Thyrotropin-releasing hormone effects on guinea pig antrum.

Thyrotropin-releasing hormone transiently increased resting tension and spontaneous contraction amplitude more potently in longitudinal muscle than in circular muscle. The frequency of the contraction did not change. Excitation was enhanced by eserine, and blocked by atropine and tetrodotoxin; this excitation was, therefore, probably mediated by acetylcholine release from cholinergic neurons in the myenteric plexus. In the presence of atropine, thyrotropin-releasing hormone (10(-10) M) inhibited spontaneous contraction, particularly in longitudinal muscle. This was accompanied by decreased resting tension at and above 10(-8) M. Because contraction was not blocked by adrenergic blockade, indomethacin, methysergide, or hexamethonium, but was abolished by tetrodotoxin, it was postulated that thyrotropin-releasing hormone might stimulate nonadrenergic, noncholinergic inhibitory neurons. There was no cross-tachyphylaxis between 5-hydroxytryptamine and thyrotropin-releasing hormone. Results show that 5-hydroxytryptamine does not interact with thyrotropin-releasing hormone.