Effects of rocuronium and vecuronium on initial rundown of endplate potentials in the isolated phrenic nerve diaphragm preparation of rats

Abstract

Rocuronium and vecuronium, two non-depolarizing neuromuscular blockers, have been widely used in surgery procedures. However, their electrophysiological properties need to be more widely explored. We examined the effects of rocuronium and vecuronium on initial rundown of endplate potential amplitudes in the non-uniform stretched muscle preparation of the rat isolated phrenic nerve diaphragm. More specifically, the endplate potentials were recorded with one microelectrode from a single endplate. The effects of rocuronium or vecuronium each at 4 concentrations (0.5 ×, l ×, 2 ×, 4 × EC95; EC95 = concentration of the drug required to produce the inhibitory effect by 95%) on the amplitude of endplate potentials and its rundown were observed. Treatment of the isolated rat phrenic nerve-diaphragm preparation with rocuronium (2.5-20 μg/ml) or vecuronium (0.5-4 μg/ml) decreased the amplitude of endplate potentials and inhibited its rundown in a concentration-dependent manner. At the concentration (2.5 μg/ml for rocuronium and 0.5 μg/ml for vecuronium) that did not alter the endplate potential amplitude, the onset of reduced endplate potential rundown was 3 and 5 min after administration of rocuronium or vecuronium, respectively. The results suggest that rocuronium and vecuronium block the neuromuscular junction presynaptically and that rocuronium does it faster than vecuronium.

Keywords: Diaphragm; Motor endplate; Neuromuscular junction; Rocuronium; Vecuronium.

Figure 1

Effects of rocuronium and vecuronium on the rundown rate of trains of endplate potentials (EPPs) in the isolated phrenic nerve diaphragm preparation of rats. (A) Examples of the single normal EPP (a) and trains of EPPs (b) in the isolated phrenic nerve diaphragm preparation. The second wave of the normal EPP was higher than the first wave (a). In the trains of EPPs (b), the amplitude of EPP1 was 32.4 mV; the average amplitude of EPP11-50 was 27.8 mV; the decay rate (R) = [1−(EPP11-50 average quantum content/EPP1 quantum content)] × 100% = 74%. n = 2 (a) or 50 (b). (B) The effect of rocuronium (2.5 μg/ml) on the rundown rate of trains of EPPs at 3 min after drug administration. (a) Normal control; (b) post-treatment of rocuronium. (C) The effect of vecuronium (0.5 μg/ml) on the rundown rate of trains of EPPs at 5 min after drug administration. (a) Normal control; (b) post-treatment of vecuronium.