The influence of the age of the rat on the neuromuscular response to mivacurium in vitro.

The effective concentrations of mivacurium induced neuromuscular block were studied in the phrenic nerve-hemidiaphragm preparations of forty male Sprague-Dawley rats in four groups according to body weight, 10 g, 25 g, 100 g, and 200 g. Mivacurium was added to the Krebs' solution to obtain an initial concentration of 0.275 microgram.ml-1 (2.5 x 10(-7) mole.litre-1). Thereafter, the concentration of mivacurium was increased in increments to obtain more than 90% neuromuscular twitch inhibition at 0.1 Hz. Data were analysed by probit and logistic models. The two statistical techniques yielded very similar results, almost identical values of EC50 and EC95. There were significant decreases in the effective concentrations of mivacurium needed to depress the twitch response in neonatal rats (10 g) compared to older rats. No significant differences were observed between 25, 100 and 200 g rats. We conclude that the neonatal neuromuscular junctions of rats show increased sensitivity to mivacurium related to pharmacodynamic factors.