E. L. de Beer

Effect of sarcomere length and filament lattice spacing on force development in skinned cardiac and skeletal muscle preparations from the rabbit

SummarySkinned cardiac and skeletal muscle freeze-dried preparations were activated in solutions strongly buffered for Ca2+. The response of single skeletal muscle fibres or thin strips of papillary muscle was investigated in relation to changes in Ca content of the perfusate. Sarcomere length was set and controlled during the experiments. The relation between the negative logarithm of the Ca concentration, the pCa, and the normalized developed force proved to be sigmoidal. The exact position of these curves proved to be dependent upon both sarcomere length and the distance between the filaments. The latter was shown by means of osmotic compression of the fibres using dextran. As a consequence of these observations. it was concluded that the length-tension relation is dependent upon the actual Ca concentration. The results are discussed in terms of cross-bridge interaction.

The length dependency of calcium activated contractions in the femoral artery smooth muscle studied with different methods of skinning

The relationship between the calcium concentration and the isometric tension obtained with different techniques of skinning provides information on the biochemical events of contraction in vascular smooth muscle. Muscle preparations of the rabbit femoral artery were skinned with triton X-100, saponin, β-escin and α-toxin and the relationship between the calcium concentration and isometric tension was determined at different preparation lengths. We determined the calcium sensitivity as a function of muscle length with different techniques of skinning. At a pCa of 6.0, triton X-100 skinned smooth muscle of the femoral artery generated 50% of the maximal tension. In α-toxin skinned preparations, this calcium sensitivity was shifted to a pCa of 5.6. The sensitivity of the saponin and β-escin skinned preparations were in between those of the triton X-100 and the α-toxin skinned preparations. The cooperativity of the regulation of contraction varied among the differently skinned preparations between 3 (α-toxin) and 6 (triton X-100). The relationships between the calcium concentration and the isometric tension of the differently skinned preparations up to the optimal length for tension generation did not exhibit any length dependency. The length tension relationship, obtained from the maximal response at the highest calcium concentration is in line with that from other studies. The presence of intracellular proteins and membranes affects the regulation of contraction in smooth muscle of the femoral artery.

The effect of actin filament compliance on the interpretation of the elastic properties of skeletal muscle fibres.

Recently, X-ray diffraction studies provided direct evidence for an appreciable length change in the actin filament upon activation. This finding has profound implications on the interpretation of the elastic properties of skeletal muscle fibre. In this study we determined the compliance of the actin filament during activation, using the data obtained previously from quick stretch and release experiments on skeletal muscle fibres of the frog. The effects of filament compliance are demonstrated clearly in the elastic properties of partially activated fibres. The low- frequency elasticity increases linearly with tension, reflecting an increase in the number of force-producing cross-bridges. At higher frequencies, this linearity is lost. In this study we describe the data consistently in terms of a cross-bridge stiffness increasing linearly with tension and a constant Youngs modulus for the actin filament of 44 MN m−2. This corresponds to a compliance of 23 pm μm−1 per kN m−2 tension developed. Using this value for the actin filament Youngs modulus, its contribution to the elastic properties of skeletal muscle fibre of the frog is considered in rigor and relaxation. The filament compliance hardly affects the overall elasticity of the musle fibre in relaxation. In contrast, it contributes to a large extent to the overall elasticity in rigor. Taking account of the filament compliance, we find that the Youngs modulus in rigor exhibits an increase from 14 MN m−2 at frequencies below 500 Hz to 55 MN m−2 above 40 kHz

Tetragonal deformation of the hexagonal myofilament matrix in single skinned skeletal muscle fibres owing to change in sarcomere length

SummarySingle skinned skeletal muscle fibres were immersed in solutions containing two different levels of activator calicium (pCa: 4.4; 6.0). Sarcomere length was varied from 1.6 to 3.5 Μm and recorded by laser diffraction. Slack length was 2.0 Μm. Small-angle equatorial X-ray diffraction patterns of relaxed and activated fibres at different sarcomere lengths were recorded using synchrotron radiation. The position and amplitude of the diffraction peaks were calculated from the spectra based on the hexagonal arrangement of the myofilament matrix, relating the position of the (1.0)- and (1.1)-diffraction peaks in this model by √3. The diffraction peaks were fitted by five Gaussian functions (1.0, 1.1, 2.0, 2.1 and Z-line) and residual background was corrected by means of a hyperbola. The coupling of the position of the (1.0)- and (1.1)-peak was expressed as a factor: FAC=[d(1.0)/d(1.1)]/√3. In the relaxed state this coupling factor decreased at increasing sarcomere length (0.9880±0.002 at 2.0 Μm; 0.900±0.01 at 3.5 Μm). The coupling factor tends toward the one that will be obtained from the squared structure of actin filaments near the Z-discs. At shorter sarcomere lengths a decrease of the coupling factor has also been seen (0.9600±0.005 at 1.6 Μm), giving rise to an increased uniform deformation of the hexagonal matrix, when sarcomere length is changed from slack length. From these experiments we conclude that a change in sarcomere length (from slack length) increases the deformation of the actin-myosin matrix to a tetragonal lattice.

The influence of velocity of length change on tension development in skeletal muscle: Model calculations and experimental results

Force responses obtained during constant velocity length changes on skeletal muscle tissue are simulated by means of two cross-bridge models proposed by Huxley and Simmons (1971, Nature 233, 533-538) and by Julian et al. (1974, Biophys. J. 14, 546-562). An implicit method was used for the numerical approximation in the simulations. The simulated force transients due to constant velocity length changes are found to be in qualitative agreement with re-investigated experimental results obtained from the whole sartorius muscle of the frog. A non-linear tension transient is observed, dependent both on amplitude and on velocity of release revealing an inflexion which gives the transient a shoulder shape. When velocity is increased the inflexion occurs earlier and at a lower tension value. A non-linear transient is observed during stretches performed at moderate velocities. Force responses are found to deviate concavely downwards from a linear time course. Simulations, however, predict a rather linear tension transient for comparable velocities. Implications of the experimental findings are discussed for both models.

Effects of Physiological Doses of Female Sex Hormones on the Mechanical and Electrical Behaviour of the Heart

The effect of physical exercise on female sex hormone metabolism has recently been studied (1, 2, 3). It was found that the plasma concentrations of estradiol and progesterone rapidly increased during exercise. It was recently established (4, 5) that the heart can act as a target organ for these hormones via receptor sites within the cytosol. However, a fast reaction of the heart via a receptor mechanism seems to us unlikely since a protein synthesis would have to be involved. Therefore, we investigated the short term effects of these hormones, in the physiological range of concentrations, on the electrical and mechanical activity of isolated rat and rabbit hearts.

High-speed transient-recording microprocessor system for the analysis of asymmetrical diffraction spectra from straited muscle

A microprocessor based digital recording system has been developed to study the fine structure and asymmetry of diffraction spectra from striated muscle during contraction. Two linear 256-element photodiode arrays provide analog videosignals of the diffraction lines imaged onto these charged coupled devices. The photodiode arrays are alternately read and the videosignals can be digitized and stored within 1.36 ms (two images of 256 points) with a spatial resolution of 5 nm. (In this paper the spatial resolution is considered to be the standard deviation of the first-order maximum of a monochromatic wave of the He/Ne laser measured from the diode-arrays, using ideal gratings with a spacing between 1.6 and 3.6 microns.) The systems memory with a capacity of 192 pairs of images of 256 points can be optimized by means of a threshold to contain about 2000 images without any loss of information. A transient recording approach makes the system capable of recording long term slow phenomena of up to 5 s as well as fast events and the combination of fast events within slow processes. The system presented here has a significantly improved time resolution and storage capacity when compared to other systems and is more versatile. This is the first system which enables the simultaneous examination of the fine structure and asymmetry of diffraction spectra.

The effect of controlled changes in volume on the active state of the rabbit isolated left ventricle

SummaryThe contractile properties of isovolumically contracting isolated rabbit left ventricles are studied under the influence of controlled rapid volume changes during systole and diastole. The time integral of the pressure curve (TTI), representing the active state, is used to quantify the energy consumption of the ventricle. Steady state conditions resulting from an introduced volume change show a TTI/EDV relation which represents the Starling curve. However, immediately after a quick volume increase (decrease) introduced in diastole, the TTI/EDV ratio has a higher (lower) value than indicated by the Starling relation. This shows a volume dependent activation (deactivation), related to changes in the inotropic state of the heart muscle cells within the ventricular wall.A volume increase at a later moment (in systole) always produces a lower rate of activation. Indeed, if the rapid volume change, is introduced at moments later than 70% of time to peak pressure, TTI is less than observed from the Starling mechanism, indicating a deactivation When comparing the decreasing effect on the active state introduced by volume decrease during systole, it is shown that this effect is not only a function of the amplitude of the decrease itself but is highly dependent upon the way EDV is reached.

Non-linear pressure development during velocity controlled volume releases in isolated contracting rabbit left ventricle

Pressure responses obtained during steady-volume releases performed on isolated contracting rabbit left ventricle reveal a typical non-linear time course, dependent on velocity of volume release (VVR) and on amplitude of volume release (AVR). For values of VVR = 4.8 ml/s and of AVR = 0.28 ml (about 20% of the EDV at preload 0.5 kPa) a shoulder shaped pressure-time course is observed. The shoulder is seen during releases independently of the time during systole at which the release starts. When releases are compared which start at the same time during systole but have different velocities then the shoulder appears at higher amplitudes for the higher velocities. The shoulder can be explained by an active actin-myosin interaction within the scheme of a multi-state contraction model.

Force generation and shift of mass between myosin and actin in skinned striated muscle fibres at low calcium concentrations

Abstract Skinned muscle fibres from the gracilis muscle of the rabbit were used to record small angle X-ray diffraction spectra under various contractile conditions. The intracellular calcium concentration, expressed as pCa, was varied between 8.0 and 5.74. Equatorial diffraction spectra were fitted by a function consisting of five Gaussian curves and a hyperbola to separate the (1.0), (1.1), (2.0), (2.1) and Z-line diffraction peaks. The hyperbola was used to correct for residual scattering in the preparation. The ratio between the intensities of the (1.1) and (1.0) peaks was defined as the relative transfer of mass between myosin and actin, due to crossbridge formation after activation by calcium. The relation between the ratio and the relative force of the fibre (normalized to the force at pCa 5.74 and sarcomere length 2.0 μm) was linear. At high pCa (from pCa 6.34 to 8.0) no active force was observed, while the ratio still decreased. Sarcomere length was recorded by laser diffraction. The laser diffraction patterns did not show changes in sarcomere length due to activation in the high pCa range (between 8.0 and 6.34). From these results the conclusion is drawn that crossbridge movement occurs even at subthreshold calcium concentrations in the cell, when no active force is exerted. Since no force is generated this movement may be related to crossbridges in the weakly bound state.