Functional Implications of the Resting Sarcomere Length‐Tension Curve in Living Heart Muscle
1
Citation
15
Reference
10
Related Paper
Abstract:
The sarcomere pattern and tension of atrial trabeculae isolated from frog hearts have been monitored. The sarcomere length at zero tension varied with the size of the trabeculae but was never less than 1.88 pm, at which length the ends of the thin filaments are at the centre of the A band. Resting tension became large at sarcomere lengths greater than 2.3 m̈m. It was difficult to stretch the trabeculae to produce sarcomere lengths greater than 2.7 m̈m and doing so generally resulted in irreversible changes. Sarcomeres as long as 3.2 m̈m were seen, however, in cells in series with spontaneously contracting fibres. Broadening of the A band at larger sarcomere lengths was interpreted as indicating misalignment of thick filaments and suggests that thick and thin filaments interact in the resting heart. The entire change in length of the central undamaged half of the trabeculae during stretching could be accounted for by the change in sarcomere length.Keywords:
Tension (geology)
Cardiac muscle
Cite
Citations (0)
Muscle fibre
Cite
Citations (59)
The goal of this study was to evaluate if isolated sarcomeres and half-sarcomeres produce a long-lasting increase in force after a stretch is imposed during activation. Single and half-sarcomeres were isolated from myofibrils using micro-needles, which were also used for force measurements. After full force development, both preparations were stretched by different magnitudes. The sarcomere length (SL) or half-sarcomere length variations (HSL) were extracted by measuring the initial and final distances from the Z-line to the adjacent Z-line or to a region externally adjacent to the M-line of the sarcomere, respectively. Half-sarcomeres generated approximately the same amount of isometric force (29.0 ± SD 15.5 nN·μm(-2)) as single sarcomeres (32.1 ± SD 15.3 nN·μm(-2)) when activated. In both cases, the steady-state forces after stretch were higher than the forces during isometric contractions at similar conditions. The results suggest that stretch-induced force enhancement is partly caused by proteins within the half-sarcomere.
Cite
Citations (38)
We studied sarcomere performance in single isolated intact frog atrial cells using techniques that allow direct measurement of sarcomere length and force. The purpose of this investigation was to determine whether length-dependent alterations in contractile activation occur in the single isolated cardiac cell. This was accomplished by determining the effect of initial sarcomere length on the time course of sarcomere shortening and force development during auxotonic twitch contractions. The results presented in this paper demonstrate that the velocity of sarcomere shortening, the rate of force development, and the magnitude of force development during auxotonic twitch contractions all increase as initial sarcomere length increases over the range of about 2 micrometers to greater than 3 micrometers. These results indicate that the level of contractile activation increases as initial sarcomere length increases. Also, results are presented that indicate that the rate of increase of contractile activation during a twitch contraction also increases as initial sarcomere length increases. These length-dependent effects on contractile activation in conjunction with the slow time course of contractile activation cause the force-velocity-length relationship to be time-dependent: i.e., the velocity of sarcomere shortening at a given sarcomere length and load depends on the time during the contraction when the sarcomere reaches that length. The results suggest that length-dependent alterations in contractile activation may play a major role in the improved contractile performance that accompanies an increase in initial sarcomere length in cardiac muscle.
Cardiac muscle
Cite
Citations (18)
The sarcomere pattern and tension of atrial trabeculae isolated from frog hearts have been monitored. The sarcomere length at zero tension varied with the size of the trabeculae but was never less than 1.88 pm, at which length the ends of the thin filaments are at the centre of the A band. Resting tension became large at sarcomere lengths greater than 2.3 m̈m. It was difficult to stretch the trabeculae to produce sarcomere lengths greater than 2.7 m̈m and doing so generally resulted in irreversible changes. Sarcomeres as long as 3.2 m̈m were seen, however, in cells in series with spontaneously contracting fibres. Broadening of the A band at larger sarcomere lengths was interpreted as indicating misalignment of thick filaments and suggests that thick and thin filaments interact in the resting heart. The entire change in length of the central undamaged half of the trabeculae during stretching could be accounted for by the change in sarcomere length.
Tension (geology)
Cite
Citations (1)
In cardiac muscle, contraction is triggered by sarcolemmal depolarization, resulting in an intracellular Ca2+ transient, binding of Ca2+ to troponin, and subsequent cross-bridge formation (excitation–contraction [EC] coupling). Here, we develop a novel experimental system for simultaneous nano-imaging of intracellular Ca2+ dynamics and single sarcomere length (SL) in rat neonatal cardiomyocytes. We achieve this by expressing a fluorescence resonance energy transfer (FRET)–based Ca2+ sensor yellow Cameleon–Nano (YC-Nano) fused to α-actinin in order to localize to the Z disks. We find that, among four different YC-Nanos, α-actinin–YC-Nano140 is best suited for high-precision analysis of EC coupling and α-actinin–YC-Nano140 enables quantitative analyses of intracellular calcium transients and sarcomere dynamics at low and high temperatures, during spontaneous beating and with electrical stimulation. We use this tool to show that calcium transients are synchronized along the length of a myofibril. However, the averaging of SL along myofibrils causes a marked underestimate (∼50%) of the magnitude of displacement because of the different timing of individual SL changes, regardless of the absence or presence of positive inotropy (via β-adrenergic stimulation or enhanced actomyosin interaction). Finally, we find that β-adrenergic stimulation with 50 nM isoproterenol accelerated Ca2+ dynamics, in association with an approximately twofold increase in sarcomere lengthening velocity. We conclude that our experimental system has a broad range of potential applications for the unveiling molecular mechanisms of EC coupling in cardiomyocytes at the single sarcomere level.
Myofibril
Calcium in biology
Myofilament
Troponin C
Cite
Citations (21)
Cardiac muscle
Cite
Citations (0)
We examined length changes of individual half-sarcomeres during and after stretch in actively contracting, single rabbit psoas myofibrils containing 10-30 sarcomeres. The myofibrils were fluorescently immunostained so that both Z-lines and M-bands of sarcomeres could be monitored by video microscopy simultaneously with the force measurement. Half-sarcomere lengths were determined by processing of video images and tracking the fluorescent Z-line and M-band signals. Upon Ca2+ activation, during the rise in force, active half-sarcomeres predominantly shorten but to different extents so that an active myofibril consists of half-sarcomeres of different lengths and thus asymmetric sarcomeres, i.e. shifted A-bands, indicating different amounts of filament overlap in the two halves. When force reached a plateau, the myofibril was stretched by 15-20% resting length (L0) at a velocity of approximately 0.2 L0 s(-1). The myofibril force response to a ramp stretch is similar to that reported from muscle fibres. Despite the approximately 2.5-fold increase in force due to the stretch, the variability in half-sarcomere length remained almost constant during the stretch and A-band shifts did not progress further, independent of whether half-sarcomeres shortened or lengthened during the initial Ca2+ activation. Moreover, albeit half-sarcomeres lengthened to different extents during a stretch, rapid elongation of individual sarcomeres beyond filament overlap ('popping') was not observed. Thus, in contrast to predictions of the 'popping sarcomere' hypothesis, a stretch rather stabilizes the uniformity of half-sarcomere lengths and sarcomere symmetry. In general, the half-sarcomere length changes (dynamics) before and after stretch were slow and the dynamics after stretch were not readily predictable on the basis of the steady-state force-sarcomere length relation.
Myofibril
Cite
Citations (97)
Myofibril
Cite
Citations (5)
When a stretch is imposed to activated muscles, there is a residual force enhancement that persists after the stretch; the force is higher than that produced during an isometric contraction in the corresponding length. The mechanisms behind the force enhancement remain elusive, and there is disagreement if it represents a sarcomeric property, or if it is associated with length nonuniformities among sarcomeres and half-sarcomeres. The purpose of this study was to investigate the effects of stretch on single sarcomeres and myofibrils with predetermined numbers of sarcomeres ( n = 2, 3. . . , 8) isolated from the rabbit psoas muscle. Sarcomeres were attached between two precalibrated microneedles for force measurements, and images of the preparations were projected onto a linear photodiode array for measurements of half-sarcomere length (SL). Fully activated sarcomeres were subjected to a stretch (5–10% of initial SL, at a speed of 0.3 μm·s −1 ·SL −1 ) after which they were maintained isometric for at least 5 s before deactivation. Single sarcomeres showed two patterns: 31 sarcomeres showed a small level of force enhancement after stretch (10.46 ± 0.78%), and 28 sarcomeres did not show force enhancement (−0.54 ± 0.17%). In these preparations, there was not a strong correlation between the force enhancement and half-sarcomere length nonuniformities. When three or more sarcomeres arranged in series were stretched, force enhancement was always observed, and it increased linearly with the degree of half-sarcomere length nonuniformities. The results show that the residual force enhancement has two mechanisms: 1) stretch-induced changes in sarcomeric structure(s); we suggest that titin is responsible for this component, and 2) stretch-induced nonuniformities of half-sarcomere lengths, which significantly increases the level of force enhancement.
Cite
Citations (54)