Figure 7. Effect of nanomolar dantrolene sodium on caffeine-induced tensions in human single skinned fiber. Isometric tension recorded from human skeletal single skinned fiber. Both records were obtained from same fiber. Control: sarcoplasmic reticulum was loaded during 1 min pCa 6.6 solution. After two washes (in washing solution), tension was challenged by caffeine. To investigate the effect of low concentration of dantrolene sodium (DS) on Calcium²+ sequestration by SR, fibers were pretreated by 5 nM of DS during 5 min, and then control protocol was repeated. Bar = exposure time for each concentration of caffeine tested. Calibrations: vertical, 15 mg; horizontal, 40 s. Pooled data (N = 10, mean+/- SE) of:(A) peak tension and (B) dT/dt induced by incremental concentration of caffeine using protocol described above. Ordinate of A and B represent, respectively, percent of maximal tension and percent of dT/dt maximal induced by 20 mM of caffeine. *P < 0.05. **P < 0.01.