Range Of Sarcomere Lengths Research Articles

Length-Dependent Active Tension Development In Single Intact Cardiomyocytes, Isolated From Different Regions Of Guinea Pig Heart

Introduction: Information on the force-length relation of intact myocytes from different regions of the heart is scarce. We therefore studied myocytes, isolated from apical and basal areas of guinea pig left and right ventricles (cell numbers: LVA 22, LVB 29, RVA 11, RVB 12). Methods: Force-length relations were measured by attaching carbon fibers to myocytes, allowing application of diastolic stretch while measuring passive and active force.1 Cells were kept at 36±1°C and paced at 2 Hz. Recorded forces were normalized to cell cross-sectional area and used to construct end-diastolic, end-systolic and active tension-length relations (EDTL, ESTL and ATL=ESTL-EDTL; respectively). In addition, the ratio of the slopes of ESTL and EDTL was used as a cross-section independent factor to characterise the Frank-Starling Gain (FSG) in individual cells. Results: For all tissue regions ESTL, EDTL and, hence, ATL, are linear over the range of end-diastolic sarcomere lengths studied (1.88-2.15 μm). Plotting the slope values of ATL vs. EDTL for all cells shows a positive correlation (slope 1.29, R2=0.24, 74 cells). In addition, FSG is larger than one for all cells studied: RVB 3.91±0.54), RVA (2.84±0.30), LVB (2.77±0.17), and LVA (3.20±0.24). Conclusions: Using the carbon fiber technique, it is possible to probe length-dependence of passive and active tension at the single cell level, without the interference of extracellular structures. The Frank-Starling Gain varies between the different regions of the heart and the positive correlation between ATL and EDTL confirms that in intact cells passive force bearing structures (such as titin) are likely to play a role in modulating length dependent activation.[1] Iribe et al, Force-length relations in isolated intact cardiomyocytes subjected to dynamic changes in mechanical load. AJP 2007/292:1487-1497.

Effects of Length Changes on Force Produced by Ca2+ and ADP-Activated Myofibrils along the Ascending Limb of the Force-Length Relation

Isometric forces produced by skeletal muscles are higher after stretch and smaller after shortening. A few studies investigating these length-dependent changes in force were conducted on the ascending limb of the force-length (FL) relation, showing conflicting results with elusive mechanisms. The purposes of this study were: (i) to evaluate the effects of muscle stretching and shortening on forces along the ascending limb of the FL relation, (ii) to evaluate if sarcomere length dispersion changes after the imposed length changes, and (iii) to assess if cross-bridges play a role in the length-induced force changes. Rabbit psoas myofibrils were attached between two pre-calibrated micro-needles, and their images were projected into a photodiode array for measurements of individual sarcomere length (SL). Myofibrils were activated by Ca2+ or ADP - the later induces cross-bridge attachment to actin independently of Ca2+. After activation myofibrils were subjected to three stretches or shortenings (∼4%SL), with isometric periods allowed between length changes so that force would achieve a steady-state. Forces of ADP-activated myofibrils were greater (7-8%) than those of Ca2+-activated myofibrils at corresponding SLs (range: 2.2-2.4μm) after shortening, but forces were similar after stretch. Forces were greater (26% with ADP and 15% with Ca2+, SL: 2.2μm) after stretch than after shortening. Sarcomere dispersion was similar after stretch or shortening in Ca2+ and ADP-activated myofibrils. The results suggest that stretching and shortening affects isometric forces on the ascending limb of the FL relation through different mechanisms, and are not associated with SL dispersion. While cross-bridges seem to be involved in force depression, they are likely not involved in force enhancement.

Architectural Analysis and Intraoperative Measurements Demonstrate the Unique Design of the Multifidus Muscle for Lumbar Spine Stability

Muscular instability is an important risk factor for lumbar spine injury and chronic low-back pain. Although the lumbar multifidus muscle is considered an important paraspinal muscle, its design features are not completely understood. The purpose of the present study was to determine the architectural properties, in vivo sarcomere length operating range, and passive mechanical properties of the human multifidus muscle. We hypothesized that its architecture would be characterized by short fibers and a large physiological cross-sectional area and that it would operate over a relatively wide range of sarcomere lengths but would have very stiff passive material properties. The lumbar spines of eight cadaver specimens were excised en bloc from T12 to the sacrum. Multifidus muscles were isolated from each vertebral level, permitting the architectural measurements of mass, sarcomere length, normalized fiber length, physiological cross-sectional area, and fiber length-to-muscle length ratio. To determine the sarcomere length operating range of the muscle, sarcomere lengths were measured from intraoperative biopsy specimens that were obtained with the spine in the flexed and extended positions. The material properties of single muscle fibers were obtained from passive stress-strain tests of excised biopsy specimens. The average muscle mass (and standard error) was 146 +/- 8.7 g, and the average sarcomere length was 2.27 +/- 0.06 microm, yielding an average normalized fiber length of 5.66 +/- 0.65 cm, an average physiological cross-sectional area of 23.9 +/- 3.0 cm(2), and an average fiber length-to-muscle length ratio of 0.21 +/- 0.03. Intraoperative sarcomere length measurements revealed that the muscle operates from 1.98 +/- 0.15 microm in extension to 2.70 +/- 0.11 microm in flexion. Passive mechanical data suggested that the material properties of the muscle are comparable with those of muscles of the arm or leg. The architectural design (a high cross-sectional area and a low fiber length-to-muscle length ratio) demonstrates that the multifidus muscle is uniquely designed as a stabilizer to produce large forces. Furthermore, multifidus sarcomeres are positioned on the ascending portion of the length-tension curve, allowing the muscle to become stronger as the spine assumes a forward-leaning posture.

Abstract 1581: Extension of Titin as a Function of Filling Pressure and Sarcomere Length in the Porcine Left Ventricle

The giant sarcomeric protein titin is a molecular spring that is the chief source of cardiomyocyte passive tension and a major determinant of myocardial stiffness. The spring portion is located in the I band and consists of PEVK, Ig repeat and N2B and N2A elements. Titin occurs as two isoforms. N2B is a smaller and stiffer isoform that contains only the N2B element and predominates in the left ventricle (LV) of rodents. N2BA titin contains both N2A and N2B elements. N2B and N2BA are co-expressed in the sarcomere of large mammals (~60:40 ratio). As a result, passive cardiomyocyte and myocardial stiffness in large mammals is less than in rodents. Details of titin extension as a function of sarcomere length (SL) have been elucidated in rodents but not in large mammals, where the presence of both isoforms would be expected to modify extension and passive tension. Accordingly, we studied titin extension in miniswine. We first established the relation between filling pressure and SL in the anterior LV wall of the in situ, freshly arrested (KCl) heart. SL was determined over a range of filling pressures using a light microscopic method that minimizes shrinkage. At equilibrium volume (transmural pressure 0 mmHg), SL was between 2.00–2.10 μm, longer than slack length of ~1.85 μm in muscle strips. SL reached a maximum of ~2.50 βm when the LV was over-distended (filling pressure >40 mmHg). We then examined extension of titin in myocardial strips using electron microscopy and immuno-labeling of selected epitopes. The chief difference between isoforms was that the N2B-Us epitope segment in N2B titin lengthened ~four times more than the N2B-Us segment in N2BA titin over SLs from ~1.80 to ~2.50 μm. This difference remained large over the SL range present in the in situ LV. Linear fits of the measured end-to-end length of N2B-Us segments were used to estimate the force-SL relation of single N2B and N2BA molecules. This analysis predicted a much steeper relation for N2B titin. Thus, over the range of SLs present in the in situ LV the most prominent difference in extension of N2B and N2BA titin is greater lengthening of the N2B segment of N2B titin. This predicts a much greater in situ stiffness for N2B titin and demonstrates how passive stiffness can be exquisitely controlled by varying isoform expression.

Physiological Functions of the Giant Elastic Protein Titin in Mammalian Striated Muscle

The striated muscle sarcomere contains the third filament comprising the giant elastic protein titin, in addition to thick and thin filaments. Titin is the primary source of nonactomyosin-based passive force in both skeletal and cardiac muscles, within the physiological sarcomere length range. Titin's force repositions the thick filaments in the center of the sarcomere after contraction or stretch and thus maintains sarcomere length and structural integrity. In the heart, titin determines myocardial wall stiffness, thereby regulating ventricular filling. Recent studies have revealed the mechanisms involved in the fine tuning of titin-based passive force via alternative splicing or posttranslational modification. It has also been discovered that titin performs roles that go beyond passive force generation, such as a regulation of the Frank-Starling mechanism of the heart. In this review, we discuss how titin regulates passive and active properties of striated muscle during normal muscle function and during disease.

Length-dependent activation and auto-oscillation in skeletal myofibrils at partial activation by Ca2+

The length-dependent activation of skeletal myofibrils was examined at the single-sarcomere level with phase-contrast microscopy at sarcomere length (SL) >2.2μm. At the maximal activation by Ca2+ (pCa 4.5) the active force linearly decreased with increasing SL, while at partial activation by Ca2+ (pCa 6.1–6.5) the larger active force was generated at longer SL. Throughout these experiments, the distribution of SL was kept homogeneous upon activation. In addition, we found that the spontaneous oscillation of force and SL frequently occurs in the SL range 2.2–2.6μm at pCa 6.1–6.2. Either changes in [Ca2+] or osmotic compression of the myofilament lattice induced by the addition of dextran T-500, affected both the length dependence of activation and the occurrence of auto-oscillation. These results suggest that the force-generating properties of sarcomeres in striated muscle are determined not only by [Ca2+], but also by the lattice spacing as a function of SL.

Muscle anatomy is a primary determinant of muscle relaxation dynamics in the lobster (Panulirus interruptus) stomatogastric system

We stained sarcomere thin filaments with fluorescently labeled phalloidin, measured sarcomere and muscle length, and calculated sarcomere number in pyloric and gastric mill muscles. A wide range of sarcomere lengths (3.25-12.29 microm), muscle lengths (5.9-21.1 mm), and sarcomere numbers (648-3,036) were observed. Sarcomere number differences occurred both because of changes in sarcomere length and muscle length, and sarcomere and muscle length varied independently. This independence, the wide range of sarcomere numbers present, and the muscles being all 'slow', graded muscles allowed us to use these data to test Huxley and Neidergerke's (1954) hypothesis that muscle dynamics depend on sarcomere number. The time constants of exponential fits to contraction relaxations were used to measure muscle dynamics, and comparison of theoretical predictions and experimental results quantitatively confirm the predicted dependence. The differing dynamics of the various pyloric muscles are likely functionally important, and the dependence of muscle dynamics on sarcomere number implies that sarcomere number is likely closely regulated in these muscles. The stomatogastric system may thus be an excellent model system for studying the mechanisms regulating muscle sarcomere number.

When fibres go slack and cross bridges are free to run: a brilliant method to study kinetic properties of acto‐myosin interaction

When fibres go slack and cross bridges are free to run: a brilliant method to study kinetic properties of acto‐myosin interaction

Comparison of the tension responses to ramp shortening and lengthening in intact mammalian muscle fibres: crossbridge and non-crossbridge contributions

We examined the tension responses to ramp shortening and lengthening over a range of velocities (0.1-5 L(0)/s) and at 20 degrees C and 30 degrees C in tetanized intact fibre bundles from a rat fast (flexor hallucis brevis) muscle; fibre length (L(0)) was 2.2 mm and sarcomere length approximately 2.5 microm. The tension change during ramp releases as well as ramp stretches showed an early transition (often appearing as an inflection) at 1-4 ms; the tension change at this transition and the length change at which it occurred increased with velocity. A second transition, indicated by a more gradual reduction in slope, occurred when the length had changed by 14-28 nm per half-sarcomere; the tension at this transition increased with lengthening velocity towards a plateau and it decreased with shortening velocity towards zero tension. The velocity dependence of the time to the transitions and the length change at the transitions showed some asymmetries between shortening and lengthening. Based on analyses of the velocity dependence of the tension and modelling, we propose that the first transition reflects the tension change associated with the crossbridge power stroke in shortening, or with the reversal of the power stroke in lengthening. Modelling shows that the reduction in slope at the second transition occurs when most of the crossbridges (myosin heads) that were attached at the start of the ramp become detached. After the second transition, the tension reaches a steady level in the model whereas the tension continues to increase during lengthening and continues to decrease during shortening in the experiments; this continuous tension change is seen at a wide range of initial sarcomere lengths and when active force is reduced by the myosin inhibitor, BTS. The continuous tension decline during shortening is not abolished by caffeine, but the rate of decline is reduced when the active force is depressed by BTS. We propose that stiffening of non-crossbridge visco-elastic elements upon activation contributes to the continuous tension rise during lengthening and the release of such tension and Ca-insensitive deactivation contribute to the tension decline during shortening in muscle fibres.

Intraoperative Single-Site Sarcomere Length Measurement Accurately Reflects Whole-Muscle Sarcomere Length in the Rabbit

To compare single-site intraoperative sarcomere length values with sarcomere lengths measured from systematic sampling of the entire transferred muscle. The tendon of the rabbit second toe extensor muscle was transposed to the ankle extensor retinaculum under levels of stretch over the sarcomere length range of about 2.5 microm to about 4.0 microm. Intraoperative sarcomere length was measured at a single site with a laser diffraction device. Whole-muscle sarcomere length measurement was then determined by sampling across the muscle in the proximal, middle, and distal regions. Linear regression analysis and intraclass correlation coefficients were used to validate single intraoperative sarcomere lengths relative to whole-muscle sarcomere lengths. Single intraoperative sarcomere lengths correlated strongly with average whole-muscle sarcomere length, although there was a systematic tendency to overestimate intraoperative sarcomere length. Intraoperative sarcomere length also matched well with all regions sampled, indicating that there was no tendency for intraoperative sarcomere length to better represent one region of the muscle compared with another. These results show that intraoperative sarcomere lengths accurately represent the entire muscle. The relatively small sarcomere length variations validate the use of intraoperative sarcomere length measurement during tendon transfer in which the entire muscle is not available for measurement because of limited surgical exposure.

Local control in thin filament activation of cardiac muscle

In both cardiac and skeletal muscles, contraction begins when Ca2+ binds to a regulatory site or sites in the N-lobe of troponin (Tn) C. Ca2+ binding to TnC induces sequential protein–protein interactions between TnC and TnI, TnI and TnT, and TnI–TnT and tropomyosin (Tm). As a result, Tm moves along actin (A) molecules exposing the sites for myosin S1 to attach thus leading to contraction. Recent protein biochemical, protein crystal structural, and cell physiology studies have advanced our understanding of the structure and function of myofilament proteins, especially the thin filament proteins (reviewed in Gordon et al. 2000; Kobayashi & Solaro, 2005). It has been demonstrated that for each seven actin monomers, there is one Tm and one Tn (7: 1: 1, A: Tm: Tn), which makes up one regulatory unit (RU). This structural arrangement forms the basis for cooperative interactions not only between Tn–Tm along the thin filament within the unit but also cooperative interactions between near-neighbouring regulatory units when cross-bridges are formed. In fact, it has been hypothesized that 12–14 actin monomers are controlled by one Tn. This hypothesis has recently been supported by Regnier et al. (2002) who have shown that in skeletal muscle, Ca2+-triggered activation of thin filament spread beyond the regulatory unit up to 10–12 actins which compose one functional unit (FU). What about cardiac muscle activation? The sequential events after Ca2+ binding to TnC are very similar in both types of muscles. However, several lines of evidence may indicate that cardiac muscle activates in a unique fashion. (1) The Ca2+ transient during systole may not be long enough to fully activate the thin filament. (2) Cardiac muscle must have a greater sarcomere length dependence of force development due to the narrow range of sarcomere lengths the heart normally works. (3) Cardiac muscle is more responsive to neuro/hormonal stimulation. In this issue of The Journal of Physiology, Gillis et al. (2007) have demonstrated that a cardiac FU in thin filament activation is limited within a RU in that Tn activates ≤ 7 actins upon Ca2+ activation, and that the behaviour of the cardiac FU depends on bound cross-bridges within the RU and is relatively independent of near-neighbour FUs (i.e. greater local control of activation). Although the molecular mechanism for the local control of thin filament activation is not precisely known, these findings indeed highlight the fact that different isoforms of regulatory proteins may be responsible for the differences in thin filament activation between cardiac and fast skeletal muscles. For example, as discussed by Gillis et al. structural and kinetic differences in Ca2+ binding to cTnC, cTnI–cTnC interactions between fast skeletal and cardiac isoforms, as well as more flexible Tm in cardiac muscle could contribute to the differences in RUs between skeletal and cardiac muscle. The findings that cardiac muscle activation is more limited within the RU and has a greater reliance on crossbridge attachment, thus allowing greater and finer local control of activation and force generation, has important implications for pathophysiological states. For example, hypertrophic and dilated cardiomyopathies can be caused by mutations in genes which encode thick or thin filament proteins (Morita et al. 2005). These mutant proteins impact on Ca2+ sensitivity, sarcomere length dependence and also cross-bridge behaviour of the cardiac muscle. Post-translational modifications also impact on thin filament proteins in disease states including degradation and oxidation of myofilament proteins in ischaemia/reperfusion injury which are functionally significant. Finally, cardiac muscle activation is influenced by neuro/hormonal modulation via protein kinase signalling pathways. In heart failure, altered phosphorylation of myofilament proteins may alter cross-bridge dynamics and decrease force production. Precisely how of each of the myofilament proteins contribute to the limited near-neighbour interactions and greater cross-bridge attachment control of activation in cardiac muscle is not known and should be the focus of future investigations. Understanding the molecular properties of these regulatory proteins will no doubt help us to better appreciate the nature of cardiac contraction at both whole organ and cellular level in health and disease.

Force-length relations in isolated intact cardiomyocytes subjected to dynamic changes in mechanical load

We developed a dynamic force-length (FL) control system for single intact cardiomyocytes that uses a pair of compliant, computer-controlled, and piezo translator (PZT)-positioned carbon fibers (CF). CF are attached to opposite cell ends to afford dynamic and bidirectional control of the cell's mechanical environment. PZT and CF tip positions, as well as sarcomere length (SL), are simultaneously monitored in real time, and passive/active forces are calculated from CF bending. Cell force and length were dynamically adjusted by corresponding changes in PZT position, to achieve isometric, isotonic, or work-loop style contractions. Functionality of the technique was assessed by studying FL behavior of guinea pig intact cardiomyocytes. End-diastolic and end-systolic FL relations, obtained with varying preload and/or afterloads, were near linear, independent of the mode of contraction, and overlapping for the range of end-diastolic SLs tested (1.85-2.05 micro m). Instantaneous elastance curves, obtained from FL relation curves, showed an afterload-dependent decrease in time to peak elastance and slowed relaxation with both increased preload and afterload. The ability of the present system to independently and dynamically control preload, afterload, and transition between end-diastolic and end-systolic FL coordinates provides a valuable extension to the range of tools available for the study of single cardiomyocyte mechanics, to foster its interrelation with whole heart pathophysiology.

Comment on the Classics

We examined the architectural properties of the rotator cuff muscles in 10 cadaveric specimens to understand their functional design. Based on our data and previously published joint angle-muscle excursion data, sarcomere length operating ranges were modeled through all permutations in 75 masculine medial and lateral rotation and 75 masculine abduction at the glenohumeral joint. Based on physiologic cross-sectional area, the subscapularis would have the greatest force-producing capacity, followed by the infraspinatus, supraspinatus, and teres minor. Based on fiber length, the supraspinatus would operate over the widest range of sarcomere lengths. The supraspinatus and infraspinatus had relatively long sarcomere lengths in the anatomic position, and were under relatively high passive tensions at rest, indicating they are responsible for glenohumeral resting stability. However, the subscapularis contributed passive tension at maximum abduction and lateral rotation, indicating it plays a critical role in glenohumeral stability in the position of apprehension. These data illustrate the exquisite coupling of muscle architecture and joint mechanics, which allows the rotator cuff to produce near maximal active tensions in the midrange and produce passive tensions in the various end-range positions. During surgery relatively small changes to rotator cuff muscle length may result in relatively large changes in shoulder function.

Role of the giant elastic protein titin in the Frank-Starling mechanism of the heart.

Increased ventricular volume enhances the systolic performance, a phenomenon known as Frank-Starling's law of the heart. At its basis is the ability of cardiac muscle to produce increased active force in response to increased muscle length. Although numerous studies have been conducted to elucidate the molecular basis of length-dependent activation, the mechanism remains elusive. The giant protein titin (also known as connectin) is the third filament system in the sarcomere and is responsible for most passive stiffness of striated muscle in the physiological sarcomere length range. The force generated by titin is usually seen as passive and independent of active force generation. Recent findings, however, suggest that titin-based passive force modulates actin-myosin interaction, resulting in greater active force in response to stretch. In this short review, we discuss the molecular mechanisms of length-dependent activation, focusing on the possible role of titin in its regulation.

Association of the Chaperone αB-crystallin with Titin in Heart Muscle

alphaB-crystallin, a major component of the vertebrate lens, is a chaperone belonging to the family of small heat shock proteins. These proteins form oligomers that bind to partially unfolded substrates and prevent denaturation. alphaB-crystallin in cardiac muscle binds to myofibrils under conditions of ischemia, and previous work has shown that the protein binds to titin in the I-band of cardiac fibers (Golenhofen, N., Arbeiter, A., Koob, R., and Drenckhahn, D. (2002) J. Mol. Cell. Cardiol. 34, 309-319). This part of titin extends as muscles are stretched and is made up of immunoglobulin-like modules and two extensible regions (N2B and PEVK) that have no well defined secondary structure. We have followed the position of alphaB-crystallin in stretched cardiac fibers relative to a known part of the titin sequence. alphaB-crystallin bound to a discrete region of the I-band that moved away from the Z-disc as sarcomeres were extended. In the physiological range of sarcomere lengths, alphaB-crystallin bound in the position of the N2B region of titin, but not to PEVK. In overstretched myofibrils, it was also in the Ig region between N2B and the Z-disc. Binding between alphaB-crystallin and N2B was confirmed using recombinant titin fragments. The Ig domains in an eight-domain fragment were stabilized by alphaB-crystallin; atomic force microscopy showed that higher stretching forces were needed to unfold the domains in the presence of the chaperone. Reversible association with alphaB-crystallin would protect I-band titin from stress liable to cause domain unfolding until conditions are favorable for refolding to the native state.

Quick Review: Hemodynamics

Before discussing basic hemodynamics, we should remind ourselves of the systemic circuit: 1. Cardiac Anatomy 2. Circulatory Pathways This brief review discusses the basics of hemodynamics. CARDIAC ANATOMY The Heart: 2 Separate Volume Pumps ! The in-series nature of these two systems implies that the output of the Right Heart becomes the input of the Left Heart, and therefore, the output of the Left Heart becomes the input of the Right Heart FLOW VIA SERIES: DEMONSTRATED BY WILLIAM HARVEY, 1628 Desaturated Blood returns from the Systemic Vessels via the SVC & IVC Saturated Blood is then returned to the Left Atrium via the Pulmonary Veins ! MYOCARDIAL PERFUSION OCCURS PRIMARILY DURING DIASTOLE MYOCARDIAL BLOOD FLOW: MYOCYTE CONTRACTION At the cellular level, electrical depolarization of the myocardial cell membrane allows ionized calcium flux into the cytoplasm leading to hydrolysis of ATP by Myosin. This leads to a conformational change in the Actin-Myosin Cross Bridge producing sliding of myosin filaments relative to actin & overall shortening of the sarcomere [Sliding Filament Theory] Calcium is then removed from the cell by Active Transport in the Sarcoplasmic Reticulum allowing Relaxation, while ATP is regenerated by Metabolic Processes Over the physiologic range of sarcomere length (1.6 2.0 um), the amount of metabolic energy converted to mechanical work is dependent on the available Surface Area of Cross-Bridge Interactions ! Work is directly proportional to End-Diastolic Sarcomere Length This “Length Dependency” is the fundamental basis for the Frank-Starling Law ! Otto Frank, 1885 (Frog Heart Preparations): “the output of a normal heart is influenced primarily by the volume of blood in the ventricle at the end of diastole” Figure 1 Quick Review: Hemodynamics 2 of 3 CARDIAC OUTPUT CO = HR x SV “the amount of blood pumped by the heart per unit time” NORMAL C.O. : 3.5 8.5 L/MIN Manipulation of the factors can lead to augmentation of CO at the lowest possible energy cost ! DETERMINANTS OF CARDIAC PERFORMANCE & OUTPUT Preload: EDV (the load that stretches a muscle prior to contraction) Afterload: SVR (the load that must be moved during muscle contraction) Contractility: the velocity of muscle shortening at a constant preload and afterload Compliance: the length that a muscle is stretched by a given preload. Determined by the inherent Elasticity ! Heart Rate: several effects on overall Cardiac Function : Tachycardia/Bradycardia PRELOAD PCWP = LAP = LVEDP (best approximation) But Remember, the relationship between LVEDP & LVEDVis NOT Linear!! PCWP is by definition an ESTIMATE of EDV& thus, an ESTIMATE of Preload Elevation of CVP to Equal PAD& PCWP Square Root Sign : characteristic RA waveform in patients with Constrictive Pericarditis AFTERLOAD The impedance to LV Ejection and is usually estimated by the Systemic Vascular Resistance. Remember: changes in afterload have no effect on the contractility of a normal heart SVR = {(MABP CVP)/CO} x 80 MBAP (Mean Arterial Blood Pressure) = DBP + [1/3(SBP DBP)] SVR units: dynes-second/cm Decreasing Afterload exchanges Pressure Work for Flow Work and serves to increase vital organ perfusion ! Pressure Work Flow Work Plus, since pressure work is more costly than flow work in terms of myocardial oxygen consumption, by decreasing afterload you also decrease the overall energy requirement PVR = {(MPAP PWP)/CO} x 80 Remember: CONTRACTILITY the inotropic state: an intrinsic property of myocardial muscle which is manifested as a greater force of contraction for a given preload By increasing intropic state, you increase both Pressure Work & Flow Work thus, the cost in myocardial oxygen consumption may be high !! COMPLIANCE & ELASTICITY “Compliance”: the tendency of an object to return to it's original shape when it has been deformed or altered (Compliance = change in Volume / change in Pressure) HEART RATE Heart Rate can Influence Cardiac Function in Several Ways: Cardiac Physiology is based on a thorough understanding of the underlying mechanics ! References Quick Review: Hemodynamics 3 of 3 Author Information Tisha K. Fujii, DO Dept. of Trauma & Critical Care , Boston University School of Medicine , Boston Medical Center Bradley J. Phillips, MD Dept. of Trauma & Critical Care , Boston University School of Medicine , Boston Medical Center

Molecular Mechanics of Cardiac Titin's PEVK and N2B Spring Elements

Titin is a giant elastic protein that is responsible for the majority of passive force generated by the myocardium. Titin's force is derived from its extensible I-band region, which, in the cardiac isoform, comprises three main extensible elements: tandem Ig segments, the PEVK domain, and the N2B unique sequence (N2B-Us). Using atomic force microscopy, we characterized the single molecule force-extension curves of the PEVK and N2B-Us spring elements, which together are responsible for physiological levels of passive force in moderately to highly stretched myocardium. Stretch-release force-extension curves of both the PEVK domain and N2B-Us displayed little hysteresis: the stretch and release data nearly overlapped. The force-extension curves closely followed worm-like chain behavior. Histograms of persistence length (measure of chain bending rigidity) indicated that the single molecule persistence lengths are approximately 1.4 and approximately 0.65 nm for the PEVK domain and N2B-Us, respectively. Using these mechanical characteristics and those determined earlier for the tandem Ig segment (assuming folded Ig domains), we modeled the cardiac titin extensible region in the sarcomere and calculated the extension of the various spring elements and the forces generated by titin, both as a function of sarcomere length. In the physiological sarcomere length range, predicted values and those obtained experimentally were indistinguishable.

Invertebrate connectin spans as much as 3.5 microm in the giant sarcomeres of crayfish claw muscle.

In crayfish claw closer muscle, the giant sarcomeres are 8.3 microm long at rest, four times longer than vertebrate striated muscle sarcomeres, and they are extensible up to 13 microm upon stretch. Invertebrate connectin (I-connectin) is an elastic protein which holds the A band at the center of the sarcomere. The entire sequence of crayfish I-connectin was predicted from cDNA sequences of 53 424 bp (17 352 residues; 1960 kDa). Crayfish I-connectin contains two novel 68- and 71-residue repeats, and also two PEVK domains and one kettin region. Kettin is a small isoform of I-connectin. Immunoblot tests using antibody to the 68-residue repeats revealed the presence of I-connectin also in long sarcomeres of insect leg muscle and barnacle ventral muscle. Immunofluorescence microscopy demonstrated that the two repeats, the long spacer and the two PEVK domains contribute to sarcomere extension. These regions rich in charged amino acids, occupying 63% of the crayfish I-connectin molecule, may allow a span of a 3.5 microm distance as a new class of composite spring.

Sarcomere Length-Induced Alterations of Capillary Hemodynamics in Rat Spinotrapezius Muscle: Vasoactive vs Passive Control

Skeletal muscle blood flow is reduced as fibers are stretched longitudinally. Neither the underlying cause(s) of this decrement in blood flow nor the consequences in terms of capillary red blood cell (rbc) hemodynamics has been established clearly within the physiological range of muscle sarcomere length. Using intravital microscopy, this investigation determined arteriolar diameter and capillary rbc velocity (Vrbc), flux (Frbc), and hematocrit (Hctt) in the rat spinotrapezius muscle at shortened/resting (2.6 μm) and physiological extended (3.2 μm) sarcomere lengths under control (c) and local maximally vasodilated (v, phentolamine, 1 μmol/L; prazosin, 0.1 μmol/L; nitroprusside, 10 μmol/L) conditions. The hypothesis tested was that muscle stretch would reduce Vrbc and Frbc proportionally such that Hctt would remain unchanged and that these reductions in Vrbc and Frbc would be attenuated following maximal vasodilation. Vrbc and Frbc were increased significantly following maximal vasodilation at 2.6-μm (59 and 84%) and 3.2-μm (64 and 104%) sarcomere lengths, respectively. Irrespective of sarcomere length, Hctt was elevated significantly following vasodilation (c, 0.20 ± 0.01; v, 0.27 ± 0.01). At 3.2 μm compared with the 2.6-μm sarcomere length, Vrbc and Frbc were both reduced significantly under control and vasodilated conditions as expected. However, the percent reduction in either Vrbc (c, 27%, and v, 29%) or Frbc (c, 26%, and v, 33%) was not significantly different between the 2.6- and 3.2-μm sarcomere lengths. In addition, arteriolar diameter was not altered discernably as sarcomere length was increased from 2.7 μm (c, 29.0 ± 4.5; v, 37.9 ± 6.7 μm) to 3.2 μm (c, 29.4 ± 4.5; v, 37.3 ± 6.2 μm). These data suggest that increasing sarcomere length from resting to the upper extreme of the physiological range in the rat spinotrapezius muscle reduces Vrbc and Frbc (at constant hematocrit) by a mechanism that is independent of stretch-activated arteriolar vasoconstriction.

Sarcomeric visco-elasticity of chemically skinned skeletal muscle fibres of the rabbit at rest.

The giant muscle protein titin (connectin), contained in the gap filament that connect a thick filament to the Z-line in a sarcomere, is generally considered to be responsible for the passive force (tension) and visco-elasticity in resting striated muscle. However, whether it can account for all the features of the resting tension response remains unclear. In this paper, we examine the basic features of the 'sarcomeric visco-elasticity' in a single resting mammalian muscle fibre and attempt to account for various tension components on the basis of known structural features of a sarcomere. At sarcomere length of approximately 2.6 microm, the force response to a ramp stretch of 2-5% is complex but can be resolved into four functionally different components. The behaviour displayed by the components ranges from pure viscous type (directly proportional to stretch velocity, ranging from 0.1 to 30 lengths s(-1)) to predominantly elastic type (insensitive to stretch velocity at 1-2 s time scale); simulations show two components of visco-elasticity with characteristically different relaxation times. The velocity-sensitive components (only) are enhanced by filament lattice compression (dextran - 500 kD) and by increased medium viscosity (dextran - 12 kD); also, the relaxation time of visco-elasticity is longer with increased medium viscosity. Amplitude of all the components and the relaxation time of visco-elasticity are increased at longer sarcomere length (range approximately 2.5 - 3.0 microm). The study, and quantitative analyses, extend our previous work on intact muscle fibres and suggest that the velocity-sensitive tension components in intact sarcomere arise from interactions between sarcomeric filaments, filament segments and inter-filamentary medium; the two components of visco-elasticity arise from distinct regions of titin (connectin) molecules.