The importance of stereoisomerism in muscarinic activity

The muscarinic activities and rates of hydrolysis by acetylcholinesterase of the optical isomers of the acetyl-α- and acetyl-β-methylcholines of known configuration have been determined. The results have been correlated and the possible characteristics of muscarinic receptors outlined.

Ganglionic stimulant action of muscarine

Ganglionic stimulant action of muscarine has been demonstrated on the inferior mesenteric ganglion of the cat. The stimulant effect of muscarine on this ganglion is slow in onset and highly sensitive to atropine, thus differing from "nicotinic" ganglion stimulants such as 1,1-dimethyl-4-phenylpiperazinium iodide. The racemic form of muscarine is less potent than the natural l(+) muscarine and the d(–) form is inactive. Three stereoisomers of muscarine, inactive on the postganglionic parasympathetic receptor sites, were also found to be inactive on the ganglion. The existence of ganglionic receptors functionally similar to postganglionic parasympathetic receptors is proposed.

Facilitation of neuromuscular transmission by anticholinesterase drugs

A close correlation has been shown to exist between the in vitro anticholinesterase potencies of ambenonium, neostigmine, methoxyambenonium and edrophonium chloride and their abilities to increase muscle contractions produced by close-arterial injections of acetylcholine. No correlation was found between the anticholinesterase potencies of the drugs and their potentiations of the maximal twitch in response to electrical stimulation, or their antagonisms of tubocurarine. It is concluded that some action, in addition to inhibition of cholinesterase, contributed to their facilitation at the neuromuscular junction, and it is suggested that this action may be at the prejunctional site.

A PRELIMINARY INVESTIGATION OF THE PHARMACOLOGY OF THE HUMAN ISOLATED TAENIA COLI PREPARATION

The effects of drugs on smooth muscle strips of human taenia coli, obtained from operation specimens, were studied in vitro. Both nicotinic and muscarinic sites of action of acetylcholine were demonstrated, the nicotinic effect being a relaxation. The sympathomimetic amines, adrenaline, noradrenaline, and isoprenaline produced a relaxation of the tissue by an action on adrenaline alpha- and beta-receptors. The presence of both types of receptor was demonstrated by selective adrenergic blockade with pronethalol or Hydergine. Pronethalol in high concentrations gave a nonselective adrenergic blockade. The ganglion-stimulating agents nicotine and dimethylphenylpiperazinium produced a relaxation of the tissue in all concentrations. This relaxation was inhibited by pronethalol or physostigmine but no contractile component to ganglion stimulation was revealed when these two drugs were present together. These results indicate the presence of either sympathetic ganglia in the intrinsic nerve plexuses, or adrenergic stores in the bowel wall. There is no pharmacological evidence for parasympathetic ganglia in human sigmoid colon. Histamine produced relaxant, contractile or biphasic responses. The type of response was independent of the "tone" of the preparation. The responses were not modified by procaine, hyoscine or pronethalol, which result indicates that both the contractile and relaxant responses to histamine were due to a direct action of the drug on smooth muscle. 5-Hydroxytryptamine produced either a contraction or a relaxation of the tissue. The relaxation was due to a direct effect of the drug, since hexamethonium, procaine or pronethalol did not affect the response. No conclusions have been drawn regarding the mechanism of the contractile response to 5-hydroxytryptamine. The nature of the responses of the tissue to drugs was independent of the disease for which the specimen of colon was removed.

AN ATTEMPT TO STUDY THE EFFECTS OF CHEMICAL STRUCTURE ON THE AFFINITY AND EFFICACY OF COMPOUNDS RELATED TO ACETYLCHOLINE

Two sets of series of compounds, RN(+)Me(3), RN(+)Me(2)Et, RN(+)MeEt(2), RN(+)Et(3), and R'N(+)Me(3), R'N(+)Me(2)Et, R'N(+)MeEt(2), R'N(+)Et(3), have been prepared, in which R is a 2-(diphenylacetoxy)ethyl, 2-(benziloyloxy)ethyl, 2-(2,2-diphenylethoxy)ethyl, 3-(diphenylmethoxy)propyl or 3,3-diphenylbutyrylmethyl group, and R' is a 2-acetoxyethyl, 2-ethoxyethyl, 3-methoxypropyl or butyrylmethyl group: compounds of the first set therefore differ from those of the second set in that they contain a diphenylmethyl group (or a benziloyl group) in place of a methyl group. The former compounds are antagonists of acetylcholine whereas most of the latter act like acetylcholine. The affinity constants of the former compounds for the acetylcholine receptors of the guinea-pig ileum have been determined and the equipotent molar ratios relative to acetylcholine have been measured for the latter compounds. The variation of the affinity constant with the constitution of the onium group in the antagonists (the diphenylmethyl compounds) was sufficiently consistent from one series to another for it to seem likely that corresponding changes in affinity with the constitution of the onium group would occur in the agonists. From the relative activity of the agonists and with this knowledge of relative affinity it was possible to assess the effects of their structure on efficacy. Substitution of one methyl in the onium group by an ethyl group in these compounds increased affinity but decreased efficacy. The replacement of a second methyl by a second ethyl group had little effect on affinity but decreased efficacy still further. The replacement of the ester link in acetylcholine by a 4-ether oxygen atom (as in the diphenylmethoxypropyl and methoxypropyl compounds) did not appreciably reduce affinity but markedly reduced efficacy, whereas the replacement of the ester link by a 3-ether oxygen atom (as in the diphenylethoxyethyl and ethoxyethyl compounds) markedly reduced affinity but did not reduce efficacy. The diphenylbutyrylmethyl compounds had low affinity and the butyrylmethyl compounds had low efficacy. We conclude that the action of acetylcholine at the postganglionic parasympathetic receptors in the guinea-pig ileum depends upon the presence of the 4-carbonyl group (and presumably the onium group) for affinity and on the 3-ether oxygen atom and the trimethylammonium group for efficacy.