Effect of pyridine aldoximes on response of frog rectus muscle to acetylcholine

The effects of pyridine-2, 3- and 4-aldoxime methiodide (PAM) on the isotonic contracture of the isolated frog rectus abdominis muscle elicited by acetylcholine have been studied. The potentiation to acetylcholine resulting from incubation with PAM isomers is reversible and may be related to the weak anticholinesterase property of these compounds. Concentrations of the isomers higher than those needed for maximal potentiation produce a reversible block of the muscle to acetylcholine. pH studies suggest that cations are responsible for the potentiating and blocking properties of the PAM isomers.

Potentiation of the response of frog rectus muscle to acetylcholine by isopropyl methyl phosphonofluoridate and its modification by pyridine-2-aldoxime methiodide

Pyridine-2-aldoxime methiodide (P2AM) was used to study the relation between the recovery of cholinesterase activity of isolated frog rectus abdominis muscle and the change of isotonic response to acetylcholine after previous treatment with the anticholinesterase, isopropyl methyl phosphonofluoridate (sarin). Addition of P2AM to muscle which had been incubated with sarin produced an 88% decrease in potentiation to acetylcholine. This was accompanied by 71% and 35% recoveries of the cholinesterase activity of the intact and finely ground muscle respectively compared with controls from the contralateral muscle. Following pre-treatment with sarin, a two-hour rinsing with acetylcholine (3 mug./ml.) produced a 61% decrease in potentiation to acetylcholine accompanied by 24% and 4.5% recoveries of cholinesterase activity in intact and in ground muscle respectively. Since control experiments showed absence of uncombined sarin in the muscle after rinsing with acetylcholine solution, the results indicate a greater effectiveness of P2AM and acetylcholine in reactivating superficially situated cholinesterase of the frog rectus abdominis as compared with enzyme within the interior of the muscle.

The stimulatory action of acetylcholine on isolated rabbit atria

Quinidine (2.5 to 5x10(-6) g./ml.) decreased the maximum rate of depolarization of the action potentials (AP) of both isolated rabbit atrial and ventricular fibres recorded with an intracellular microelectrode and finally abolished electrical excitability and spontaneous rhythmicity. Acetylcholine (ACh) 10(-6) restored these properties in the atrium (but not ventricle) without changing the membrane resting potential (MRP). Adrenaline 10(-6) to 10(-4) at the same time was without effect. The maximum rate of depolarization of an AP in response to an extra shock interposed between the 8th and 9th driving stimuli was increased by ACh 10(-6) in atria which had received quinidine to a greater extent than that of the normal AP. It is suggested that ACh restarted atria arrested by quinidine by an action on the sodium "carrying system" (excitatory mechanism) and this was not the result of an increase in MRP. An hypothesis is put forward to explain the stimulatory and inhibitory effects of ACh on rabbit atria.

A depressant effect of acetylcholine on the idio-ventricular pacemaker of the isolated perfused rabbit heart

In Langendorff preparations of the isolated rabbit heart, excision of the atria and severance of the atrio-ventricular bundle resulted in the onset of idio-ventricular rhythm at a rate much below the original sino-atrial rate. The mean decrease in rate in twelve experiments was 61%. Acetylcholine perfusion decreased the idio-ventricular rate still further. Physostigmine augmented this effect while atropine prevented or abolished it. Similar results were obtained with a rat heart. The results indicate that a cholinergic receptor mechanism is present at pacemaker sites in mammalian ventricles. However, when rabbit ventricles, beating under atrio-ventricular nodal rhythm or idio-ventricular rhythm, were cooled to the point of cardiac arrest, acetylcholine failed to cause reappearance of the cardiac beat.

Effects of cross-suture and of injected acetylcholine on the supersensitivity of denervated striated muscle

The sensitivity of the tongue muscles to acetylcholine was examined in repeated experiments on cats anaesthetized with hexobarbitone. The supersensitivity which developed after section of the hypoglossal nerve was found to decrease when the muscles, after cross-suture, became re-innervated from the vagus or the chorda-lingual nerve. The supersensitivity caused by denervation could also be reduced by injecting acetylcholine repeatedly during six hours in acute experiments.

Action of acetylcholine, choline and D-tubocurarine on the membrane of frog sartorius muscle fibers

Resting potentials of frog sartorius muscle fibers were taken with microelectrodes at different distances along the length of muscles before and after adding acetylcholine or choline. The mean membrane potential drop and scatter of the potentials recorded in the relatively nerve-free, and in the more densely innervated parts of the muscle, were similar. The loss of direct excitability to electrical and mechanical stimulation was correlated with the concentration of choline or acetylcholine presented. d-tubocurarine added beforehand protected against the depolarizing effect of acetylcholine and choline everywhere along the length of the muscle. A generalized action of acetylcholine and choline and also of d-tubocurarine all along the muscle fibers was inferred. This generalized action at higher concentrations of acetylcholine and choline is believed to be additional to a more specific end plate action.

Effects of acetylcholine on single fibers of the atrioventricular node

The action of acetylcholine on atrioventricular nodal fibers has been studied with the aid of intracellular microelectrodes. It has been found that during total atrioventricular dissociation no block of conduction occurs at the junction between the A-V node and the bundle of His or within fibers of the atrium. Failure of impulse transmission occurs in fibers in the atrial margin of the A-V node. The application of acetylcholine produces characteristic changes in the action potentials of fibers in the atrial margin of the A-V node: depolarization becomes slow, amplitude falls and notching and slurring appear in the upstrokes. These observations support the concept that transmission of excitation through the A-V node is decremental and that the excitatory process in nodal fibers consists of more than one excitatory event.