Relationship Between Inhibition of Acetylcholinesterase and Response of the Rat Phrenic Nerve‐diaphragm Preparation to Indirect Stimulation at Higher Frequencies

Abstract

I Rat isolated diaphragm preparations were stimulated indirectly either intermittently at 20, 50 or 100 Hz or continuously at 0.2 Hz. 2 Addition of 1.8 gM paraoxon (which inhibits acetylcholinesterase by forming a phosphorylated enzyme which undergoes slow spontaneous reactivation) for 5 min to the organ bath produced a failure of the muscle to maintain tetanic tension (tetanic fade, Wedensky inhibition) and potentiated the neuromuscular blocking activity of exogenous acetylcholine. The rates of recovery from both these effects were recorded. 3 In a series of experiments with dyflos (which inhibits acetylcholinesterase by forming a phosphorylated enzyme which does not undergo spontaneous reactivation) the relationship between functional acetylcholinesterase activity and neuromuscular blocking activity of exogenous acetylcholine was also determined. 4 From the data obtained, the relationship between functional acetylcholinesterase activity and tetanic fade was calculated. These calculations show that (i) a considerable reduction in functional acetylcholinesterase activity is required before the diaphragm loses its ability to respond with a sustained tetanus to indirect stimulation at higher frequencies, (ii) the minimum (critical) level of functional acetylcholinesterase activity required for a normal tetanic response is directly related to the frequency of stimulation and (iii) once functional acetylcholinesterase activity has been reduced to the critical level, a very small further reduction leads to a complete tetanic fade. 5 The meaning of functional acetylcholinesterase assays and of conclusions which can be drawn . from them, is discussed,

Mechanisms of the inhibition by neostigmine of tetanic contraction in the mouse diaphragm

“…Inhibition of acetylcholinesterase (AChE) in the normal neuromuscular preparation with anticholinesterase agents, including the neostigmine-like carbamates and organophosphorus compounds, is known to enhance twitch responses to single stimuli, but on the contrary, to cause a rapid fade (Wedensky inhibition) of the tetanic contraction evoked by repetitive stimulation of the nerve (Morrison, 1977;Heffron & Hobbiger, 1979;. Inhibition of butyrylcholinesterase, however, has no effect on neuromuscular transmission (Heffron, 1972).…”

Twitch potentiation by organophosphate anticholinesterases in rat phrenic nerve diaphragm preparations

“…In spite of this, substantial reversal of the twitch potentiation occured within 1 h of removal of paraoxon from the organ bath, which, if a marked decline of twitch potentiation had occurred, involved a transient phase of further potentiation. Thus, as is the case with the tetanic fade produced by the organophosphate anticholinesterases (Berry & Lovatt-Evans, 1951;Barnes & Duff, 1953;Fleisher, Hansa, Kilos & Harrison, 1960;Van der Meer & Wolthuis, 1965;Heffron & Hobbiger, 1979), small changes in the level of enzyme inhibition can produce marked changes in the muscle response to nerve stimulation.…”

Effects of nicotine receptor agonists on acetylcholine release from the isolated motor nerve, small intestine and trachea of rats and guinea-pigs

“…A second population of neuronal nicotine receptors at motoneurones is localized at the last node of Ranvier (preterminal nicotine receptors) [3,9,11,18,19]. Also non-myelinated axons of afferent and efferent nerves are endowed with nicotine receptors (for literature see references 2 and 3).…”