An inhibitory action of histamine on the guinea-pig ileum.

1. In atropinized, plexus-containing preparations of the longitudinal muscle from the guinea-pig ileum, in which histamine contractions were abolished by mepyramine or diphenhydramine, an inhibitory action of histamine was revealed on the "tetanic spasms" produced by field stimulation.2. The inhibitory action of histamine on the atropine-resistant tetanic spasms, which are due to the excitation of non-cholinergic neurones in Auerbach's plexus (Ambache & Freeman, 1968a, b), was reversible. It is specific for the tetanic spasms, because histamine did not reduce contractions elicited by bradykinin, 5-hydroxytryptamine, prostaglandin E(2), nicotine or dimethylphenylpiperazinium.3. L-Histidinol and 2-mercaptohistamine exerted a considerably weaker inhibitory effect upon the tetanic spasms than histamine. Four other imidazoles tested, L-histidine, murexine, dihydromurexine and imidazolecarboxylcholine, were ineffective; so was the pyrazole ring isomer of histamine, betazole.4. The inhibitory action of histamine persisted after adrenoceptor blockade by phentolamine and pronethalol and after prior reserpinization of the guineapigs.5. The inhibitory action of histamine was also obtained after ganglionic paralysis by hexamethonium or dimethylphenylpiperazinium but was antagonized specifically by nicotine.6. On atropinized preparations of the longitudinal muscle from the guinea-pig descending colon histamine exerted, at most, an insignificant inhibitory effect on the tetanic spasms.