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

Single skinned skeletal muscle fibres were immersed in solutions containing two different levels of activator calcium (pCa: 4.4; 6.0). Sarcomere length was varied from 1.6 to 3.5 microns and recorded by laser diffraction. Slack length was 2.0 microns. 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 square root of 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)]/square root 3. In the relaxed state this coupling factor decreased at increasing sarcomere length (0.9880 +/- 0.002 at 2.0 microns; 0.900 +/- 0.01 at 3.5 microns). 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 microns), 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.

Length-dependent activation by Ba2+ and Sr2+ of skinned cardiac and skeletal muscle of the rabbit.

Over a wide range of sarcomere lengths, force activation by Ca2+, Ba2+, and Sr2+ was studied in papillary muscle and in fast skeletal fibers of the gracilis muscle of the rabbit, both skinned by means of freeze drying. The length-tension relations of Ba2+ activation differ significantly from those of Sr2+ and Ca2+ activation with respect to both the value and the position of the maximum. At (almost) full activation, force induced in gracilis muscle by Ba2+ was 50% of the developed force induced by Ca2+. The position of the Sr2+ sensitivity curve for papillary muscle preparations is independent of sarcomere length, in contrast to the position of the Ca2+ sensitivity curves. The binding of Sr2+ to the papillary preparation proves to be very stable as observed from the long-lasting relaxation after activation. Immersion of the papillary preparation in the relaxation fluid after activation with Ba2+ results in a tension transient: a rise in tension followed by a decrease was observed. The maximal value of the tension transient was up to twice the steady tension, dependent on Ba2+ concentration. The steady-state tension was approximately 50% of the Ca2(+)-induced tension. Ba2+ sensitivity curves are not sigmoidal but show a maximum. Above [Ba2+] greater than 10(-5) to 10(-4) M (dependent on sarcomere length) tension decreased. These observations suggest that two counteracting processes govern Ba2+ contraction in papillary muscle preparations, namely activation and inhibition.

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

Skinned 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.

Caffeine suppresses length dependency of Ca2+ sensitivity of skinned striated muscle.

Freeze-dried skinned cardiac and skeletal muscle preparations of the rabbit were immersed in Ca2+-containing solutions with different concentrations of caffeine. The relation between the negative logarithm of the Ca2+ concentration (pCa) and normalized developed force was studied. The exact position of these Ca2+-sensitivity functions proved to be dependent on both the sarcomere length (monitored by means of laser diffraction) and caffeine concentration. High concentrations of caffeine induce a reversible fall in tension, particularly at low binding site saturation (low pCa) and long sarcomere lengths. At a concentration of 10 mM caffeine, the sarcomere length dependency of the Ca2+-sensitivity curves is markedly reduced for the rising part of the curve. Only the depressive effect of caffeine at high pCa remains. A possible mechanism of caffeine action is discussed.