Search

1. By applying the X‐ray diffraction method to chemically skinned papillary muscles of the rat, the transfer of myosin heads from the thick to the thin filaments was studied as a function of Ca2+ concentration. 2. No significant transfer of the heads occurred when the Ca2+ concentration was below the threshold of contraction (pCa 6.2). 3. During the maximum isometric contraction at pCa 4.4, 80% of the myosin heads were transferred to the thin filament. 4. When the muscle was activated isometrically at low Ca2+ concentrations (pCa 6.2‐5.8), where the average tension was less than 20% of the maximum, a disproportionately large number of myosin heads were transferred to the thin filament. 5. It was concluded that a significant fraction of the heads transferred at the low Ca2+ concentrations does not produce tension.

1. Fast (posterior latissimus dorsi, PLD) and slow (anterior latissimus dorsi, ALD) muscles of the chick were studied by time‐resolved X‐ray diffraction using a synchrotron radiation source. 2. In both muscles and at both 20 and 30 degrees C, intensities of the X‐ray equatorial reflections changed faster than tension at the beginning of tetanus. When the intensity change was converted into the mass transfer from the thick to the thin filament, the difference between the half‐rise times of the transfer and tension development at 20 degrees C was 140 ms in ALD and 37 ms in PLD. At 30 degrees C it was 110 ms and 10‐20 ms for ALD and PLD respectively. 3. These results indicate that in the early stage of contraction, some of the myosin heads in the vicinity of the thin filament are developing little or no tension, and suggest that the fast and slow muscles differ in the transition rate of myosin heads from the state of attachment with low tension to that with high tension.

1. A position sensitive X‐ray counter was connected to a data‐collection system which registered the outputs of the counter as a function of time. This enabled us to study time‐dependent changes in the intensities of the equatorial reflexions from frog sartorius muscle during an isometric twitch. 2. The intensity ratio of the 1.0 and 1.1 equatorial reflexions (I1,0/I1,1) decreased during a twitch. The time at which the intensity ratio started to decrease coinsided approximately with the onset of tension development. No detectable change in the intensity ratio occurred during the latency relaxation. 3. The intensity ratio reached a minimum level approximately 90 msec before the peak of tension, and stayed at that level until the peak tension. 4. The intensity ratio returned to the 'resting' value 0.7‐‐1.1 sec after the twitch tension had fallen to zero. 5. The observed decrease in the intensity ratio was interpreted as being caused by radial movements of the myosin projections from the vicinity of thick filaments to that of thin filaments. This leads us to conclude that, early in the rising phase of twitch tension, the number of myosin projections in the vicinity of thin filaments reaches the value obtained during a maximum isometric tetanus.

A mouse toe muscle was chemically skinned with saponin and the 1,0 spacing of the hexagonal myofilament lattice at a sarcomere length of 2.5 micron was measured with the X‐ray‐diffraction method. In the relaxed state, the 1,0 spacing was 40.8 nm. When the muscle was maximally activated at pCa 4.4, the spacing decreased to 38.4 nm. During contractions at lower calcium concentrations, the spacing decreased less. In rigor, the spacing decreased to almost the same extent as during maximum contraction, although the rigor tension was only 8% of the maximum tension. When the spacing in relaxed muscle had been adjusted osmotically to about 38 nm, activation caused no further decrease in the spacing. The results support the view that the force responsible for the lattice shrinkage during contraction is produced by cross‐bridges displaced from their optimum lateral positions.

The cross‐bridge movements underlying the tension responses of active muscle to slow length changes were studied by a time‐resolved X‐ray diffraction method. During an isometric tetanus at 2 degrees C, the meridional reflexion at 1/14.3 nm‐1 was 55% more intense than in the resting state, suggesting that the myosin heads maintain the 14.3 nm periodicity of the thick filament. When active muscle was stretched by 7% at a constant speed of 0.03‐0.70 muscle lengths s‐1, the intensity of the meridional reflexion decreased progressively as the tension increased continuously during the stretch. This suggests that the myosin heads spread out along the thick filament. During stress relaxation after a stretch, the intensity returned gradually toward the active isometric level, suggesting a rearrangement of the myosin heads. The meridional intensity changed in a similar manner when active muscle was released by 7% at the same speeds; it decreased progressively during the release and returned gradually to the isometric level after completion of the release. The intensity decrease during a release was smaller than that during a stretch, provided the speed was low (0.03‐0.09 muscle lengths s‐1). It was concluded that the tension responses to slow length changes are due to shifts of the myosin heads along the thick filament, and that the elastic element responsible for tension production is located in the myosin molecules.

1. The equatorial X‐ray diffraction pattern was recorded from a papillary muscle of a cross‐circulated canine heart at different phases of the cardiac cycle. The intensity ratio of the 1, 0 and the 1, 1 reflexions (I1, o/I1,1) was 0‐79 in the systolic phase and 1‐19 in the diastolic phase. 2. Using the intensity ratio obtained, the approximate proportion of the myosin projections present in the vicinity of the thin filaments was calculated. This was 70‐71% in the systolic phase and 51‐52% in the diastolic phase of the total myosin projections. 3. The peak systolic tension was roughly proportional to the proportion of the projections present in the vicinity of the thin filaments during systole. 4. The projections which stayed in the vicinity of the thin filaments during diastole did not produce significant contractile force.