The action of drugs on the circular muscle strip from the guinea-pig isolated ileum

The circular muscle strip is a new preparation for examining the action of drugs on the circular muscle of the guinea-pig isolated intestine. The preparation differed from the longitudinal muscle in that it was insensitive to drugs which act on autonomic effector tissues but, after inhibition of cholinesterase, it responded readily to choline esters, 5-hydroxytryptamine, histamine and nicotine. This behaviour necessitated the treatment of each strip with the anticholinesterase NN-diisopropylphosphodiamidic fluoride (mipafox) before each experiment. The contractions of the strip by 5-hydroxytryptamine, histamine and nicotine were abolished by procaine, botulinum toxin (Type A), morphine and hemicholinium, whilst the actions of acetylcholine and methacholine were unaffected. Contractions of the strip in response to each of the drugs were abolished by atropine and hyoscine. The action of nicotine was specifically antagonized by hexamethonium, that of 5-hydroxytryptamine by desensitization of the tissue to 5-hydroxytryptamine, and that of histamine either by desensitization of the tissue to histamine or by mepyramine. It is postulated that 5-hydroxytryptamine, histamine and nicotine stimulate specific receptor sites within the intramural nerve plexuses of the guinea-pig isolated ileum. Finally, botulinum toxin (Type A), morphine or hemicholinum, acting on the neuronal elements of the intramural plexuses, depressed the contractions of the circular muscle strip due to histamine or nicotine more readily than those due to 5-hydroxytryptamine.

The action of substances which block sympathetic postganglionic nervous transmission

The substances which block sympathetic postganglionic transmission, xylocholine, bretylium and guanethidine, also block neuromuscular and sympathetic ganglionic transmission. To see if these last properties were related to the sympathetic blocking property, phenyltrimethylammonium, which blocks the neuromuscular junction (Riker, 1953), was used. It blocked the inhibition of the rabbit ileum produced by stimulating the periarterial nerves in the mesentery, though with higher concentrations the effect of stimulation was initially increased. The action was not modified by the presence of hyoscine. The blocking action was exerted on the response to stimulation of the highest frequency first, and on the response to stimulation of the lowest frequency last. This relation of block to stimulus frequency is similar to that at the neuromuscular junction when tubocurarine is used. Nine compounds have now been shown to block responses to sympathetic postganglionic stimulation, and seven of these are onium compounds. They are, however, mon-onium compounds, and not bis-onium compounds like hexamethonium and decamethonium, so that they can probably enter the postganglionic fibre, which bis-onium compounds (having a charged group at each end of the molecule) may not be able to do. Since these mon-onium compounds have some blocking action at neuromuscular junctions and at sympathetic ganglia, their block of postganglionic transmission may be essentially similar to that by hexamethonium at ganglia and to that by decamethonium at neuromuscular junctions. It is known that acetylcholine releases noradrenaline from sympathetic postganglionic terminations, and xylocholine and bretylium block this release in the vessels of the rabbit ear and in the rabbit isolated atria.