PubMed · 9291795
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P V Scott. 1997. Halothane or halitosis?. https://pubmed.ncbi.nlm.nih.gov/9291795/
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Volatile anesthetic concentrations have been difficult to measure, but are an important experimental parameter for in vitro studies of anesthetic actions. Calcium sensitive electrodes were investigated as a means of continuously monitoring anesthetic concentrations in artificial cerebrospinal fluids (ACSF). Anesthetic-induced Ca2+ electrode signals were compared at room (22 degrees C) and physiological (35 degrees C) temperatures. Electrophysiological measures of anesthetic effects on synaptic potentials provided a bioassay. Halothane and isoflurane produced negative changes in calcium electrode potentials which were linearly related to concentrations over a clinically useful range (0.5-1.5 MAC). Anesthetic-induced voltages persisted in nominally zero Ca2+ ACSF and even in deionized water. A good correlation (r>0.9) was found for calcium electrode measures of anesthetic concentration and synaptic response depression produced by halothane, at both 22 and 35 degrees C. These results support three conclusions: (1) calcium sensitive electrodes provide a useful measure of volatile anesthetic concentrations in aqueous solution. (2) Care must be taken when using these electrodes for Ca2+ concentration measurements, if a volatile anesthetic is also to be used, since the anesthetic could introduce an appreciable error (>50%). (3) A temperature change of 13 degrees C had surprisingly little effect on Ca2+ electrode responses or on synaptic depression produced by anesthetics.
We investigated the effects of intrathecal (i.t.) lidocaine, inhalation sevoflurane, and a combination of i.t. lidocaine and sevoflurane on the formalin test in rats. Group 1 (control) received i.t. saline 10 microl. Groups 2 and 3 received i.t. lidocaine 200 microg and 400 microg, respectively. Groups 4-6 received 1.2%, 2.4% and 3.6% sevoflurane, respectively. Interaction of drugs was analyzed using a dose addition model. Group 7 received i.t. lidocaine 200 microg and 1.2% sevoflurane. The biphasic behavioral activity of the hindpaw of rats was observed. This study showed that i.t. lidocaine or inhalation sevoflurane before formalin injection, significantly suppressed the behavioral activity of the hindpaw of rats, and that this suppression was significantly potentiated by the co-administration of i.t. lidocaine and inhalation sevoflurane.
Firefly luciferase (FFL) has been used as a lipid-free protein model to study direct interaction of anesthetics with proteins. FFL emits a burst of light when luciferin and ATP are added in the presence of oxygen. Volatile anesthetics inhibited FFL at mM ranges, while myristic acid inhibited it at microM range. Despite the large difference, octanol/water partition coefficients of both myristic acid and halothane are 199. Differential scanning calorimetry (DSC) showed that thermal transition occurred at 38.5 degreesC with excess enthalpy of denaturation of 91.9 kcal mol-1. The transition, however, was irreversible. According to the irreversible transition kinetics, the anesthetic effects were evaluated by the temperature where the irreversible transition is half completed (T1/2). Volatile anesthetics decreased T1/2 at mM ranges, while myristic acid and oxyluciferyladenylate (luciferin competitor) increased it at microM ranges. Luciferin is a heterocyclic carboxylate and acylates AMP. Carboxyl group of myristic acid appears to make a high affinity contact to the luciferin-recognition sites. The induced-fit theory states that binding of substrates induces the enzyme into high-energy transition state. Myristic acid stabilized FFL at the transition state, which resisted thermal denaturation. Anesthetics destabilized FFL by reversibly unfolding the protein into less active intermediate states and promoted irreversible transition when the temperature is elevated.