Pharmacology of local anesthetics

-Effects of Local Anesthetics on Smooth Muscle Tissue in Rat Trachea: An In Vitro Study

OBJECTIVES

We evaluated the muscle responses of rat trachea to LA drugs, such as lidocaine and prilocaine, in terms of airway spasms.

MATERIALS AND METHODS

A total of 16 male rats were used. After ketamine anesthesia, the tracheal ring of each rat was removed and placed in the organ bath in the Krebs solution. The rat tracheal veins were randomly divided into two groups based on the LA applied at the basal tonus level: group 1 (n=8), lidocaine; group 2 (n=8), prilocaine. Second, the baths were washed. Supramaximal contraction was obtained by applying acetylcholine to the tracheal rings (n=16) at a basal tonus level. The rat tracheas with supramaximal contraction were randomly divided into two groups: group 3 (n=8), lidocaine; group 4 (n=8), prilocaine. The contraction responses of each group were recorded and statistically compared.

RESULTS

Lidocaine constituted a significant relaxation response in the tracheal tissue in both basal tonus and supramaximal tonus levels. Moreover, it was observed that the relaxation of lidocaine was higher in the supramaximal contraction than in the basal tonus tension level. However, for prilocaine, no significant change was observed in both tonus levels.

CONCLUSION

This study suggests that lidocaine as a LA drug should be preferred as the first choice in patients with respiratory risk, and that its use over prilocaine should be preferred, if supported by advanced clinical studies.

Cytotoxic effect and possible mechanisms of Tetracaine on human corneal epithelial cells in vitro

AIM

To demonstrate the cytotoxic effect and possible mechanisms of Tetracaine on human corneal epithelial (HCEP) cells in vitro.

METHODS

In vitro cultured HCEP cell were treated with Tetracaine hydrochloride at different doses for different times, and their morphology, viability, and plasma membrane permeability were detected by light microscopy, methyl thiazolyl tetrazolium (MTT) assay, and acridine orange (AO)/ethidium bromide (EB) staining, respectively. Their cell cycle progression, phosphatidylserine orientation in plasma membrane, and mitochondrial membrane potential (MTP) were assessed by flow cytometry. DNA fragmentation, ultrastructure, caspase activation, and the cytoplasmic apoptosis inducing factor (AIF) and cytochrome c (Cyt. c) along with the expression of B-cell lymphoma-2 (Bcl-2) family proteins were examined by gel electrophoresis, transmission electron microscope, enzyme linked immunosorbent assay (ELISA), and Western blot, respectively.

RESULTS

After exposed to Tetracaine at doses from 10.0 to 0.3125 g/L, the HCEP cells showed dose- and time-dependent morphological abnormality and typical cytopathic effect, viability decline, and plasma membrane permeability elevation. Tetracaine induced phosphatidylserine externalization, DNA fragmentation, G1 phase arrest, and ultrastructural abnormality and apoptotic body formation. Furthermore, Tetracaine at a dose of 0.3125 g/L also induced caspase-3, -9 and -8 activation, MTP disruption, up-regulation of the cytoplasmic amount of Cyt. c and AIF, the expressions of Bax and Bad, and down-regulation of the expressions of Bcl-2 and Bcl-xL.

CONCLUSION

Tetracaine above 0.3125 g/L (1/32 of its clinical applied dosage) has a dose- and time-dependent cytotoxicity to HCEP cells in vitro, with inducing cell apoptosis via a death receptor-mediated mitochondrion-dependent pathway.

Topical anesthesia

Topical anesthetics are being widely used in numerous medical and surgical sub-specialties such as anesthesia, ophthalmology, otorhinolaryngology, dentistry, urology, and aesthetic surgery. They cause superficial loss of pain sensation after direct application. Their delivery and effectiveness can be enhanced by using free bases; by increasing the drug concentration, lowering the melting point; by using physical and chemical permeation enhancers and lipid delivery vesicles. Various topical anesthetic agents available for use are eutectic mixture of local anesthetics, ELA-max, lidocaine, epinephrine, tetracaine, bupivanor, 4% tetracaine, benzocaine, proparacaine, Betacaine-LA, topicaine, lidoderm, S-caine patch™ and local anesthetic peel. While using them, careful attention must be paid to their pharmacology, area and duration of application, age and weight of the patients and possible side-effects.

Intravenous lipid emulsion for local anesthetic toxicity: a review of the literature

INTRODUCTION

The use of intravenous lipid emulsion (IVLE) has been proposed as a new potential treatment for local anesthetic toxicity. Local anesthetics work through reversible binding at sodium channels, and signs and symptoms of toxicity include central nervous system and cardiovascular effects. Cardiovascular collapse is a potential result of local anesthetic toxicity, and is generally resistant to resuscitation efforts with standard measures.

DISCUSSION

Various animal studies have been published investigating the use of IVLE in local anesthetic toxicity. IVLE has been shown to increase the lethal dose of bupivacaine required, resuscitate animals that underwent local anesthetic-induced cardiovascular collapse, and more quickly reduce the amount of local anesthetic in the myocardium (compared to administration of control solution). Four human case reports utilizing IVLE for presumed local anesthetic toxicity, with varying presentations, are reviewed.

CONCLUSIONS

IVLE has shown to be an interesting prospect for local anesthetic toxicity. Human case reports have shown successful resuscitation with use of IVLE, using varying dosing regimens. More studies are needed to determine the optimal dosing regimen, as well as to determine the potential adverse effects of IVLE when used in this setting.