Contribution of direct actions of the oxime HI-6 in reversing soman-induced muscle weakness in the rat diaphragm

Abstract

The actions of the bispyridinium oxime HI-6 ([[[(4-aminocarbonyl)pyridino]-methoxy]methyl]-2-[(hydroxyimino)methyl]-pyridinium dichloride) were investigated in vitro on rat phrenic nerve-hemidiaphragm preparations. Isometric twitch and tetanic tensions were elicited at 37°C with supramaximal nerve stimulation at frequencies of 20 and 50 z. To approximate normal respiration patterns, trials consisting of 30 successive 0.55 s trains were alternated with 1.25 s rest periods. Under control conditions, the above stimulation pattern generated tensions that were well maintained at both frequencies. In contrast, a marked depression of muscle tension was observed in diphragms removed from rats administered 339 μg/kg soman (3 LD 50) and tested in vitro. Addition of HI-6, 4 h after soman exposure, led to a nearly complete recovery of muscle tension at 20 Hz. At 50 Hz, muscle tensions still declined especially when trains were elicited at 1.25 and 3 s intervals. The recovery by HI-6 observed in this study appears to be mediated by mechanisms unrelated to acetylcholinesterase reactivation since no increase of enzymatic activity was detected and the effect was reversed by a brief washout in oxime-free physiological solution. The results suggest that the direct action of HI-6 may play a role in restoring soman-induced diaphragmatic failure but this effect woul be significant primarily under low use conditions.