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 mu g . 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 i nhibition at 0.1 Hz. Data were analysed by probit and logistic models. The two statistical techniques yielded very similar results, almost i dentical values of EC(50) and EC(95). There were significant decreases in the effective concentrations of mivacurium needed to depress the t witch response in neonatal rats (10 g) compared to older rats. No sign ificant differences were observed between 25, 100 and 200 g rats. We c onclude that the neonatal neuromuscular junctions of rats show increas ed sensitivity to mivacurium related to pharmacodynamic factors.