Neuromuscular Blocking Properties of beta-Bungarotoxin, Hexamethonium and Verapamil in the Rat Phrenic Nerve-Hemidiaphragm Preparation

Background: β-Bungarotoxin irreversibly changes the presynaptic membrane, hexamethonium acts on the presynaptic nicotinic receptor, and verapamil blocks the ion channels on the presynaptic membrane. The effect of these drugs on twitch height and train of four (TOF) ratio were investigated, as well as the reversal effects of neostigmine, pyridostigmine or 4-aminopyridine (4-AP) on the partial neuromuscular blockade induced by these drugs. Methods: Square wave, 0.1 Hz supramaximal stimuli or 2 Hz, 0.2 ms train of four stimuli, was applied to the phrenic nerve-hemidiaphragm preparation of the rat, and the twitch height response was recorded mechanomyographically. The cumulative concentration effects and TOF ratios at each point of twitch depression after β-bungarotoxin, hexamethonium or verapamil were measured. TOF ratios were observed at 75, 50 and 25% of the control twitch height value during observation of the concentration effect. The EC50 and EC95 of β-bungarotoxin, hexamethonium or verapamil were calculated using an inhibitory sigmoid Emax model. The reversal effect of some doses of neostigmine, pyridostigmine or 4-aminopyridine to the partial neuromuscular block produced by EC50 of βbungarotoxin, hexamethonium or verapamil was determined. Results: The EC50 and EC95 of β-bungarotoxin, hexamethonium and verapamil were 0.0695 and 0.1160μg/ml, 1267.0 and 2033.5μg/ml and 29.45 and 37.99μg/ml respectively. TOF fade was marked with hexamethonium or verapamil but small with β-bungarotoxin. Neostigmine or pyridostigmine did not reverse the partial neuromuscular block induced by β-bungarotoxin, hexamethonium or verapamil. However, 4-AP produced a dose-dependent recovery of the twitch response (P < 0.05). Conclusions: β-Bungarotoxin, hexamethonium and verapamil produced different degree of TOF fade, and this may be due to different sites of action of these drugs. 4-AP reversed effectively the partial