ON THE RELAXING FACTOR SYSTEM IN RABBIT RED MUSCLE

Abstract

Microsomal fractions (between 12, 400×g and 67, 500×g) were obtained from rabbit red (soleus, semitendinosus) and white (psoas) muscle. The relaxing activity, the ATPase activity and the calcium uptake of the microsomal fractions of these muscles were studied under various experimental conditions.
The following results were obtained.
1. The microsomal fraction of red muscle inhibited the mixed myofibrillar ATPase activity, i. e. relaxing activity was about 60% at the Q value 10, and it relaxed also the single glycerol-extracted muscle fiber prepared from rabbit psoas. The relaxation was reversed by addition of 20μM calcium and was spontaneously recovered by the further incubation.
2. The ATPase activity of red microsomal fraction was remarkably high and was inhibited about 50% on aging for one week at 0°C or by 5 to 20 mM azide, and it was not affected by 0.5 mM EGTA. This ATPase activity was not activated by addition of 20 to 200μM calcium. The effect of calcium on the ATPase was not shown, even when the ATPase of mitochondria presumably contaminating the red microsomal fraction was inhibited on aging or by azide.
3. The red microsomal fraction was able to accumulate calcium; the amount of calcium uptake was shown to be about 0.7μmoles/mg of protein/20 min at 200μM added calcium concentration. At 20μM added calcium concentration, more than 90% of the added calcium was removed from the reaction medium by the red microsomal fraction within 10 minutes.