Caffeine sensitivity of sarcoplasmic reticulum of fast and slow fibers from normal and malignant hyperthermia human muscle.

We have carried out a comparative study of caffeine sensitivity of the sarcoplasmic reticulum (SR) of fast and slow normal human fibers chemically skinned. Human slow-fiber SR is more sensitive to caffeine than fast fiber SR; however, it releases less calcium and at a lower rate than the SR of fast fibers when exposed to threshold concentrations of caffeine. These results indicate that the SR calcium release mechanisms of SR of fast and slow human fibers are homologous but not identical. An increased sensitivity of SR to caffeine is found in both fast and slow fibers from human malignant hyperthermia muscle. However, fast fibers seem to be the most affected, since their caffeine threshold for contraction is very close to that of slow fibers.