Relative contribution of pre- and post-synaptic effects to the neostigmine-induced recovery of neuromuscular transmission blocked by vecuronium

The relative contribution of the pre- and post-synaptic effects to the neostigmine-induced recovery of neuromuscular transmission blocked by vecuronium was studied. A conjunction of myographical and electrophysiological techniques was employed. The preparation was the sciatic nerve-extensor digitorum longus muscle of the rat, in vitro. The physiological variables recorded were nerve-evoked twitches (generated at 0.1 Hz), tetanic contractions (generated at 50 Hz) and end-plate potentials (epps), generated in trains of 50 Hz. The epps were analyzed in: amplitude of first epp in the train; mean amplitude of the 30th to the 59th epp in the train (epps-plateau); half-decay time of the epp; early tetanic rundown of epps in the train; plateau tetanic rundown of epps in the train; quantal content of the epps and quantal size. In myographical experiments, a concentration of vecuronium was found (0.8 mu m) that affected both twitches and tetanic contractions and a concentration of neostigmine was found (0.048 mu m) that completely restored the twitch affected by vecuronium. The cellular effects of vecuronium and neostigmine, studied alone or in association, in the above-mentioned concentrations, were scrutinized by means of electrophysiological techniques. These showed that vecuronium alone decreased the peak amplitude, the quantal content of epps and the quantal size and reinforced the tetanic rundown of epps. Neostigmine alone increased the peak amplitude, the quantal content and the half-decay time of the epps. When employed in the presence of vecuronium, neostigmine increased both the quantal content of the epps (via a presynaptic effect) and the half-decay time of the epps (via a postsynaptic effect). Seeing the pre- and the post-synaptic effects of neostigmine were of similar magnitude, they permit to conclude that both these effects contributed significantly to the restoration by neostigmine of the neuromuscular transmission blocked by vecuronium.