Formaldehyde, 0.5-4.5 mM, increased the threshold for electrical excitation of the nerve, and led to a partial and reversible inhibition of the compound action potential (cAP). The depression was not enhanced by high frequency stimulation. At 8.9 mM or higher, the depression of the nerve excitability could not be reversed. The inhibition of the nerve developed more slowly than that of the muscle, and the nerve was unaffected after 10 min exposure to 2.2 mM. Formaldehyde, 2.2 mM, caused an immediate depression of the indirectly (through the nerve) and directory (at the muscle) elicited twitch tension. After 10 min the tensions were reduced to respectively, 56% and 49% of control. However, the electromyogram was not changed, indicating that the effect was localized to the excitation-contraction coupling. Tetanic tension (100 Hz in 5 sec) was inhibited more than twitch tension during indirect stimulation, whereas the opposite was found during direct stimulation of the muscle. Thus, during high frequency stimulation, formaldehyde must have an additional effect on the neuromuscular transmission. This effect was localized presynaptically since a fall out of endplate potentials was observed in the formaldehyde-treated diaphragm. In 6.7 mM or higher concentrations the directly or indirectly induced contractions were irreversibly blocked. The resting membrane potential of the muscle cells was unchanged after exposure to formaldehyde. Formaldehyde caused myotonia-like contractions of the diaphragm, occasionally after exposure to low concentrations (2.2 mM), and always after exposure to higher concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)
Action Potentials Animals