An inhibitory action of histamine on the guinea‐pig ileum

Abstract

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 E₂, 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 guinea-pigs. 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.