Effect of transverse tubule-disruption on 14C-caffeine influx in frog skeletal muscle

Effect of transverse tubule-disruption (T-disruption) on 14C-caffeine influx was studied using small bundles, consisting of 40 to 50 fibers, of frog semitendinosus muscle. The peak tension of caffeine contracture was inhibited by about 69% at 5 mM caffeine and by about 31% at 7 mM caffeine by T-disruption. In addition, the amount of caffeine influx in a small bundle was inhibited by about 52% at 5 mM caffeine and by about 28% at 7 mM caffeine by T-disruption, corresponding to the inhibition of the peak tension of caffeine contracture by T-disruption. These findings support our previous suggestion that caffeine enters the muscle fibers through the surface membrane and transverse tubuler membrane (T-membrane), and that the T-membrane contributes to the entry of caffeine more greatly at 5 mM caffeine than at 7 mM caffeine. Moreover, at 5 mM caffeine it was noted that there is a slight difference between the extent of inhibition of the caffeine influx and that of the peak contracture tension. From these results, it could be concluded that the inhibition of peak tension of caffeine contracture by T-disruption is mainly due to the inhibition of caffeine influx which occurs through the T-membrane and is partly due to the functional disturbance of the sarcoplasmic reticulum which is probably induced by T-disruption.