BackgroundAn increase in masseter muscle tone in response to halothane or succinylcholine anesthesia (or both) can be observed in healthy persons. Thus the authors compared the fiber‐type halothane and succinylcholine sensitivities in human masseter and vastus lateralis muscles.MethodsMasseter and vastus lateralis muscle segments were obtained from 13 and 9 healthy persons, respectively. After chemical skinning of a single fiber and loading the sarcoplasmic reticulum with Ca++0.16 [micro sign]M solution, halothane (0.5‐4 vol% bubbled in the incubating solution), succinylcholine (0.1 [micro sign]M to 10 mM), or both sensitivities were defined as the concentration inducing more than 10% of the maximum tension obtained by application of 16 [micro sign]M Ca++solution. The myofilament response to Ca++was studied with and without halothane by observing the isometric tension of skinned masseter fibers challenged with increasing concentrations of Ca++. Muscle fiber type was determined by the difference in strontium‐induced tension measurements.ResultsA significant difference in halothane sensitivity was found between type 1 masseter fibers (0.6 +/− 0.2 vol%; mean +/− SD) versus type 1 (2.7 +/− 0.6 vol%) and type 2 vastus lateralis muscle (2.5 +/− 0.4 vol%). Succinylcholine did not induce Ca++release by the sarcoplasmic reticulum. In the masseter muscle, 0.75 vol% halothane decreased the maximal activated tension by 40% but did not change the Ca++concentration that yields 50% of the maximal tension.ConclusionsThe very low halothane threshold for Ca++release from the masseter muscle usually could be counteracted by a direct negative inotropic effect on contractile proteins. However, halothane may increase the sensitivity of the sarcoplasmic reticulum Ca++release to succinylcholine‐induced depolarization, leading to an increase in masseter muscle tone.