Pharmacological actions of some cyclic analogues of choline

Two cyclic choline analogues (3-hydroxy-N,N- dimethylpiperidinium and 2-hydroxymethyl-N,N- dimethylpiperidinium ) and two cyclic homocholine analogues (4-hydroxy-N,N- dimethylpiperidinium and 3-hydroxymethyl-N,N- dimethylpiperidinium ) have been studied with regard to their actions at the cholinergic synapse. All the analogues had some direct depolarizing activity on the frog rectus abdominis muscle but they were less potent in this respect than acetylcholine. Compared to physostigmine, the analogues were weak inhibitors of cholinesterase enzymes. All the analogues were found to have a presynaptic blocking action on the rat phrenic nerve-hemidiaphragm preparation, which was reversed by choline. In addition, they all inhibited the high affinity transport of choline into synaptosomes but only the cyclic choline analogues were found to be acetylated by soluble choline acetyltransferase in vitro. We conclude that the hydroxypiperidinium analogues caused the presynaptic block seen at the neuromuscular junction by inhibiting acetylcholine synthesis.

Some pharmacological actions of acetylsecohemicholinium

The effects of the acetylated derivative of HC-3 (acetylsecohemicholinium; AcHC-3) have been studied at cholinergic nerve terminals and compared with the effects of the parent compound. AcHC-3 blocked neuromuscular transmission in nerve-muscle preparations; it was shown to be less effective than HC-3 in producing a pre-junctional block in the rat diaphragm but was more effective than HC-3 in eliciting a post-junctional blocking effect in the chick biventer muscle. On the frog rectus abdominis muscle AcHC-3 caused a substantial potentiation of the contractures elicited by acetylcholine but did not by itself cause a contracture of the muscle. AcHC-3 inhibited the synthesis of acetylcholine by cholinergic nerve ending particles and inhibited the uptake of [14C]choline into brain synaptosomal fractions to a similar extent to HC-3. AcHC-3 was shown to be a substrate for cholinesterase enzymes although the rate of hydrolysis was much less than the rate of hydrolysis of acetylcholine. It is concluded that AcHC-3 is effective in inhibiting cholinergic transmission and this action is exerted by the open chain (seco) compound and is not due to the hydrolysis of the AcHC-3 by cholinesterases to form the active HC-3 molecule.

Some pharmacological properties of the false cholinergic transmitter acetylpyrrolidinecholine and its precursor pyrrolidinecholine

The acetylchline analogue acetylpyrrolidinecholine as well as the choline analogue pyrrolidinecholine were synthesized and the cholinergic properties of both substances were investigated on the guinea-pig ileum, rat blood pressure and frog rectus abdominis muscle. Acetylpyrrolidinecholine was 3-5 times less potent than acetylcholine on the three preparations tested. The dose-response curves to acetylpyrrolidinecholine were shifted to the right in a parallel manner by atropine and (+)-tubocurarine. The dissociation constants for atropine and (+)-tubocurarine obtained with acetylpyrrolidinecholine as agonist were not different from those obtained with acetylcholine. This indicates that acetylpyrrolidinecholine specifically stimulates muscarine and nicotine receptors. Eserine potentiated the effects of acetylcholine more than those of acetylpyrrolidinecholine. Pyrrolidinecholine was only a weak agonist on the guinea-pig ileum. It caused a rise of rat blood pressure in doses higher than 10 mumol per rat. Neuromuscular transmission of the phrenic nerve-diaphragm preparation of the rat was not impaired during a 150 min incubation period with 1 mM pyrrolidinecholine. It is suggested that the possible formation and release of acetylpyrrolidinecholine as a false cholinergic transmitter does not modify neuromuscular transmission in skeletal muscle.

Methylmercury and the skeletal muscle receptor

Methylmercury at bath concentration of 2 X 10(-5) M was capable of inhibiting muscular contractions of the isolated rat phrenic-nerve hemidiaphragm preparation. At the height of inhibition, nerve action potential could still be recorded and the muscles continued to respond to direct stimulation. The inhibition was not reversible with L-cysteine or D-penicillamine but limited protection was possible by prior treatment with (+)-tubocurarine. Treatment of frog rectus muscles with methylmercury (0-2 mM for 15 min) resulted in a shift to the right of 1 log unit in the dose response curve to acetylcholine and a reduction in the maximum response of the tissue. The observed inhibitory action of methylmercury on neuromuscular transmission may be explained by an action on the disulphide bond believed to be present on a cholinergic receptor.

The pharmacological actions of some polymethylene-bis-(hydroxyethyl)-dimethyl-ammonium compounds on cholinergic transmission

Some polymethylene-bis-(hydroxyethyl)-dimethyl-ammonium (dicholine) compounds containing from 7 to 10 methylene groups between the quaternary nitrogen atoms, have been studied for activity at various sites of cholinergic transmission. The post-junctional activity of the compounds was investigated on the frog rectus abdominis muscle. Small doses of C8-, C9-, and C10-dicholine potentiated ACh contractions, however larger doses of all the dicholine compounds blocked the nicotinic receptor sites of the frog rectus muscle. The dicholine compounds blocked neuromuscular transmission in the rat phrenic nerve diaphragm. These actions at the rat neuromuscular junction are compared with the purely post-junctional actions on the frog rectus muscle. Although the compounds exert anticholinesterase activity in vitro it is suggested that this effect plays little part in the action of the drugs at the neuromuscular junction. It is concluded that the dicholine compounds have both pre- and post-junctional activity at the neuromuscular junction. The dicholine compounds are acetylated at varying rates by partially purified choline acetyltransferase (ChAc) although all are acetylated less readily than choline. The rate of acetylation of the dicholine compounds by ChAc parallels their activity in blocking neuromuscular transmission and it is suggested that other quaternary ammonium compounds containing hydroxyl or hydroxyethyl groupings may be acetylated by ChAc and this may affect their blocking action at the neuromuscular junction.

Renal tubular excretion of triethylcholine (TEC) in the chicken: enhancement and inhibition of renal excretion of choline and acetylcholine by TEC

1. [3H]-triethylcholine (TEC) was actively transported by the renal tubule of the chicken at a rate 85% that of simultaneously administered p-aminohippuric acid (PAH). 2. TEC was demonstrated to be transported by the organic cation transport system in the kidney through inhibition with quinine and the bio-cation choline. 3. When the infusion of TEC was increased to 2 times 10(-6) mol kg(-1) min(-1) reaching the infused kidney, the transport of [3H]-TEC was inhibited, suggesting that an excretory transport maximum for TEC in the renal tubules had been reached. 4. The excretion of both choline and acetylcholine was enhanced by TEC loads as low as 1 times 10(-18) mol kg(-1) min(-1). Enhancement continued as TEC infusion was increased up to approximately 1 times 10(-7) mol kg(-1) min(-1) at which point this enhancement was converted to inhibition. 5. Possible mechanisms for the biphasic effect of TEC on organic cation transport are discussed.

The ganglion blocking and vagolytic actions of three short-acting neuromuscular blocking drugs in the cat

Three recently-introduced short-acting neuromuscular blocking drugs with non-depolarizing mechanisms of action, stercuronium, dacuronium and AH8165 (1,1′-azobis [3-methyl-2-phenyl-1H-imidazo (1, 2a) pyridinium] dibromide) have been tested in the anaesthetized cat on the responses of the nictitating membrane to stimulation of the cervical nerve, and of the heart to vagal stimulation. The effects of the three drugs at the superior cervical ganglion and at the cardiac neuroeffector junction have been compared with their neuromuscular blocking effects. At doses lower than those required to block neuromuscular transmission all three compounds possessed a selective atropine-like action at the cardiac vagus neuroeffector junction in that they inhibited the bradycardia produced by vagal stimulation and by acetyl-β-methylcholine, whilst the depressor action of acetyl-β-methylcholine was unaffected. The ratios of the doses of the drugs to block the responses of the preganglionically-stimulated nictitating membrane and of the tibialis anterior muscle were 16ṁ7 for stercuronium, 8ṁ5 for dacuronium and 3ṁ8 for AH8165. The greater ganglion-blocking activity of AH8165 was reflected in the depressor action of the compound, whereas the weak ganglion-blocking actions of stercuronium and dacuronium were insufficient to mask the tachycardia and pressor effect caused by their blocking action on the cardiac vagus neuroeffector junction.

Neuromuscular blocking activity of a new series of quaternary N-substituted choline esters

1. The neuromuscular blocking activity of a new group of mono- and diquaternary N-substituted choline esters of several carboxylic acids has been evaluated in various whole animal preparations in the mouse, rat and cat.2. A basic non-depolarizing mechanism was present in all but one compound-a depolarizing agent, which was studied for comparison. There was evidence of facilitatory activity and nicotinic stimulation in the monoquaternary compounds. These effects were diminished in the diquaternary compounds.3. The presence of a bulky aromatic ring system on the nitrogen atom appeared to increase both neuromuscular blocking potency and facilitatory activity in the experimental animal. A similar relationship had previously been demonstrated in vitro in another study.4. The duration of action, although short in most compounds, did not correlate well with previously determined in vitro hydrolysis rates, possibly because of species differences.5. The general pharmacology of each compound appeared to depend considerably upon the structure of the choline moiety.

Effects of some polymethylene bis(hydroxyethyl) dimethylammonium compounds on acetylcholine synthesis

1. The effects of some polymethylene bis(hydroxyethyl)dimethylammonium compounds have been studied on acetylcholine (ACh) synthesis and as substrates for choline acetyltransferase (ChAc).2. The decamethylene analogue (C(10)-dichol) inhibited ACh synthesis by mitochondrial (P(2)) fractions of guinea-pig cerebral cortex suspended in Tris buffer but had no effect on ACh synthesis by P(2) fractions when the membranes surrounding the ChAc enzyme were broken down by homogenization in Triton X-100.3. C(10)-Dichol was acetylated by ChAc almost to the same extent as choline. The initial rate of acetylation, at a concentration of 10(-3)M, was more rapid than for choline; however, the apparent Michaelis-Menten constant for C(10)-dichol was greater than the K(m) for choline, showing a loewr affinity for the ChAc enzyme.4. All the dicholine compounds were acetylated to some extent by ChAc and the rate of acetylation increased with an increase in the length of the methylene chain between the two quaternary nitrogen atoms in each dicholine molecule.5. The rate of acetylation of the dicholine compounds paralleled the activity of these analogues at the prejunctional site at the neuromuscular junction. The possibility is suggested that part of the pharmacological activity of these compounds may be due to their incorporation into cholinergic nerve endings followed by acetylation by ChAc before subsequent release as a false transmitter.

A comparison between the blocking actions of 2-(4-phenylpiperidino) cyclohexanol (AH 5183) and its N-methyl quaternary analogue (AH 5954)

1. The neuromuscular blocking activities of AH 5183 (2-(4-phenylpiperidino) cyclohexanol) and its N-methyl quaternary analogue (AH 5954) were compared in rapidly stimulated nerve-skeletal muscle preparations of the rat, chicken and cat.2. The evidence indicated that in isolated preparations the neuromuscular block produced by both AH 5183 and AH 5954 was primarily pre-junctional in origin. That produced by AH 5954 was readily reversible either by washing the tissue or by reducing the stimulation frequency, whereas that produced by AH 5183 was difficult to reverse in these ways.3. Low doses of AH 5954 sensitized the rat hemidiaphragm preparation to the neuromuscular blocking action of choline. The neuromuscular block produced by choline was reversible by tetraethylammonium but not by neostigmine. This suggested that the blocking action of choline is at least partly pre-junctional in nature.4. In anaesthetized cats AH 5954 possessed a biphasic neuromuscular blocking action. The initial phase was rapid in onset, suggestive of a post-junctional action, whereas the second phase was prolonged and reversible by choline, suggestive of a prejunctional inhibitory action on the choline transport mechanism. AH 5183 produced no initial blocking action and was irreversible by choline.5. Both AH 5183 and AH 5954 possessed local anaesthetic and alpha-adrenoceptor blocking actions. These actions and the neuromuscular blocking action were affected to different degrees by quaternization, suggesting that the three main actions of the two drugs are independent.6. It was concluded that AH 5954 and AH 5183 act at different pre-junctional sites at the neuromuscular junction, AH 5954 acting extraneuronally by inhibiting choline transport and AH 5183 intraneuronally at the level of the synaptic vesicle membrane.

Studies on the blocking action of 2-(4-phenyl piperidino) cyclohexanol (AH5183)

1. AH5183 (2-(4-phenyl piperidino) cyclohexanol) produced neuromuscular block of slow onset in rapidly stimulated nerve-skeletal muscle preparations of the rat, chicken and cat.2. The neuromuscular block was not antagonized by neostigmine, tetraethylammonium (TEA) or choline. The rate of onset of transmission failure was enhanced by factors which increase the release of acetylcholine.3. It was concluded that the neuromuscular blocking activity was primarily pre-junctional in origin, being due either to a non-competitive action on the choline transport mechanism, or to an intracellular action on acetylcholine metabolism.4. In high doses AH5183 possessed local anaesthetic activity, but this was considered insufficient to bring about the failure of neuromuscular transmission.5. AH5183 also produced a block of sympathetically innervated preparations that was indistinguishable from that produced by an alpha-adrenoceptor blocking drug.

Effects of the NN'-triethyl analogue of suxamethonium on neuromuscular transmission

The effects of succinyltriethylcholine on neuromuscular transmission in the cat and chick, and on the output of acetylcholine from the isolated rat hemidiaphragm, are described. Succinyltriethylcholine exhibits an initial post-junctional non-depolarizing blocking action. A secondary prejunctional inhibitory action on acetylcholine output, is due to succinyltriethylcholine rather than to the hydrolysis products. The compound also exhibits a facilitatory action which may be due to anticholinesterase activity.

The neuromuscular blocking action of a series of bicyclic bis-oniu esters

1. Thirteen bicyclic dicholine esters have been tested on mammalian and avian skeletal muscle preparations.2. One of the compounds exhibited depolarizing activity in all the preparations.3. Three of the compounds exhibited depolarizing or dual-blocking activity in avian and denervated mammalian preparations, but exhibited non-depolarizing blocking activity in innervated mammalian preparations.4. The remaining compounds exhibited non-depolarizing blocking activity with evidence of an additional facilitatory action.5. The activity exhibited was dependent upon the onium substituents and the structure of the bicyclic ring.6. All the compounds exhibited a choline-reversible block in the rapidly stimulated rat diaphragm preparation and six of them exhibited a secondary choline-reversible block of the rapidly stimulated cat tibialis anterior muscle.