The role of isometric muscle tension in the production of muscle toxicity by phencyclidine and restraint stress.

A restraint cage with a strain gauge attached to a movable roof to record the isometric force exerted by rats during restraint is described. The isometric activity score, an integral of the total upward force exerted during restraint, correlated significantly with plasma creatine phosphokinase (CPK) activity in rats following restraint. Phencyclidine 5 mg/kg administered intraperitoneally to rats 15 min prior to 1/2 hr restraint has been shown to produce muscle damage associated with large increases of plasma CPK activity. Dose-response curves for the effects of phencyclidine on isometric activity score and plasma CPK activity were essentially parallel, and isometric activity scores were linearly related to plasma CPK activity. Rats restrained and painfully stimualted on the tail developed both elevated isometric activity scores and elevations of plasma CPK activity. Slopes of the curves relating isometric activity to plasma CPK activity were identical for painfully stimulated and phencyclidine treated rats. Phencyclidine has been reported to produce large increases in locomotor activity. Thus, increased isometric muscle tension developed during phencyclidine plus restraint is related to the production of muscle damage and increased efflux of CPK in rats.