LABORATORY REPORT

Local Anesthetic-Sensitive Electrodes: Preparation of Coated-Wire Electrodes and Their Basic Properties In Vitro Atsuko Yokono, MD, Hiromu Satake, Kenji Ogli, MD

PhD,

Shoji Kaneshina,

PhD,

Satoshi Yokono,

MD,

and

Department of Anesthesiology and Emergency Medicine, Kagawa Medical School, Kagawa, Japan, and Department of Biological Science and Technology, Faculty of Engineering, The University of Tokushima, Tokushima, Japan

Coated-wire electrodes with local anesthetic (LA) cation-selective membranes were prepared, and their properties in vitro were investigated. Copper wires (0.8-mm diameter) were coated with gel membranes of 110 mg of poly(viny1 chloride), 5 mg of ion pairs of tetraphenylborate anion with LA cation, 100 mg of dioctylphtalate, and 1.5 mL of tetrahydrofuran. This was the composition determined to be most suitable. Their electromotive force relative to an Ag/AgCl electrode was measured in LA solutions. The lidocaine, dibucaine, and mepivacaine electrodes all showed good Nernstian response at 25°C in aqueous solutions in the concentration ranges of 1 x lop4 to 1 x lo-' mol/L, 4 x to 1 x lo-' mom, and 5 x mom, respectively. The response to 1 x time was within 10 s . The electrode potential decreased as the pH in the solution increased, with a

T

he concentration of local anesthetics in plasma, like that of many other drugs, is usually assayed by high-performance liquid chromatography, gas chromatography, radioimmunoassay, or fluorescence polarization immunoassay . These methods require special techniques and equipment and are expensive for routine use. They also require the sampling of a certain amount of blood. As a result, it is difficult to discern the detailed pharmacokinetics concerning the effects or intoxication of local anesthetics. We have developed coated-wire electrodes sensitive to local anesthetic cations that are small and feasible for future use in vivo. This report describes the basic principle of the sensor system, preparation, and some properties of the electrodes in vitro. Supported in part by Grants-in-Aid of Scientific Research (No. 02455015 and No. 03857197) from the Ministry of Education, Science and Culture, Japan. Accepted for publication July 21, 1992. Address correspondence to Dr. Yokono, Department of Anesthesiology and Emergency Medicine, Kagawa Medical School, 1750-1, Ikenobe, Miki-Cho, Kita-Gun, Kagawa, 761-07, Japan. 81992 by the International Anesthesia Research Society 0003-2999/92'$5.00

corresponding decrease of the protonated form of LA. The hydrophobic nature of the LA was closely related to the electromotive force and to the selectivity of the electrode toward various LA cations. Dibucaine, the most hydrophobic, had the highest electrode potential. The more hydrophobic the LA of the electrode, the less it is interfered with by other LA molecules. The more hydrophobic the interferent cation, the more it acts on the electrode potential. The electrode system could also measure LA in human plasma at 37"C, although the responsiveness was depressed in the low concentration range owing to binding of LA to the serum protein. Its small size, convenience of handling, good response, and simplicity of measurement indicate the feasibility of the electrode system for in vivo application. (Anesth Analg 1992;75:1063-9)

Principle Local anesthetics used clinically are basically tertiary amine compounds. Most of the molecules exist in a protonated form (cation) in vivo, according to the Henderson-Hasselbalch equation. In general, the activity (which is nearly equal to "concentration" in dilute solutions) of a cation or anion can be determined electrochemically. When two solutions with different concentrations of an ion are separated by a certain ion-selective membrane, ions keep moving through it until equilibrium is attained, where the chemical diffusion force balances the electromotive force. This is just the basic concentration cell. The Nernst equation shows that the ratio of the ion activity of both solutions corresponds to the electric potential difference between them; pWmV meters in general use are based on this principle. The main problem with this method is how to make the membrane selective only to the particular ion concerned. Local anesthetics are amphiphilic. The amine end is hydrophilic and can be positively charged, whereas the aromatic end is greatly hydrophobic. If a cationAnesth Analg 1992;75:106X9

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exchange membrane (e.g., poly(viny1chloride) [PVC] membrane) incorporates a certain kind of local anesthetic cation with some hydrophobic anions as ion carriers, the hydrophobic nature enables this membrane to react only to the local anesthetic cations concerned in the surrounding solution. Thus, a local anesthetic cation-selective electrode can be obtained that consists of this ion-selective membrane and an inner solution of a known concentration of the local anesthetic. When this electrode is immersed in an ionic solution of unknown concentration (the outer solution), the ratio of the two concentrations can be calculated from the electromotive force between the inner and outer solutions. This type of sensor is easily made, and its use in potentiometry exhibits good response; however, it is still too large to insert into blood vessels for the purpose of in vivo investigations. We developed another type of electrode: a coatedwire electrode that operates as described earlier. This sensor simply consists of a copper wire coated with the membrane described previously and has no inner solution. The details of the mechanism by which the activity of local anesthetic cations surrounding the sensor membrane is converted to the electric potential of the wire are stiIl unknown. However, this simple sensor also works in accordance with the Nernst equation (Appendix 1) (1). Because it needs no inner solution and can be miniaturized, it may be applicable to in vivo use. In the following, we report the preparation of the coated-wire electrodes sensitive to some amide-type local anesthetics: lidocaine electrodes, dibucaine electrodes, and mepivacaine electrodes, and their basic properties for potentiometric analysis in vitro.

Methods Construction of Electrodes The coated-wire electrodes were prepared by the same method described previously (2). As shown schematically in Figure 1, copper wires (0.8 mm diameter) were coated to a thickness of about 0.4 mm with membranes of PVC matrix that included the ion pairs, dioctylphtalate (DOE'), and tetrahydrofuran (THF) in various compositions. A high-molecular-weight PVC (Wako, Japan) was used. To make the ion exchangers of the membrane, local anesthetic-tetraphenylborate ion pairs were prepared from hydrochlorides of local anesthetics and sodium tetraphenylborate (NaTPB) (Aldrich) according to the same method described previously (2) for other ion pairs. Hydrochlorides of local anesthetics used were purchased from Sigma Chemical Co. (St. Louis, Mo.), except for lidocaine/HCl (Fujisawa

+-Copper wire (0.8 mm OD)

Copper wire

Ag / AgCl 3M KCI Glass tube

(2.0mm OD)

3M KCI - agar bridge Fluoropore ( 0 . 4 5 ~ )

Araldite

t

Normal saline solution (NS)

Ion Selective Membrane

c Araldite

NS - agar

Figure 1. The structure of the coated-wire electrode (left) and the reference electrode (right).

Yakuhin, Japan). Dioctylphtalate (Tokyo Kasei, Japan) was used to plasticize the PVC membrane. Tetrahydrofuran (Wako, Japan) was used as a common solvent for the ion pair, DOP, and PVC. Glass and epoxy resin (Araldite) covered the surface of the wires to maintain isolation, except for the membrane portion. The electrodes were allowed to stand for at least 10 h in contact with air, and afterward were stored by immersion in molL solutions of the local anesthetics for which they were selective.

Potential Measurement The coated-wire electrodes were used as the indicator electrodes, whereas an Ag/AgCl electrode connecting a normal saline agar bridge was the reference electrode (Figure 1).Immersing both the electrodes in test solutions (i.e., local anesthetics to which the electrode was selective in 0.15 M, pH 7 phosphate buffer), we recorded the electromotive force. The electrochemical system was as follows: local anesthetic coated-wire electrode/test solutiodnormal saline agar bridge/3 moYL KC1 solutiodAg/AgCl electrode. The electromotive force was measured with a multi-ion monitor (Yamashita Giken, Tokushima, Japan) at 25°C in the constantly stirred solution. A circulating water bath controlled the temperature of the test solution. To construct calibration curves (electromotive force vs concentration), suitable increments of local anesthetic solution were added so as to cover the concentration range to lo-' moUL. We prepared five coated-wire electrodes, each with 8 to 13 types of membrane compositions. We investigated the effect of membrane composition on the responsiveness, stability, and reproducibility of the electrode potential and determined the suitability of the composition according to these points. Using

ANESTH ANALG 1992;75:1063-9

LABORATORY REPORT

the electrodes with the most suitable composition, we performed the following tests.

pH change of the solution. We examined the effect of pH of the test solution on the electrode potential for mollL solutions of local anesthetics in normal saline at 25°C. The pH was changed repeatedly in the range 4-11 by the addition of lo-’ m o m hydrochloric acid or sodium hydroxide solutions, or both, containing lop4 mol/L of the corresponding local anesthetic. Selectivity of the electrodes. The influence of other local anesthetic cations toward each electrode potential was investigated. Selectivity coefficients (Appendix 2) were determined by the separate solution method (3,4) using lo-’ mol/L of local anesthetic solutions. Response in plasma. The response of the electrodes in human plasma was also examined to ascertain the feasibility of their use in vivo. Sensitivity, stability, and reproducibility of the electrode potential were checked at 37°C in constantly stirred Plasmanate (Miles Inc.) with the same electrode system.

Results Composition of Electrode Membrane The best results were obtained with an electrode with a membrane prepared from mixtures of 100 mg of DOP, 110 mg of PVC, and 1.5 mL of THF for all local anesthetic ion-selective membranes. As for ion pairs, the potential response curves shifted in a parallel manner to the negative side as the membrane content of the local anesthetic-TPB ion pairs increased. However, the slope of the linear part of the curve did not change when the range of the ion pair content was within 1-25, 1-10, and 1-25 mg for lidocaine, mepivacaine, and dibucaine, respectively, the mixtures DOP, PVC, and THF being held constant as above. When the contents of DOP (

Local anesthetic-sensitive electrodes: preparation of coated-wire electrodes and their basic properties in vitro.

Coated-wire electrodes with local anesthetic (LA) cation-selective membranes were prepared, and their properties in vitro were investigated. Copper wi...
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