Effects of thyrotropin-releasing hormone (TRH) on the isolated small intestine and taenia coli of the guinea pig

Abstract

Thyrotropin-releasing hormone (TRH) produced a contraction in the isolated segment of duodenum and taenia coli of the guinea-pig (pA2, 8.0 and 8.9). TRH induced a contraction, a relaxation, or a contraction followed by relaxation in the jejunum and ileum. All the responses to TRH of the small intestine and the taenia coli were abolished in the presence of tetrodotoxin but not affected by hexamethonium. The contractile response to TRH of the small intestine was abolished and replaced by a relaxation in the presence of hyoscine. This relaxant response was not affected by guanethidine. The taenial response to TRH was partially inhibited by either hyoscine or methysergide and markedly diminished by the two together. These findings indicate that TRH acts on the myenteric neurons of the small intestine and taenia coli of the guinea pig. The contractile response of small intestine is likely to be induced through cholinergic nerves while cholinergic, serotonergic and unidentified excitatory neurons seem to be involved in the taenial response. These neurogenic actions of TRH on the guinea-pig intestine are in contrast with the myogenic natur of the response to TRH in the duodenum of the rat.