An analysis of the direct and indirect actions of drugs on the isolated guinea-pig ileum

Abstract

The responses of the isolated guinea-pig ileum to coaxial stimulation of its nerves, to histamine, acetylcholine, bradykinin, nicotine, tetramethylammonium, 1,1-dimethyl-4-phenyl-piperazinium iodide and 5-hydroxytryptamine were studied, before and during anoxia, cooling, or exposure to hyoscine, phenoxybenzamine hydrochloride, morphine or hexamethonium. Dose ratios were used to determine the amount of block induced by these procedures. With the response to coaxial nerve stimulation as an indication of the excitability of the nervous tissue, it was found that anoxia or cooling abolished the response to single shocks. Under these conditions the response of the ileum to histamine, acetylcholine and bradykinin was hardly affected, indicating a direct action of these substances on the muscle fibres. The effects of nicotine, tetramethylammonium, dimethylphenylpiperazinium and 5-hydroxytryptamine were reduced to various degrees, and we have concluded that their main actions are indirect, through stimulation of cholinergic nerve fibres. When these indirect actions were prevented, increasing the dose revealed a direct action, a larger increase in dose being required for 5-hydroxytryptamine and dimethylphenylpiperazinium than for tetramethylammonium and nicotine. Exposure of the ileum to hyoscine and phenoxybenzamine showed that these direct actions of nicotine and tetramethylammonium were not only on acetylcholine receptors but also on receptors insensitive to hyoscine but sensitive to phenoxybenzamine. The main action of 5-hydroxytryptamine was on nervous elements, yet treatment of the ileum with phenoxybenzamine gave a higher dose ratio for 5-hydroxytryptamine than did treatment with morphine. The meaning of this result is discussed in relation to the general belief that receptors sensitive to morphine are in nervous tissue and receptors sensitive to phenoxybenzamine are in smooth muscle. We have concluded that morphine is only a partial antagonist of 5-hydroxytryptamine receptors in nervous tissue and that phenoxybenzamine antagonizes more 5-hydroxytryptamine receptors than those in smooth muscle.