ABSTRACT
Background: This study was designed to determine whether presynaptic receptor blockade could be differentiated from postsynaptic receptor blockade by examining the effect of increasing frequencies of indirect stimulation on partial twitch depression in vitro rat phrenic nerve hemidiaphragm preparations. Methods: After isolating rat phrenic nerve hemidiaphragm preparation, T200/T1 ratio (twitch height of the 200th stimuli divided by that of the 1st stimuli) at frequencies of 0.2, 0.5, 1.0, and 2.0 Hz using a drug concentration which provided approximately 20% twitch depression at 0.1 Hz was calculated. To compare T200/T1 ratios with TOF ratios, 2.0 Hz TOF response was measured immediately after 200th stimuli at either frequency of stimulation. Results: Hexamethonium caused a marked decrease in T200/T1 ratio at 0.5~2.0 Hz of stimulation, whereas alpha-bungarotoxin caused no change in T200/T1 ratios at up to 2.0 Hz of stimulation. The T200/T1 ratios produced by d-tubocurarine, vecuronium, mivacurium, and rocuronium located intermediate between alpha-bungarotoxin and hexamethonium, however significant differences among four drugs were found at 2.0 Hz. The propensity for decrease in T200/T1 ratios at 2.0 Hz might differ from this study: hexamethonium >d-tubocurarine >rocuronium >mivacurium = vecuronium >alpha-bungarotoxin. T200/T1 ratios at 2.0 Hz were not different from TOF ratios. Conclusions: When the observed effects in this study were provided with result of alpha-bungarotoxin acting predominantly at postsynaptic receptors and hexamethonium acting predominantly at presynaptic receptors, the effects of nondepolarizing muscle relaxants at each binding site could be differentiated by examining the T200/T1 ratios at 2.0 Hz.