Effect of fiber types, fascicle size and halothane on caffeine contractures in rat muscles.

Caffeine contractures were recorded from thin bundles and whole extensor digitorum longus (EDL) and soleus muscles of rat, and correlated with preparation size and fiber types. Thin bundles were more sensitive to caffeine and halothane than whole muscles, and bundles of 100% type I fibers were more sensitive than bundles of 100% type II fibers. Magnitude of contracture had significant correlation with maximal tetanic tension, total number of fibers, thickness of the preparation, and proportion of type I fibers. These results suggest that fascicle size and fiber types significantly affect results of in vitro contracture test for susceptibility to malignant hyperthermia.