PHARMACOLOGICAL PROPERTIES
Pharmacotherapeutic group: Anaesthetics, anaesthetics general, other general anaesthetics ATCvet code: QN01AX93
Pharmacodynamic properties
Tricaine methanesulfonate has properties slightly different from, but similar to, both ester and amide anaesthetics, acting as a general anaesthetic or narcotic. It is more water-soluble than benzocaine, lending it to fish application. The drug causes reduced blood flow through the gills and reduced oxygen consumption. The rate at which narcosis is induced depends upon the concentration of the product in water and also upon the water temperature. At higher temperatures onset or narcosis is more rapid; however the safety margin is less. Immersion of fish in unmedicated water reverses narcotic effects.
Pharmacokinetic particulars
Fish are normally immersed in solutions and both absorption and excretion occur through the gill epithelium. It is soluble in lipids, which probably accounts for its rapid diffusion across gills in both directions, with rapid anaesthesia and rapid recovery. Excretion occurs mainly across the gill epithelium. Non-polar ethyl metaaminobenzoate and its N-acetyl derivative are both excreted across the gills, whereas the polar meta-aminobenzoic acid and its N-acetyl derivative are excreted via the kidneys. All species tested appear to produce an acetylated derivative, to the extent normally of less than 20% of the original anaesthetic. The hydrolysis to produce the free acid also varies with species, so the kidney excretion varies with species. However, the effectiveness varies less between species owing to the free movement of the drug across the gills.
The concentration in salmonid muscle, whilst the fish is under anaesthetic, ranges from 9.4 to 72.0 mg/kg. The half life of the anaesthetic in muscle on withdrawal is approximately 70 minutes. Thus 24 hours gives 20 half lives. The highest concentrations found in salmonid muscle after 24 hours have been 2.6 to 3.2 mg/kg (the oral LD in a 30kg dog is 30,000 x 4mg of the anaesthetic).