Caffeine Contracture in Rat Soleus Muscle

Caffeine contracture was investigated in the small bundle (0.3 mm in width .0.1 mm in thickness) of the rat soleus muscle at the solution temperature of 37℃. The threshold concentration of caffeine was 1mM. The contracture tension was increased with rising the concentration of caffeine. The contracture with two peaks was obtained in the standard solution containing more than 10mM caffeine. The first peak appeared immediately after the addition of caffeine and the second peak was produced on the early falling phase of the first peak. The second peak could never be observed with less than 10mM caffeine. The tetanic tension caused by the electrical stimulation (80 Hz, 4 sec) could be superimposed on the contracture with exception of the falling phase of the second peak. The tensons of two peaks were not affected with the sustained depolarization by high K^+ (about 216mM), while 10mM procaine, added into the caffeine containing solution, decreased the tensions of two peaks in the contracture. The tension of the second peak was reduced in the Ca^<2+>-free solution with caffeine. Furthermore, this was completely suppressed by the addition of 1 mM EGTA to the Ca^<2+>-free solution containing caffeine. However, no change in the tension of the first peak was found with the deprivation of Ca^<2+> in the external solution. These results suggest that the second peak might result from caffeine-induced Ca^<2+> release from the sarcoplasmic reticulum which was replenished by the external Ca^<2+> after the first peak was developed.