Tugging force: A new objective index for evaluating acute changes in neuromuscular function in mice.

INTRODUCTION: Many agents, including muscle relaxants, influence neuromuscular function. Although in vitro and in situ measurement systems of skeletal muscular contractility have been developed and in use for years, no convenient device and parameter are available to examine the time course of the acute effects of such agents on neuromuscular function in conscious mice.

METHODS: We created a two-compartment device consisting of a transparent, wide, acrylic chamber with a foot shock grid, and an attached, opaque narrow tunnel to measure neuromuscular performance. A mouse placed into the wide chamber, quickly enters the narrow tunnel. We attached a string to the tail of the mouse to measure the developed tugging force when the mouse entered the narrow tunnel.

RESULTS: After administering the muscle relaxants suxamethonium, vecuronium, and pancuronium, the peak tugging force decreased maximally at 3 or 5 min after administration, and recovered within 20 min. These responses to the muscle relaxants were dose-dependent.

DISCUSSION: Tugging force is an objective and reproducible parameter for examining the time course of an acute change in neuromuscular function.