Pharmacodynamic properties
Benzocaine inhibits the conduction of nerve impulses by blocking the sodium channels. Cationic movements across the membranes are decreased or even abolished. Resting potential remains stable, but action potentials are decreased as a function of the concentration of the active substance around the nerve fibre. As long as the fish remains in the anaesthetic bath (30- 40mg/l), the absorption continues until a lethal level is reached.
In Salmonidae, deep anaesthesia occurs at concentrations of 9 – 14mg benzocaine per kg body weight. The induction time before optimal anaesthesia varies with the rearing conditions, concentration of the anaesthetic bath and the water temperature.
A normal time period to anaesthetise Salmonidae at a temperature of 10 – 15°C and at a concentration of 30 – 40mg/l benzocaine (15 – 20ml Optomease Vet/100 litres water), would be 2 – 5 minutes.
Increasing the concentration of the active substance results in a shorter anaesthetic induction time. At the same concentration of benzocaine, the anaesthetic induction time decreases with an increase in water temperature.
Pharmacokinetic properties
Absorption
Mainly across the gills
Tissue Distribution
After absorption, benzocaine concentrates in the plasma and rapidly reaches the central nervous system. After 2 minutes exposure, the plasma concentration of benzocaine is variable.
Biotransformation
Benzocaine is metabolised by acetylation and de-ethylation.
Elimination
Benzocaine and its acetylated metabolites are rapidly eliminated across the gills. Polar metabolites, such as polar de-ethylated metabolites are excreted at slower rates in the urine. Benzocaine is rapidly eliminated from plasma within the first 20 minutes after dosing. The plasma concentration decreases rapidly within the first 10 minutes. Thereafter, the plasma concentration decreases at a slower rate with a half-life of 89 – 109 minutes.
In a study with C14- traced benzocaine, brachial elimination accounted for 59.2% of the dose during the first 3 hours. The kidney excreted 2.7% of the dose after 3 hours, and 9.0% after 24 hours. 2.0% of the dose was excreted across the gallbladder after 24 hours.