Halothane induces calcium release from human skinned masseter muscle fibers

Background: An increase in masseter muscle tone in response to halothane or succinylcholine anesthesia (or both) can be observed in healthy persons. T hus the authors compared the fiber-type halothane and succinylcholine sensi tivities in human masseter and vastus lateralis muscles. Methods: Masseter and vastus lateralis muscle segments were obtained from 1 3 and 9 healthy persons, respectively, After chemical skinning of a single fiber and loading the sarcoplasmic reticulum with Ca++ 0.16 mu M solution, halothane (0.5-4 vol% bubbled in the incubating solution), succinylcholine (0.1 mu M to 10 nM), or both sensitivities were defined as the concentratio n inducing more than 10% of the maximum tension obtained by application of 16 mu M Ca++ solution. The myofilament response to Ca++ was studied with an d 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 . Results: A significant difference in halothane sensitivity was found betwee n 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%), Succi nylcholine did not induce Ca++ release by the sarcoplasmic reticulum. in th e masseter muscle, 0.75 vol% halothane decreased the maximal activated tens ion by 40% but did not change the Ca++ concentration that yields 50% of the maximal tension. Conclusions: The very low halothane threshold for Ca++ release from the mas seter muscle usually could be counteracted by a direct negative inotropic e ffect on contractile proteins. However, halothane may increase the sensitiv ity of the sarcoplasmic reticulum Ca++ release to succinylcholine-induced d epolarization, leading to an increase in masseter muscle tone.