ABSTRACT
BACKGROUND: Succinylcholine causes phase II block by a large dose or a prolonged exposure. There are rare data in a pharmacodynamics of phase II block. The purpose of this study was to investigate a concentration-response relationship, the occurrence and recovery of phase II block, and to clarify whether pretreatment of succinylcholine potentiates muscle relaxation caused by rocuronium. METHODS: Hemidiaphragm-phrenic nerve preparations were dissected from male Sprague-Dawley rats (150-250 g). Preparations were bathed in Krebs' solution, then maintained at 32oC, and saturated with a mixture of 95% O2 and 5% CO2. Isometric forces made with supramaximal stimulations (0.1 Hz, and 50 Hz for 1.9 s) to the phrenic nerve, were measured with a force transducer, before and after each treatment. Succinylcholine, 10, 20, 40, 60 and 80microM were cumulatively added to the bath. Succinylcholine 80microM (for 100 min) or succinylcholine 300microM (for 20 min) was washed out. After succinylcholine 300microM (for 20 min), or 0microM as pretreatment was washed out, rocuronium, 2, 4, 8, 12, 16, 20microM were cumulatively added to the bath. RESULTS: The potencies of single twitch, peak tetanic tension and tetanic fade for succinylcholine were 36.1, 26.0, 20.7microM. Irrespective of dose or exposure duration, the recovery of muscle relaxation caused by succinylcholine was almost complete around 20 min after succinylcholine was washed out. CONCLUSIONS: Tetanic fade occurred at a lower concentration of succinylcholine than single twitch. The recovery from phase II block was relatively rapid if the concentration of succinylcholine is sufficiently low.