Mechanical measurements from isolated cardiac myocytes using a pipette attachment system

Longitudinal diffusion barriers imposed by myofilaments and mitochondria in murine cardiac myocytes

Using optical and electrical methods, we document that diffusion in the cytoplasm of BL6 murine cardiomyocytes becomes restricted >20-fold as molecular weight increases from 30 to 2,000, roughly as expected for pores with porin channel dimensions. Bodipy-FL ATP diffuses >40-fold slower than in free water at 25°C. From several fluorophores analyzed, bound fluorophore fractions range from 0.1 for a 2 kD FITC-labeled polyethylene glycol to 0.93 for sulforhodamine. Unbound fluorophores diffuse at 0.5-8 × 10-7 cm2/s (5-80 μm2/s). Analysis of Na/K pump and veratridine-modified Na channel currents suggests that Na diffusion is nearly unrestricted at 35°C (time constant for equilibration with the pipette tip, ∼20 s). Using multiple strategies, we estimate that at 35°C, ATP diffuses four to eight times slower than in free water. To address whether restrictions are caused more by protein or membrane networks, we verified first that a protein gel, 10 g% gelatin, restricts diffusion with strong dependence on molecular weight. Solute diffusion in membrane-extracted cardiac myofilaments, confined laterally by suction into large-diameter pipette tips, is less restricted than in intact myocytes. Notably, myofilaments extracted similarly from skeletal (diaphragm) myocytes are less restrictive. Solute diffusion in myocytes with sarcolemma permeabilized by β-escin (80 µM) is similar to diffusion in intact myocytes. Restrictions are strain-dependent, being twofold greater in BL6 myocytes than in CD1/J6/129svJ myocytes. Furthermore, longitudinal diffusion is 2.5-fold more restricted in CD1/J6/129svJ myocytes lacking the mitochondrial porin, VDAC1, than in WT CD1/J6/129svJ myocytes. Thus, mitochondria networks restrict long-range diffusion while presumably optimizing nucleotide transfer between myofilaments and mitochondria. We project that diffusion restrictions imposed by both myofilaments and the outer mitochondrial membrane are important determinants of total free cytoplasmic AMP and ADP (∼10 μM). However, the capacity of diffusion to deliver ATP to myofilaments remains ∼100-fold greater than ATP consumption.

Pore-like diffusion barriers in murine cardiac myocytes

Using both optical and electrical methods, we document that solute diffusion in the cytoplasm of BL6 murine cardiac myocytes becomes restricted >30-fold as molecular weight increases from 30 to 2000, roughly as expected for pores with dimensions of cardiac porin channels. The Bodipy-FL ATP analogue diffuses ∼50-fold slower in BL6 cardiac cytoplasm than in free water. From several fluorophores analyzed, our estimates of bound fluorophore fractions range from 0.1 for a 2 kD FITC-labeled polyethylene glycol to 0.93 for sulforhodamine. We estimate that diffusion coefficients of unbound fluorophores range from 0.5 to 8 x 10 -7 cm 2 /s. Analysis of Na/K pump and veratridine-modified Na channel currents confirms that Na diffusion is nearly unrestricted (time constant for equilibration with the pipette tip, ∼20 s). Using three different approaches, we estimate that ATP diffuses 8 to 10-times slower in the cytoplasm of BL6 myocytes than in free water. To address whether restrictions are caused more by cytoplasmic protein or membrane networks, we verified first that a protein gel, 10 gram% gelatin, restricts solute diffusion with strong dependence on molecular weight. Solute diffusion in membrane-extracted cardiac myofilaments, confined laterally by suction into large-diameter pipette tips, is however less restricted than in intact myocytes. Notably, myofilaments from equivalently extracted skeletal (diaphragm) myocytes restrict diffusion less than cardiac myofilaments. Solute diffusion in myocytes with sarcolemma permeabilized by β-escin (80 µM) is similarly restricted as in intact myocytes. Diffusion restriction in cardiac myocytes is strain-dependent, being about two-fold greater in BL6 myocytes than in myocytes with a CD1/J6/129svJ background. Furthermore, diffusion is 2.5-fold more restricted in CD1/J6/129svJ myocytes lacking the mitochondrial porin, Vdac1, than in WT CD1/J6/129svJ myocytes. We conclude that both myofilaments and mitochondria networks restrict diffusion in cardiac myocytes. As a result, long-range solute diffusion may preferentially occur via passage through porin channels and intramembrane mitochondrial spaces, where diffusion is less restricted than in myofilament spaces.

3D Cardiac Cell Culture: A Critical Review of Current Technologies and Applications

Three-dimensional (3D) cell culture is often mentioned in the context of regenerative medicine, for example, for the replacement of ischemic myocardium with tissue-engineered muscle constructs. Additionally, 3D cell culture is used, although less commonly, in basic research, toxicology, and drug development. These applications have recently benefited from innovations in stem cell technologies allowing the mass-production of hiPSC-derived cardiomyocytes or other cardiovascular cells, and from new culturing methods including organ-on-chip and bioprinting technologies. On the analysis side, improved sensors, computer-assisted image analysis, and data collection techniques have lowered the bar for switching to 3D cell culture models. Nevertheless, 3D cell culture is not as widespread or standardized as traditional cell culture methods using monolayers of cells on flat surfaces. The many possibilities of 3D cell culture, but also its limitations, drawbacks and methodological pitfalls, are less well-known. This article reviews currently used cardiovascular 3D cell culture production methods and analysis techniques for the investigation of cardiotoxicity, in drug development and for disease modeling.

Insect Muscular Tissue-Powered Swimming Robot

Bio-actuators that use insect muscular tissue have attracted attention from researchers worldwide because of their small size, self-motive property, self-repairer ability, robustness, and the need for less environment management than mammalian cells. To demonstrate the potential of insect muscular tissue for use as bio-actuators, three types of these robots, a pillar actuator, a walker, and a twizzer, have been designed and fabricated. However, a model of an insect muscular tissue-powered swimming robot that is able to float and swim in a solution has not yet been reported. Therefore, in this paper, we present a prototype of an insect muscular tissue-powered autonomous micro swimming robot that operates at room temperature and requires no temperature and pH maintenance. To design a practical robot body that is capable of swimming by using the force of the insect dorsal vessel (DV), we first measured the contraction force of the DV. Then, the body of the swimming robot was designed, and the design was confirmed by a simulation that used the condition of measured contraction force. After that, we fabricated the robot body using polydimethylpolysiloxane (PDMS). The PDMS body was obtained from a mold that was fabricated by a stereo lithography method. Finally, we carefully attached the DV to the PDMS body to complete the assembly of the swimming robot. As a result, we confirmed the micro swimming robot swam autonomously at an average velocity of 11.7 μm/s using spontaneous contractions of the complete insect DV tissue. These results demonstrated that the insect DV has potential for use as a bio-actuator for floating and swimming in solution.

Inferring intra-cellular mechanics using geometric metamorphosis: A preliminary study

Mechanotransduction plays an important role in sub-cellular processes and is an active area of research. Determining the forces/strains that the intra-cellular structures experience is vital for developing quantitative models of cellular behavior. Established techniques such as traction force microscopy, digital image correlation etc. track surface forces and kinematics of intra-cellular structures. However, difficulties arise when cells cannot be seeded on micro-patterned substrates or the intra-cellular structures vary (unstable landmarks). Here, we applied geometric metamorphosis, a global image registration method, to determine the kinematic profile of a cell during cell division. The method does not require stable landmarks, the registration is non-local in nature and constraints such as volume conservation can be enforced. The cell wall was tracked over time and a sequence of transformations relating the cell wall at the start of cytokinesis to the configuration prior to the daughters completely separate was determined. These transformations are associated with a scalar metric and a statistical atlas describing the wall kinematics from multiple tracking's of the wall shape is constructed. Using these transformations, the cellular kinematics can be described using a Lagrangian frame of reference and the evolution of a material point property can be easily modeled. To demonstrate this, we use the kinematic data derived from the atlas along with a model of stress-fiber (de)formation dynamics to simulate the stress-fiber configuration as the cell domain deforms.

Relevance of cardiomyocyte mechano-electric coupling to stretch-induced arrhythmias: optical voltage/calcium measurement in mechanically stimulated cells, tissues and organs

Stretch-induced arrhythmias are multi-scale phenomena in which alterations in channel activities and/or calcium handling lead to the organ level derangement of the heart rhythm. To understand how cellular mechano-electric coupling (MEC) leads to stretch-induced arrhythmias at the organ level, we developed stretching devices and optical voltage/calcium measurement techniques optimized to each cardiac level. This review introduces these experimental techniques of (1) optical voltage measurement coupled with a carbon-fiber technique for single isolated cardiomyocytes, (2) optical voltage mapping combined with motion tracking technique for myocardial tissue/whole heart preparations and (3) real-time calcium imaging coupled with a laser optical trap technique for cardiomyocytes. Following the overview of each methodology, results are presented. We conclude that individual MEC in cardiomyocytes can be heterogeneous at the ventricular level, especially when moderate amplitude mechanical stretches are applied to the heart, and that this heterogeneous MEC can evoke focal excitation that develops into re-entrant arrhythmias.

Real-time determination of sarcomere length of a single cardiomyocyte during contraction

Sarcomere length of a cardiomyocyte is an important control parameter for physiology studies on a single cell level; for instance, its accurate determination in real time is essential for performing single cardiomyocyte contraction experiments. The aim of this work is to develop an efficient and accurate method for estimating a mean sarcomere length of a contracting cardiomyocyte using microscopy images as an input. The novelty in developed method lies in 1) using unbiased measure of similarities to eliminate systematic errors from conventional autocorrelation function (ACF)-based methods when applied to region of interest of an image, 2) using a semianalytical, seminumerical approach for evaluating the similarity measure to take into account spatial dependence of neighboring image pixels, and 3) using a detrend algorithm to extract the sarcomere striation pattern content from the microscopy images. The developed sarcomere length estimation procedure has superior computational efficiency and estimation accuracy compared with the conventional ACF and spectral analysis-based methods using fast Fourier transform. As shown by analyzing synthetic images with the known periodicity, the estimates obtained by the developed method are more accurate at the subpixel level than ones obtained using ACF analysis. When applied in practice on rat cardiomyocytes, our method was found to be robust to the choice of the region of interest that may 1) include projections of carbon fibers and nucleus, 2) have uneven background, and 3) be slightly disoriented with respect to average direction of sarcomere striation pattern. The developed method is implemented in open-source software.

Contractility assessment in enzymatically isolated cardiomyocytes

The use of enzymatically isolated cardiac myocytes is ubiquitous in modern cardiovascular research. Parallels established between cardiomyocyte shortening responses and those of intact tissue make the cardiomyocyte an invaluable experimental model of cardiac function. Much of our understanding regarding the fundamental processes underlying heart function is owed to our increasing capabilities in single-cell stimulation and direct or indirect observation, as well as quantitative analysis of such cells. Of the many important mechanisms and functions that can be readily assessed in cardiomyocytes at all stages of development, contractility is the most representative and one of the most revealing. The purpose of this review is to provide a survey of various methodological approaches in the literature used to assess adult and neonatal cardiomyocyte contractility. The various methods employed to evaluate the contractile behavior of enzymatically isolated mammalian cardiac myocytes can be conveniently divided into two general categories-those employing optical (image)-based systems and those that use transducer-based technologies. This survey is by no means complete, but we have made an effort to include the most popular methods in terms of reliability and accessibility. These techniques are in constant evolution and hold great promise for the next generation of breakthrough studies in cell biology for the prevention, treatment, and cure of cardiovascular diseases.

Modeling of cardiac muscle thin films: pre-stretch, passive and active behavior

Recent progress in tissue engineering has made it possible to build contractile bio-hybrid materials that undergo conformational changes by growing a layer of cardiac muscle on elastic polymeric membranes. Further development of such muscular thin films for building actuators and powering devices requires exploring several design parameters, which include the alignment of the cardiac myocytes and the thickness/Young's modulus of elastomeric film. To more efficiently explore these design parameters, we propose a 3-D phenomenological constitutive model, which accounts for both the passive deformation including pre-stretch and the active behavior of the cardiomyocytes. The proposed 3-D constitutive model is implemented within a finite element framework, and can be used to improve the current design of bio-hybrid thin films and help developing bio-hybrid constructs capable of complex conformational changes.

Micromechanical regulation in cardiac myocytes and fibroblasts: implications for tissue remodeling

Cells of the myocardium are at home in one of the most mechanically dynamic environments in the body. At the cellular level, pulsatile stimuli of chamber filling and emptying are experienced as cyclic strains (relative deformation) and stresses (force per unit area). The intrinsic characteristics of tension-generating myocytes and fibroblasts thus have a continuous mechanical interplay with their extrinsic surroundings. This review explores the ways that the micromechanics at the scale of single cardiac myocytes and fibroblasts have been measured, modeled, and recapitulated in vitro in the context of adaptation. Both types of cardiac cells respond to externally applied strain, and many of the intracellular mechanosensing pathways have been identified with the careful manipulation of experimental variables. In addition to strain, the extent of loading in myocytes and fibroblasts is also regulated by cues from the microenvironment such as substrate surface chemistry, stiffness, and topography. Combinations of these structural cues in three dimensions are needed to mimic the micromechanical complexity derived from the extracellular matrix of the developing, healthy, or pathophysiologic heart. An understanding of cardiac cell micromechanics can therefore inform the design and composition of tissue engineering scaffolds or stem cell niches for future applications in regenerative medicine.

Contractile tension and beating rates of self-exciting monolayers and 3D-tissue constructs of neonatal rat cardiomyocytes

The CellDrum technology (The term 'CellDrum technology' includes a couple of slightly different technological setups for measuring lateral mechanical tension in various types of cell monolayers or 3D-tissue constructs) was designed to quantify the contraction rate and mechanical tension of self-exciting cardiac myocytes. Cells were grown either within flexible, circular collagen gels or as monolayer on top of respective 1-mum thin silicone membranes. Membrane and cells were bulged outwards by air pressure. This biaxial strain distribution is rather similar the beating, blood-filled heart. The setup allowed presetting the mechanical residual stress level externally by adjusting the centre deflection, thus, mimicking hypertension in vitro. Tension was measured as oscillating differential pressure change between chamber and environment. A 0.5-mm thick collagen-cardiac myocyte tissue construct induced after 2 days of culturing (initial cell density 2 x 10(4) cells/ml), a mechanical tension of 1.62 +/- 0.17 microN/mm(2). Mechanical load is an important growth regulator in the developing heart, and the orientation and alignment of cardiomyocytes is stress sensitive. Therefore, it was necessary to develop the CellDrum technology with its biaxial stress-strain distribution and defined mechanical boundary conditions. Cells were exposed to strain in two directions, radially and circumferentially, which is similar to biaxial loading in real heart tissues. Thus, from a biomechanical point of view, the system is preferable to previous setups based on uniaxial stretching.

A novel functional assessment of the differentiation of micropatterned muscle cells

Duchenne muscular dystrophy (DMD) is a lethal disease characterized by rapid, progressive atrophy of muscle tissues. Timely screening of therapeutic interventions is necessary for the development of effective treatment approaches for DMD. We have developed an in vitro model using a combination of micropatterning of C2C12 skeletal muscle cells and cell traction force microscopy (CTFM). In this model, C2C12 cells were micropatterned on a highly elongated adhesive island such that the cells assumed a shape typical of a myotube. During differentiation, these cells gradually fused together and began expressing dystrophin, a structural protein of myotubes, meanwhile, their contractile forces, represented by cell traction forces, continually increased until the myotubes reached maturation. In addition, the high-degree alignment of cells favored myotube differentiation and dystrophin expression. Since the fundamental structural unit of muscle tissue is myofiber, which is responsible for muscle contraction, such a technology that can directly quantify the contractile forces of the myotube, a precursor of myofiber, may constitute a fast and efficient screening approach for DMD therapies.

Dynamical stress characterization and energy evaluation of single cardiac myocyte actuating on flexible substrate

Contractility estimation of cardiac myocyte is important for power evaluation of the cell in heart performance. In this paper, we used digital image correlation (DIC) method to obtain dynamic deformation field of the flexible substrate distributively actuated by single cardiac myocyte, which resulted in the dynamic history of traction forces of the cell during contraction-relaxation cycles. Furthermore, the resultant work and power of the single neonatal cardiac myocyte was evaluated to show the energy characteristics of the cells and the responses to the stiffness variation of the substrate. The method provides a useful tool to study behaviors of the cardiac myocytes interacted with the substrates.

Carbon fiber technique for the investigation of single-cell mechanics in intact cardiac myocytes

This protocol describes a method for attaching single isolated cardiac myocytes to carbon fibers for mechanical manipulation and measurement. This method relies on cell-adhesive carbon fibers that attach easily to the cell membrane without causing damage, and is thus applicable to intact myocytes. To connect the carbon fiber to micromanipulators, a fiber holder with glass capillaries must first be fabricated. After connection of the fibers to the micromanipulators, firm attachment is easily established by gently pressing the fiber tip onto the cell membrane. Unlike other methods, this technique does not require vast technical expertise, and therefore greatly facilitates experiments. This method enables detection of the effect of drugs, genetic defects or the expression of exogenous proteins on both active and passive properties of cardiac myocytes. In combination with other experimental procedures, this technique can also be applied to the study of mechano-transduction. This protocol can be completed in 3.5 h.

Advances in electrical and mechanical cardiac mapping

Cardiac mapping--recording cardiac activity during electrophysiological testing--has evolved into an indispensable tool in studying the cardiac excitation process, analysing activation patterns, and identifying arrhythmogenic tissue. Cardiac mapping is a broad term that is used here to encompass applications that record electrical or mechanical activity of the heart or both. In recent years, simultaneous and sequential electrical mapping methods have been combined with direct mechanical measurements or imaging techniques to acquire information regarding both the electrical and mechanical activity of the heart (electromechanical mapping) during normal and irregular cardiac behavior. This paper reviews the emerging area of electromechanical mapping from the point of view of the applicable technology, including its history and application.

Single cell mechanics of rat cardiomyocytes under isometric, unloaded, and physiologically loaded conditions

One of the most salient characteristics of the heart is its ability to adjust work output to external load. To examine whether a single cardiomyocyte preparation retains this property, we measured the contractile function of a single rat cardiomyocyte under a wide range of loading conditions using a force-length measurement system implemented with adaptive control. A pair of carbon fibers was used to clamp the cardiomyocyte, attached to each end under a microscope. One fiber was stiff, serving as a mechanical anchor, while the bending motion of the compliant fiber was monitored for force-length measurement. Furthermore, by controlling the position of the compliant fiber using a piezoelectric translator based on adaptive control, we could change load dynamically during contractions. Under unloaded conditions, maximal shortening velocity was 106 +/- 8.9 microm/s (n = 13 cells), and, under isometric conditions, peak developed force reached 5,720 nN (41.6 +/- 5.6 mN/mm(2); n = 17 cells). When we simulated physiological working conditions consisting of an isometric contraction, followed by shortening and relaxation, the average work output was 828 +/- 123 J/m(3) (n = 20 cells). The top left corners of tension-length loops obtained under all of these conditions approximate a line, analogous to the end-systolic pressure-volume relation of the ventricle. All of the functional characteristics described were analogous to those established by studies using papillary muscle or trabeculae preparations. In conclusion, the present results confirmed the fact that each myocyte forms the functional basis for ventricular function and that single cell mechanics can be a link between subcellular events and ventricular mechanics.

Unloaded shortening increases peak of Ca2+ transients but accelerates their decay in rat single cardiac myocytes

It is of paramount importance to investigate the relation between the time-dependent change in intracellular Ca2+ concentration ([Ca2+]i) (Ca2+ transients) and the mechanical activity of isolated single myocytes to understand the regulatory mechanisms of heart function. However, because of technical difficulties in performing mechanical measurements with single myocytes, the simultaneous recording of Ca2+ transients and mechanical activity has mainly been performed with multicellular cardiac preparations that give conflicting results concerning Ca2+ transients during isometric twitches and during twitches with unloaded shortening. In the present study, we coupled intracellular Ca2+ measurement optics with a force measurement system using carbon fibers to examine the relation between Ca2+ transients and the mechanical activity of rat single ventricular myocytes over a wide range of load. To minimize the possible load dependence of sarcoplasmic reticulum Ca2+ loading, contraction mode was switched at every twitch from unloaded shortening to isometric contraction. During a twitch with unloaded shortening, the Ca2+ transients exhibited a higher peak and a higher rate of decay than transients during an isometric twitch. Similarly, when we changed the contraction mode in every pair of twitches, Ca2+ transients were dependent only on the mode of contraction. Mechanical uncoupling with 2,3-butanedione monoxime abolished this dependence on the mode of contraction. Our results suggest that Ca2+ transients reflect the affinity of troponin C for Ca2+, which is influenced by the change in strain on the thin filament but not by the length change per se.

Miniature heart cell force transducer system implemented in MEMS technology

A fully submersible force transducer system for use with isolated heart cells has been implemented using microelectromechanical systems (MEMS) technology. By using integrated circuit fabrication techniques to make mechanical as well as electrical components, the entire low-mass transducer is only a few cubic millimeters in size and is of higher fidelity (approximately 100 nN and 13.3 kHz in solution) than previously available. When chemically activated, demembranated single cells attached to the device contract and slightly deform a strain gauge whose signal is converted to an amplified electrical output. When integrated with a video microscope, the system is capable of optical determination of contractile protein striation periodicity and simultaneous measurement of heart cell forces in the 100-nN to 50-microN range. The average measured maximal force was Fmax = 5.77 +/- 2.38 microN. Normalizing for the cell's cross-sectional area, Fmax/area was 14.7 +/- 7.7 mN/mm2. Oscillatory stiffness data at frequencies up to 1 kHz has also been recorded from relaxed and contracted cells. This novel MEMS force transducer system permits higher fidelity measurements from cardiac myocytes than available from standard macro-sized transducers.

Extent of radial sarcomere coupling revealed in passively stretched cardiac myocytes

The complex geometry of the heart leads to a time-varying and nonuniform distribution of stress and strain on the myocardium during the cardiac cycle. This study investigated the sarcomere length and striation registration behavior resulting from a gradient of strain imposed on the cytoskeleton of isolated cardiac myocytes. Within a myocyte, sarcomeres are organized into domains separated by strips of mitochondria and nuclei. Sarcomeres are interconnected radially at the Z-disc by the exosarcomeric cytoskeleton, composed primarily of the intermediate filament desmin. Chemically skinned myocytes isolated from adult rat hearts were attached using to concentric double-barreled pipettes in such a way as to induce a gradient of applied strain across the width of the cell. At rest lengths, there was variation in the sarcomere length between domains of attached cells. Upon stretch, most domains exhibited proportional increases in sarcomere length, with the initial differences being maintained. However, some domains did not behave in synchrony with the others at shorter sarcomere lengths. Thus, the coupling between sarcomeric domains can allow for a non-linear relationship between sarcomere length and strain. Sarcomeres were tightly coupled in the radial direction within a single domain, but striation mis-registration of up to 0.20 micron between domains was induced by stretch. This indicates the looser nature of the cytoskeletal coupling between domains compared to that within domains. Thus, cardiac myocytes are not rigid functional units, but composite structures whose components consist of functionally semi-independent domains tethered by the cytoskeletal intermediate filament lattice.

Focal extracellular potential: a means to monitor electrical activity in single cardiac myocytes

The focal extracellular potential (FEP) described in this study is an electrophysiological signal related to the transmembrane potential (V(m)) of cardiac myocytes that avoids the mechanical fragility, interference with contraction, and intracellular contact associated with conventional whole cell recording. One end of a frog ventricular myocyte was secured into a glass holding pipette. The FEP was measured differentially between this pipette and a bath pipette while the cell was voltage- or current-clamped by a third whole cell pipette. The FEP appeared as an amplitude-truncated action potential, while FEP duration accurately reflected the action potential duration (APD) at 90% repolarization (APD(90)). FEP magnitude increased as the holding pipette K(+) concentration ([K(+)]) was increased. The FEP-voltage relation was quasi-linear at negative V(m) with a slope that increased with elevated holding pipette [K(+)]. Increasing the membrane conductance inside the holding pipette by adding amphotericin B or cromakalim linearized the FEP-voltage relation across all V(m). The FEP accurately reported electrical activation and APD(90) during changes of stimulation frequency and episodes of cellular stretch.

Sarcomere length-tension relationship of rat cardiac myocytes at lengths greater than optimum

The study was aimed at determining both passive and Ca(2+)-activated forces of single skinned rat cardiac cells. Particular attention was paid to the descending limb of the active length-tension curve while the sarcomeric order of stretched cells was investigated before and during contraction. To analyse sarcomere length and sarcomere-length inhomogeneity, a fast Fourier transform (FFT) was employed. The fundamental frequency in the FFT spectrum is a measure of sarcomere length. The full-width-half-maximum of the first-order line is a measure of sarcomere-length inhomogeneity. In relaxing buffer, the sarcomere-length inhomogeneity of skinned cells increased linearly with mean sarcomere length. Upon Ca(2+)-dependent activation of skinned cells contracting isometrically, mean sarcomere length decreased slightly and inhomogeneity increased; both effects were greater at higher Ca(2+)concentrations. Maximum activation was reached at sarcomere lengths between 2.2 and 2.4 microm, whereas the descending limb of the active length-tension curve approached zero force already at approximately 2.8 microm. This steep force decline could not be explained by overly inhomogeneous sarcomere lengths in very long, contracting cells. Rather, the results of mechanical measurements on single cardiac myofibrils implied that high stretching is accompanied by irreversible structural alterations within cardiac sarcomeres, most likely thick-filament disarray and disruption of binding sites between myosin and titin due to changes in titin's tertiary structure. Loss of a regular thick-filament organization may then impair active force generation. We conclude that the descending limb of the cardiac length-tension curve is determined both by the degree of actin-myosin overlap and by the intrinsic properties of titin filaments.

Stretch-activated whole cell currents in adult rat cardiac myocytes

Mechanoelectric transduction can initiate cardiac arrhythmias. To examine the origins of this effect at the cellular level, we made whole cell voltage-clamp recordings from acutely isolated rat ventricular myocytes under controlled strain. Longitudinal stretch elicited noninactivating inward cationic currents that increased the action potential duration. These stretch-activated currents could be blocked by 100 microM Gd(3+) but not by octanol. The current-voltage relationship was nearly linear, with a reversal potential of approximately -6 mV in normal Tyrode solution. Current density varied with sarcomere length (SL) according to I (pA/pF) = 8.3 - 5.0 SL (microm). Repeated attempts to record single channel currents from stretch-activated ion channels failed, in accord with the absence of such data from the literature. The inability to record single channel currents may be a result of channels being located on internal membranes such as the T tubules or, possibly, inactivation of the channels by the mechanics of patch formation.

A newly designed tensile tester for cells and its application to fibroblasts

A tensile test system for cells has been designed and applied to fibroblasts from the rabbit patellar tendon. It consists of a thermostatic test chamber, an inverted fluorescence microscope, micromanipulators, a direct drive linear actuator, a cantilever-type load cell, and a video dimension analyzer (VDA). The test chamber and the microscope are mounted on a vibration isolator. A cell floated in Hanks' balanced salt solution of 37 degrees C is gripped with a pair of micropipettes which have very fine tips (outer diameter = 20 approximately 30 microm, inner diameter = 3 approximately 5 microm) and are coated with a cell adhesive, Cell-Tak, at their ends. One of the micropipettes is fixed to the load cell; the other one is attached to the linear actuator which is used to stretch the cell. Load applied to the cell is measured with the load cell, while elongation of the cell is determined with the VDA using the images of the ends of the micropipettes as markers. The measurement accuracy of the load cell was +/-0.05 microN. All the fibroblasts tested were firmly attached to the micropipettes during tensile testing, and showed local non-uniform deformation. The maximum load and elongation to failure of the cells were 0.9+/-0.2 microN and 86+/-24 microm, respectively.

The trabecula culture system: a novel technique to study contractile parameters over a multiday time period

In the intact heart, various triggers induce alterations in gene expression that impact on contractile function. Because changes in gene expression reflect altered protein expression patterns after 12-48 h, we developed a system in which intact twitching cardiac trabeculae can be studied for multiday periods. Right ventricular trabeculae from pentobarbital sodium anesthetized rabbits were mounted in a sterile, closed muscle chamber. Over the first 48 h, developed force (Fdev) did not significantly change: 102.3 and 98.9% of the initial Fdev was observed after 24 and 48 h, respectively (n = 8). Also, neither diastolic force, time from peak to 50% relaxation (RT50), nor protein synthesis measured by a [3H]leucine incorporation assay changed significantly over time. Contractile response after > 48 h to an increase in extracellular calcium concentration (1.8 to 2.5 mM; Fdev increased 43.5%, n = 2) or to 1 microM isoproterenol (Fdev increased 138.6% and RT50 decreased 34.9%, n = 2) was similar to those observed in freshly dissected preparations. In conclusion, this system can investigate contractile function of multicellular preparations under well-defined physiological conditions after events that alter gene and consequent protein expression.

Active and passive forces of isolated myofibrils from cardiac and fast skeletal muscle of the frog

1. Force measurements in isolated myofibrils (15 degrees C; sarcomere length, 2.10 microns) were used in this study to determine whether sarcomeric proteins are responsible for the large differences in the amounts of active and passive tension of cardiac versus skeletal muscle. Single myofibrils and bundles of two to four myofibrils were prepared from glycerinated tibialis anterior and sartorius muscles of the frog. Skinned frog atrial myocytes were used as a model for cardiac myofibrils. 2. Electron microscope analysis of the preparations showed that: (i) frog atrial myocytes contained a small and variable number of individual myofibrils (from 1 to 7); (ii) the mean cross-sectional area and mean number of myosin filaments of individual cardiac myofibrils did not differ significantly from those of single skeletal myofibrils; and (iii) the total myofibril cross-sectional area of atrial myocytes was on average comparable to that of bundles of two to four skeletal myofibrils. 3. In maximally activated skeletal preparations, values of active force ranged from 0.45 +/- 0.03 microN for the single myofibrils (mean +/- S.E.M.; n = 16) to 1.44 +/- 0.24 microN for the bundles of two to four myofibrils (n = 9). Maximum active force values of forty-five cardiac myocytes averaged 1.47 +/- 0.10 microN and exhibited a non-continuous distribution with peaks at intervals of about 0.5 microN. The results suggest that variation in active force among cardiac preparations mainly reflects variability in the number of myofibrils inside the myocytes and that individual cardiac myofibrils develop the same average amount of force as single skeletal myofibrils. 4. The mean sarcomere length-resting force relation of atrial myocytes could be superimposed on that of bundles of two to four skeletal myofibrils. This suggests that, for any given amount of strain, individual cardiac and skeletal sarcomeres bear essentially the same passive force. 5. The length-passive tension data of all preparations could be fitted by an exponential equation. Equation parameters obtained for both types of myofibrils were in reasonable agreement with those reported for larger preparations of frog skeletal muscle but were very different from those estimated for multicellular frog atrial preparations. It is concluded that myofibrils are the major determinant of resting tension in skeletal muscle; structures other than the myofibrils are responsible for the high passive stiffness of frog cardiac muscle.

Titin develops restoring force in rat cardiac myocytes

When relaxed after contraction, isolated cardiac myocytes quickly relengthen back to their slack length. The molecular basis of the force that underlies passive relengthening, known as restoring force, is not well understood. In a previous study of titin's elasticity in cardiac myocytes, we proposed that titin/connectin develops restoring force, in addition to passive force. This study tested whether titin indeed contributes to the restoring force in cardiac myocytes. Skinned rat cardiac myocytes in suspension were shortened by approximately 20%, using Ca(2+)-independent shortening, followed by relaxation. Cells were observed to relengthen until they reached their original slack sarcomere length. However, the ability to relengthen was abolished after cells had been treated for 12 minutes with trypsin (0.25 microgram/mL, 20 degrees C). Gel electrophoresis showed that this treatment had degraded titin without clearly affecting other proteins, and immunoelectron microscopy revealed that the elastic segment of titin in the I band was missing from the sarcomere. Restoring force was also directly measured, before and after trypsin treatment. Restoring force of control cells was -61 +/- 20 micrograms (per cell) at a sarcomere length of 1.70 microns. Comparison of our results with those of activated trabeculae indicated that a large fraction of restoring force of cardiac muscle originates from within the myocyte. Restoring force of myocytes was found to be depressed after titin had been degraded with trypsin. We conclude that cardiac, titin indeed develops restoring force in shortened cardiac myocytes, in addition to passive force in stretched cells, and that titin functions as a bidirectional spring. Our work suggests that at the level of the whole heart, part of the actomyosin-based active force that is developed during systole is harnessed by titin, allowing for elastic diastolic recoil and aiding in ventricular filling.