抄録
Some properties of the relaxing factor systems and the calcium sensitivity of myofibrils prepared from rabbit red muscle were studied with the following results.
1. The mixed myofibrillar ATPase activity was inhibited by the microsomal fraction and the effective eluate prepared from red muscle by about 60 and 40%, respectively. These relaxing activities were removed by caffeine depending on the concentrations.
2. The supernatants prepared from red muscle homogenates inhibited the mixed myofibrillar ATPase activity by about 15% at the 10 vol.%.
3. Five mM azide did not affect the calcium uptake of the red microsomal fraction or the inhibitory effect of the red mitochondrial fraction on the mixed myofibrillar ATPase activity.
4. Over the range of pCa from 7.9 to 5.2, the ATPase activities of red and white myofibrils increased with decreasing pCa and reached a maximum at pCa 5.5. The maximum ATPase activity of red myofibrils was about one fourth that of the white ones. The ATPase activity of white myofibrils was more remarkably activated than that of red myofibrils by decreasing pCa at lower than 6.5.
5. The ATPase activity of red myofibrils was less inhibited by EGTA than that of white ones; for 50% inhibition, 30 μM EGTA was required in red myofibrils and 10 μM in white ones. In the presence of 5 mM azide, the inhibitory effect of EGTA on red myofibrils increased only about 15%.
6. There was no difference in the calcium contents between red myofibrils and white ones.
On the basis of these results, the relaxation mechanism of living red muscle was discussed.
