superfused with a modified Krebs-Henseleit solution at 25 degrees C. Sarcomere length was measured by laser diffraction techniques. Force was measured with a silicone strain gauge. Unloaded velocity of sarcomere shortening (V0) was measured by the 'isovelocity release' technique. 2. At [Ca2+]o = 1.5 mM and SL below 1.9 microns, V0 increased in proportion to SL, while V0 was independent of SL above 1.9 microns. At [Ca2+]o = 0.5 mM, V0 was proportional to SL up to 2.2 microns. At [Ca2+]o = 0.2 mM, V0 was proportional to SL up to 2.3 microns which is the longest SL that we were able to study in our trabeculae. 3. A unique relationship was observed between V0 and F0, irrespective of whether F0 was altered by variation of [Ca2+]o or sarcomere length above slack length. 4. Passive viscosity (Fv) was measured during the pause between contractions in the presence of 1.5 mM [Ca2+bdo and in the range SL = 2.0-2.1 microns by applying 0.1 micron stretches at various velocities up to v = 30 microns s-1. The force response to stretch, corrected for the contribution of parallel elastic force, showed viscoelastic characteristics with an exponential increase to a maximum (Fv) during stretch and an exponential decline after the end of the stretch. Fv increased, by 0.3%F0 microns-1 s-1, in proportion to v < 5 microns s-1; the increase of Fv was smaller at higher v, suggesting non-Newtonian viscous properties. 5. The time constant of the increase of force during the stretch decreased (tau rise congruent to 7 ms to tau rise congruent to 4 ms) with increases in v (congruent to 4 microns s-1 to v congruent to 10 microns s-1; P = 0.02). The time constant of decay of force at the end of the stretch also decreased with increases in v (tau decay congruent to 8 ms at v congruent to 4 microns s-1 to tau decay congruent to 3 ms at v congruent to 30 microns s-1; P < 0.001). Calculated stiffness of the elastic term of the viscoelastic element was independent of v, i.e. 45-50 N mm-3.(ABSTRACT TRUNCATED AT 400 WORDS)

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.